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Load Cases for Pipe Stress Analysis: Caesar II Load Cases

Written by Anup Kumar Dey in Caesar II,Piping Stress Analysis,Piping Stress Basics

Piping Stress Analysis is simply creating the load cases required for analysis and studying the impact of the same on the behavior of the critical piping systems. A load case can be defined as a set of loads (Weight, Pressure, Temperature, External Forces, Displacements, etc) and boundary conditions for defining a particular loading condition. So Stress Analysis can not be thought of without proper load case creation. Sometimes these load cases are mentioned in the piping stress analysis design basis. In this article, we will learn the basic load cases that are required for stress analysis activity.

Objectives of Pipe Stress Analysis:

The main objectives of stress analysis are to ensure:

  1. Structural Integrity (Design adequacy for the pressure of the carrying fluid, Failure against various loading in the life cycle, and Limiting stresses below code allowable.)
  2. Operational Integrity (Limiting nozzle loads of the connected equipment within allowable values, Avoiding leakage at joints, Limiting sagging & displacement within allowable values.)
  3. Optimal Design (Avoiding excessive flexibility and also high loads on supporting structures. Aim towards an optimal design for both piping and structure.)

What is a Load Case in Pipe Stress Analysis?

In the context of pipe stress analysis, a load case refers to a specific scenario or combination of loads that the piping system may experience during its lifecycle. These loads can arise from various sources such as internal pressure, temperature changes, external forces, and more. By analyzing these load cases, pipe stress engineers can predict how the piping system will behave under different conditions, allowing them to design systems that can withstand these stresses without failure.

A typical example of a load case in Caesar II pipe stress analysis may constitute the thermal, deadweight, and pressure loads together known as the operating load case. Similarly, a sustained load case is composed of dead weight and pressure load. Again, a load case can also be formed by combining the results of other load cases. For instance, a load case might represent the difference in displacements between the operating condition and the installed condition.

Notations Used for Load Cases in Caesar II

To meet these objectives several load cases are required during stress analysis. In this article we will use the following notations for building load cases:

  • WW=water filled weight of the piping/pipeline system,
  • HP=Hydrotest Pressure,
  • W=Weight of pipe including content and insulation,
  • P1=Internal Design pressure,
  • T1=Maximum Design temperature,
  • T2=Maximum Operating temperature,
  • T3= Minimum Design temperature,
  • WIN1, WIN2, WIN3, WIN4: wind loads acting in some specific direction,
  • U1, U2, U3, U4: uniform (seismic) loads acting in some specific direction.

Basic Load Cases for Caesar II Pipe Stress Analysis:

For Stress analysis in Caesar II, Various Load case combinations are used which serve several purposes. While analysis at a minimum, the stress check is required for the below-mentioned cases:

a. Hydrotesting case:

Piping/ Pipeline systems are normally hydro-tested (sometimes pneumatic tested) before the actual operation to ensure the absence of leakage. Water is used as the testing medium. So during this situation pipe will be subjected to water-filled weight and hydro-test pressure.
Accordingly, our first load case will be as mentioned below

1WW+HP HYD
Load Case for Hydrotest Case

b. Operating and ALT Sustained load cases:

When the operation starts working fluid will flow through the piping at a temperature and pressure. Alt Sustained cases are used as Hot Sustained cases which means sustained stress that the system carries during operation. So accordingly our operating load cases will be as mentioned below:

2W+T1+P1 OPE For operating temperature case at maximum design temperature
3W+P1 SUS Alt Sustained case based on operating case 1 (T1)
4W+T2+P1 OPE For a maximum system operating temperature case
5W+P1 SUS Alt Sustained case based on operating case 1 (T2)
6W+T3+P1 OPE For minimum system temperature case
7W+P1 SUS Alt Sustained case based on operating case 1 (T3)
Operating and Alt-Sustained Load Cases

c.  Sustained Load Case:

Sustained loads will exist throughout the plant operation. Weight and pressure are known as sustained loads.  So our sustained load case will be as follows:

8. W+P1 SUS
Sustained Load Case

d. Occasional Load Cases: 

Piping may be subjected to occasional wind and seismic forces. So to check stresses in those situations we have to build the following load cases:

9W+T2+P1+WIN1 OPEConsidering wind from +X direction
10W+T2+P1+WIN2 OPEConsidering wind from -X direction
11W+T2+P1+WIN3 OPEConsidering wind from +Z direction
12W+T2+P1+WIN4   OPEConsidering wind from -Z direction
13W+T2+P1+U1 OPEConsidering seismic from +X direction
14W+T2+P1-U1 OPEConsidering seismic from -X direction
15W+T2+P1+U2   OPEConsidering seismic from +Z direction
16W+T2+P1-U2 OPEConsidering seismic from -Z direction
Occasional Load Cases in Operating Condition

While stress analysis the above load cases from load case 9 to load case 16 are generated only to check loads at node points. Figure 1 shows typical load cases that should be generated during stress analysis

Typical Load Cases
Fig. 1: Typical Load Cases

To find occasional stresses we need to add pure occasional cases with sustained load and then compare them with code allowable values. The following sets of load cases are built for that purpose.

