A pipefitter is a professional with assembly, fabrication, installation, and repair experience in piping systems. All industrial plants involve kilometers of the piping network is the result of thousands of pipefitters’ hard work. So, in the piping industry, pipefitters play a responsible role. As most of the piping systems operate under high pressure, to maintain the system integrity, the work performed by pipefitters must be of high quality. Pipefitters are also known as steamfitters.
What do pipefitters do? Pipefitters Job description
Most of the time the question arises in our mind is that “what do pipefitters do?” in actual practice. Broadly, a pipefitter performs all piping-related construction jobs under the supervision of a construction piping engineer. The major job description of a pipefitter can include (but is not limited to) the following:
Preparing the workplace, materials, and equipment after Inspecting the workplace and clearing the obstructions.
Proper planning of piping system and equipment installations in consultation with the piping engineer.
Modify pipes as per specifications, codes, standards, or design drawing requirements using a variety of tools known as pipefitters tools.
Measuring and marking the pipes for cutting and threading operations to suit the assembly requirement.
Welding pipes with several pipe components to form piping assembly and systems.
Using brackets, clamps, and welding equipment to secure pipes to walls and fixtures.
Collecting and ordering required materials like pipes, brackets, hydraulic cylinders, hangers, etc. for erecting at the site.
Maintaining and Repairing piping systems, supports, and connected equipment.
Testing the functionality of the piping system.
Basic Requirements for being a Pipefitter
To be suitable for a pipefitter job application, the candidate has to be
Education: High school/ Diploma/ITI.
Training: Pipefitters training/Trade school education /apprenticeship.
Experience: Proven working experience as a Pipe Fitter.
Excellent troubleshooting skills.
Ability to plan, prioritize, and maintain strong attention to minute job details.
Good communication and managerial skills.
Sometimes state licensure requirements are also required for specific job requirements.
Pipefitters Job/ Jobs for Pipefitters
Pipefitter’s work opportunities are broad. There is a huge demand for experienced pipefitters all over the world. The industries where a pipefitter can expect a job are:
Process Piping Industry
Refineries
Chemical Plants
Petrochemical Plants
Power Piping industry
Pharmaceutical Plants
Nuclear Industry
Steel industry
Oil and Gas
Offshore industries
Onshore industries
Pipeline Industries, etc
Food Industries
Marine Industries
Overall, wherever pipes are used for fluid transport or processing, pipefitters can get a job during the construction and shut-down maintenance stage of all those industries.
Pipefitters Salary / Salary for Pipefitters
The pipefitters industry is ever-growing. As various industries mentioned above employ pipefitters, they get a handsome package. The salary of pipefitters varies depending on the region. Pipefitters’ salary is completely different in Asia with respect to the USA or UK. An experienced pipefitter can expect $10 to $25 per hour depending on the USA or Europe, but $6 to $15 in Asia.
Pipefitters Tools
Typical Pipefitters tools
In the day to day work, every pipefitter uses various kinds of tools, popularly known as pipefitters tools. All these tools make a pipefitter’s life easy. A few of such commonly used pipefitters tools are listed below for reference:
Pipefitters normally get their huge experience from apprentice on-the-job training programs. The formats of the pipefitter’s training programs vary a little from country to country but most of them cover the basic piping details. Most of this training discusses the following:
Basic Technical mathematics for pipefitters
Inch and Metric systems of measurement
Pipe system types and design
Equipment, Tools, and materials used in Piping
Technical Drawings and Symbols and interpret them for pipe fabrication
Pipefitters Handbook explains all the important aspects of a pipefitter’s work. This book is a ready reference to gain knowledge to work in the pipefitters trade. Even though on-the-job training is the main aspect of a pipefitters experience, this pipefitters handbook is a good companion for a pipefitter. Various authors have shared their lifelong experiences to make the reader more knowledgeable and learn from the author’s viewpoint.
Difference Between Plumber and Pipefitter
The term plumbing and pipefitting are sometimes used interchangeably by many users, but both are not the same even though both work with piping systems. Both Plumber and Pipefitter a profession are different as mentioned below:
Plumber
Pipefitter
A plumber’s work revolves around public utility systems.
Pipefitters handle a wide range of complex piping systems.
Their main product is water, sewer, and wastewater systems
Pipefitter work with pipes handling steam, crude, and various other process fluids at high temperatures and pressure.
They work in buildings; both residential and commercial.
Pipefitters work in industries.
Lower end skills
Higher-end skills like pipe bending, threading, welding, grinding, etc.
Less Hazardous
Highly Hazardous
Less challenging job
Highly challenging and risky jobs.
Salary less
More Salary
Materials are normally copper and PVC
Carbon steel, stainless steel, and other alloys.
Plumber vs Pipefitter
Overview of Lateral Buckling and Upheaval Buckling of Pipelines
Long pipelines are often subjected to lateral and upheaval buckling. Both upheaval and lateral buckling can induce excessive bending stresses in the pipeline that may lead to pipeline failure. In January 2000, the 1.3 million liters of oil spill in Guanabara Bay, Brazil was initiated due to the lateral buckling phenomenon that caused the rupture of the offshore pipeline wall. Such consequences can be prevented, if the pipeline (both onshore and offshore) is designed to take care of the lateral and upheaval buckling phenomenon.
What are Lateral Buckling and Upheaval Buckling?
A pipeline traveling long distances transporting fluids under pressure and temperature is a slender structure. Both, temperature and internal pressure cause pipeline longitudinal expansion. Surface friction acts against this expansion restraining it which generates compression stresses in the pipeline wall that eventuates in buckling. There are two types of Buckling; Lateral Buckling and Upheaval Buckling.
A surface-laid pipeline resting on the sand or seabed buckles laterally in the horizontal plane and this is called lateral buckling. On the other hand, a pipeline in a trench or sometimes buried pipeline undergoes buckling in the vertical plane, known as upheaval buckling. Both lateral buckling and upheaval buckling are highly sensitive in presence of local geometric imperfection.
As you might have gone through the previous article regarding the “Stress Analysis for Surface laid pipelines”, one of the Points of consideration during the analysis is the LATERAL / UPHEAVAL BUCKLING of the pipelines. The problem of upheaval / lateral buckling of the pipelines not only occurs naturally for the offshore (subsea) pipelines but also for the onshore pipelines at the Gathering and Injection lines of Oil and Gas production fields. In this article, we will explore the causes and the optimizations which could be implemented to avoid Upheaval / Lateral Buckling.
Lateral Buckling and Upheaval Buckling
Causes of Lateral and Upheaval Buckling
When a pipeline is under operation at a temperature and pressure higher than ambient, it will try to expand. If the line is not free to expand, the pipe will develop an axial compressive force. If the force exerted by the pipe on the soil exceeds the vertical restraint against uplift movement created by the pipe’s weight, its bending stiffness, and the resistance of the soil cover, the pipe will tend to move upward, and considerable vertical displacement may occur. The pipeline response may then be unacceptable because of excessive plastic yield deformation. Upheaval buckling is hence a failure mode that has to be taken into account in the design of trenched and buried pipelines.
The buckling in the lateral direction is most probable when the pipelines are laid on a flat area or bund area (SABKHA area as it is also known in a desert) for sabkha areas and when it is not provided with sufficient lateral resistance. However, when the pipelines are buried in a normal trench, the soil constraints in the lateral direction are high due to a large mass of soil, and chances of lateral buckling are not present.
