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Common Causes of Piping Vibration: How to Reduce Pipe Vibration?

Written by Anup Kumar Dey in Mechanical,Piping Stress Analysis,Piping Stress Basics

Piping Vibration can be defined as a continuous to-and-fro motion from an equilibrium position. Piping vibration problems cause serious integrity risks to operating plants; both onshore and offshore production facilities. The vibration of the Piping System can cause fatigue failure on process piping and small branch connections and reliability problems on equipment. Equipment nozzles, Relief lines, instrumentation ports, drain connections, and valves can also be subjected to Piping Vibration. Data provided by the UK Health & Safety Executive shows that around 21% of hydrocarbon releases are due to vibration-induced fatigue failures.

Due to the fact that most of the piping design codes (ASME B31.3, B31.1, B31.4, B 31.8, etc.) do not address the vibration issues in a detailed fashion, the damaging effect is normally ignored during the design stage and simple static analysis without attention to vibration is performed on piping systems.

At the same time, the piping vibration tendency is increasing to a great extent due to increased flow rates of process industries through pipes and the usage of high-strength thin-walled piping (flexible) material during design. It is seen that Piping Vibration causes many problems in operating plants and the problem should be solved during the design phase. Major of the damaging effects of vibration can be mitigated if proper design philosophy is taken while designing the system. This article highlights the major causes of piping vibration and their effects in short.

There are many reasons which can cause vibration in a piping system.

Cause of Piping Vibration

There are a variety of excitation mechanisms that can be present in a piping system and can produce piping vibration and finally, failure resulting from fatigue. Some of those causes are listed below:

  • Flow-induced Vibration: Caused by the turbulence of the flowing fluid.
  • Mechanical forces from Equipment: Caused by the excitation forces of reciprocating and rotary equipment like pumps, compressors, etc.
  • Pressure Pulsations from reciprocating equipment.
  • High-frequency Acoustic excitations are generated by high-pressure drops at relief valves, control valves, or orifice plates.
  • Water Hammer (Pressure Surge) or Momentum changes due to sudden valve closure.
  • Cavitation or vapor bubble collapse due to localized pressure drop.
  • Due to the sudden flashing of fluid.
  • Periodic pressure disturbances during a flow past the dead-end of branch connection/ instrumental items.

Effects of Piping Vibration

Data has shown that out of all failures and downtimes in any individual plant around 10-15% are because of vibration-induced fatigue. The major effects of piping vibration are as follows:

  • Piping Vibration causes dynamic stresses (fatigue) in a piping system. If this stress is more than the critical value it will initiate a crack that will propagate slowly and end in the failure of the item in concern. The more fatigue-sensitive places are the weld point connections where the branch and header are joined together.
  • In addition to dynamic stresses, vibration results in wearing surfaces in contact due to cyclical relative motion between them. This phenomenon is known as Fretting.

The vibration of plant piping is a significant risk to asset integrity and safety. So, must be addressed. To manage the risk of piping vibration, various analysis and measurement services are performed.

Every piping system has the tendency to vibrate at certain frequencies, called natural frequencies. Every natural frequency is associated with a definite and unique shape, called a mode shape. The natural frequencies and modes depend on the distribution of mass and stiffness throughout the piping system, and the distribution is influenced by piping diameter, material properties, wall thickness, location of lumped masses (such as valves), piping supports, and fluid density.

A mode shape has the locations of zero motion (node) and maximum motion (anti-nodes). The response of the piping to an applied excitation depends on the relationship between the frequency and pattern of the excitation and the piping system’s natural frequencies/modes.

When a piping system is excited by a dynamic excitation with a frequency that coincides with one of its natural frequencies, the system undergoes great displacements and stresses. This phenomenon is known as resonance, and it can cause high vibration, even fatigue, and subsequently, failure. Vibration generated in the piping work may lead to high-cycle fatigue of components, such as small-bore connections, or the failure at welds in the main piping itself.

Vibration Problems Due to Two-Phase Flow

Two-phase flow refers to the interactive flow of two distinct phases with common interfaces in a piping system, with each phase representing a mass or volume of matter. The two phases can exist as combinations of solid, gas, and/or liquid phases. Although multiphase flow involving three phases can also exist, most multiphase engineering applications are two-phase flows.

Two-phase flow exists in many process piping and power piping components. The flowing fluid is a source of energy that can induce small-amplitude subcritical oscillations and large-amplitude dynamic instabilities. In fact, many practical system components have experienced excessive flow-induced vibrations. To prevent unacceptable flow-induced vibration, Users must understand excitation mechanisms, develop analytical and experimental techniques, and provide reliable design guidelines.

How to Reduce Pipe Vibration?

There are a number of ways that can be implemented to reduce pipe vibration. Some of the common ways to reduce pipe and pipeline vibration at operating plants are outlined below:

Support Addition:

Adding pipe supports makes the pipe or pipeline system stiffer which increases the pipe’s natural frequency. Small forces can not easily vibrate a stiff or rigid piping system. The addition of guides, line stops, and hold-down supports in most situations arrest the piping vibration. If guide and line stops have sufficient gaps, tightening up those clearances will also provide a nice effect on pipeline vibration elimination. In reciprocating systems, the support span is reduced to lower values than the standard pipe spacing. Note that, adding support will make the pipe system more rigid. So, It is always suggested to check the thermal stress after the addition of new pipe supports. To reduce/eliminate piping vibration, it is a general practice to keep the natural frequency of the piping system in excess of 4 Hz.

