Introduction To Pressure Surge In Liquid Systems

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Process Engineering Guide: GBHE-PEG-FLO-305

Introduction to Pressure Surge in Liquid Systems Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Introduction to Pressure Surge in Liquid Systems

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 3 3 DEFINITIONS 3 4 CAUSES OF PRESSURE SURGE 3 4.1 Start-up 4 5 CONSEQUENCES OF PRESSURE SURGES 5 6 PRELIMINARY CALCULATIONS 5

6.1 Estimation of the Sonic Velocity 5 6.2 Pipeline Period 7

7 CALCULATION OF PEAK PRESSURES 8 7.1 Rigid Liquid Column Theory 8 7.2 Sudden Changes in Flowrate 9 7.3 Moderately Rapid Changes in Flowrate 9 7.4 Reflections and Attenuations 10 7.5 Vapor Cavity Formation 10 7.6 Complex Piping Systems 10 8 FORCES ON PIPE SUPPORTS 10 .



9 METHODS OF REDUCING THE EFFECTS OF PRESSURE SURGE 13 9.1 Flowrate 13 9.2 Pipe Diameter 13 9.3 Valve Selection and Operation 13 9.4 Pump Start-up/Shut-down 16 9.5 Surge Tanks and Accumulators 16 9.6 Vacuum Breakers 17 9.7 Changes to Equipment 18 10 DETAILED ANALYSIS 18 10.1 Data Requirements 18 10.2 Interpretation of Results 19 11 GUIDELINES FOR CALCULATIONS 21 12 EXAMPLES OF PRESSURE SURGE INCIDENTS 21 12.1 Caustic Soda Pipeline Movement 21 12.2 Ammonia Pipe Movement 22 12.3 Propylene Reactor Start-up 23 12.4 Cooling Water Failure 23 12.5 Dry Riser Fire Sprinkler Systems 23 12.6 Cast Iron Fire Main Pressurization 23



13 REFERENCES 24 14 NOMENCLATURE 25 TABLES 1 BULK MODULUS OF ELASTICITY AND SONIC

VELOCITY FOR SOME LIQUIDS 26 2 YOUNG'S MODULUS AND POISSON'S RATIO OF

SOME PIPE MATERIALS 30 3 GUIDELINES FOR PRESSURE SURGE ANALYSIS

OF PIPING SYSTEMS 31 FIGURES 1 WAVESPEED IN PIPES FILLED WITH WATER 7 2 FORCES ON A PIPING SYSTEM IN STEADY

STATE FLOW 11

3 FORCES ON A PIPING SYSTEM IN TRANSIENT FLOW 11

4 EFFECT OF VALVE CLOSURE TIME ON OUT-OF -BALANCE FORCES 13

5 EFFECT OF VALVE TYPE ON FLOWRATE DURING

TRANSIENT FLOW 15

6 EFFECT OF VALVE TYPE ON SURGE PRESSURE 15 7 CAUSTIC SALES PIPELINE 22



0 INTRODUCTION/PURPOSE This document is one of a series on fluid flow produced by GBH Enterprises. If a rapid change is made to the flowrate of a liquid in a piping system, for example by the operation of a valve, a transient pressure change will be propagated through the system. This transient pressure may be significantly greater than either the initial pressure or the final steady state pressure which the system reaches. This may cause damage to the piping system; either by exceeding the design pressure of the piping system with consequent risk of rupture, or by producing out of balance forces which are greater than can be sustained by the pipe supports. The phenomenon known as 'water hammer' is a special case of 'pressure surge'. 1 SCOPE This Guide provides an introduction for process engineers to the problems associated with pressure surge. It describes the causes of pressure surge and indicates how to make preliminary estimations of the likely magnitude of its effects. Methods of reducing the magnitude of the effects are discussed. Guidelines are given to help in assessing which piping system should be analyzed in detail and the data requirements to perform a detailed computer analysis of the system are given. Some examples are given of problems encountered by GBH Enterprises due to pressure surge. Tables 1, 2 and 3 contain some useful data to assist in surge calculations. This Guide is not a comprehensive treatise on pressure transients. More detailed information is available in standard texts on the subject, for example references 1, 2 and 3. This Guide does not give advice on the mechanical design of piping systems subjected to pressure surge, although it does indicate how to calculate the loads arising from pressure surge. 2 FIELD OF APPLICATION This Guide is of use to all process engineers and plant operating personnel in the GBH Enterprises world-wide, who may be involved in the specification, design or operation of equipment involving the flow of liquids in pipelines.



3 DEFINITIONS For the purposes of this Guide, the following definition applies: Joukowski Pressure or The initial change in pressure or liquid head Joukowski Head associated with an instantaneous change in

liquid velocity. 4 CAUSES OF PRESSURE SURGE Any operation which can result in a rapid change in velocity is a potential cause of serious pressure surge. Typical operations include: (a) Rapid change in position, either closing or opening, of a control or

isolation valve. (b) Opening of a safety valve or rupture of a bursting disc. (c) Starting or stopping of a pump. (d) Priming of an empty pipeline. (e) Unsteady flow generated by a reciprocating pump. (f) Rapid phase change, due to thermal effects, rapid chemical reaction or

the collapse of a vapor cavity. Traditionally, process engineers have associated pressure surge problems almost exclusively with UPSTREAM effects following the RAPID closure of valves at the DOWNSTREAM end of LONG pipelines. It will be shown later that the magnitude of the initial surge resulting from a rapid valve closure is independent of the pipe length. The pipe length only influences what is meant by 'rapid'. The pressure rise associated with a slow closure of a valve does, however, depend on the pipe length. The effects DOWNSTREAM of a rapidly closed valve are also very important. The initial result is a reduction in pressure. This may be sufficient to reduce the absolute pressure below the vapor pressure of the liquid, whereupon a vapor cavity will form.