Upheaval Buckling Calculation
The calculation for the upheaval buckling is based on the technical paper “About Upheaval and Lateral buckling of embedded pipelines” by Dr. K. Peters (3R International Edition 2006) for the underground pipelines and flowlines.
Based on the calculations, the maximum allowable overbend angle, in 12m of pipe length (Cold bend length considered in Pipelines), along the pipeline route and other calculation parameters are specified in the applicable calculation reports and the drawings. The calculations can also be done with the development of programs in ‘Mathcad,’ or equivalent software based on Dr. K. Peter’s technical paper as mentioned above.
The calculations model is carried out in 2- phases,
Phase 1: Calculation is done based on the topographically surveyed pipeline corridor profile
Phase 2: The verification phase for the design model is based on the actual survey for the top of the pipe in the trench after lowering and before back-filling.
Lateral Buckling Resistance in SABKHA
A lateral force that required resisting lateral buckling can be calculated as per calculations mentioned in “Technical Paper about Upheaval & Lateral Buckling of Embedded Pipelines” while the allowable lateral bend angle can be also calculated in a similar way to the allowable over-bend angle as explained above with the difference being using the lateral soil resistance. However, in most cases, the change in direction will be exceeding any allowable lateral bend angle, and hence the lateral berm reinforcement at the change of direction shall be provided based on the study done by K. TERZAGHI in Theoretical Soil Mechanics.
Extra Soil Cover/Berm Reinforcement Material
In case of upheaval buckling analysis proves that an extra soil cover is required at any location on the pipeline route and also if berm reinforcement is required to resist lateral buckling, gatch material shall be used with the required stabilization and proper slope (1:2) so that it shall not be blown away by wind effect across pipeline design life. Gatch material shall be the same as the approved for pipeline berm stabilization.
Guidelines and Recommendations
Suitable notes shall be included in the alignment sheets and the applicable drawings to take care of the maximum allowable over-bend angle, during construction. The alignment sheets also shall indicate the minimum elastic bend radius.
The buried pipelines shall be laid in such a way that the profile of the pipeline is smooth and without steep direction change. To achieve this cutting and filling or other suitable methods shall be done during pipeline construction.
For a given design parameter, there are two primary options to control the upheaval buckling. The first one is to ensure that the change of angle is within the calculated maximum allowable overbend angle limit for the given soil cover. The second one is to increase the soil resistance to increase the allowable overbend angle. To increase the soil resistance, the most direct method is to increase the soil cover
If the profile is such that the angle exceeds this limit when routing the pipeline, then suitable grading shall be done to keep the change of angle within limits, or alternatively, the allowable angle can be increased by increasing soil cover
Based on upheaval buckling calculations, the maximum allowable over-bend angle per 12 m pipe length shall be defined while pipeline corridor profile grading shall be carried out to keep the change of angle within such allowable over-bend angle limits.
The calculations for normal terrain sections in sandy areas shall be carried out for an effective height of 1 meter. Though the total effective height would be considered 1.9 m with 1 m as the normal depth of cover and 0.9 m soil cover in lieu of a total berm height of 1.0 meters.
In addition to the requirement in item 9.4 above during corridor grading, the construction team shall survey the top of pipe elevations immediately after lowering at intervals of 100 meters and 10 meters intervals at a change of directions for a reasonable distance on both sides. This top of the pipe shall survey shall be carried out for the full entire pipeline in sections before backfilling and shall be completed for the full pipeline length and submitted to the stress engineer for upheaval buckling verification after laying.
Along with this document, the construction shall refer to relevant documents regarding stress analysis and the maximum allowed angle for field bends.
If two or more pipelines are laid in the same trench or are in the same corridor with the same grading, the maximum allowable overbend angle shall be the lowest of all the individual maximum overbend angles.
During the construction, the contractor shall give special attention to rough sections of the route identified in the alignment sheets and shall grade such rough sections suitably in line with the calculation results. This will make sure that during final buckling checks additional cover if required shall be minimum.
Conclusions
As per past experience, upheaval buckling would be expected at locations with less vertical resistance to axial compressive force in the pipeline sections such as straight sections between nearby road crossings, vertical/ horizontal overbend angles higher than allowed buckling angles, interface between sabkha and non- sabkha areas, at both sides of road crossings, etc.
An actuator is a machine component that is used for moving and controlling a system or mechanism. To perform its operation, An actuator needs a control signal and a power source. They are widely used in valves, gates, conveyors, automatic control systems, etc. A valve actuator is a pneumatic, hydraulic, or electrically powered device that supplies force and motion for opening and closing a valve. The actuators can only open and close the valve or enable intermediate positioning. Some valve actuators contain switches or other means of remotely displaying the valve position. They are available in a variety of sizes. Commercial actuators basically perform any of the two functions listed below:
Applying a force or torque for lifting, turning, or forming.
Types of Actuators
Depending on the motion that actuators provide to the valves, two types of actuator mechanisms are available:
Rotary Actuators, and
Linear Actuators
Rotary actuators produce the rotating motion to operate valves like a ball, butterfly, and plug valves. On the other hand, Linear actuators convert hydraulic, pneumatic, or electric energy into linear motion to operate valves like gate valves, globe valves, pinch valves, etc.
Based on the power source the actuators use, four types of actuators are available in industrial applications:
Manual
Pneumatic
Hydraulic and
Electric Actuators
Fig. 1: Types of Valve Actuators
Manual Actuators
Manual Actuators are mechanical devices consisting of hand-operated knobs, levers, or wheels. They are unpowered tools and are used primarily in commercial applications for precise positioning.
Pneumatic actuators
Pneumatic actuators use the energy of compressed air to generate rotary and linear movements to operate valves and dampers. In the broadest sense, an air cylinder is a pneumatic actuator. Most pneumatic actuators use a few standard components which are easy for technicians to maintain
Motion control applications that use rotary pneumatic actuators generally fall in rack and pinion or paddle designs. Double-acting rack and pinion arrangements are often used. Multi-position, three, four, or five-stop actuators are often used for sequential assembly operations. The rotary actuators can also be used for indexing, stepping, picking up, and placing movements.
Pneumatic linear actuators are used in ascending stem valves to directly actuated gate valves, globe, etc. Two types are usually used, the diaphragm and the piston. Membrane styles are popular because their large surface area can generate tremendous force at moderate air pressure. The membrane is a rubber membrane, the edge of which is sealed with the outer housing of the actuator.
The air pressure moves the membrane up or down against the spring pressure, depending on whether the actuator is designed in such a way that it cannot be opened or closed. The stroke lengths are generally shorter than for piston valves, where the strokes depend only on the length of the cylinder and not on the stretch that the diaphragm can tolerate. Piston actuators can be sized to produce an adequate actuation force based on the available air pressure and can be made in double-acting spring return types. Some linear actuators use the familiar air springs instead of membranes. Pneumatic modulating valves are particularly effective because their speed is adjustable.
Advantages of Pneumatic Actuators:
The main advantages of pneumatic actuators are:
Provides more power than electric actuators.
An ideal choice for a site with many modulating valves
Medium fast to fast response. Normally they are faster than most electric motor actuators.
Safe type of power with a very low explosion risk.
Disadvantages of Pneumatic Actuators:
There are a few drawbacks of pneumatic actuators as listed below:
The requirement of an air compressor can increase the cost.
Compressors, dryers, and pneumatic actuators need to be maintained from time to time.