Adding Vibration Damping to the Main Piping:

Installing vibration dampeners in the main pipe near the vibration source can easily dampen the pipe vibration. In highly critical systems, many a time hydraulic cylinders and dynamic dampers are installed to reduce pipe vibration.

Adding viscous dampers or sway braces is a good solution to dampen the pipe vibration while allowing the required thermal movement.

Adding reinforcement to vents, drains, and small bore pipe connections:

To increase the stiffness of small bore pipe connections, thicker wall pipes are used. Most of the time, reinforcement by adding bracing from the main piping system reduces the vibration in small bore pipe connections.

Supporting Rigid Bodies (Valves):

Heavy rigid objects like valves, flanges, or other items must be independently supported to reduce the potential of vibration of those elements.

Increase Pipe Size:

Sometimes if the flow velocity is very high, pipe size can be increased to reduce the vibration possibility due to flow-induced vibration. The same must be discussed with the concerned process engineer for hydraulic recalculation and confirmation. Flow smoothening can also be tried to convert turbulence flow into a smooth flow which in turn will reduce the vibration tendency.

Increasing the pipe wall thickness increases the pipe rigidity which in turn reduces the vibration possibility of the pipe.

Piping Vibration due to Seismic Event:

Snubbers are used in piping systems to reduce the vibration generated due to seismic events.

Acoustic Silencers:

To reduce the vibration generated due to acoustic-induced vibration, acoustic silencers can be used. Sometimes, using multi-stage pressure reduction inside the orifice or pressure control valves proves to be beneficial in piping vibration reduction.

Isolation of Vibration Source:

Isolating the vibration source from the pipework is an effective method to control pipe vibration. Expansion bellows, PTFE pads, Anti-vibration mounts, etc can be used to isolate the piping system from the vibrating source.

Reducing Vibration due to Pulsating equipment:

To prevent the occurrence of resonance, the piping natural frequency is kept +/-20% away from the pulsating equipment frequency. Pulsation bottles are widely used for reciprocating devices to reduce the pipe vibration potential.

Changing Pipe/Pipeline Routing:

Minimizing bends and avoiding abrupt directional changes reduces the dynamic pressure reaction which in turn reduces pipeline vibration tendency.

Pipe Vibration due to Surge:

To reduce the piping vibration due to surge/ water hammer, the rapid changes in fluid velocity must be prevented. Use of soft start pumps, increasing valve opening/ closing time, using surge pressure relief valves, Surge arrestors, etc can be beneficial.

Piping Vibration Standards

Piping vibration standards are guidelines or specifications that establish limits and criteria for acceptable levels of vibration in industrial piping systems. Excessive vibration in piping systems can cause damage, failure, or reduced operational efficiency, which can lead to safety hazards, downtime, and costly repairs. So, there are several piping vibration standards that address the issue of piping vibration and provide certain guidelines. Some of the common vibration standards are:

  • Guidance for the Avoidance of Vibration-Induced Fatigue Failure in Process Pipework by Energy Institute
  • ASME OM-3: This Standard provides general requirements for the assessment of piping system vibration for nuclear power plants.
  • VDI 3842: Vibrations in piping systems
  • ISO 10816-3: Mechanical vibration – Evaluation of machine vibration by measurements on non-rotating parts.
  • ISO 17.160: Vibrations, shock, and vibration measurements.
  • ISO 2017: Mechanical vibration and shock
  • ISO 2041: Mechanical vibration, shock, and condition monitoring
  • ASTM D999: Product Vibration Testing
  • NUREG-1061
  • IEC 60068
  • Gost PTM 38.001
  • API 618: Reciprocating Compressors for Petroleum, Chemical, and Gas Industry Services: This standard provides guidelines for the design and installation of reciprocating compressors, including requirements for minimizing vibration.
  • API 610
  • ASME B31.3: Process Piping: This standard provides guidelines for the design, fabrication, installation, inspection, and testing of process piping systems, including requirements for vibration control.
  • EEMUA 145: Vibration of pipework: A guide to the design and protection of pipework against vibration and fatigue: This standard provides guidance on the design and protection of pipework against vibration and fatigue.

These standards provide criteria for vibration levels, methods for measuring and analyzing vibration, and recommendations for mitigating excessive vibration. Compliance with these standards can help ensure the safe and reliable operation of piping systems.

Piping Vibration Severity Chart

A piping vibration severity chart is a tool used to assess the severity of vibration in piping systems. It provides a graphical representation of the vibration level in relation to established standards or criteria, such as those outlined in the piping vibration standards mentioned in my previous answer.

The chart typically plots the measured vibration amplitude (usually in units of millimeters per second or inches per second) on the vertical axis and the frequency of the vibration (usually in Hertz) on the horizontal axis. The chart may also include different colored zones or bands that indicate different levels of severity, ranging from acceptable to potentially damaging or unsafe.