The subsequent collapse of this cavity can give rise to severe problems, with high transient pressures and out of balance forces. Vapor cavity collapse is one of the most intractable problems in pressure surge analysis, and any system with the potential for cavity formation should be viewed with caution. 4.1 Start-up It is important to realize that during start-up and filling of piping systems the liquid velocities may be considerably greater than the maximum which may occur during normal operation, due to the reduced frictional resistances which may occur during this period. The associated potential for surge problems may be greater than during normal operation. 4.1.1 Vapor cavities At start-up it is quite possible that vapor cavities will be present at the high points of the system, which will collapse as the system is pressurized. 4.1.2 Gas pockets Gas pockets may reduce fluid friction and allow much higher velocities than occur in normal full liquid operation. The deceleration of the liquid by the gas compression can lead under certain circumstances to very high gas pressures and temperatures. Explosive ignition of flammable mixtures has been known. 4.1.3 Venting The volumetric flow of a gas through an orifice is considerably greater than that of a liquid for the same pressure drop. When venting gas from a system, high flowrates can occur, which cannot be maintained when the gas has been expelled; a sudden flow reduction occurs, resulting in pressure surge. Specific examples include priming distributors in columns and batch blowing in systems with valves at the downstream end of the line. 4.1.4 Fast Pressurization Fast pressurization of a closed system can double the pressure rise at the far end of the system as the pressure wave is reflected from the closed end. This can arise either from the fast opening of a valve at the inlet end of the system or pump start-up with the pump discharge valve open.



5 CONSEQUENCES OF PRESSURE SURGES Usually, the initial reason for investigating the possibility of pressure surge is a concern that the design pressure of the piping system may be exceeded. Another problem, not always appreciated, is that even if the surge pressure is within the design pressure, a surge may result in the creation of large out-of-balance forces within the piping system. These may damage the supports, or in the extreme, cause the collapse of pipe bridges. The resulting displacement of the pipes may be sufficient to cause failure through bending, or might result in a branch pipe being torn off due to impact when the main pipe is displaced. This may extend to other pipes on the same pipe bridge which are not themselves subject to pressure surge. Experiences indicate that failure of pipework supports as a result of pressure surge is more likely than pipeline rupture due to over-pressure. Some examples of pressure surge incidents are given in Clause 12. 6 PRELIMINARY CALCULATIONS Detailed pressure surge analysis of most piping systems is a complex and lengthy operation, usually involving the use of a computer program, and is beyond the scope of this Guide. However, some simple preliminary calculations are possible in order to estimate the likely magnitude of any effects. These may show that no significant problem exists, or may indicate the need for a more detailed study. Guidelines for determining whether piping systems require detailed analysis are given in Table 3. 6.1 Estimation of the Sonic Velocity Pressure surges are propagated through a piping system at the local sonic velocity. An estimation of this is basic to all calculations. In a rigid pipe, pressure disturbances will propagate at the basic sonic velocity for the fluid, which is given by:



Dissolved gases have little effect on the sonic velocity, provided they remain in solution. Even small traces of undissolved gas reduce the sonic velocity dramatically, and will greatly reduce the magnitude of pressure surges. However, it is very difficult to quantify such effects, and it is recommended that such benefits be ignored when assessing a system for potential surge damage. The bulk modulus of elasticity is not readily obtainable for many liquids. It is not an item which is stored in the GBH Enterprises; “The VAULT” physical properties data bank. Some commercially available programs purport to calculate the speed of sound in liquids, but the method used is of doubtful validity. It should not be used. Table 1 gives typical values of bulk modulus of elasticity and sonic velocity for some liquids. Note that different sources may give significantly different values for these properties. The most extreme case known to the author is for liquid HF, where the sonic velocity calculated from the bulk modulus obtained from one source differs by a factor of 3 from another source's claimed direct measurement of sonic velocity. In general, a higher assumed value for the sonic velocity can be expected to lead to predictions of higher surge pressures. The velocity of propagation of a pressure wave in a thin walled elastic pipe is lower than the sonic velocity in a rigid pipe, and can be calculated from:



The value of B depends on the method of pipe support and the Poisson ratio of the pipe material. When the Young's Modulus, E, is large, as for metal pipes, the numerical value of B may be taken as unity without significant error. In other cases, B may be calculated from:

Values of Young's Modulus and Poisson's ratio for some common materials are given in Table 2. The reduction in velocity due to the elasticity of the pipe is normally relatively small for metal pipes, but can be considerable with plastic pipes. Figure 1, which is taken from reference 4, shows the effects of pipe material and wall thickness on the velocity of propagation of a pressure wave in a water filled pipe.