17L9-L4 OCC Pure wind from +X direction
18L10-L4 OCC Pure wind from -X direction
19L11-L4 OCC Pure wind from +Z direction
20L12-L4 OCC Pure wind from -Z direction
21L13-L4 OCC Pure seismic from +X direction
22L14-L4 OCC Pure seismic from -X direction
23L15-L4 OCC Pure seismic from +Z direction
24L16-L4 OCC Pure seismic from -Z direction
25L17+L8 OCC Pure wind+Sustained
26L18+L8 OCC Pure wind+Sustained
27L19+L8 OCC Pure wind+Sustained
28L20+L8 OCC Pure wind+Sustained
29L21+L8 OCC Pure seismic+Sustained
30L22+L8 OCC Pure seismic+Sustained
31L23+L8 OCC Pure seismic+Sustained
32L24+L8 OCC Pure seismic+Sustained
Occasional Load Case Sets

Load cases from 25 to 32 will be used for checking occasional stresses with respect to the code ASME B31.3 allowable (=1.33 times Sh value from code). Use scalar combination for load cases 25 to 32 above and algebraic combination for others as shown in Fig. 2 attached below:

Load Case Combination
Fig. 2: Load Case combination

e. Expansion Cases:

Following load cases are required for checking the expansion stress range as per the code

33L2-L8 EXP
34L4-L8 EXP
35L6-L8 EXP
36L2-L6 EXP
Expansion/Thermal Load Cases

The above load cases (from 33 to 36) are used to check the expansion stress range

The above-mentioned load cases are the minimum required load cases to analyze any stress system. Out of the above load cases, the load cases mentioned in load case numbers 1, 3, 5, 7, 8, and 25-36 are used for stress checks. The load cases mentioned in load case numbers 1, 2, 4, and 6 to 16 are used for checking restraint forces, displacements, and nozzle load checking.

Some additional load cases may be required for PSV-connected systems, systems having surge or slug forces, and rotary equipment-connected systems.

Seismic and Wind analysis may not be required every time. So those load cases can be deleted if the piping system does not fall under the purview of wind and seismic analysis by project specification. However, to perform wind and seismic analysis proper related data must have to be entered in the Caesar II spreadsheet.

If the stress system involves the use of imposed displacements (D) and forces (F) then those have to be added with the above load cases in the form of D1, D2, or F1, F2 as applicable.

Better Engineering Practices for Stress Analysis

It is a better practice to keep:

  • Hydro and sustained stresses below 60% of the code allowable
  • Expansion and occasional stresses below 80% of the code allowable
  • Sustained and Hydrotest sagging below 10 mm for process lines and below 3 mm for steam, two-phase, flare lines, and free-draining lines.
  • Design/Maximum displacement below 75 mm for unit piping and below 200 mm in rack piping.

Video Tutorial for Load Case Creation in Caesar II

The following video tutorial explains the load case creation steps with an example

Caesar II Load Case Video tutorial

Online Course on Pipe Stress Analysis with Practical Example

Complete Pipe Stress Analysis using Caesar II Online Course (30+ Hours)

References (External Links):

Introduction to Pressure Surge Analysis

Written by Anup Kumar Dey in Process

What is Pressure Surge or Water Hammer?

A pressure surge is a pressure wave that is caused by the kinetic energy of the moving fluid when there is a sudden change in flow velocity. Due to the instantaneous conversion of momentum to pressure when flowing liquid stopped quickly this sudden increase or surge of pressure is experienced. Pressure surge is popularly known as Water Hammer, Fluid Hammer, or Hydraulic Surge.

In Piping/Pipeline system networks this phenomenon is a major concern for Piping/Pipeline/Process engineers. As noticed in the below graph (Fig. 1), the pressure spike will continue hitting the pipe/pipeline trying to release the generated excessive energy and therefore the system will be at high risk.

Typical Pressure Surge Curve
Fig. 1: Typical Pressure Surge Curve

ASME B31.3 defines that the pressure rise due to pressure surge and other normal operation variations shall not exceed the internal design pressure at any point in the piping system and equipment by more than 33%.

What Can Cause Pressure Surge?

Pressure Surge or the sudden change in velocity and or pressure can arise due to various reasons. Hydraulic transients occur at changes in flow in piping/pipelines and this could be due to the:

  • Pump start & stop, specifically due to load shedding or sudden power failure
  • Quick operation of Valve (Sudden closure/opening)
  • Sudden closure of the check valve
  • Presence of Air pockets inside piping/ pipeline systems, especially during pump start
  • A sudden release of Air
  • Quick Pipeline filling
  • Pressure Surges can occur in open channels and partly liquid-filled pipes, as well

All of the above causes will generate high-pressure waves that can travel both upstream and downstream from point of origin. Please note that

  • Some pipelines are in transient operations over 75% of the time.
  • Pressure Surge (pressure rise) increases as the pipeline straight length increases since the contained momentum within two direction changes (elbows/Tee) will be higher (more volume).
  • A pressure surge normally consists of multiple events, resulting in up to ten times the normal pipeline pressure. When a surge relief valve opens, it vents the pressure to a safety system.
  • Surge pressure is created during the last 20% of valve closure.

Is Water Hammer Dangerous?

Refer to Fig. 2 to understand what a pressure surge can cause to a Piping System. Pressure Surge of Significant nature creates high pressure and velocity rise that can lead to:

  • Failure of pipe/pipeline fittings
  • Bursting of pipes
  • Damage to the Pump/pumping system
  • Deformation of valves and piping supports
  • Vibration or shaking of the piping/pipeline system
Consequences of Pressure Surge
Fig. 2: Consequences of Pressure Surge

Basic Definitions concerning Pressure Surge:

  • Pressure Surge:– It is basically a pressure wave caused due to a sudden change in flow velocity.
  • Wave speed or acoustic velocity:– The velocity at which pressure waves travel through the liquid/fluid.
  • Joukowsky equation:– Relationship relating head change to velocity change and acoustic velocity.
  • Pipeline Period:– Time required for a pressure wave to traverse the pipe/pipeline length and come back.
  • Pressure Head:– Pressure is measured as the height of fluid (10 m head of water is roughly 1 atmosphere)
  • Effective Valve closure Time: The period over which a Valve reduces the flow from 90% of its steady state to zero. In relation to Total Valve Closure Time, this is typically the last 15% opening for butterfly valves, 25% opening for ball valves, and 30% opening for plug valves. This can be used as a rule of thumb during the initial assessment phases.