By comparing the measured vibration amplitude and frequency to the severity chart, engineers can quickly determine whether the vibration is within acceptable limits or requires further investigation and corrective action. This can help prevent equipment damage, reduce maintenance costs, and improve safety and reliability in piping systems.

It is important to note that different standards may have different severity charts, so it is important to refer to the appropriate standard when interpreting vibration data using a severity chart.

Video Tutorial on Solving Vibration Problem Due to two-phase flow

In this video collection presented by the Dynaflow research group, the basics of vibration because of two-phase flow, are clarified by the presenter. I hope, most of you will get a good insight to solve the two-phase vibration problems. If you wish to add something more please write in the comments section.

Video: Solving Vibration Problems Due to Two-Phase Flow

Few more Useful Resources for you.

Vibration Related
Pump Related
Piping Design and Layout Basics
Heat Exchanger Related
Vessel Related

Introduction to Marine Loading Arm

Written by Anup Kumar Dey in Pipeline,Piping Design Basics,Piping Interface

A Marine Loading Arm is used to load or unload the ship or vessel carrying Petroleum products, chemicals, etc. Marine loading arms are made up of several sections of pipe (quick-connect fittings), connected by swivel joints.  The section on the shore side of the ‘apex’ of the loading arm is known as the inboard arm and the section on the tanker side of the ‘apex’ is known as the outboard section.

Marine loading arms are flexible enough to accommodate any movement of the vessel during loading/discharging. These hydraulically operated marine loading arms are fitted with emergency release couplings and emergency release systems and support faster and safety loading/unloading requirements.

A marine loading arm is an alternative to direct hose hook-ups. A loading arm must be drained before breaking off the connection. This is done in any one of the two ways mentioned below:

  • For fuels like gas oil and diesel, high-pressure air can be used to blow out the oil traces.
  • In the case of fuels such as kerosene or petrol, the lines can be stripped using pumps.

Loading arms have the capability to handle both liquids and gases, in a wide range of viscosities and temperatures. Cargoes from liquid sulfur to liquefied natural gas can be moved through marine loading arms.

Features of Marine Loading Arm

Marine loading arms (Fig. 1) must be specified/designed to allow for movement of the tanker during loading/unloading, due to tides/waves/wind and due to the tanker’s cargo increasing/decreasing.  Each loading arm is designed with a certain operating ‘envelope’.  If a connected loading arm is forced outside this envelope, it should be disconnected immediately (manually or, for some systems, automatically). The main factors that affect the required operating envelope are tanker DWT, tidal range, maximum wave height, the elevation of the jetty structure, and the size/number of loading arms at the berth.  

In general, marine loading arms are hydraulically powered.  A hydraulic system, which often serves all the loading arms at a berth, is used to move the arms into position at the tanker’s rail, and to retract the arms back into the stowed position after use.  It is common for loading arms to be one (or even two) sizes smaller than the associated pipelines.

When loading a small coastal tanker (up to say 5,000 DWT), it is normal to use only one marine loading arm (subject to loading arm diameter), whereas when unloading a large vessel (eg a crude oil VLCC in excess of 160,000 DWT), it is common to use up to four loading arms in parallel, in order to achieve the required flowrate.

There are a number of systems for connecting marine loading arms to tankers.  The simplest connection is made using a flanged coupling.  It is common practice for loading arms to be fitted with powered universal couplers.  Marine Loading arms handling LPG (or any other pressurized/refrigerated material) should have a powered emergency release coupling (PERC), connected to the ESD system.

If the occupancy of a berth is very low by design, it may make sense economically to use hoses rather than loading arms.  But there are factors that may outweigh the cost-benefit of hoses relative to loading arms, as follows:

  • In order to handle hoses with a diameter of 4” and above, a crane is required 
  • In order to prevent snagging, hoses often need support that has to be adjusted during loading/unloading, for which a crane has required the environmental performance (in terms of emissions to air and to water) of a berth with hoses is generally not as good as that of a berth with loading arms

There are a number of standards relating to loading arm design and hose manufacture. Loading arms can be divided into two categories;

  • those designed to handle liquefied gas (pressurized, refrigerated or semi-refrigerated) and
  • those designed to handle Refinery products which are stored at ambient conditions.
Typical Marine Loading Arm
Fig. 1: Typical Marine Loading Arm

Marine Loading Arms for Refinery Products

For berth handling Refinery products that are stored at ambient conditions, it is common to have two pairs of loading arms to handle all non-refrigerated/pressurized products.  Each pair of arms has a manifold such that one pair is dedicated to ‘white oils’ and one pair is dedicated to ‘black oils’. Product contamination is avoided by stripping and draining as part of the loading process, as follows:

  • After pumping to the tanker is complete, the MOV upstream of the manifold is closed.  These MOVs are generally the double seal design, to ensure product segregation.  
  • The liquid remaining in the manifold and the inboard section of the loading arm is then pumped out by the stripping pump.  The stripping pump discharges either into the outboard section of the selected loading arm (i.e. onto the tanker) via a small ‘piggyback’ line, back into the supply pipeline upstream of the MOV, or to the on-shore slops system. 
  • Any un-pumpable material in the manifold is drained into an on-shore slops drum at the berth, leaving the manifold and loading arms empty.