FIGURE 1 WAVESPEED IN PIPES FILLED WITH WATER



6.2 Pipeline Period The magnitude of the initial pressure transient associated with a change in flowrate depends on whether the change can be regarded as 'fast' or 'slow'. In this context, the terms 'fast' and 'slow' are relative rather than absolute, and are related to a quantity known as the 'pipeline period', t*. This is the time that elapses between initiation of the change and the return of the reflected surge from the end of the pipeline to the point of initiation (see below). Thus:

where L is the length of the pipeline from the source of the disturbance to the end of the pipe (m) and c is the velocity of propagation of the pressure wave (m/s). There will be two pipeline periods associated with a single pipe having a valve part way down it; one will be based on the upstream length and one on the downstream length. 7 CALCULATION OF PEAK PRESSURES 7.1 Rigid Liquid Column Theory If the velocity of a liquid in a pipeline is changed gradually and steadily, such that the change takes place over more than about 10 pipeline periods, the simplifying assumptions can be made that the rate of change of velocity is the same at all points along the pipeline. Compressibility effects can be ignored and the liquid treated as a rigid column. Pressure changes can then be calculated by considering the momentum effects. For the upstream side of the disturbance:



Similarly, for the downstream side of the disturbance: Note: The above approach assumes that the rate of change of velocity is reasonably uniform. Most types of valve have non-linear characteristics, some very much so such that most of the effect occurs over the last 10-20% of closure. The effective closure times of these valves are thus considerably less than the nominal times. This has to be remembered when determining if the closure can be considered 'slow'. Note also that the effects are very much dependent on valve size, with much better performance being obtained when small valves are used in large lines. See sub clauses 9.3.2.1, 9.3.2.2 and figures 5 and 6. 7.2 Sudden Changes in Flowrate. Any change in flowrate which is completed in under one pipeline period can be considered instantaneous as far as the initial pressure transient is concerned. Note that for a long pipeline, this time could be quite significant. For example, if the sonic velocity is 1000 m/s and the pipeline is 10 km long the pipeline period will be 20 seconds. A valve which closes in 15 s will have the same effect as one which closes in 1 s. Rigid column theory can only safely be applied to this system if the closure time exceeds 200 s.



When the velocity at any point in a pipe carrying liquid is suddenly changed, for example by the closure of a valve, a pressure wave is propagated from that point along the pipe. The magnitude of the initial pressure change is given by the formulae:

This pressure change is usually known as the Joukowski Pressure, or, if expressed in terms of a head of the liquid, the Joukowski Head. Note that the equations do not include the length of the pipeline. The same value will be obtained in a short pipe as in a long one, provided the closure is sufficiently rapid to occur within one pipeline period. If the disturbance is caused by the sudden closure of a valve, the upstream pressure will initially increase by the Joukowski Pressure, whereas the downstream pressure will decrease by this amount. Conversely, the sudden opening of a valve will produce a fall in upstream pressure and a rise in downstream pressure. When a liquid is flowing down a pipeline there is a reduction in pressure along the line due to frictional effects. When the liquid has been brought to rest following the closure of a valve at the end of the line this frictional loss no longer occurs. The pressure at the valve initially rises rapidly by the Joukowski Pressure. Then, over the next pipeline period, it continues to rise gradually by an amount equal to the original frictional loss.



7.3 Moderately Rapid Changes in Flowrate If a change in flowrate takes place over an interval between one and ten pipeline periods, neither the rigid liquid column method nor the Joukowski Head calculation is strictly applicable. For simple unbranched systems it is possible to estimate the magnitude of the peak pressure by a stepwise manual method, as explained in reference 6, but the procedure is quite involved. If the calculation of the Joukowski Head indicates the possibility of problems, it is generally preferable to perform a detailed computer analysis. 7.4 Reflections and Attenuations When a pressure surge reaches the end of a pipeline, it is reflected back down the line. If the end of the pipe is open, the magnitude of the reflection is the same as the incident surge, but the sign is reversed. Thus a positive pressure surge is reflected as a rarefaction, and vice versa. When a surge reaches a closed end, the reflected surge has double the incoming magnitude. Because of these reflections, alternate pressure and rarefaction surges pass up and down the line, being reflected from each end. Due to friction effects, the magnitude of the surges gradually dies away. 7.5 Vapor Cavity Formation If the static pressure in the system falls below the vapor pressure at any point, a vapor cavity will tend to form, which will continue to grow all the while the pressure remains low. Subsequent positive pressures will cause the cavity to collapse. This will lead to a large abrupt pressure surge as the cavity is condensed. Experience shows that this is one of the most severe problems to deal with. If preliminary calculations indicate that cavity formation is likely, a more detailed analysis is recommended. 7.6 Complex Piping Systems For a branched piping system, or one including several high points where vapor cavity formation may occur, reflections will occur off the ends of each pipeline, and vapor cavities will collapse in differing times. The result is a very complex pressure/time history, which cannot sensibly be analyzed by hand calculations. Computer analysis of the system is then essential if preliminary calculations give any indication of potential problems.



8 FORCES ON PIPE SUPPORTS A flowing fluid has the potential to exert a force on account of its pressure and its momentum. The potential force of a fluid in a pipe, acting in the direction of the flow, is given by:

When a fluid is flowing in steady flow through a pipeline, as it approaches a bend, it will exert a force on that bend, equal to the potential force in the direction at which it approaches the bend. After flowing round the bend, it will exert a backward force of the same magnitude in the new direction of flow. The resultant force acts outwards on the bend in a direction bisecting the angle of the bend. Thus for a 90° bend, the resulting force (F') is given by: .

The force on the bend is restrained by the tension in the pipe. As the fluid flows along the straight lengths between bends, its pressure falls due to the effects of friction, thus reducing F, but the corresponding reduction in tension in the pipe is balanced by the shear forces on the wall. There is thus no net force on the pipework, except on the last bend before the discharge from the pipe, where there is a backward force F along the direction of the pipe. See Figure 2.