Analysis of Water Hammer/Pressure Surge

The most important parameters to estimate the magnitude of transient pressures is:

  • Acoustic wave speed, a
  • Pipe/Pipeline period, T
  • Joukowsky head, Δh

The acoustic wave speed formula depends on the fluid and the pipe characteristics expressed as:

acoustic wave speed formula
  • a = Velocity of the pressure wave
  • K = Bulk modulus of the fluid
  • ρ = Liquid density
  • D = Internal diameter of the pipe
  • E = Young’s modulus of the pipe material
  • e = Wall thickness
  • ϕ = restraint factor (usually taken as 1)
Variation of wavespeed with pipeline characteristics
Fig. 3: Variation of wave speed with pipeline characteristics

The time that a pressure wave takes to travel from its origin through the system and back to its source is defined as the pipe period. For a single pipeline with pipeline Length, L this is provided as given below:

  • T = Critical period
  • L = Length of the pipe
  • a = Velocity of the pressure wave

Events that take place in less than T are called ‘fast’ events and these are likely to cause pressure surge issues.

Joukowsky formula
Fig. 4: Joukowsky formula

As per the Joukowsky formula, the pressure head change (Δh) due to an instantaneous velocity change (ΔV) is expressed as shown above. Here,

  • Δh = head rise
  • ΔV = change in velocity
  • a = wave speed
  • g = acceleration due to gravity

This is a very useful guide that explains the likely severity of a pressure surge event but is not a replacement for a proper surge analysis!

Limitations of the Joukowsky formula

Joukowsky formula is applicable to a limited set of fluid systems.

  • Its application should be limited to situations matching the following criteria:
    • Simple ‘linear’ piping systems i.e. there are no branches by which pressure waves can be reflected back and cause constructive interference in the main line.
    • Valve closure time is significantly shorter than the pressure wave communication time.
    • System frictional losses are similar to that of a water transport system.
  • Joukowsky equation does not consider column separation in its analysis of fluid hammer. Column separation can often result in surge pressures exceeding those predicted by the Joukowsky equation and therefore the Joukowsky equation should not be applied when analyzing a system in which the pipeline pressure can rapidly drop below the fluid vapor pressure.

How to Avoid Pressure Surge

To avoid pressure surge system must be protected. Protection of systems against water hammer can be parted into three groups:

1.0: System Design Solutions:

  • Use of pipework with a higher pressure rating i.e to make the pipework stronger to withstand the effects of surge pressure (Normally followed for radioactive, highly corrosive, or lethal fluids, where no fluid is allowed to escape.)
  • Rerouting of the pipeline avoiding high/low points
  • Changing of piping material, thus altering the wave speed
  • Increase the pipe diameter, thereby reducing the velocity
  • Increase pump inertia by incorporating a flywheel
  • Adding bypass lines
  • Providing Additional Pipe supports: By adding more supports in the piping system, the natural frequency of the system is increased. So, vibration tendency will reduce. Also, providing support near concentrated mass will reduce high local stresses.

2.0 Active Protection:

Piping/Pipeline systems can be protected against Surge impact by using devices during pipeline normal operation like:

  • Variable speed pumping: Variable speed drives provide a reliable means of preventing damage from pressure surge events.
  • Soft starters: The primary purpose of Soft starters is to reduce the electrical load on the power supply to a facility.
  • Slow closing and opening valves: A common form of pressure surge initiation is due to the rapid closing of a valve. Extending the closure times attenuates the pressure surge possibility.

Be informed that these devices require power and during load shedding or power failure cannot be of use.

3.0: Passive Protection (Fig. 5):

Passive Equipments for Surge Protection
Fig. 5: Passive Equipment for Surge Protection

There are several passive protection equipments available in the market that operates without the need for additional power. A few examples of these are:

  • Surge Vessels
  • Surge Shafts
  • Air Valves
  • Vacuum Breakers
  • Pressure Relief Valves/Surge relief Valve: Click here to know more about Surge relief valves
  • Surge Anticipation Valves: A surge anticipation valve is specially designed to provide a diversionary fluid flow during a pressure surge event.
  • Intermediate Check valves: In a long pipeline, an intermediate check valve has the ability to prevent the damaging reverse velocity from reaching a pump station. It effectively reduces the pressure surge to half.
  • Gas Accumulators: The gas accumulator is particularly effective in pressure surge scenarios due to a loss of power situation when downstream of the pump check valve, a negative pressure wave develops immediately. The deceleration of the liquid column is reduced by the residual pressure in the gas accumulator and prevents column separation. However, the gas accumulator should be located close to the boundary element that causes the transient event.
  • Liquid Accumulators: A liquid accumulator is a vessel that has lower elasticity than the pipe itself. The vessel will exhibit strain to a higher degree than the pipe and thus mitigate pressure transient.
  • Using low-modulus thermoplastic materials in combination with ferrous materials can mitigate a pressure surge.

Selection of System for Surge Protection

Refer to Fig. 6 below which provide a flowchart for Surge Protection System Selection.