For hydrocarbons stored at ambient conditions, the stripping pump should discharge into the outboard section of the loading arm because generally speaking the material has already been metered and therefore belongs to the customer.

Loading Arms for LPG at Ambient Temperature

For a berth handling LPG or any other material stored under pressure, it is common to have a vapour return system to take the vapours displaced from a tanker during loading.  The flow of returned vapour should be metered. The vapour return can be a separate loading arm, but it is common for an LPG loading arm to be designed with a ‘piggy-back’ vapour return line i.e. two loading arms in one. 

The returned vapour can be routed to the storage vessel from which product is exported.  This enables high loading rates to be achieved but can be impractical/uneconomic over large distances, particularly if fans/blowers are required to boost the pressure in the vapour return pipeline.  Returning vapours to the storage vessel give rise to the risk of a storage vessel being sent off-spec by contaminated returned vapours. 

An alternative is to route the returned vapor to a local flare/vent system, with a pressure controller in the vapor return line to back-pressure the tanker. 

When pumping LPG onto a tanker is complete, stripping of the loading arm(s) is usually achieved as follows:

  • The loading arm/manifold is de-pressured into a dedicated closed sump, from which the drainings vaporize (with heating as required) into the vapor return system.  
  • The loading arm/manifold is purged with nitrogen to remove hydrocarbon vapors. Nitrogen purging is often carried out before connecting the loading arm, in order to remove air.

Loading Arms for Refrigerated LPG

Many of the considerations applicable to loading arms for products stored under pressure also apply to the design of loading arms for refrigerated (or semi-refrigerated) LPG, and for other products stored at below ambient temperature.

Additional design consideration for loading arms handling (semi) refrigerated products is the provision of a liquid return line to permit the chill down of the supply system prior to loading.

Design Code for Marine Loading Arm

Marine Loading Arms are Designed as per OCIMF (Oil Companies International Marine Forum) guidelines.

Main Components of a Marine Loading Arm

The main components of a typical marine loading arm is shown in Fig. 2 below.

  • Riser Pipe
  • Inboard Arm
  • Outboard Arm
  • Counter balance
  • Connection Flange / QCDC
  • Apex Swivel Joints
  • Emergency Release Coupling
  • Hydraulic System
Components of Marine Loading Arm
Fig. 2: Components of Marine Loading Arm

Working of Marine Loading Arm

The working philosophy of a marine loading arm is shown in Fig. 3:

  • Marine loading arm is designed based on its Operating envelop
  • Marine Loading Arms are operated by using the hydraulic system.
  • Heart of the marine loading arm is swivel Joint. Which plays an important role in operating the Marine loading arms.
Working of Marine Loading Arm
Fig. 3: Working of Marine Loading Arm

Swivel Joints used in Marine Loading Arm

The main functions of the swivel joints (Fig. 4) in a marine loading arm are mentioned below

  • Enabling the loading arm once connected, to follow the movements of the vessel within the working envelope.
  • Allow the rotation between two items of a product line whilst ensuring no product leakage, even under external pressure.
  • In a standard marine arm, there are six swivel joints with the direction of rotation in three planes giving the arm six degrees of freedom. This allows the arm to be maneuvered to and from the vessel and, once connected, allows the arm to follow all the motions of the vessel.
  • Each swivel joint is made up of a male and female part joined by two or three bail bearing raceways allowing the free rotation of the joint. A compression type packing seal is used to seal the two parts of the swivel joint.
  • Swivel Joints are available in various materials including carbon steel, LTCS, Hastelloy, Titanium, etc.
Swivel Joint of a marine loading arm
Fig. 4: Swivel Joint of a marine loading arm

End Connection of Marine Loading Arm

  • The End connection ((Fig. 5)) of the Marine Loading Arm is a standard ASME B16.5 flange.
  • It is available in various sizes and ratings.
  • It is also called a Quick connect / disconnect coupling (QCDC)
  • End connections are of two types:
  • Manually operated.
  • Hydraulically Operated.
End Connection of Marine Loading Arm
Fig. 5: End Connection of Marine Loading Arm

Emergency Release Coupling

Emergency Safety Systems are used to ensure the best possible safety in fluid loading/unloading operations with Marine Loading Arms.

The system allows a fully automatic and safe disconnection of the loading arm from the ship without product spillage when the arm exceeds the operating envelope’s limit line.

Hydraulic/Control System in Marine Loading Arm

Hydraulic Control System in Marine Loading Arm
Fig. 6: Hydraulic Control System in Marine Loading Arm

Erection of Marine Loading Arms

Marine loading arms are normally shipped out fully pre-assembled with all their electronics and hydraulics systems already installed. When assembling the marine loading arms this saves considerable time and money providing economy to the end-user.