FIGURE 2 FORCES ON A PIPING SYSTEM IN STEADY STATE FLOW

However, if a pressure transient is passing through the piping system, the forces on the pipework are no longer in balance. Consider the section of pipe shown in FIGURE 3 FORCES ON A PIPING SYSTEM IN TRANSIENT FLOW



Assume that the valve is closed suddenly. A transient will pass back up the pipe from the valve at the sonic velocity. On the valve side of the transient, the fluid will have been brought to rest and the pressure will be greater than the initial pressure by the Joukowski Pressure for the system, ρ.c.v. Upstream of the transient, the fluid will still be travelling with the original velocity and pressure. Consider the length of pipe between the two bends in Figure 3, when the transient is somewhere between the bends, and ignore the effects of friction. The force along the pipe from the upstream bend is then:

In the simple system considered here, where the pressure remains at the higher value for a significant time after the transient has passed, this force will act for a time Δt, the time taken for the pressure transient to pass along the length of pipe between the two bends:

For more complex systems, involving multiple reflections and vapor cavity collapses, for example, a very complex pressure/time response may occur, and the duration of a high pressure may be shorter than that taken for the transient to pass between the two bends. When analyzing the forces on the piping system it is necessary to consider both the magnitude and duration of the forces.



For a sudden pressure rise, the effect of these forces cannot be considered as equivalent to steady state forces. It is generally necessary to apply a multiplying factor, the Dynamic Load Factor or DLF, to the calculated shock loading when performing a dynamic pipeline support analysis. The magnitude of this factor depends on the nature of the pipe supports, and the duration of the force in relation to the natural frequency of the piping system. The DLF can be up to a value of 2 for rigidly anchored systems with a relatively long force duration. Conversely, for short duration pressure pulses in flexible (low frequency) pipes the dynamic load factor on supports can be much less than unity. Consult a piping engineer for advice. For more information on the mechanical aspects of designing pipework for pressure surge, Visit us at www.gbhenterprises.com. If the valve is closed gradually rather than suddenly, not only will the magnitude of the surge pressure be reduced, but the pressure will rise gradually rather than in a stepwise fashion. The effect of this is to reduce significantly the magnitude of the out of balance forces. This is illustrated in Figure 4. FIGURE 4 EFFECT OF VALVE CLOSURE TIME ON OUT-OF-BALANCE

FORCES

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9 METHODS OF REDUCING THE EFFECTS OF PRESSURE SURGE Pressure surges occur because a rapid change in liquid flowrate is imposed on the system. Surge reduction methods work by reducing the magnitude of this change or increasing the time over which it occurs. They usually rely either on reducing the rate at which the primary event causing the surge (e.g. valve closure) occurs, or providing an alternative source or sink for the liquid in the form of a surge tank or accumulator. 9.1 Flowrate For most pipelines, the required flowrate is determined by the process. However, for some operations, such as the batchwise transfer of liquid from one vessel to another, it may be possible to increase the transfer time, thus reducing the flow velocity. Closure of the system valves will then produce a smaller change in velocity, and hence a reduced surge pressure. 9.2 Pipe Diameter Increasing the diameter of a pipeline for a given flowrate will reduce the velocity, and hence the peak surge pressure. However, this approach is likely to be expensive compared with the alternatives. Moreover, although it will reduce the magnitude of the peak pressure, the out-of balance forces will remain substantially the same, as the reduction in pressure is balanced by an increase in pipe cross sectional area. This method will not be considered further here. 9.3 Valve Selection and Operation

9.3.1 Isolation valves

The unthinking operation of an isolation valve when the liquid is flowing in the pipeline can have serious consequences, especially as many types of isolation valve can be closed very rapidly, for example with a quarter turn of a handle. If the surge analysis indicates that this is unacceptable, some action will be necessary to prevent this happening. An operating procedure may be considered adequate for this purpose, but in general some physical constraint is preferable. It may be necessary to remove the operating handles from key valves, or provide them with locks. An alternative is to fit them with gear drives with low ratios, such that very many turns are required to operate the valves.



For some designs of butterfly valve, the forces on the disk from the flowing liquid tend to turn the valve into the closed position, particularly when the valve is initially in the partially closed position. This may result in the inadvertent rapid closure of the valve, and it may be necessary to provide some physical constraint to prevent this happening.

9.3.2 Control valves

9.3.2.1 Stroke time

Where the process demands permit it, the speed of closure of a control valve may be reduced by selection of a suitable actuator. Certain types of valve, particularly butterfly and ball valves, have unfavorable characteristics from the point of pressure surge, as most of their effect occurs over the last few percent of the closure. For these valves, a split range actuator may be useful. This will have a rapid rate of closure over the part of the characteristic where flowrate does not vary much with valve position, and a slower response during the last stages of closure.

9.3.2.2 Valve type

As has been stated before, some valve types have better characteristics than others from the point of view of pressure surge. Where piping specifications permit, a change in valve type may have significant effects on the magnitude and rapidity of pressure changes in the system.

In order to illustrate this, a model of a simple piping system has been set up using the “The Vault” (see Clause 10). The system modeled consists of a feed tank, a pump, a length of pipeline with a pipeline period of 2 seconds, a control valve and a receiving tank. Five different valve characteristics (loss coefficient as a function of valve position) have been simulated, with a valve closure time of 10 seconds in each case. The valve types used were:

• A diaphragm valve (Saunders type K).