Selection of System for Surge Protection
Fig. 6: Selection of System for Surge Protection

Pressure Surge Modelling Software

There are currently various software packages that can be used for analysis:

  • HyTran
  • Flowmaster
  • WANDA
  • Hammer
  • AFT Impulse
  • PIPENET
  • PTRAN
  • PASS/Hydro system
  • Flownex Simulation Environment

Methodology (Fig. 7):

Surge Analysis Methodology
Fig. 7: Surge Analysis Methodology

Designing a Pressure Surge Relief System

  • Consideration of a complex range of factors like the potential for pressure increases, the volumes to be passed by the surge relief equipment in operation, and the capacity of the system to contain pressures, etc are required for the design of a complete surge relief system.
  • Control or ESD valve closure times can also affect surge pressures in a pipeline. By increasing the valve closure time, a gradual flow decay can be achieved which will reduce the potential for pressure surge.
  • Control narrative and system interlocks to ensure Staged pump shutdown sequence and linked ship/shore ESDs when your facility is linked to loading berths/jetties.
  • Carry out transient / surge analysis using detailed computer modeling using the software mentioned above to simulate the complex interactions of equipment, pipelines, and fluid to normal, fault, and emergency events.
  • Design piping to withstand maximum surge pressure – MSP.
  • Although many design approaches can help reduce surge pressures in pipelines, going for a higher pipe rating or massive support arrangements aren’t recommended for the associated significant cost, and a surge relief valve was found to be the most feasible option to protect the system.
  • A correctly designed surge relief system will include components to dampen or slow the relief valve on closing, and this often requires sophisticated reverse flow plots.
  • In nitrogen-loaded Surge Relief valves, attention must be paid to the nitrogen gas system. The nitrogen system must supply a constant pressure (set point) to the modulating valve, even under conditions of varying ambient temperatures. Normally, the system is designed to use standard gas bottles and has its own control system to regulate the nitrogen supply pressure.

Conclusions:

  • The Pressure Surge phenomena during transient events are very important as they can put the system’s integrity at high risk.
  • During risk and HAZOP analysis, Pressure Surge Events and the corresponding mitigation devices should be always taken into account.
  • System operations staff must be trained in order to prevent operations likely to damage the system’s integrity.
  • Surge protection equipment must be maintained periodically.
  • It is highly possible to increase the reliability and life expectancy of systems by taking preventive measures for reducing the risk of failure due to pressure surge events,
  • The pipe/Pipeline system should be properly supported with the hold downs, guides, and line stops and the supports along with supporting structures must be designed considering dynamic forces during a Surge event.

Some more Resources for you..

Understanding Centrifugal Compressor Surge and Control
Water Hammer Basics in Pumps
Pipe Stress Analysis from Water Hammer Loads

References:

Frequently Asked Questions

What is Pressure Surge?

Pressure Surge is a pressure wave that is caused by the kinetic energy of the moving fluid when there is a sudden change in flow velocity.

What is the pressure surge in piping?

If the high-velocity flow in a pipe is forced to stop or change direction suddenly, a pressure wave generates and moves back at the speed of sound in the liquid. This can produce huge forces in the piping or pipeline system. This is called Pressure Surge in Piping

What is the difference between Pressure Surge and Water Hammer?

Pressure Surge, Water Hammer, Fluid Hammer or Hydraulic Surge, all these refer to the same event. There is no difference.

What Can Cause Pressure Surge?

The Pressure Surge in a Piping system can be caused by any of the following Events:
1. Pump start & stop, specifically due to load shedding or sudden power failure
2. Quick operation of Valve (Sudden closure/opening)
3. Sudden closure of the check valve
4. Presence of Air pockets inside piping/ pipeline systems, especially during pump start
5. A sudden release of Air
6. Quick Pipeline filling

How to Avoid Pressure Surge?

Pressure Surge can be avoided by the following methods:
1. Rapid Changes in fluid velocity occurs when valves are opened or closed suddenly. So by reducing the fluid velocity or by increasing the time taken for closing/opening the valve it can be avoided.
2. Surge can be avoided by installing Surge Relief Valve, Surge Tank, Viscous Damper, etc in the system.
3. The impact of surge can be reduced by reducing the number of elbows.
4. Eliminate the Presence of Air

What is Surge Analysis?

Surge Analysis is the analysis of pressure changes in the piping system, normally performed by Process Engineers for proper pipe sizing or finding the peak surge pressure.

Types of Structural Platforms

Written by Anup Kumar Dey in Civil,Piping Interface

A structural platform or civil platform is normally a horizontal surface at an elevated level usually provided for maintenance and easy access (operation) to required items such as valves, instruments, etc. Sometimes pipes and piping items can be supported from platforms. There is an abundant use of structural platforms in the Oil & Gas and Power Plant Industries.

Piping loads have to be transferred to the Civil and Structural Department after pipe stress analysis for taking consideration in platform design. However, heavy loads are not normally supported by platforms. At the same time, platforms are generally not designed for high horizontal loads. In such situations, some alternate structural arrangement is recommended.