Materials of Marine Loading Arms

All Marine Loading arms can be made of various materials like

  • Carbon Steel
  • Stainless Steel 316L
  • Stainless Steel 304L
  • LTCS or Low-temperature Carbon steel

Centrifugal Pumps with Speed Control

Written by Anup Kumar Dey in Mechanical,Piping Interface

How Electrical Motor Works

  • The rotor has fixed magnetic fields
  • The stator receives current from the drive which creates a magnetic field.
  • This rotating magnetic field moves the rotor
  • The frequency is how often the current flows through the stator.
  • Synchronous Speed = (120 * frequency) / # Poles =(120 * 60Hz ) / 4          = 1800 rpm
Schematic of Electric Motor
Schematic of Electric Motor

Methods of Pump speed control

Variable Frequency Drive (VFD)-

  • It converts fixed frequency three-phase Power input to variable frequency three phase output to Motor.
  • The output frequency is greater or less than the supply frequency.
  • Generally, the frequency is lower or the same as the supply frequency due to motor constraints.
  • Motor Speed = (120 * frequency) / # Poles
Variable Speed Drive
Variable Speed Drive

Basis of Centrifugal Pump Operation:

Pump H-Q Curve
Pump H-Q Curve

The conventional way of Flow control:

  • By installing a Control valve at the pump discharge will be throttled to control the required flow.
  • The pump will follow the pump curve
Varying pump capacity
Varying pump capacity

Flow Control using speed control:

  • By changing the pump speed using VFD / VSD, the Pump flow rate will change.
  • The pump will follow the system resistance curve.

The behavior of pump speed control :

  • VFD/VSD follows the system curve by changing the pump speed.
  • VFD/VSD pump retains its characteristic performance curve shape.
  • % change in speed changes the same % of flow rate if no static head is available.
Effects of Changing Speed of a Centrifugal Pump
Effects of Changing Speed of a Centrifugal Pump
  • The pump can be operated within the operating envelope provided by the pump vendor, and flow and corresponding head can be achieved within the operating envelope along with the system resistance curve.

Specifying Speed control pumps:

  • Like any fixed-speed pump VS pumps also required proper sizing.
  • To achieve all desired head Vs flow ranges for VS pump all anticipated cases shall be provided to the vendor for all Sp.Gr. (fig. shows a pump can not reach point B)
Pump sizing

Specifying VFD pumps:

  • NPSHr and Motor HP shall be adequate for all flow and head ranges.
  • NPSHr also depends on the viscosity and its effects shall be confirmed with the pump vendor.
  • At lower flow rates generally, HPSHr increases, following the bathtub curve and the same shall be confirmed with the vendor.

Benefits of Speed control:

  • No need for a throttling valve to operate the pump at desired flow and head.
  • Control By-pass valve / MCF valve can be avoided.
  • Reduced capital and maintenance cost of throttling valve and bypass valve.
  • Reduction in the required head (Delta P required for throttling valve)
  • Power saving at reduced flow requirements
  • Reduced heat dissipated to pumping fluid for fixed speed pump as efficiency is lower at reduced flow.
  • Start-up of high HP machine i.e compressor, Pump, etc. at a lower speed.

Why different operating points?

  • Oversizing of the pump by adding a safety margin.
  • Reduced production over the life cycle.
  • Different destinations- one at a time.
  • Changes in physical properties of fluids. (emulsions with different water cuts)
  • Changes in piping roughness over a life.
  • The single pipeline is used for multiple products.
  • Fluctuations in destination pressure (MOL line)

When VFD/VSD application is economical?

  • If the static head is lower than 50 % of the total head required. The required flow is 50% of the design value and may require 20% of the time.
  • If reduced duty is between 60-80% of BEP (Proper sizing of pump for required duty) Only VFD application may not be sufficient for the control range

Additional requirements for Motor with VFD:

  • Motor operated on VFD generates higher Temp. due to the irregular shape of electrical waveforms produced by VFD. Thus motor efficiency will be reduced due to high operating temperature
  • To ensure the motor will not overheat, the motor is typically de-rated at full load from 3 to 10%.
  • Increase heat can lead to environmental hazards as motor skin temperature may increase. A specially designed motor or de-rated motor shall be used.
  • Sometimes special motor bearings shall be proposed to work in a high-temperature environment.
  • Generally, the motor shall be specified as suitable for VFD and shall purchase from the pump supplier.
  • May need an additional cooling fan for motor cooling at low motor speed.
  • Additional Capex for VFD
  • Need additional space in the control room for VFD installation
  • Additional Air conditioning requirement.
  • If installed outdoors cost of additional enclosure and additional air conditioning is very high.

VSD vs VFD?

  • VSD is economical up to 80% of speed. losses increase below 80% speed and VFD becomes the only economical option compare to VSD.
  • VFD can be configured with a fail-safe characteristic and the pump can run at full speed, however, VSD can not.
  • VFD can be bypassed if a full-speed operation is required, however, the VSD is dedicated to the machine and can not be bypassed.

Is it worth it?

  • Despite the need for additional Capx and special requirements, VFD/VSD pumping can save money. But System economics shall be studied properly.
  • VFD/ VSD eliminates the requirement of a throttling valve, Bypass valve, and Motor starter and reduces power requirements shall be properly considered while analyzing economics.
  • For start-ups of high HP machines – VFD/VSD eliminates the requirement of an additional power turbine in some cases.