Leakage can occur at compressed air fittings
Hydraulic Actuators
Hydraulic actuators are devices that use the pressure of a liquid to operate the equipment. The normal liquid is a fire-resistant and stable oil that can work over a wide temperature range. Hydraulically powered devices and systems vary from very simple ones to others that are complex.
Working of Hydraulic Actuators:
All hydraulically operated devices do one of two things:
move fluid within a space with two or more movable surfaces, like in a hydraulic jack; or
confine it inside a system where hydraulic fluid is pumped so it can do useful work.
They rely on a basic fact about liquids: the volume occupied by the trapped hydraulic fluid cannot change because it is incompressible. Hydraulic actuators provide more power than any other type of actuator. Also, they provide a faster response and are compact in design. Hydraulic actuators and essential components like pumps, connected pipes, and fluid tanks are often part of a “turnkey” system.
Advantages of Hydraulic Actuators:
They can generate substantial power
Fast-acting
Compact Design
Depending on the application, hydraulic actuators need to be sized.
Disadvantages of Hydraulic Actuators:
Frequent maintenance required
Energy inefficient
Expensive
The operation can be noisy
Electric actuators or Electric Motor Actuators
Electric Actuatorsare electromechanical devices used to remotely control quarter-turn valves such as ball and butterfly valves. Compared to their pneumatic and hydraulic counterparts, electric valve actuators offer a more efficient, cleaner, and quieter method of valve control. They can be purchased as a package with the valve or as a separate unit.
Electric Motor Actuators are self-contained units used to operate a final control element or load. They serve various purposes:
Convert the rotation of the motor’s shaft to a straight-line movement of a final control element or load through a gearbox, or a mix of gears and linkages.
Convert the rotation of the motor’s shaft to a lower speed through a gearbox for the final control element or operated device.
Position the vanes of a damper through a connecting linkage between the motor and the damper. Sometimes more than one linkage is used.
Electric motor actuators have many of the same components, such as a starter and an overload with motor cutoff contacts, that Motor Control Center compartments have for loads like pumps. Most electric motor actuators have external controls such as Open, Close, and Stop push buttons, and a Local-Off-Remote switch.
Fig. 2: Typical Electric Actuators
Common electric valve actuators have a 2-point control or a 3-point control, but both have 3 wires.
Types of electric valve actuators
1. 2 Point Control valve Actuators: The three wires are for +, -, and one control wire. To turn the valve, the control cable must be powered to open and not to close, or vice versa. Without the power to the entire device, the valve will remain in the newest position. For example, JP Fluid Control’s AW1-R series uses this open/close wiring scheme.
2. 3-point control valve actuators: All three wires are for – and two are for +. Therefore, the two control signals can open or close the valve, depending on which one is being powered. The 3-point control also offers the option of intermediate stops (partially open). The two control cables should never be powered at the same time; otherwise, the actuator will be damaged. For example, JP Fluid Control’s AW1 series uses this 3-point wiring scheme
Advantages of Electric Actuators:
Provides good reliability
Economic
Easy installation and operation with negligible maintenance.
Accurate control mechanism.
Low power consumption
Disadvantages of Electric Actuators:
Limited power capability.
In case of power failure, the valve may not automatically revert to a safe position.
For larger control loops, they tend to be slower
Pneumatic actuators vs electric actuators
A few points to consider while comparing the pneumatic actuators with the electric valve actuators.
Temperature range:
Both pneumatic and electric actuators can be used in a wide temperature range. The standard temperature range of a pneumatic actuator is -4 to 174 ° F (-20 to 80 ° C) but can be extended to -40 to 250 ° F (-40 to 121 ° C) with optional seals, bearings, and grease. If control accessories (limit switches, solenoid valves, etc.) are used they may not have the same temperature rating as the actuator and this must be taken into account in all applications. In the case of low-temperature applications, the quality must be taken into account and the supply air must be taken into account in relation to the dew point. The dew point is the temperature at which condensation occurs in the air.
Condensate can freeze and block the air supply lines, which can cause the actuator to malfunction. Electric actuators are available in a temperature range of -40 to 65° C. When used outdoors, an electric drive must be sealed off from the environment to prevent moisture from penetrating the internal operation. Condensation may still form inside if it is removed from the power supply line, which may have caught rainwater prior to installation. Because motors heat the interior of the actuator housing during operation and cool when it do not, temperature fluctuations can cause “breathing” and condensation in the surrounding area. For this reason, all the electric actuators used outdoors must be equipped with heating.
Dangerous areas:
It is sometimes difficult to justify using electric actuators in a hazardous environment. However, if compressed air is not available, or if a pneumatic actuator does not have the required operating characteristics, an electric actuator with properly rated housing can be used.
Presentation properties:
Before deciding on a pneumatic or electric actuator for valve automation, there are a few key performance characteristics to consider.
Duty cycle:
Pneumatic actuators have a duty cycle of 100 percent. The harder you work, the better you work. Electric actuators are typically available with 25 percent duty cycle motors. This means that the motor has to be idle frequently to avoid overheating in high-cycle applications. Since most automatic on / off valves remain inactive, 95 percent of duty cycle time is typically not an issue. With optional motors and/or capacitors, an electrical actuator can be upgraded to a pulse duty factor of 100 percent.
Modulate control:
Since electric actuators are gear motors, it is impossible to drive faster without changing gears. A pulse circuit can optionally be added for slower operation. In modulating operation, an electric actuator interacts well with existing electronic control systems and makes electro-pneumatic controls superfluous. A pneumatic or electro-pneumatic positioner is used with pneumatic actuators to provide a means of controlling the position of the valve.
Torque to-weight ratio:
Electric actuators have a high torque-to-weight ratio of more than 4,000 lbf.-in. (450 Nm). Pneumatic actuators have an excellent torque to weight ratio below 4,000 lbf.-in.
Cruise control:
The ability to control the speed of a pneumatic actuator is a key design advantage. The easiest way to control the speed is to insert the actuator with a variable opening (needle valve) into the air pilot’s outlet opening.
How does an actuator work?
It is basically a motor that converts energy into torque. This torque controls a mechanism or a system where the actuator has been incorporated. It helps in introducing or preventing the motion. It runs on electricity or pressure. The control system can be controlled mechanically or electronically, software-driven, or human-operated. They work because of the work done by the rotor and stator assemblies, also known as the primary and secondary windings within the motor. Voltage is applied to the primary assembly which results in inducing the flow of current to the rotor assembly, or the secondary winding. The interaction of these two creates a magnetic field that results in motion.
The working of actuators differs slightly based on their types. Pneumatic actuators work using the pressure of air and hydraulic actuators work using liquid pressure.
A valve drive can simply be defined as a black box with a signal or a power supply via air or oil pressure that creates a stop for the valve movement as an output. The quality of a valve depends on many parameters such as metallurgy, mechanical resistance, machining, etc. The performance of a valve is highly dependent on its actuator. It is important to consider the factors you are considering: frequency of operation, ease of access, and critical features.
Parts of a Valve Actuator
An actuator is connected to and works with two parts: the valve body and the valve pilot.
It consists of several parts including A bonnet, adjusting screw, engine valve spring and diaphragm, vent plug, yoke, upper spindle, clutch block, and drive indicator.
Actuators can automate valves so that no human interaction with the valve package is required to operate the valve. They can be remotely controlled and act as shutdown mechanisms in an emergency that would be dangerous for humans. It is a mechanism for controlling the energy supply. The source can be hydraulic pressure, pneumatic pressure, or electrical current.