Types of Structural Platforms:

Refer to Fig. 1, 2, and 3 for visualization of structural platforms. Normally 3 types of structural platforms are used in Oil and Gas/ Power Plant Industry, and those are:

  • OPERATING: Used for personnel movement for operating the valves/switch panels only
  • MAINTAINANCE: Used for maintenance of valves/equipment
  • MANIFOLDS: number of pipes are branching in one line
Typical Platform 1
Fig. 1: Typical Structural Platform 1
Typical Platform 2
Fig. 2: Typical Platform 2
Typical Platform 3
Fig. 3: Typical Platform 3

Inputs for Designing Structural platform:

  • PIPING LAYOUT: The piping layout will reflect the needs of the platforms.
  • SIZE AND ELEVATIONS will be provided
  • PIPE SUPPORT LOADING (IF ANY)
  • NORMAL LIVE LOADS
    • Access walkways: 250 kg/sq.m
    • Operating floors: 500 kg/sq.m
    • Special case(heat exchanger): 750 kg/sq.m

Approach to Structural Platforms:

Depending on the use of structural platforms and space availability, ladders or staircases are used for ascending and descending purposes,

Standard drawings for:

  • ladders
  • staircase

Covering of Platform:

  • Gratings and chequered plates as the covering of platforms depending on the use
  • Standard drawing: Grating

Protection in Platform:

  • Handrails to protect the personnel from falling.
  • Standard drawing: Handrails

Some More Resources for You

Piping Design and Layout,
Piping Stress Analysis,
Piping Interface,
Piping Material,
Piping Design Software ,
Civil Structural

References (External Link):

https://www.sciencedirect.com/topics/engineering/platform-design

General Requirements for Field Welding

Written by Anup Kumar Dey in Construction,Maintenance,Mechanical,Pipeline

Field Welding, as the name specifies, is done outside the manufacturing shop where the outside environment is not controlled. As most of the time it is done on construction sites, it becomes very challenging to maintain the quality of the welded product. In this article, I will cover a few important points for the requirements of field welding of various components like piping, tanks, structures, etc.

Documents Applicable for Field Welding:

  • ASME Code SectionⅨ
  • ASME Code B31.1 / B31.3
  • AWS D1.1

General Requirements During Field Welding:

Base Metal:

Before welding, the base metal that has to be welded must be confirmed to fulfill all the requirements which are specified in the applicable codes, standards, and local/company specifications as per applicable material certificates.

Welding Consumable:

The materials of welding consumables in all applicable welding processes must meet all the requirements mentioned in the governing codes, standards, and company specifications with the applicable consumable certificates.

All Welding consumables shall be positively segregated according to grades, brands, and size. They shall be stored in facilities to prevent any moisture absorption, contamination, or rust generation. They shall be free from such harmful defects as the exfoliation or crack of covering flux, dirt, and grease.

Prior to starting the welding, covered electrodes for low-hydrogen type shall be fully dried in accordance with the manufacturer’s recommendation (e.g. between 300℃ to 350℃ for 1hr). After drying, these electrodes shall be stored in the electrode oven between 100℃ to 150℃. If 4 hours have elapsed since these electrodes were taken out of the oven, they shall be re-dried. The number of times of re-drying shall be up to 2 times.

Backing Strip:

  • A permanent backing strip shall not be used.
  • Temporary backing strips used at the welded joint shall be removed.

Shield Gas:

In accordance with applicable code and standard. Shield gas, backing gas, and trailing gas to be used for GTAW shall be Ar gas.

Requirements for Welding machine and Related Equipment:

All welding equipment such as torches, cables, grinders, and heating equipment shall be maintained in good working order and calibrated as necessary.

Earth cable for welding source shall be attached to base metal stably using cable holders. If a copper alloy cable holder is used, it shall be prevented from contact directly between the base metal and copper alloy cable holder by means of inserting a steel plate.

Field Welding at Construction Sites
Field Welding at Construction Sites

Groove:

  • The shapes and dimensions of grooves to be welded shall be in accordance with applicable codes and standards or shall be approved in WPS and drawings.
  • For carbon steel, flame cutting and beveling are acceptable only if the cut surface is reasonably smooth and sound, and all oxides are removed by grinding from the flame-cut surface.
  • For low alloy steel, flame-cut bevels are acceptable only where machine mechanical cutting is not feasible. After flame-cutting, approximately 2.0 mm of material shall be removed from the surface of the bevel by grinding.
  • For stainless steel, mechanical means or plasma cutting shall be acceptable. The grooved surface shall be smoothly finished by removing completely all burrs caused by machining, and surface oxides caused by plasma cutting.

Fitting:

Parts that have to be welded shall be fixed by jigs, clamps, bridges, or direct tack welding so that required dimensions and orientations can be obtained. The materials of jigs or bridges welded to base metals shall be the same materials as the base metals.

Tack welding shall be done only by qualified welders using the approved WPS. Welding electrodes to be used for tack welding shall be the same grades or brands as those to be used for production welding.

After completion of welding, jigs or bridges shall be removed and their vestiges shall be finished smoothly by grinders to prevent any harmful defects such as under-cut in the base metal.

The allowance of dimensions and orientations for fitting-up to be welded has to be in accordance with the applicable document and the approved WPS.

Cleaning:

  • Prior to welding, the groove surfaces and their adjacent areas shall be completely free from oil, paint, low melting point metal, oxides, water, and all other foreign matters.

Weather:

  • Temperature-Welding work shall not be allowed under –10℃ of ambient conditions.
  • Rain / Wind-Welding works shall not be allowed, while rain, snow, or wind conditions could affect the quality of welding work unless suitable protection facilities are provided.