Typical capacity control:

  • Suction pressure control as a capacity control
  • Discharge pressure control as a high-pressure control.
  • Station capacity is controlled by pressure or level which manipulates the pump speed through cascade control to maintain desired suction pressure/tank level.

End of curve control:

  • The end-of-curve operation could be a concern in pumping in large oil pipelines where the back pressure (system resistance) can vary.
  • For fixed-speed pumps, based on flow the pump minimum and end-of-curve operation can easily be defined.
  • For variable-speed pumps, the flow value is not an indication of the pump operating point in the performance curve. Hence pump parameter is defined based on flow and head.
  • Pump parameter X is defined as X= K* (flow)^2/Delta P
  • The pump Parameter is a constant line on the performance map which defines the pump’s relative operating point
  • If the system resistance curve is very steep, it is not possible to control/maintain the pump end-of-curve operation using speed.
  • Once system resistance moves away from the performance map, the control system needs another handle to increase the resistance.
  • This is generally achieved by a control valve on the common discharge line.
Pump Performance Curve
Pump Performance Curve

Overview of GRP Pipes

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

GRP Pipes or Glass Reinforced Plastics pipes are composite material pipes consisting of a polymer matrix that is reinforced with glass fibres. They have very high corrosion resistance ability and are thus used widely for low-temperature corrosion-resistant applications. In recent times, GRP pipes are slowly replacing steel in various services like fire and water services. At the same time, GRE or GRP pipes can withstand high pressures. In many places, the term FRP is used interchangeably for GRP pipes. In this article, we will explore an overview of GRP Pipes.

GRP Family

  • GRP: Glass–fibre reinforced plastic.
  • GRE: Glass–fibre-reinforced epoxy.
  • GRV: Glass–fibre vinyl ester.
  • GRUP: Glass – fibre reinforced unsaturated polyester.

The different types of pipes are selected according to the required properties like chemical resistance, temperature resistance, and mechanical properties.

Overview of GRP Pipes
Typical GRP pipes

Characteristics of GRP Pipe

  • Corrosion resistance: Corrosion resistance to both inside and outside corrosion. As a result, additional linings and exterior coatings are not required.
  • When the ratio of strength per unit of weight is considered, fiberglass composites surpass CS and SS.
  • Lightweight: Fibreglass piping is only one-sixth the weight of steel products and 10% the weight of similar concrete products.
  • Electrical properties: Standard fiberglass pipes are non-conductive. Some manufacturers offer conductive fiberglass piping systems for transporting fluids like Jet Fuel.
  • Dimensional stability: Fibreglass material meets the most stringent material stiffness, dimensional tolerance, weight, and cost criteria.
  • Low maintenance cost: Fibreglass piping is easy to maintain because it does not rust, is easily cleaned and requires minimal protection from the environment.

What are the advantages of GRP Pipes?

GRP Pipes provide various beneficial advantages as listed below:

  • Long life; highly durable.
  • Low maintenance cost.
  • High Corrosion resistance.
  • Low lifecycle cost.
  • No need for cathodic protection.
  • Less transportation and handling costs.
  • Environmental friendly.
  • Wide application range.
  • Economic when compared with DSS pipe (Duplex stainless steel)

Types of Manufacturing of GRP Pipes

Different types of manufacturing methods are applied for GRP pipes like

  • Filament winding.
  • Centrifuge
  • Continuous winding or Drostholm Method.
  • Helical Filament winding (Fig.1)
helical filament welding method
Fig. 1: helical filament welding method

Materials used:

Polyester Resins – (Temperature limit is 70 deg C.)

  • Isophthalic Polyester (Tg is 90 deg C)-Rarely used in chemical services but can be used in underground GRP gasoline storage tanks.
  • Bisphenol A Polyester (Tg is 120 deg C)- High temperature, chemically resistant resin extensively used in chemical services.
  • Chlorinated polyester (Tg is 110 deg C)-Properties same as Bisphenol A Polyester, but has inherent fire retardant characteristics. This property can be enhanced by the addition of antimony oxide particles to the resin mix.

Note: Tg is Glass transition temperature, shall be determined by either Differential Scanning Calorimetry according to ISO 11357 or Differential Thermal Mechanical Analyses according to ISO 6721.

Tg shall be greater than 30 deg C above the maximum design temperature.

  • Vinyl ester resins – (Temperature limit is 100 deg C.)-Compared to polyester resins, vinyl ester resins have improved corrosion resistance and high-temperature resistance.
  • Epoxy resins – (Temperature limit is 110 deg C.)- Epoxy resins have excellent resistance to a wide range of moderately strong acids and alkalis and most hydrocarbons. Aliphatic amines, Aromatic amines, and anhydrides are some examples of base epoxy resins.

Reinforcing materials:

  • Reinforcing material shall be a suitable grade of glass fibre having a glass finish compatible with the resin system used.
  • Glass-fibre materials typically used for GRP Pipes and vessels are E-Glass, C-Glass, ECR Glass, Synthetic etc.