Where an actuator should be used?
The actuators are ideal for installations where human interaction is impossible or dangerous, eg. space installation or any location that prevents access to humans as valve actuators.
Compact actuators are used in FPSO or other locations where space and weight are critical. These actuators are designed to offer the powerful torque and thrust of their larger counterparts, but with a smaller footprint to install.
Subsea actuators are designed to withstand the low temperatures, extremely high pressures, and remote access capabilities of subsea equipment.
Ball valve actuators
A ball valve is a shut-off valve that controls the flow of a liquid or gas by means of a rotary ball having a bore. They are characterized by a long service life and provide reliable sealing over the life span, even when the valve is not in use for a long time. They are more resistant to contaminated media than most other types of valves. In special versions, ball valves are also used as control valves.
Types of ball valve actuators
1. Standard:
These are the most common types of ball drives. They consist of housing, seats, a ball, and a lever for rotating the ball. These include ball valves with two, three, and four connections.
2. Hydraulics:
Hydraulic ball valves are specially designed for hydraulic and heating systems due to their high operating pressure and their resistance to hydraulic and heating oil. These valves are made of steel or stainless steel.
3. Flanged:
These valves offer a high flow rate because they typically have a complete connection structure. When choosing a flange ball valve, in addition to the pressure rating, you should also check the compression class of the flange, which indicates the highest pressure that this type of connection can withstand. These ball valves are equipped with two, three, or four connections.
4. Ventilated:
Ventilated ball valves look almost exactly the same in design as conventional 2-way ball valves. The main difference is that when the outlet is closed, it vents to the environment. This is achieved through a small hole drilled into the ball and valve body. When the valve closes, the openings align with the outlet opening and release the pressure. This is particularly useful in compressed air systems where pressure relief provides a safer working environment.
Control valve actuators
The purpose of a control valve actuator is to provide the motive force to operate a valve mechanism.
Types of control valve actuators–
1. Pneumatic control valve: This type has a flexible membrane with pressure applied against the spring force of the actuator. When the control system sends its signal, the actuator generates a force that exceeds the force of the spring and moves the actuator shaft
2. Electric control valve: This actuator has a motor and a gearbox to generate torque that moves the valve up and down. We can find this type in linear and rotary control valves.
3. Electro-hydraulic valve actuator: This type mixes electrical signals and hydraulic units to operate the valve. The signal controls the flow of oil to open and close the valve using a flap nozzle system similar to a pneumatic system.
4. Hydraulic control valve actuator: This works very similarly to a pneumatic actuator and can be used for both linear and rotary control valves. However, it uses liquid instead of air to create force in your system.
How to choose the right actuator?
Selection of the best actuator type for any application is dependent on many factors including:
Operational characteristics and functions like actuation speed, cycle life, the requirement of fail-safe, etc.
Parameters to consider while selecting an actuator are,
1. Valve Size and Torque: Large, high-pressure class valves require high torque to operate. Choosing a very large pneumatic actuator for such a large valve is not economical. In this case, a hydraulic drive is recommended.
2. Failure mode: In contrast to electrical actuators, pneumatic and hydraulic actuators remain in the open or closed position during a power failure. These types of valves are spring-loaded, i.e. in the event of a power or signal failure, the spring returns the valve to a predefined safe position. Therefore, for example, electric actuators are not suitable for shut-off valves that must be completely closed when power is available.
3. Operating Speed - Electric actuators operate valves more slowly than pneumatic and hydraulic actuators. Therefore, an electric actuator may not be suitable if the valve is expected to operate at 1 in / sec or greater.
4. Frequency and Ease of Use: It is common to use electric actuators for certain large valves that are operated frequently rather than manually to facilitate operation. For example, it is proposed to equip a manual 20-inch class 300 ball valve with frequent operation with an electric actuator for ease of use only.
5. Control accessories: In contrast to pneumatic and hydraulic actuators, the control accessories for electric actuators are built into the actuator. In fact, electrical actuators do not require space for control accessories, which is an advantage. Hydraulic actuators have larger control panels compared to pneumatic actuators.
6. Hazardous Areas: In some cases, the use of electric actuators may be restricted in a hazardous environment. Different hazard zones and classes are defined depending on the presence of flammable gases or vapors that can limit the use of electric actuators.
7. Cost: Electric actuators are the cheapest type of actuator, hydraulic actuators are the most expensive, and tires are in the middle.
8. Power Source Availability: A hydraulic actuator cannot be used in a facility if a high-pressure oil source is not available.
Meaning of Ultrasonic Testing | Ultrasonic Testing of Welds
Ultrasonic testing, often abbreviated to either UT or Ultrasonic NDT, is an umbrella term for a number of non-destructive techniques used to detect the characteristics of a material. This type of testing involves using high-frequency ultrasound sound waves for the purposes of characteristic investigation. In this article, the characteristics of ultrasonic testing of welds will be discussed.
Non-destructive ultrasonic testing was first explored as early as 1942, by Dr. Floyd Firestone. He created a method to detect irregularities within the mass of material, even when they were invisible on the surface. Since then, other researchers have continued to refine the process, as well as expand the range of its utility.
What is Ultrasound?
Ultrasound refers to a frequency level that human beings are incapable of hearing. The overall ultrasound frequency range is 20,000 Hertz and above. In welding ultrasonic testing, the frequency used typically falls within the range of 500 Kilohertz to 20 MegaHertz.
Types of Ultrasonic Waves
There are four main types of waves used in non-destructive techniques like ultrasonic testing
1. Longitudinal waves
Longitudinal waves oscillate in the same direction as the propagation of the wave. The density of these waves fluctuates as they move. These waves can travel through liquids, gases, and solids through movements of compression and expansion.
2. Shear waves
Shear waves are also called transverse waves. In this type, the particles vibrate and move at a right angle to the direction of the wave’s propagation. These waves are stronger in solids than in other states of matter, though they are generally weaker when compared to longitudinal waves.
3. Surface waves
Surface waves are also called Rayleigh waves. The particle vibration of these waves is in the form of an elliptical orbit. These waves are extremely sensitive to surface irregularities and defects. They are also good at following curves and are useful in places where other waves cannot reach them.
4. Lamb waves
Lamb waves are also called plate waves. They are similar to Rayleigh waves, but cannot be generated in pieces that are thick. They require flat pieces that are only a few wavelengths thick.
Equipment/Tools Required for Ultrasonic testing
Transducer:
In ultrasonic testing, a transducer is a device that converts electrical energy into sound waves that travel through the piece being inspected and vice versa. Transducers are available in a variety of frequencies for different test pieces. There are many types of transducers, divided broadly into contact and non-contact devices.
Contact transducers require direct contact with the test piece. The shape and material of the transducer can vary. Also, the type, temperature, and thickness of the test piece will dictate the type of transducer being used, e.g. delay line transducer for thin pieces, dual line transducer for corroded or extremely hot pieces, etc. Contact transducers require couplants to enable the transmission of energy from the transducer to the surface of the test piece. Couplants are used to displace the air; they are usually in liquid or gel form.
Non-contact transducers or electromagnetic acoustic transducers do not require direct contact with the test piece to inspect it thoroughly. These transducers can be used in harsh temperatures and on most metals. As there is no physical contact, these transducers do not require couplants to facilitate transmission. However, unlike contact transducers, these devices are electromagnetic in nature and can therefore only be used on metallic or magnetic test pieces. Non-contact transducers are often used in welding ultrasonic testing.