• A reduced bore ball valve.

• A butterfly valve.

• A control valve with nominally linear characteristics.

• A control valve with nominally equal percentage characteristics.



Figure 5 shows the variation of flowrate with time for the five valve types. It can be seen that over most of the closure, the equal percentage valve gives the most gradual change in flowrate. However, towards the end of the closure, there is a rapid change in flow. This is because the characteristic used represents a real valve, (Taylor Instruments, ideal rangeability 40:1, practical rangeability 20:1) which only exhibits equal percentage characteristics over the major portion of its range. Indeed, an ideal equal percentage valve would never shut off completely.

At the other extreme the 'K' type diaphragm valve has very little effect on the flow for the first 90% of the closure, followed by a very rapid reduction in flow, giving an effective closure time of only about 1 second in this case.

FIGURE 5 EFFECT OF VALVE TYPE ON FLOWRATE DURING TRANSIENT

FLOW

Figure 6 shows the corresponding pressures resulting from the different valve closures. It can be seen that the diaphragm valve gives both the highest peak pressures, and the most rapid rate of change. The equal percentage valve gives the lowest pressure rise, but note the rapid rate of rise occurring at the end of the closure, as the valve departs from ideal behavior.



FIGURE 6 EFFECT OF VALVE TYPE ON SURGE PRESSURE

It should be emphasized that these two Figures are illustrative only. The actual magnitude of the differences between different valve types will depend on the system. The effects of valve size should also be considered; a large valve will have worse characteristics than a small one of the same type, as the initial stages of closure of a large valve result in little extra flow resistance and hence little change in flow rate. It cannot be concluded that an equal percentage valve will always give the best results.

9.3.3 Emergency shut-down valves

Emergency shut-down valves may be required to operate very rapidly. However, it is always worth considering the safety consequences of increasing the closure time. Alternatively, a change in valve type, as discussed above, may produce a reduced pressure surge with the same overall closure time.



9.4 Pump Start-up/Shut-down It is in general bad practice to start up a pump with an open delivery valve into a long piping system. The preferred arrangement is to have a recycle line from the pump delivery to the feed tank sized to give the minimum acceptable flowrate for the pump (a 'kick-back' line), and an isolation valve at the start of the main system. When the pump is up to speed, this valve may be opened in a slow, controlled fashion to divert the flow into the main piping system. For critical applications, where there are likely to be vapor cavities present at start-up, it is recommended that a small bore bypass be installed round the main discharge valve to ensure that the line is started up under well defined low flow conditions. The practice of 'cracking valves' open to restrict flowrate is unsafe. Pump shut-downs should also be done in a controlled manner. Unfortunately, this is not always possible, as power failures, for example, can result in a pump trip. It may be possible to increase the run-up time of a pump by choice of a suitable motor. There are now control systems available which allow the start-up to take place over an extended period, say 2 minutes. These have been used successfully in many locations. Alternatively, addition of a flywheel to the pump drive will increase both run-up and run-down times. The motor will obviously have to be suitable to deal with the higher inertia. Unfortunately, this approach is not possible with a canned pump. Note: The use of a non-return valve on a pump delivery may itself be a cause of pressure surges. 9.5 Surge Tanks and Accumulators If the methods described above are not suitable to prevent surge problems, the use of a surge tank or accumulator may be required. These devices operate by providing an alternative source or sink for the liquid, enabling, for example, a key valve to be opened or closed rapidly while allowing the flow in the pipeline to be increased or reduced in a more gradual and controlled manner. To be most effective, these devices should be connected to the piping system close to the source of the flow disturbance, e.g. the control valve which has been identified as the cause of the surge. If there is more than one potential cause of surge, several surge tanks or accumulators may be needed, one at the location of each potential source.



9.5.1 Surge tanks

A surge tank consists of a vertical cylindrical tank mounted at some suitable point along the line. The tank is open to the atmosphere at the top, and is placed so that its top is higher than the steady state head at that point. It has to have sufficient capacity such that it will not be drained or over-filled by the highest flow that might occur during a transient. Because they are open to the atmosphere, surge tanks are not generally used in the chemical industry, but they are common in the water supply and hydro-electric industries.

9.5.2 Accumulators

An accumulator is a closed vessel, partly filled with gas, which is connected to the pipeline at a suitable point. The gas acts as a cushion which absorbs some of the pressure surges passing along the pipe by allowing some of the flow to be diverted into or out of the accumulator. Accumulators are common on reciprocating pumps, where they reduce the pressure fluctuations. They may be located on the suction side, delivery side or both, depending on the perceived problems. Like surge tanks, accumulators have to be sized to allow for the magnitude of the flows. This may make them large and prohibitively costly for high flow lines.

The main disadvantage of accumulators apart from cost is the problem of ensuring that they are in a suitable condition to operate when required. If the gas is in direct contact with the liquid it may dissolve over a period of time, and some means of checking for this is necessary. Some designs of accumulator get round this by the use of a bladder or diaphragm. However, there is always a danger that the diaphragm may rupture. It may not be easy to determine if this has happened. It may also be difficult to find a material for the diaphragm which is compatible with the chemicals being handled.