Codes and Standards for GRP Pipes

The common codes and standards for GRP piping systems are

  • BS EN ISO 14692: Petroleum and natural gas industries – Glass-reinforced plastics (GRP) piping.
  • AWWA M45:Fibreglass Pipe Design.
  • SHELL DEP 31.40.10.19:GRP Pipelines and Piping Systems (Supplements to ISO 14692)
  • UKOOA: United Kingdom Offshore Operator Association.

GRP Pipes and Fittings

Refer to Fig. 2 which shows some typical GRP pipes and fittings.

GRP Fittings
Fig. 2: GRP Fittings

GRP Pipe Joining methods

  • Adhesive Joint (Fig. 3)
  • Flange Joint (Fig. 5)
  • Lamination Joint (Fig. 5)
  • Rubber Seal Lock Joint (Fig. 4)
Adhesive joints
Fig. 3: Adhesive joints
Rubber Seal Lock Joint
Fig. 4: Rubber Seal Lock Joint
Flanged and lamination joint
Fig. 5: Flanged and lamination joint

Applications of GRP and GRE Pipes

GRP Pipes are widely used in the following plants:

  • Chemical process.
  • Desalination.
  • Industrial effluents.
  • Mining.
  • Oil fields.
  • Potable water.
  • Power plant cooling and raw water.
  • Seawater intake and outfalls.
  • Metal Production
  • Slurry piping.
  • Water distribution.
  • Pulp and Paper Mills.
  • Sewerage & Drainage
  • Transportation of oils, alcohols, fats, disposal, and solutions for food industries.

GRP Pipe Failures

Fig. 6 shows a typical failed GRP pipe.

GRP pipe failures
Fig. 6: GRP pipe failures

Metallic-GRP Piping interface

  • In order to have reliable flange sealing, generally, steel ring elastomer gaskets are used.
  • Gasket material should match with pressure, temperature, and chemical resistance requirements
  • PTFE-envelope-type gaskets should be avoided for large sizes and high pressure.
  • Refer to Fig. 7 which clearly shows the GRP-Metallic interface in a piping system.
Metallic GRP Interface
Fig. 7: Metallic GRP Interface

GRP/GRE pipe Supporting

Refer to Fig. 8 to Fig. 10 for typical GRP piping supports.

Clamped shoe support on GRP Pipes
Fig. 8: Clamped shoe support on GRP Pipes
  • Standard CS support may not match GRP as pipe’s Outer Diameter may be different.
  • The use of Saddles & Elastomeric pads may allow the use of standard support.
  • The support design should be as per the vendor catalog.
  • GRE pipes are generally rested with clamp & shoe for all supports.
  • Heavy in-line items like valves and strainers should be independently supported.
  • Parasite support (pipe to pipe) is not allowed.
  • Supports on fittings to be avoided.
  • Excessive clamping forces can cause pipe crushing.
  • GRP spans are much less than the CS spans, in absence of vendor data ISO 14692 can be used.
  • Piping should be supported for shock loadings
  • Special care is to be taken in a freezing environment.
Flange Support on GRE Pipe
Fig. 9: Flange Support on GRE Pipe
GRE Pipe support with Guide and Line Stop
Fig. 10: GRE Pipe support with Guide and Line Stop

Few more related articles to give you more insights into the subject.

An Article on HYDROSTATIC FIELD TEST of GRP / GRE lines
Stress Analysis of GRP / GRE / FRP piping system using Caesar II
Basic Principles for an aboveground GRP piping system
Buried GRP/FRP pipe Laying and Installation Procedure
Stress Analysis of GRP / GRE / FRP Piping using START-PROF

Stress Analysis of the GRP Piping System

In piping stress analysis guides or flexibility specifications, GRP/GRE composite lines are considered critical irrespective of their sizes. So, a formal stress analysis must be performed to investigate the stresses, loads, displacements, supports, etc. to decide if the GRP piping system will work smoothly throughout its design life. I have developed an online course explaining step-by-step procedures for GRP piping stress analysis. You can check it out here.

Advantages of using Coordinate System in Caesar II

Written by Anirban Datta in Caesar II,Piping Stress Analysis,Piping Stress Basics

As Pipe Stress Engineers, we all, in some way or other, have handled the assignment of modeling and analyzing parallel lines routed over pipe racks or sleepers. Often these lines require Expansion Loops to have sufficient flexibility. However, although may be routed kilometers, these lines often do not have any interconnection between them. Therefore, normal practice is to model these lines in separate ‘.C2’ files and carry out stress analysis and expansion loops sizing and positioning separately. Now, suppose if we could model all lines running parallel to each other in a corridor and with the same design code (e.g. B31.3) in a single ‘.C2’ file, then not only we could view and review all the lines together, but also we could size and locate expansion loops for each line with respect to the others. This would not only reduce the modeling and analysis efforts but would also enable us to handle a lesser number of CAESAR II native files.