Diagnostic machine:
A machine that records the signal of the pulse and the resulting echoes. These machines are also used to analyze the received data in different ways. The machine used will depend on the type of inspection being conducted, e.g. thickness gauging, flaw detection, etc. These machines can be either manual or automated, and either portable or not. Different types of diagnostic machines will be required for contact or immersion testing. The information presented on the equipment will also differ. There may or may not be a digital graphic of the signal. The machine must be calibrated on the basis of the properties of the test piece before it can be used for inspection.
Pulser and Receiver:
This is a device that can produce and receive ultrasonic energy of high frequencies. It is the connection between the transducer and the diagnostic machine. The pulsar emits controlled bursts of energy. The receiver section transports the echoes produced by the transducer to the diagnostic machine.
Ultrasonic Testing Procedure
Once the pulse receiver is connected to the transducer and a diagnostic device, the process of inspection begins. The active element of the transducer is passed over the piece being inspected.
There are multiple methods to carry out ultrasonic testing. These methods can be classified into three main categories:
1. Method of receiving ultrasound
a. Reflected transmission
In the reflected transmission method, a pulse of energy is sent through the test piece that continues until it hits a different medium, i.e. a border. It is usually the back wall of the piece. A reflected pulse or an echo is then emitted from the back wall. The intensity of the pulse and the echo are noted. In case of a crack or imperfection, the pulse will not reach the back wall, it will be reflected from the location of the imperfection, resulting in an echo signal of lower intensity.
b. Through/attenuated transmission
In this method, an ultrasound is sent through one surface of the test piece and is received by a separate receiver on the other side of the piece. In case of imperfection, the intensity of the sound received will be lower than usual. For this method, at least two sides of the test piece must be available. Also, this type is better for pinpointing the location of the crack, but the depth at which it is located can remain unclear.
2. Angle of sound waves
a. Normal beam
This refers to the angle at which the pulse is introduced to the test piece. A normal beam is a 90-degree angle. The normal beam method can be used for test pieces that are flat or smooth, with no unwieldy obstructions.
b. Angle beam
In this method, the pulse is emitted into the test piece at any angle other than 90 degrees. For this method, an angled transducer will be required for an easier sound introduction. This method is useful when the best way to get the largest reflection is at a diagonal. It is also useful in case some part of the test piece is obstructed, and normal beam inspection is not possible. This method is useful for determining the thickness of the test piece and the depth of the imperfection.
3. Method of coupling
a. Contact
In contact testing, a couplant is applied between the transducer and the test piece to reduce the air and increase the intensity of the sound waves.
b. Immersion
In immersion testing, the test piece and the transducer are both immersed in a bath. In this type of testing the water acts as the couplants. The movement of the transducer is smoother in this method of ultrasonic testing.In this method, direct physical contact between the transducer and the test piece is not required. Also, a specific immersion transducer is to be used for this method.
c. Non-contact
This is a type of testing that does not require contact and operates on principles of electromagnetism mechanics.
Fig. 1 shows the principle of Ultrasonic Testing.
Fig. 1: Principle of Ultrasonic Testing
An ultrasound transducer or ultrasonic probe sends a sound wave into a test material. Two indications are received from each probe; the first one is from the initial pulse of the probe, and the second one is from the back wall echo. If there is any defect, it creates a third indication (See the right side display in Fig. 1) and simultaneously reduces the amplitude of the back wall indication. The depth at which the defect is present can easily be calculated by dividing the length D by the length between MS and BW.
Common Data Formats for Ultrasonic Testing
1. A Scan
The A-Scan presents the data as the strength of the signal received and the time it took to receive it. The depth and size of the flaw can be determined using this type of scan by comparing the size and position of the signal on the scan.
2. B Scan
The B Scan is a cross-sectional scan of the test piece. In this type of presentation, the time taken for travel is presented with its distance to the transducer.
3. C Scan
The C Scan is a plane view of the test piece, with a sign indicating the position of the detected flaw. The plane view can be from the top, bottom, or side, depending on the position of the transducer.
With automated diagnostic machines and interfaces, all these types of charts are created automatically to provide an accurate analysis of the test piece. With progress in the field of ultrasound imaging, the quality of data presented has become more and more detailed. Now, it is possible to spot even minuscule irregularities.
Materials to be used on
Ultrasonic testing can be conducted on a variety of materials. It is applicable to most metals, alloys, and composites, as well as concrete, ceramics, plastic, and even wood. Ultrasonic testing is even used in the medical field, e.g. sonography.
Ultrasonic Testing of Welds
Welding is the process of fusing at least two parts by using methods like heat and/or pressure. Though welding is most commonly used on metals, it is also possible to weld thermoplastics and wood. A welding project that is complete is called a weldment. A weldment mainly consists of two materials: a parent component and a consumable. A parent component is the material of the parts that are going to be joined. A consumable, sometimes called a filler, is the material that is used to fuse the parts together.
Weldments can be either homogeneous or heterogeneous. Homogenous welds are those where the material of the consumable is compositionally similar to the material of the parent component. Consequently, in heterogeneous welds, the two materials are compositionally different. While homogenous weldments are generally preferred, heterogenous weldments have to be constructed in cases when the parent component is brittle or otherwise unstable.
Since welding is a high-heat and pressure process, it requires skill and practice to produce smoothly fused weldments that are flawless. Mistakes like hurrying the process, or letting the metal cool too soon can lead to cracks or other imperfections, which are sometimes located inside the weldment. They cannot be spotted from the surface but can compromise the strength of the welded product. This is why ultrasonic testing of welds is a good way to determine the quality and craftsmanship of a weldment without disassembling it.
Fig. 2: Ultrasonic Testing of Welds
1. Ultrasonic Testing of Welds for Flaw detection
Ultrasonic testing in welding can be used to detect defects and irregularities in test pieces. As a non-destructive method, it is completely non-invasive. However, it can still provide extremely accurate readings of flaws that lie beneath the surface of the item:
a. Porosity
Pores are extremely small voids in a component. They are formed when gas gets trapped in welding metal as it is solidifying. Pores are either distributed evenly throughout an item or are concentrated on one part. Pores are mostly spherical, but they can also look elongated.
b. Slag inclusions
Slag inclusions are non-metallic solid substances that are stuck within the welded metal. If welding is done too fast, or at the wrong angle, slag inclusions are likely to occur. Unless they are at the surface, these inclusions are impossible to spot without testing.
c. Lack of sidewall fusion
Lack of the smoothening of the fusion between the metal used for welding and the parent metal is called lack of sidewall fusion. This can happen if the arc length is too big and the metal melts over the side.
d. Lack of inter-run fusion
This is when the weld metal does not fuse the previous weld bead adequately.
e. Lack of root penetration
When both sides of the joint’s root region remain unfused, it is called a lack of root penetration.
f. Undercutting
An undercut is a type of welding defect where the cross-sectional thickness of the metal is reduced, thus lowering the strength of the weld and of the item itself. In industrial settings, equipment that has undercutting can be a safety concern and needs to be replaced.
g. Longitudinal or transverse cracks
Longitudinal cracks run across the center of the weld, while transverse cracks run perpendicular through the axis of the weld.