An alternative to the diaphragm is to separate the liquid and gas by a piston. Here there is a danger that corrosion of the piston or cylinder, or dirt in the process fluid, may cause the piston to stick. The gas space above the cylinder will normally be pre-pressured to a pressure above the normal operating pressure of the system, resulting in the piston being in its fully extended position. It is normal to mount a pressure gauge on the gas side of the piston to ensure that it is properly charged. Regular checking of this pressure and topping up when necessary is essential to maintain the integrity of the system.



Note: A piston type accumulator may itself be a cause of vapor cavity formation. The gas behind the piston has to be initially pressurized above the system pressure or the position of the cylinder is indeterminate. However, following a surge incident the liquid will be forced out of the cylinder by the gas pressure. This liquid outflow may stop abruptly when the piston reaches its lower limit, giving a negative pressure wave.

It is a regrettable fact that the maintenance of accumulators is often neglected. For this reason, they are not recommended as the primary safety device where the consequences of failure to operate are unacceptable. If it does prove unavoidable to use them in such circumstances, it is essential that suitable maintenance procedures are implemented and can be demonstrated. Inspection of accumulators should be included in safety inspection routines.

9.6 Vacuum Breakers If a pipeline passes through a high point, during shut-down conditions the pressure at that point may fall below the vapor pressure and a vapor cavity may form. When flow is re-started in the line, this cavity will collapse and initiate a pressure surge. Ideally, pipelines should be routed to avoid such high points, but this is rarely possible in practice. If a vacuum breaking valve is installed at the high point, the vapor cavity will be replaced by an air pocket. When the flow is re-started in the line, this air pocket will compress, gradually accelerating the liquid column ahead of it and reducing the surge. The air pocket acts in effect like an accumulator. This approach is obviously only applicable where ingress of air into the liquid line is acceptable. It has been recommended to alleviate surge problems on aqueous caustic soda lines. The approach could in principle be modified to let an inert gas such as nitrogen into the liquid line, provided that the gas does not cause problems when it is passed through the system. This obviously results in a more complex system, and the observations on maintenance given in 9.5.2 also apply. Note: Air venting valves, which are sometimes installed at the high points of pipelines to remove air during start-up, are themselves a potential cause of surge. See 4.1.3.



9.7 Changes to Equipment As an alternative to reducing the magnitude of a surge, it may be worth considering whether the design rating of the piping system could be increased. The design pressure of a piping system may be dictated not by the pipe wall thickness or the flange rating, but by the pressure rating of a valve or instrument in the pipeline. Changes here, particularly if made at the design stage, may enable rapid valve closures to occur without exceeding the design pressure. However, remember that not only the piping but also the supports have to be designed to handle the effects of the surge. Conversely, applying one of the surge alleviation techniques above may permit the use of a lower design pressure for the pipeline. Analysis on a new boat loading installation in 1987, involving a long 10" stainless steel line, showed that the slowing of a valve closure would enable thinner piping to be used, with a saving of $36,000. 10 DETAILED ANALYSIS The preliminary hand calculations described in Clauses 6-8 can give an estimate of the likely magnitude of a pressure surge, but if these indicate a possible problem, (for example, predicted Joukowski Pressure close to the design pressure, the potential to form vapor cavities, or large predicted out of balance forces), expert advice should be sought. The analysis of all but the simplest systems requires the use of a computer program. Experience is necessary to obtain the best results from these programs, and to interpret the answers. There are sources of specialist advice and assistance with modeling pressure surge within GBH Enterprises.



10.1 Data Requirements In order to perform a complete pressure surge analysis, a significant quantity of data is required. Much of this will be readily available for a normal project, but certain items are less likely to be known. The following is a list of the ideal data requirements. In practice, not all the data may be available, and reasonable guesses may have to be made for the missing items, based on typical values for similar items. Reference 7 is a good source of pressure drop data for many pipeline components. (a) Density of the process liquid at operating conditions. (b) Vapor pressure of the liquid at normal operating temperature. This is

needed to determine the conditions under which vapor cavity formation is likely.

(c) Bulk modulus of elasticity of the liquid. This is the inverse of the

compressibility. Combined with information on the pipe wall thickness, it is used to determine the speed of propagation of a pressure wave in the piping system, using equation (2). This property is not easy to find. Values for some liquids are given in Table 1, but for other liquids it will probably be necessary to consult a physical properties specialist for assistance. In the absence of any reliable data, an estimate of the wave speed will have to be made by analogy with similar liquids.

(d) An isometric drawing of the pipeline installation, including all valves and

other fittings, and giving pipe lengths and elevations. It is not necessary for lengths to be highly accurate. When doing the analysis, the pipeline will be divided up into sections corresponding to the distance travelled by a pressure wave in one time-step, and some rounding of the data will be necessary to ensure an integer number of these sections. Section lengths in a typical analysis might be 10 to 50 meters. The relative elevations should be given with reasonable accuracy, as they influence the formation of vapor cavities.

(e) The pipe diameter, wall thickness and Young's modulus of the pipe

material. These items are needed, along with the liquid compressibility, to determine the speed of propagation of a pressure wave (equation (2)).

(f) The liquid level and pressure above the liquid in all tanks connected to the

piping system.



(g) For each valve which is to be considered as a potential cause of pressure surge, the valve resistance (K value), as a function of position. Ideally, the data should be obtained from the manufacturer, especially for control valves. Standard data are available for certain valve types, which can be used in the absence of manufacturer's data.

(h) For each valve, the position as a function of time. Often only a closure

time is known. In such cases, the position has to be assumed to vary linearly with time. It is possible to provide a valve actuator with a non-linear characteristic; this can be useful for valves such as butterfly and ball, where most of the effect occurs in the last part of the closure.