Requirements for Field Welding:

  • Welding shall be performed according to the approved WPS by qualified welders/welding operators.
  • The Arc starting point shall be in the bevel surface or tab plates. Arc termination and starting point shall not overlap those of the previous layer.
  • After completion of welding from the first side of double butt welds, the initial root pass, including root tack welds, shall be chipped, ground, or gouged to sound metal. The back-chipped welds, welding groove, and plate edges shall be examined to ensure that all cracks, laminations, pinholes, and other defects have been removed prior to commencing welding on the second side
  • For stainless steel, the width of weaving shall be less than approximately 2.5 times the diameter of the electrode.
  • Except for austenitic stainless steel, welding passes shall be performed consecutively as a rule so that there is no interruption between the start and finish of welding. The number of welding layers shall be two or more as a rule, including socket welding for piping.
  • Welding may be interrupted after a joint has been welded 1/2 thickness of base metal or 3rd layer or more. In the case of Cr-Mo steel, post-heating after interruption and pre-heating before restarting shall be applied in accordance with the applicable code, standard, specification, and WPS.
  • After completion of welding, the surface of welds shall be smooth, and shall not be allowed to include harmful welding defects such as excessive irregularity surface, under-cut, overlap or spatter

Heat Treatment requirements for Field Welding:

Pre-heating:

  • When the ambient temperature is not more than 5℃, pre-heating shall be applied within the temperature range of 30℃ to 50℃
  • Pre-heating temperature shall be measured by the use of temperature-indicating crayons, thermocouple pyrometers, or other suitable methods to assure that the required pre-heating temperature is obtained prior to and uniformly maintained during the welding operation.
  • For Cr-Mo steel, according to the level of Cr contents, the pre-heating temperature shall be in accordance with the following.
    • 1/2% < Cr ≦ 2% : 150℃ or more
    • 2% < Cr ≦ 9% : 200℃ or more
  • For austenitic stainless steel, pre-heating shall not be applied as a rule.
  • For stainless steel except for austenitic stainless steel, pre-heating shall be applied in the temperature range of 150℃ to 350℃.

Interface Temperature:

  • For carbon steel, the interface temperature shall be between 100℃ to 350℃.
  • For Cr-Mo steel, the interface temperature shall be between the temperature of pre-heating and 350℃.
  • For austenitic stainless steel, the interface temperature shall not be over 150℃.
  • For stainless steel except for austenitic stainless steel, the interface temperature shall be between 150℃ to 350℃.

Post-heating:

If the final Post Weld Heat Treatment (PWHT) cannot be carried out immediately on completion of welding, The post-heating temperature which is over its preheating temperature shall be applied.

PWHT:

All applicable codes, standards, and specifications shall be followed during the post-weld heat treatment. It must be done once all welding is completed.

PWHT has to be performed locally by means of electric induction heating, electric resistance heating, or high-frequency electric heating. Alternatively, furnace heat treatment using gas or electricity can be also acceptable. However, the use of fixed or handheld-type gas burners shall not be acceptable.

Inspection:

Governing codes, standards and specifications are to be followed while Tests and inspections are conducted at each stage of field welding.

Repair:

Before starting the repair weld, The WPS for repair weld and production WPS shall be approved. Once repair welding is finished, the repaired area has to be re-inspected by the same means previously used.

Special Field Welding Requirements for Piping:

Welding consumable:

When the operating temperature is above 345 ℃ for 2-1/4Cr-1Mo steel, each batch or heat of welding consumable and covered electrodes, including the wire flux combinations used in welding, shall be analyzed for P, Sn, Sb, and As. This analysis has to be done on the weld metal deposits. The Temper Embrittlement Factor, X-bar, must be maintained as below;

X-bar = (10P + 4Sn + 5Sb + As) / 100 ≦ 15 (PPM)

Element concentrations are in PPM i.e, parts per million.

Welding:

  • The welding position should be a 1G position as much as possible.
  • Fillet welding to pressure retaining components shall be ground to a smooth, concave contour.
  • Peening shall not be permitted.
  • Each welder/ welding operator shall mark his proper identification mark on the pipe surface near his executed welding.

 Pre-heating:

  1. For carbon steel with an operating temperature above 100℃, pre-heating shall be required for the below-mentioned services;
    • The thickness of the base metal is 25 mm or more.
    • The tensile strength of the base metal is 490N/mm2 or more.
    • Ceq ( = C + Mn/6 + Si/24 ) of base metal is 0.45% or more.
  2. The width of pre-heating shall be 3 times of base metal or 25mm at both sides of the welding bead which is larger.

Post-heating:

The joint that has been welded shall not be allowed to be cooled below 149℃ before heat treatment. In case this is not practical, the weld along with the adjacent pipe shall be heated uniformly to 316℃ for 15 minutes wrapped with insulation, and then allowed to cool. Then PWHT may be performed later.

PWHT:

PWHT has to be performed for the following services;

  • Cr-Mo steel piping for all services
  • Carbon steel piping containing amines at a concentration greater than 2.0 eight %
  • Any other service specified in the applicable code, standard, and specification.

Few more welding articles for you.

Welding Galvanized Steel
Overview of Pipeline Welding
Welding Positions: Pipe Welding Positions
Welding Defects: Defects in Welding: Types of Welding Defects
Welding Inspector: CSWIP and AWS-CWI
General requirements for Field Welding
Underwater Welding & Inspection Overview
Methods for Welding Stainless Steel

References:

Video Courses in Welding

To learn more about welding the following video courses you can refer to:

Fixing Location of Various Static Equipment: Fired Heater, Reactor, Exchanger, Drums

Written by Anup Kumar Dey in Piping Design Basics

Process Industries uses various types of equipment for different process reasons. The main purpose of this presentation is to discuss the basic aspects related to fixing an appropriate location for the piping design of static equipment.  Static equipment generally found in process plants are as follows:

  • Fired Heaters
  • Reactors
  • Exchangers
  • Air-cooled Heat Exchangers
  • Drums
  • Towers