2. Ultrasonic Testing in Welding for Thickness detection
An ultrasonic thickness gauge is used to determine the thickness of an item by using sound waves to determine the time it takes for the sound waves to produce an echo. The thickness of an item is an important characteristic: the thickness of medical tubes and contact lenses needs to be standardized, and the thickness of heavy machinery is not only a matter of having no variables, but it is also a matter of safety. Also, ultrasonic testing of welds to determine their thickness is a great way to keep track of the corrosion levels of pipes and tanks.
Fig. 3 below shows ultrasonic testing in the welding of pipe to detect any flaw in the piping weld.
Fig. 3: Ultrasonic Testing of welding in pipes
Advantages of Ultrasonic Testing
Detection of deep-rooted invisible flaws: Some flaws are not on the surface of an item. With ultrasonic testing, it is possible to identify flaws deep within the mass of the test piece.
Detection of minuscule flaws: Ultrasonic testing can identify the presence of tiny flaws as well as big ones. With the equipment required for this method being upgraded constantly, the preciseness level is only increasing further.
Possible even with only one surface: Some items being tested may be too corroded to move, welded into the wall, etc. In cases like this, it is not possible to move the item for testing. Thankfully, ultrasonic testing is possible even when the examiner only has access to one surface of the test piece.
Accuracy at determining depth and thickness: Ultrasonic testing equipment can identify the precise location of a crack, as well as determine how big it is, without having to use invasive methods.
May even determine the nature of the flaw: In some cases, it is also possible for an experienced examiner to identify the cause or origin of the flaw by analyzing the readings on the testing equipment.
A non-destructive and safe method for inspectors: It is a completely non-destructive method and does not cause any harm to the items being tested. Additionally, it is a safe method for workers to examine the item, as it does not pose any health or safety concerns for the examiner.
Highly automated: Diagnostic machines are now highly automated and capable of performing any calculations the examiner might need. Also, the machines not only organize the data but also present it in all three data formats commonly used in ultrasonic testing.
Immediate results for quick decisions: The results of ultrasonic testing are instantly available to the examiner. This means that flaws are identified immediately, and consequent decisions can be taken without delay.
Disadvantages of Ultrasonic Testing
Manual operation of the sensitive probe requires practice: As the probes are sensitive, they need to be handled with extreme care. This is because, even with couplants, probes can pick up a significant level of noise, which can hinder results.
Technical knowledge required: Ultrasonic testing can be confusing without at least a basic understanding of the topic. To carry out this task, personnel will have to be given some technical training in this field. They will also need to practice their application methods a lot.
Water-based couplants pose a danger to tested items, and need anti-freeze liquids: Water can be a danger to many items that are regularly tested using ultrasonic NDT. While it would be ideal for all test pieces to be water-resistant, this is not the case. And unfortunately, water-based couplants are the most common type of couplants. If they are to be used, it is advisable to use antifreeze on the item beforehand to prevent damage.
Equipment requires calibration, and thick test pieces will need multiple setups: The diagnostic equipment has to be calibrated according to the features of every test piece, which can be very time-consuming. In cases where the piece is too thick, the equipment will have to be set up multiple times for a single test piece.
What is Response Spectrum? | Steps for Earthquake Response Spectrum Analysis
A response spectrum is a graphical plot of the frequency of an oscillator and its damping. The response spectrum plot represents the peak or steady-state response (velocity, displacement, or acceleration) of a series of oscillators of varying natural frequencies. In the vibration analysis of any system, the response spectrum is very useful as the resulting plot can provide the response of any linear system with respect to its natural frequency. The response spectrum finds its usage in interpreting seismic or earthquake events and slug flow events.
Response Spectrum Analysis is a scientific method for estimating the structural response of dynamic vibration events. To perform the response spectrum analysis, the first job is to define the response spectrum of the system. In this article, we will explore the basics of the response spectrum and learn the steps for performing seismic/earthquake analysis using the response spectrum analysis method.
What is an earthquake?
Earthquakes are random ground motion that produces inertial loads in structures built on them. The ground motion of the earthquake can be attenuated by the building’s resonant response producing much larger motions at higher levels. That leads to major damage to the structure. Hence, structures need to be designed to withstand the earthquake’s ground motion.
Static vs Dynamic Seismic Analysis
This earthquake analysis of piping systems can be performed in two ways: by static method or dynamic way
Static Equivalent Seismic Analysis:
Static equivalent analysis, for most non-critical piping systems, the earthquake is treated as a static load, which is proportional to the weight of the piping and components. The magnitude of the load is generally determined by the ‘g’ factor according to respective codes, for example, UBC, IBC, ASCE, IS, etc. This load is applied statically in the vertical and two horizontal directions.
Dynamic Earthquake Analysis
Dynamic Analysis, for critical piping systems, a dynamic analysis is generally preferred because that produces more realistic and accurate results compared to an equivalent static analysis. The random ground motion can be recorded using accelerometers and applied to the structure or piping model through all the ground supports as time histories and the effect can be assessed. These random ground motions are converted to response spectra to simplify earthquake analysis.
For seismic analysis of piping and structures, the earthquake response spectrum is the most popular tool. For predicting forces and displacements of pipes and structures, the response spectrum method provides various computational advantages. The main benefit of the seismic response spectrum method is the calculation of only the maximum displacement and force values in each mode of vibration using smooth design spectra that are the average of several earthquake motions.
Response Spectrum Analysis Method
Response spectrum plot gives the maximum response (that maybe maximum displacement, maximum velocity, maximum acceleration, or any other parameter of interest) to the natural frequency (or natural period) subjected to specified excitation for linear single-degree-of-freedom system oscillators. These plots are subjected to specific damping and it changes as damping changes. Refer to Fig. 1 shown below:
Fig. 1: Definition of Response Spectrum
Here abscissa is the natural frequency (or period) of system and ordinate is the maximum response.
The plot of this type is shown here in the figure, in which a one-story building is subjected to a ground displacement indicated by us(t) and u indicates deflection.
For any linear single degree of freedom system, the response spectrum curve shown in the figure gives the maximum displacement of the mass m relative to the displacement at the support which is (us-u) here.
Thus, to determine the maximum response of a linear single degree of freedom system from the available spectral chart, for specified earthquake excitation, one needs only to know the natural frequency of the system and damping.
If the natural frequency of the structure coincides with the frequency of earthquake ground motion, it leads to a resonance condition, which creates substantial damage to the system. That’s the main reason, not all buildings collapse during an earthquake. The natural frequency of building/structure is a property of height, stiffness, material, etc. Buildings whose natural frequency matches with earthquake frequency collapse during an earthquake while remaining are not experiencing major damage.
The response spectrum is using the same principles as time history. Only instead of using time history, it is using maximum values of the response. When the time history profile is not available for a particular dynamic event, then the response spectrum is used. Response spectrum analysis provides more conservative results than time history.
Parameters affecting Response Spectra
The response spectral values are dependent on various factors like,
Soil condition
Energy release mechanism
Damping in the system
Epicentral distance
Focal depth
Richter magnitude
The time period of the system
Construction of Response Spectrum Plot
The construction of a Response Spectrum requires the solution of a single degree of freedom system, for a sequence of natural frequency values and damping ratio in the range of interest. Every solution provides only one point (with the maximum value) of the response spectrum. All of these maximum response values are plotted against natural frequency to construct a single response spectrum.