(j) For all other fittings, including isolation valves which are not operated

when fluid is flowing, a resistance expressed as a K value (which has to include the diameter on which the K value is based). Note that the experts running the analyses can usually provide these data for most common fittings, provided the type is known.

(k) Pump data. During the course of a pressure transient, the differential

pressure across a pump, and hence the flowrate through it, will change. If the effects of pump start-up and shut-down are not to be modeled, only the pump characteristic curves (flowrate against differential head) are necessary. If start-up and shut-down are to be modeled, several other items of data are required, which are less readily available. These are: the moment of inertia of the pump and drive assembly; the friction torque; the speed/torque characteristics of the motor. Machines Sections should be approached for these data.

The boundaries for an analysis are normally vessels which either feed the piping system or receive liquid from it, and which can be regarded as regions of constant pressure. It is not possible to analyze a section of pipework in the middle of an existing piping system without also considering the pipes to which it is connected. Thus if a section of a pipe is to be re-routed, for example, the complete system from supply vessel to outlet has to be analyzed.



10.2 Interpretation of Results A computer simulation of a surge problem generates a large quantity of data. A typical relatively simple analysis might have 20 piping components and 15 nodes. The analysis might run for 20 seconds with a time step of 0.005 seconds, producing results at 4000 time steps. It is usual to produce most of the answers in graphical form, giving values of the parameters of interest as a function of time. When considering the mechanical design of the piping system, the items of most interest are the maximum (and for lined pipes, the minimum) pressures, and the out-of-balance forces. Pressures are obtainable directly from “The VAULT” results. The estimation of the magnitude and effect of the out-of-balance forces cannot be obtained directly from the “The VAULT” output, but can be inferred from the results. Vapor cavity collapse generally results in a step change in pressure, whereas other surge events may give a more gradual, but still rapid, change. Because of the calculation methods used by “The VAULT”, any pressure change which takes place over a single time step is considered as instantaneous. For an instantaneous pressure change, the magnitude of the out-of-balance forces at each pipe bend can be estimated as the product of the pressure change and the pipe cross sectional area.

This force will act along the axis of the pipe. In considering the anchoring requirements, this has to be regarded as a shock loading, and the appropriate factor DLF on the steady state value used.



For a more gradual pressure change, it is necessary to estimate the rate of change of pressure. The out of balance force along a length of pipe between two bends is then given by:

Note: Although this force is proportional to the pipe length, it is subject to a maximum value of the total pressure change × the cross sectional area. In many cases, the frictional resistance between the pipe and the supports will be significant compared with the above force. The frictional resistance between a pipe and skid supports is proportional to the weight of the pipe plus contents.



Equating equations 10 and 11, the maximum allowable rate of pressure rise to avoid pipe movement is then given by:

A typical value for the coefficient of friction between the pipe and its supports is 0.2. For a pipe supported on hangers rather than skids, there is no significant frictional force, and any out of balance force will tend to result in some movement. However, as the pipe swings on the hangers they will move away from the vertical and thus exert a restraining force on the pipe. For a discussion on the behavior of pipes supported on hangers when subjected to dynamic forces is given contact GBH Enterprises.. 11 GUIDELINES FOR CALCULATIONS It is obviously impractical to perform a detailed surge analysis of every pipeline on a plant. However, it is not always obvious which lines should be analyzed. The guidelines in Table 3 were developed during discussions between the author and other members of GBH Enterprises, and are proposed for use throughout the company. In developing these guidelines, consideration was also given to the practices within other major national/multinational organizations. Table 3 should be read in conjunction with the more detailed explanations in the main body of this Guide. The ultimate responsibility for determining whether a surge analysis should be carried out lies with the responsible engineer. If in doubt, a specialist should be consulted for advice.



12 EXAMPLES OF PRESSURE SURGE INCIDENTS

12.1 Caustic Soda Pipeline Movement

An example which illustrates the generation of large out of balance forces from vapor cavity collapse was experienced by a European Caustic manufacturer in the 80’s. The incident occurred on the last section of a 6" stainless steel line carrying aqueous caustic soda to a stock tank. The pipeline is shown schematically in Figure 7. There was an isolation valve approximately 75 m from the end of the pipe, which then ran for 35!m horizontally on sliding supports, through a 90° bend, along another 35 m of horizontal section supported on hangers and finally up 5 m before discharging into the stock tank. The pipe was anchored near to the isolation valve, and also to the tank near its discharge, but only supported between these points, without lateral restraint. Although the exact cause of the incident is not certain, the most likely explanation is that at some unknown time the isolation valve was closed while the line was discharging to the tank. This would have resulted in the formation of a vapor cavity downstream of the valve. The subsequent collapse of this cavity sent a positive pressure surge back down the line to the tank. The resulting out of balance force on the bend displaced the line by about 0.15 m, severely distorting the supports and hangers.

Piping Section estimated that the force necessary to do the observed damage was about 2 te, a figure that was in reasonable agreement with that calculated from the results of a surge analysis. Points to note from this incident are: (a) The peak pressures calculated were well within the design pressure for

the piping system. (b) The pipe length was relatively short and discharged through an open end. (c) The problem occurred on the downstream side of a valve. (d) The pipe supports were not designed to cope with lateral forces. (e) Those involved before and after the incident had some knowledge of

pressure surge, as analyses had been done for other pipelines in the area, but their state of knowledge before the incident would not have suggested any need to model the particular pipe section.



FIGURE 7 CAUSTIC SALES PIPELINE



12.2 Ammonia Pipe Movement

Liquid ammonia was held at low temperature in atmospheric pressure storage tanks. It was pumped into the distribution network which was held at 15 bar g pressure at ambient temperature. During hot weather ammonia vaporized in the unlagged pipework, forming vapor pockets. Subsequent pumping of cold liquid ammonia into the system caused the vapor to condense, resulting in a pressure wave being transmitted along the pipe. The pressure imbalance caused by the wave resulted in failure of the pipe anchors and displacement of the pipe by 0.6 m.

12.3 Propylene Reactor Start-up

Liquid propylene was fed through restriction orifices into a reactor. In order to assist start-up, propylene vapor rather than liquid was fed through a side branch, the liquid line being closed. Inadvertent opening of the liquid line caused the vapor pocket to collapse, allowing a very high liquid velocity in the feed line. This flowrate could not be sustained through the restriction orifice, and the resulting liquid hammer caused the non-return valve to slam shut. The bolts on the valve cover stretched, allowing propylene to jet from the broken joint.

12.4 Cooling Water Failure

The cooling water pump tripped on a large cooling water circuit on an ammonia plant. During the short delay between the trip and the automatic start-up of the stand-by pump a vapor pocket formed at an elevated heat exchanger. The presence of the cavity allowed high flow from the stand-by pump. Severe damage resulted from the water hammer which arose from the cavity collapse.

12.5 Dry Riser Fire Sprinkler Systems

Many fire sprinkler systems are pressurized with gas to avoid freezing during winter. In the event of a fire a sensor bulb bursts, allowing release of the gas pressure. Loss of gas pressure triggers the fire pump, and water is pumped into the system. High flows occur as the system fills with water, but these cannot be sustained due to the restriction of the sprinkler orifice. The resulting water hammer has led to the failure of a number of fire systems.



12.6 Cast Iron Fire Main Pressurization

A cast iron fire main was kept under pressure by towns water. On fall of pressure, the fire pump automatically started. The dead head pressure of the pump was below the design pressure of the main. However, the run-up time of the pump was much less than the pipeline period, so a pressure rise of almost twice the dead head pressure of the pump arose. As a result, the spigot and socket joints on the line were subject to frequent failure.



13 REFERENCES 1. Thorley A R D, Enever K J. "Control and suppression of pressure surges

in pipelines and tunnels", Construction Industry Research and Information Association. London 1979.

2. Wylie E B, Streeter V L. "Fluid Transients" FEB Press, Ann Arbor,

Michigan, USA. 3. Thorley A R D."Fluid Transients in Pipeline Systems." 1991. D & L George

Ltd. ISBN 0-9517830-0-9. 4. “The VAULT” Technical Guide. GBH Enterprises. 5. “The VAULT” User Manual. GBH Enterprises. 6. HTFS Handbook Sheet FM13. "Pressure surges in a pipeline with liquid

flow due to valve closure." Smith R A, 1990. 7. Miller D S. "Internal Flow Systems." 2nd edition (1990). BHRA. ISBN 0-

947711-77-5



TABLE 1 BULK MODULUS OF ELASTICITY AND SONIC VELOCITY FOR SOME LIQUIDS



Source References: 1. M Rama Rao. 'Velocity of sound in some liquids and Chemical

composition.' Journal of Chemical Physics. Vol 9 September 1941, pp682-5.

2. A.R.D. Thorley & K.J. Enever. 'Control and suppression of pressure

surges in pipelines and tunnels.' CIRIA, 1979. 3. J.J. Tuma. 'Handbook of physical correlations'. McGraw-Hill, 1983. 4. Journal of Chemical Engineering Data. Volume 23 (3) 1978, page 194. 5. A. Dibrov, V.P. Mashovets, R.P. Matveeva. 'The density and

compressibility of aqueous sodium hydroxide solutions at high temperatures. Zh. Prikladnoi Khimii. Volume 37 (1) 1964. pp 29-36.

6. R.T. Langman, C.H. Knowles. 'Velocity of compressional waves in liquid

hydrogen fluoride and some thermodynamic properties derived there from.' Journal of the Chemical society. Volume 32 (2) 1960. p 561.

7. The Handbook of Chemistry and Physics. The Rubber Company. 8. G.W.C Kaye & T. H. Laby. 'Tables of Physical and Chemical Constants'. 9. BHRA Report TN 411. Plinton. 10. I.S. Pearsall. 'The velocity of water hammer waves.' Symposium on

surges in pipelines. 11. F.W.Bridgeman. The physics of high pressure. The author would be grateful for any additional data to supplement the above list.



TABLE 2 YOUNG'S MODULUS AND POISSON'S RATIO OF SOME PIPE MATERIALS

Note: GRP properties will vary according to the proportion of fibre, bonding material and method of manufacture. Information from external consultants, suggests that the very low values quoted in reference 3 may be most appropriate for certain types of piping. . Source references: 1. Thorley ARD, Enever KJ " Control & suppression of pressure surges in

pipelines and tunnels." CIRA report 84. 2. Perry "Chemical Engineer's Handbook" (4th edition).



TABLE 3 GUIDELINES FOR PRESSURE SURGE ANALYSIS OF PIPING SYSTEMS

Introduction To Pressure Surge In Liquid Systems

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