Fixing the Location of Fired Heaters

  • The fired heater shall be located in the upwind direction or at least crosswind from sources of hydrocarbon leaks.
  • The minimum distance is required between the fired heater and the fractionating tower and reactor.
  • Provide tube removal/maintenance space within battery limits. Crane access may be necessary, especially for vertical tubes.
  • Provide space for removal/maintenance of burners, soot blowers, and convection sections.
  • Knockout drums for fuel gas supply to fired heaters shall be located as close as possible.
Fired Heater
Fig. 1: Figure of a typical Fired Heater

Fixing the Location of Reactors

  • The location shall be as close as possible to Fired Heaters so that piping is short and simple.
  • Reactors shall be located for ease of access during catalyst unloading and loading operations.
  • Space shall be allowed for cranes and storage of spent and new catalysts.
  • A separate structure with the platform is required for catalyst loading and unloading.
  • Space shall be allowed for TW- removal & maintenance.
Reactor
Fig. 2: Figure of a typical Reactor

Fixing the Location of Exchangers

  • Locate exchangers at grade unless the elevated location is required by the Client.
  • Shell and tube exchangers shall be located with channel end away from pipe ways to facilitate tube bundle removal.
  • For exchangers under drums or unit structures, where ever possible the channel end shall be clear of overhead structures for the handling of the channel end by mobile equipment.
  • Heat exchangers containing flammable liquids above 260 C, or their auto-ignition temperature (if lower), shall not be located beneath other equipment.
  • Exchangers shall be located close to the associated equipment and pipe rack, so that the piping is short, but has adequate flexibility.

Fixing the Location of Air-cooled Exchangers

  • Process equipment shall not be located above or below air-cooled heat exchangers.
  • Locate above the unit pipe rack.
  • Can’t locate where the discharge air from one exchanger can become the incoming air to another exchanger.
  • Provide crane access to air-cooled heat exchangers for maintenance.
  • The distance between the air condenser outlet and the receiver shall be minimized.
Air Cooled Heat Exchanger
Fig. 3: Figure of a typical Air Cooled Heat Exchanger

Fixing the Location of Drums

  • Drums are located within a processing unit either adjacent to related equipment or as a standalone operation
  • Should be positioned to facilitate an orderly and economic piping interconnection.
  • Generally, drums are located on either side of a central pipe rack serviced by an auxiliary road for maintenance access
  • Drums are generally located at all levels of enclosed or open-sided structures.

You may be interested in

Static Equipments used in Process Piping Industry
Nozzle Loading of Various Equipments and means for reducing them
Considerations for Equipment and Piping Layout of Air Cooled Heat Exchanger Piping
A Brief Presentation on Storage Tanks
Vendor Offer Review, TBE and Vendor Drawing Review
Construction and Maintenance of Fired heaters

Hot and Cold Thermal Insulation for Piping | Calculation of Thermal Insulation Thickness

Written by Anup Kumar Dey in Engineering Materials,Piping Design Basics

Piping Thermal Insulation is very important for saving energy costs and maintaining the process fluid temperature at the required level. In case, thermal Insulation is appropriately chosen and used so that it is Non-complaining, Maintenance-free, and Patient workhouse, It looks after the economy with tremendous savings in energy costs, the safety of personnel, and smoother process control.

On the other hand, insufficient or poor piping insulation or deterioration of existing thermal insulation can be a cause of huge energy loss. So most of the time, thermal Insulation is defined as, “A major tool in improving energy availability”. The thermal insulation material is also important to achieve low thermal conductivity and low thermal inertia.

The basic objective of thermal insulation is to retard the flow of heat:

  • From a hot surface to a cold environment or
  • From a warm environment to a cold surface
Insulated Pipes
Insulated Pipes

Energy Loss from Hot Surface Without Thermal Insulation

Fig. 1A and Fig. 1B below show a typical example of heat losses from the piping surface if the pipe is not insulated.

Example showing Heat Loss from Hot Surfaces
Fig. 1A: Example showing Heat Loss from Hot Surfaces
Energy loss from pipe without thermal insulation
Fig. 1B: Energy loss from the pipe without thermal insulation

The heat loss values are normally corrected by the correction factor for certain applications.

Economic Reasons for Thermal Insulation:

Piping thermal insulation

  • Reduces fuel consumption, and hence overall operational cost so day-to-day economic benefits.
  • Reduces capacity requirements for heating/cooling systems (boiler, refrigeration unit, etc)
  • Savings in Capital costs

Even though the basic requirement for providing thermal insulation is Economic, still it is not the sole criterion. The process requirement controls the usage of thermal insulation.

Process Reasons for using thermal Insulation:

  • Reduces the temperature drop of fluid in a heated system
  • Reduces temperature gain of fluid in the refrigerated system.
  • Reduces boil-off rate in a volatile liquid storage system
  • Assist in maintaining thermal balance in the reaction system
  • In the heated system, it lowers temp. of exposed surfaces-protects workmen from burn hazard
  • Provides fire protection for plant, equipment & piping
  • Reduces capacity requirements for heating/cooling systems (boiler, refrigeration unit, etc)

Economic Thickness for Thermal Insulation

The thermal insulation thickness for which the total cost (insulation material cost + energy cost) is minimum is termed as economic thickness. Refer to Fig. 2 below which shows the total cost for a typical plant. Similar curves are plotted to find out the economic thermal insulation thickness.