Since a large number of systems must be analyzed in order to fully plot each response spectrum, the task is lengthy and time-consuming. But once these curves are constructed and available for the excitation of interest, the analysis for the design of structure subjected to dynamic loading is reduced to a very simple calculation of the natural frequency of the system and the use of response spectrum to calculate the maximum response.
These response spectrum plots are created for specific areas of regions and different for different regions. The study of geographic areas combined with an assessment of historical earthquakes allows geologists to determine seismic risk and to create seismic hazard maps for respective areas, which show the likely maximum response values to be experienced in a region during an earthquake.
So, let’s look at a simplified example of how we can get a response spectrum plot for a specific area of the region.
Fig. 2: Constructing Response Spectrum Plot
Step 1: First, take the randomly measured ground motion from previous earthquake records in that area. (Fig a)
Step 2: Then sequence of tuned SDOF oscillators with some fixed damping values attached to a shaker table and measured motion used to shake the table. (Fig b)
Step 3: The response of all the SDOF oscillators is recorded and plotted for an individual oscillator. (Fig c)
Step 4: Maximum response for all individual oscillators is extracted to plot the combined response. (Fig d)
Calculating the maximum response for a range of values of frequency and damping and then plotting results graphically to get a spectrum chart that shows the maximum response for all possible single-degree-of-freedom systems to that component of the earthquake. This maximum response can be maximum displacements, maximum velocity, or maximum acceleration.
This combined response is made up of many peaks and troughs. The envelope of the broadened peaks is shown in fig (d), which is a conservative approach. The idea is that even though all earthquakes are different, the maximum response of similar earthquakes should be the same even though the time the maximum response occurs may differ i.e. timing of the event is not considered.
Seismic engineers and government planning departments use these values from the spectrum chart to determine the appropriate earthquake loading for buildings in the respective zone. Earthquake load impact calculations for any structure in that area of the region are simplified into a few steps to (a) calculate the natural frequency of the system, (b) and then the maximum response found from the respective spectrum chart for calculated natural frequency.
Pseudo-acceleration and Pseudo-velocity
Response spectrum plots can be plotted as maximum relative displacement, maximum velocity, or maximum acceleration. These three quantities are also known as spectral displacement (SD), Spectral velocity (SV), and Spectral acceleration (SA) and are also proportional to each other.
The spectral displacement i.e. maximum relative displacement is proportional to spectral acceleration i.e. maximum absolute acceleration. This can be demonstrated with simple numerical iterations on the dynamic equation of motion.
And this can be demonstrated by equating the equation for potential energy and kinetic energy.
The acceleration and velocity so defined are called pseudo-acceleration and pseudo-velocity, respectively. Pseudo-acceleration is very close to absolute acceleration and is the same as absolute acceleration when there is no damping. Pseudo-velocity is the fictitious velocity associated with the apparent harmonic motion for convenience.
Tripartite Response Spectra
It is possible to plot all three responses in a single chart using a logarithmic scale and it is called the Tripartite plot (Fig. 3)
Fig. 3: Sample Tripartite Plot
Dynamic Equation of Motion and Modal Superposition
The dynamic behavior of a piping system depends greatly on the free or natural vibration of the system. SDOF system deals with one natural frequency and this system can only move in one particular direction. However, in a multi-degree of freedom (MDOF) structural system, such as a piping system, there are many natural frequencies, each with its vibration shape or mode. These MDOF structures with N degrees of freedom system transformed into the problem of solving N systems, in which each one is an SDOF system. This transformation extends the use of response spectra from a single-degree-of-freedom system to the solution of the system with any number of degrees of freedom.
The equation of dynamic equilibrium associated with the response of the structure subjected to ground motion is as below.
Due to modal orthogonality, M, C, and K matrices will become diagonal matrices. The modal superposition converts the N simultaneous differential equations of the MDOF system into N-independent SDOF systems by decoupling this equation. These N-independent SDOF systems are solved one by one using SDOF techniques.
The maximum displacement in time history response can be calculated by multiplying the maximum displacement calculated from the response spectrum with the participation factor for the respective mode shape. Each mode shape is contributing up to some extend to the total response of the structure and that depends on the participation factor. Accordingly, the maximum response is calculated for all respective modes.
The amount of displacement in one mode given by,
Accordingly, the maximum time history response is calculated for all modes and respective ground motions and combined together to get maximum earthquake response. These maximum time history responses cannot be added directly and for that special techniques called modal combinations are used.
Modal Combinations
The total response of the system is determined by combining the responses from all modes. This combination is termed a modal combination. The modal combination includes internal modal forces and internal modal moments, as well as modal displacements.
Ed Wilson and Ray Clough took the response spectra method and developed the approximate method for MDOF structures that requires the combination of the modes and proposed SRSS over 50 years ago. At that time only 3 earthquake records existed for comparison whereas now we have thousands.
The methods of combining modal results present some confusion to many piping engineers. We have the SRSS (Square Root of Sum of Square), ABS (Absolute), CQC (Complete Quadratic Combination), and a few algebraic methods. They all have been used in one situation or another. However, we do not really have a clear picture of when and why a certain method is used.
The NRC Regulatory Guide 1.92 provides further guidance for nuclear facilities. Some of the methods are from Rev 1 and some are from Rev 2 with some standard mathematical methods.
Square Root of the Sum of the Squares (SRSS)
Grouping Method
Ten Percent Method
Absolute Double Sum Method
Signed Double Sum Method
Absolute CQC
Signed CQC
Steps to perform Response Spectrum analysis in AutoPIPE
Open Model in AutoPIPE from, File>Open. Note: The first step for any Dynamic analysis is modal analysis.
Go to Tools > Edit Option and make these changes:
Mass point per span (A-Auto, 0-None): A
Cutoff frequency: 100
Then click OK to accept.
Note: Specifying ‘A’ means that the mass spacing will be applied automatically using a frequency of 100Hz. It is possible to split each length into the same number of spans by using a number in the range 1-9 instead of A, but this can lead to very closely spaced nodes in short lengths.
Go to Analysis > Dynamic analysis to set up dynamic analysis settings
Under Modal analysis, check on Analyze up to Cutoff frequency and Provide cutoff frequency value. Review other information if you want to make modifications over there. And then click OK
Fig. 4: Response Spectrum Analysis Steps in AutoPipe
Then click on Analyze all from the Analysis ribbon. Make sure that Modal analysis is selected and click ‘OK’.
Check for adequate mass participation. Go to Result > Output Report, select Frequency report and click ‘OK’ to access reports.
Fig. 5: Response Spectrum Frequencies
The next step is to create a Response spectrum. Go to Loads>Response Spectrum. Here you can provide a new Response Spectrum Or can use an existing one.
Fig. 6: Response Spectrum Generation in AutoPIPE
Note: You can provide a number of response spectra together. This spectrum data can be copy pasted from MS Excel. Also, you can construct spectrum may as an external ASCII text file using any text editor software.
Now go to Analysis>Dynamic analysis>Response Spectrum.
Create a new load case by clicking on ‘New’. Then click on Spectra>Define.
Fig. 7: New Response Spectrum Generation
Missing mass correction can be considered by checking on ZPA or Missing Mass field. Then from the dropdown select Modal Combination type for calculations. Check on ‘Print Modal Results’ and click ‘OK’.
Note: Different Response spectra can be provided for each direction. Also, scale factors can be modified.
To analyze the model for Response Spectrum, click on Analyze All in the Analysis ribbon. Please make sure the Response Spectrum is selected here.
After analysis, Go to Result>Combinations.
A new load case is created for Response Spectrum, Response 1 (R1)
Fig. 8: Response Spectrum Load combinations
Code Combinations are used to check code stresses whereas Non-Code Combinations are to check forces and moments. Code combination Sus+R1 and Non-code combination R1 are created automatically by AutoPIPE. This R1 we can further combine with other operating cases.
These results can also be checked graphically,
Code stresses, Go to Result>Code Stresses, and then select Sus+R1 as combinations.
Displacements, Go to Result>Displacement and then select Response 1 as Load combination.
Detailed text reports for Response, Mode Shapes, Restraint Reactions, and Accelerations can get from Quick reports.
Go to Result>Quick Reports>Output Report and select Frequency, Mode Shapes, Restraint,and Accelerations.
Results and Interpretations of Response Spectrum Analysis Outputs
This maximum response can be found from provided response spectrum and modal analysis results as explained in the Dynamic Equation of Motion and Modal Superposition theory.
Modal analysis results provide different modes of natural frequency and participation factors.
Fig. 9: Output result of Response Spectrum in AutoPipe
By simply, plotting the period (or natural frequency) value on spectra, the maximum response is calculated. This maximum response is multiplied by the participation factor to calculate the maximum time history response in the respective mode. Accordingly, maximum responses are calculated for all modes and combined by Modal combinations to get a maximum response due to earthquake loading at all points.
About the Author: This article is prepared by Mr. Manoj Kale, AutoPipe Expert. He presented this article in the form of a webinar. To access the recording of that webinar and learn directly from the expert, Click here and register.
What is Piping Fabrication? | Tools for Pipe Fabrication
Pipe fabrication can be defined as the process of cutting, bevelling, and welding piping components such as pipes, tees, elbows, flanges, reducers, etc., as dictated by the design documents. In the process and power piping industry, Piping fabrication is a highly critical activity as it involves hundreds of components and thousands of steps and requires a high degree of precision. In any construction project wherever piping networks are involved, piping fabrication needs to be properly planned, scheduled, and executed as per design requirements. To maintain the system integrity, and proper functioning of each item, and minimize accidents, It is required to ensure maximum quality of work during pipe fabrication. In this article, we will explore more details about piping fabrication.
Piping fabrication involves various activities like piping material storage and handling, cleaning, cutting, bevelling, welding, inspection and testing, painting, insulation installation, etc.
Types of Piping fabrication
Depending on the location of the pipe fabrication work they are classified into two groups.
Shop Pipe Fabrication and
Field Pipe Fabrication.
Shop Fabrication vs Field Fabrication
There are various factors that determine whether pipes will be shop fabricated or field fabricated or both methods will be used. In most cases, both shop and field pipe fabrication is used. The major deciding factors are profitability, type, and size of the project, piping material, and size, post-fabrication surface treatment, environmental condition, accessibility of equipment, skilled personnel availability, time requirement, and availability, etc.
In general practice, small bore pipes, threaded and socket welded pipes are field fabricated whereas butt welded pipes, pipe bending, modular items, etc are shop fabricated.
In the Shop pipe fabrication process pipe, fittings and components are assembled by welding into spool assemblies at the fabricator’s facility or a workshop normally known as a pipe fabrication shop. The spools are then labeled using an identifier and transported to the construction site for installation. Whereas in field pipe fabrication all these assemblies are done at the construction site.
Tools/Equipment used for Pipe Fabrication
During pipe fabrication, various equipment and tools are used that helps in the fabrication process. The widely used equipment is listed below:
In this stage, pipe fabricators are required to calculate various parameters from the drawing specifications. They are required to take into consideration welding and tack welding processes along with any distortion that may arise from welding. The tools required for the fitting-up of pipes and flanges, such as pipe supports and clamps will also need some thought. Then they need to prepare a simple wire model of the pipe spool from the drawings. Material take-off and the client’s fabrication specification are also supplied to the pipe fabricators.
Pipe fabrication in a Pipe Fabrication Shop
Piping Fabrication Procedure
The pipe fabrication process requires assembling pipes and pipe fittings according to the spool drawing. Pipe fabricators must take into consideration the size of the assembly, as transportation could be a problem. In such cases, Sub-assemblies are an effective way of transporting large projects. Piping fabrication is done as per the below-mentioned steps:
Marking and Cutting: As per the design drawing requirement, Marking shall be done and the same shall be verified by the concerned supervisor prior to cutting. Pipe cutting is normally done as follows:
Carbon Steel pipes – By gas cutting & grinding.
Alloy Steel pipes – By grinding or flammable cutting.
Tagging: Using dye stamping, Paint marking, or Tagging, pipe heat numbers are transferred to the cut pieces before cutting the pipe.
End Preparation: In the next step, End preparation (bevelling) and fit-up are done following an approved Specification and WPS.
Welding Pipes: Extra precautions must be exercised to ensure that longitudinal seams on the joining pipes do not come in one line in a butt-welded joint. Seams must be staggered at least 100 mm apart and also will clear the branch connections. Care is taken to make sure that longitudinal seams are not resting on the steel structure.
Welding Pipes and Fittings: Pipes and Fittings for fit-up are then placed on a temporary pipe bed and supports are properly secured properly. Next, the arrangement is inspected for quality Fit-Up. Once inspection clearance is received, Joints are welded by qualified welders.
Details Marking: Various details line pipeline No., Component Heat No., Joint No., Fit-up inspection signature, Welder No., Visual inspection signature, and welding date are marked near the joint using a metal paint marker.
The pipe Spool Number is marked with a paint marker and an aluminum tag is tied to the spool.
Heat treatment: As per project-specific requirements, Preheating and PWHT will be done at the shop or field.
Fabricated pipe spools are then shifted from the pipe fabrication shop to the laydown area.
Inspection: As per the requirement of the project specification or guidelines, NDT is performed. Once NDT clearance is received, spools are released for erection/painting with a release notice.
Spools rejected in the NDT process are identified with yellow and black tags and sent for repair work. NDT has performed again on the repaired weld areas as required.
Documentation: After painting, field inspection is executed for QC and the same is recorded in the prescribed format. After the painting inspection, the spool is released for erection.
Fabrication of Stainless Steel Pipes: Stainless Steel piping fabrication is normally done in the shop with an isolated area from carbon steel and alloy steel. The equipment and tools which are used for CS fabrication shall not be used for SS. Tools for SS must be differentiated clearly by marking “For Stainless Steel” only. For Stainless steel materials, stainless steel tools will be used for grinding, brushing and clamping, etc.
Protection: For the protection and temporary storage till the erection, all flanged raised faces of completed pipe spools are fitted with plywood blinds and spool ends shall be fitted with proper caps.
Pipe Fabrication Shop
The pipe fabrication Shop is basically a workshop where all the prefabricated pipe spools are developed. All the pipe works for spool preparation are normally performed inside this shop using highly skilled pipe fabricators. Generating pipe spools in the Pipe Fabrication shops is the best economic way for reducing site installation costs for big-size projects. All equipment, tools, and manpower are available in the pipe fabrication shop and pipe spools are produced with high quality.
Pipe Fabrication Specification
Pipe fabrication Specification is an engineering document that provides all guidelines to be followed by pipe fabricators for spool pipe fabrication. The specification for piping fabrication provides the minimum requirement of preparation of detail shop drawings and the fabrication, requirements for inspection and testing. It lists all applicable codes and standards.