This is pretty well possible with the help of the ‘Block Operation’ and ‘Coordinate’ features in CAESAR II. In my earlier post titled “ADVANTAGE OF USING ‘BLOCK OPERATION’ IN CAESAR II, “ I tried to explain ‘Block Operation’. In this post, I would attempt to highlight the effective use of the ‘Coordinate’ feature.

Let us take the case of two lines running parallel to each other over a Pipe Rack, and supported in the same locations.

Parameters for Line 1:

  • Design Code = ASME B31.3
  • MOC = ASTM A106 Gr. B
  • Size = 12”
  • Sch. = STD
  • Corrosion Allowance = 1.5 mm
  • Design Pressure = 1200 kPa(g)
  • Design Temperature = 175OC
  • Fluid Density = 900 kg/m3
  • Insulation = Mineral Wool
  • Insulation Thickness = 50 mm
  • Cladding Thickness = 0.7 mm
  • Cladding Density (Aluminium) = 2700 kg/m3

Parameters for Line 2:

  • Design Code = ASME B31.3
  • MOC = ASTM A106 Gr. B
  • Size = 10”
  • Sch. = STD
  • Corrosion Allowance = 1.5 mm
  • Design Pressure = 1800 kPa(g)
  • Design Temperature = 150OC
  • Fluid Density = 900 kg/m3
  • Insulation = Mineral Wool
  • Insulation Thickness = 50 mm
  • Cladding Thickness = 0.7 mm
  • Cladding Density (Aluminium) = 2700 kg/m3

Let us assume these two lines are having a gap of 500 mm between centerlines.

First, we model Line 1 as per the given parameters.

Modeling of line 1
Fig. 1: Modeling of line 1

Line 1 starts at Node 10 and ends at Node 370.

Now, we invoke ‘List Input’.

Invoking List Input in Caesar II
Fig. 2: Invoking List Input in Caesar II

Then, at the ‘List Input’ window, we select all elements, right-click, select ‘Block Operation’ and then select ‘Duplicate’.

Duplicating Elements in Caesar II
Fig. 3: Duplicating Elements in Caesar II

In the ‘Block Duplicate’ window, under the ‘Options’ tab, we select ‘Identical’, under the ‘Insert Copied Block’ tab, we select ‘At End of Input’, and input ‘400’ in the ‘Node Increment’ box, and click ‘OK’.

Figure – 4

Now, we input ‘500’ in the box of X coordinate in ‘Enter Global Coordinates (mm.) for Node 410’ under the ‘Global Coordinates’ window.

Figure – 5

Now, we have created the geometry of Line 2, but still with the parameters of Line 1. So, we change the parameters as applicable.

Figure – 6
Figure – 7

Then, we close the ‘List Input’ window.

Now, we are ready with two lines that are running in the same route, supported at the same locations, but still requiring some manual modifications at the elbows of Line 2.

Figure – 8

So, we reduce the lengths of elements before and after the first elbow by 500 mm. Likewise, we adjust the expansion loop of Line 2 to arrive at the following geometry in Figure – 9.

Figure – 9

Finally, we increase the lengths of elements before and after the last elbow by 500 mm.

Figure – 10

This completes the parametric and geometric adjustments of Line 2.

Now, both of the lines are ready for onward analysis.

Note: Although this is a useful and smart way of working, the stress analyst must use his/her judgment for use of this feature, particularly if the lines are to be analyzed under different codes, it is recommended not to use this feature. Also, the model shown as an example is a very simplified one. An analyst may encounter more complex problems, and the extent of manual adjustment is likely to vary from little to more on a case-by-case basis.

Telegram Channels / Telegram Groups for Piping and Related Engineering

Written by Anup Kumar Dey in Piping Design Basics,Piping Interface

Telegram Application, the best Whatsapp alternative has gained tremendous traction in just a couple of years since its launch. In February 2016, the app maker stated that the number of active monthly users surpassed the 100 million mark, adding 350,000 users each day who send 15 billion messages per day.

Advantages of Engineering Telegram Groups and Telegram Channels

In recent times, due to WhatsApp’s limitation of only 257 members, engineers and designers from EPC organizations found telegram channels and telegram groups as one of the best methods of communication, discussion, knowledge sharing, job sharing and clarifying doubts between the group and channel members. As there is no limitation of the number of members similar to WhatsApp these telegram groups become very popular. In this article, links to join a few such top engineering telegram channels and top engineering telegram groups will be provided for the readers to take benefit.

Background of Piping and Related Engineering Telegram Channels and Telegram Groups

Maximum of these telegram groups and channels were started in 2015, and these are one of the most popular groups to get correct answers to the question in different fields from industry-experienced persons. Even though the Basic language in these groups is Persian, still members are aware of the English language. Hence, anyone can use his queries in the English language and he will get answers in English.

Telegram Groups and Telegram Channels

How to join these Telegram Groups/Channels

To join any of the above-mentioned groups or channels install telegram apps in your mobile from google play store or iPhone store. After installing open the apps and register your mobile no similar to WhatsApp. Once registered, search the above groups or channels in the telegram app and the group will appear. Otherwise, click the above links on your mobile after opening the telegram app. Click on join and start following the activities of the group. Readers are requested to list down other telegram groups in the comments section to help others.