Economic Thermal Insulation Thickness
Fig. 2: Determination of Economic Thickness

By virtue, Insulation shall resist heat transfer by:

  • Radiation
  • Convection
  • Conduction

Types of Thermal Insulation

  1. Mass-type insulation: Based on interposing a mass of material with a built-in capacity to retard heat flow
  2. Reflective Insulation: Based on providing a series of the reflective surface with the intervening space s evacuated
  3. Microporous Insulation: Based on a combination of Mass & Reflective technologies.

Physical Properties of Thermal Insulation Materials

Significant physical parameters of thermal insulating materials can be divided into:

  • Thermal Properties
  • Chemical Properties
  • Commercial Factors

Thermal Properties:

The basic thermal parameters that thermal insulation materials should possess are:

  • Temperature resistance
  • Thermal conductivity
  • Thermal diffusivity, and
  • Thermal shock resistance

Chemical Properties of Insulating Material:

Major Chemical properties of insulating materials are:

  • Compatibility with the metal surface
  • Compatibility with environmental media
  • Deterioration arising out of the chemical action
  • Life of insulation material

Points to remember while selecting thermal insulating materials:

  • Alkalinity (pH) or acidity
  • Chemical Reactivity/passivity
  • Coefficient of Expansion / Contraction
  • Compressive Strength & Breaking Load
  • Abrasion Resistance
  • Combustibility
  • Most importantly, THERMAL CONDUCTIVITY

Thermal Conductivity Vs Density:

The thermal conductivity of a material provides the heat loss per unit area per unit insulation thickness per unit temperature difference. The unit of measurement is W-m2/m°C or W-m/°C. With an increase in temperature, the thermal conductivity of materials increases. That is why the thermal conductivity for thermal insulation materials is always specified at the mean temperature (mean of hot and cold face temperatures). Fig. 3A provides a curve showing the relation between thermal conductivity and density of the thermal insulation material.

Thermal Conductivity and Density
Fig. 3A: A curve showing the relation between Thermal Conductivity and Density

Refer to Fig. 3B below which provides some typical thermal conductivity values for hot and cold insulation materials.

Thermal conductivity of hot and cold thermal insulation materials
Fig. 3B: Thermal conductivity of hot and cold thermal insulation materials

Widely used Hot Insulating Material:

  • Mineral Wool
  • Ceramic Fibre
  • Calcium Silicate

Widely used Cold Insulating Materials:

  • Expanded Polystyrene Foam (EPS)
  • Extruded Polystyrene Foam (XPS)
  • Polyurethane Foam (PUF)
  • Poly-isocyanurate Foam (PIR)
  • Foam glass
  • Phenolic Foam
  • Thermocol

Among the above-listed materials, Polyurethane and Polyisocyanurate have assumed the highest importance because these possess many superiorities as compared to others. Both of these materials can be used as Pre-formed shapes or installed in-situ-by Pouring or by spraying.

Insulation Finishes:

The outer part of insulation is normally provided with:

  • Weather barriers-claddings
  • Weather and Vapor retarder, Indoor coverings, and finishes

All of these have only one basic function which is to protect the insulation material from severe external exposure media.

Thermal Insulating System Design

The insulation system should perform to the expected level, undiminished over its life.

  • This needs full data on material behavior under all conditions of exposure.
  • In particular, we need to know what would make a material lose its properties.

Thermal calculations need a representative value of Thermal Conductivity for the design

  • Standard materials like Rockwool and Calcium Silicate have well-established Design ‘k’ values.
  • Limiting Service Temperature of use
  • In pipe applications, weight becomes critical. Abrasion is also a major problem with some materials.

Calculation of Thermal Insulation Thickness

The most basic piping model with thermal insulation is shown in Fig. 4. where r1 denotes the pipe outside radius and r2 shows the radius of the Pipe including insulation.

Typical Pipe Cross Section with Insulation
Fig. 4: Typical Pipe Cross Section with Insulation

In the calculation of thermal insulation, the first step is to calculate the heat loss from the pipe.

Heat loss from a surface is expressed as H = h X A x (Th-Ta) Where

  • h = Heat transfer coefficient, W/m2-K
  • H = Heat loss, Watts
  • Ta = Average ambient temperature, K
  • Ts = Desired/actual insulation surface temperature, ºC
  • Th = Hot surface temperature (for hot fluid piping), ºC & Cold surface temperature for cold fluids piping)

For horizontal pipes, the heat transfer coefficient can be calculated by:

h = (A + 0.005 (Th – Ta)) x 10 W/m2-K

For vertical pipes,

h = (B + 0.009 ( Th – Ta)) x 10 W/m2-K

Here A, and B are coefficients that can be obtained from the table in Fig. 5

Tm = ( Th + Ts)/2

  • k = Thermal conductivity of insulation at a mean temperature of Tm, W/m-oC
  • tk = Thickness of insulation, mm
  • r1 = Actual outer radius of the pipe, mm
  • r2 = (r1 + tk)
  • Rs = Surface thermal resistance =1/h oC-m2/W
  • Rl = Thermal resistance of insulation =tk/k ºC-m2/W

The heat flow from the pipe surface and the ambient can be expressed as follows

H = Heat flow, Watts= (Th-Ta)/(Rl+Ra)=(Ts-Ta)/Rs

From the above equation, and for a desired Ts, Rl can be calculated. From Rl and the known value of thermal conductivity k, the thickness of insulation can be calculated.

Equivalent thickness of insulation for pipe, Etk = (r1+tk) X ln{(r1+tk)/r1}

Some more resources for you:

Piping insulation: Important Considerations for Piping Engineer
Corrosion under insulation: A Presentation

References: