ISGOTT Pressure Surge

Pressure surge

[Extracted from International Safety Guide for Oil Tankers & Terminals 3rd edn. pg. 149]
A pressure surge is generated in a pipeline system when there is an abrupt change in the rate of flow of liquid in the line. In tanker operations it is most likely to occur as a result of one of the following during loading:

Closure of an automatic shut down valve.
Slamming shut of a shore non-return valve.
Slamming shut of a butterfly type valve.
Rapid closure of a power operated valve.

If the pressure surge in the pipeline results in pressure stresses or displacement stresses in excess of the strength of the piping or its components there may be a rupture leading to an extensive spill of oil.
GENERATION OF PRESSURE SURGE
When a pump is used to convey liquid from a feed tank down a pipeline and through a valve into a receiving tank, the pressure at any point in the system while the liquid is flowing has three components:

The pressure on the surface of the liquid in the feed tank. In a tank with its ullage space communicating to atmosphere this pressure is that of the atmosphere.
The hydrostatic pressure at the point in the system in question.
The pressure generated by the pump. This is highest at the pump outlet, decreasing commensurately with friction along the line downstream of the pump and through the valve to the receiving tank.

Of these three components, the first two can be considered constant during pressure surge and need not be considered in the following description, although they are always present and have a contributory effect on the total pressure.
Rapid closure of the valve superimposes a transient pressure upon all three components, owing to the sudden conversion of the kinetic energy of the moving liquid into strain energy by compression of the fluid and expansion of the pipe wall. To illustrate the sequence of events the simplest hypothetical case will be considered, i.e. Then the valve closure is instantaneous, there is no expansion of the pipe wall, and dissipation due to friction between the fluid and the pipe wall is ignored. This case gives rise to the highest pressures in the system.
When the valve closes, the liquid immediately upstream of the valve is brought to rest instantaneously. This causes its pressure to rise by an amount P. In any consistent set of units:
P = wav
where

w is the mass density of the liquid
a is the velocity of sound in the liquid
v is the change in linear velocity of the liquid, i.e. from its linear flow rate before closure.

The cessation of flow of liquid is propagated back up the pipeline at the speed of sound in the fluid, and as each part of the liquid comes to rest its pressure is increased by the amount P. Therefore a steep pressure front of height P travels up the pipeline at the speed of sound; this disturbance is known as a pressure surge.
Upstream of the surge, the liquid is still moving forward and still has the pressure distribution applied to it by the pump. Behind it the liquid is stationary and its pressure has been increased at all points by the constant amount P. There is still a pressure gradient downstream of the surge but a continuous series of pressure adjustments takes place in this part of the pipeline which ultimately result in a uniform pressure throughout the stationary liquid. These pressure adjustments also travel through the liquid at the speed of sound.
When the surge reaches the pump the pressure at the pump outlet (ignoring the atmospheric and hydrostatic components) becomes the sum of the surge pressure P and the output pressure of the pump at zero throughput (assuming no reversal of flow), since flow through the pump has ceased. The process of pressure equalization continues downstream of the pump. Again taking the hypothetical worst case, if the pressure is not relieved in any way, the final result is a pressure wave that oscillates throughout the length of the piping system. The maximum magnitude of the pressure wave is the sum of P and the pump outlet pressure at zero throughput. The final pressure adjustment to achieve this condition leaves the pump as soon as the original surge arrives at the pump and travels down to the valve at the speed of sound. One pressure wave cycle therefore takes a time 2L/a from the instant of valve closure, where L is the length of the line and a is the speed of sound in the liquid. This time interval is known as the pipeline period.
In this simplified description, therefore, the liquid at any point in the line experiences an abrupt increase in pressure by an amount P followed by a slower, but still rapid, further increase until the pressure reaches the sum of P and the pump outlet pressure at zero throughput.
In practical circumstances the valve closure is not instantaneous and there is thus some relief of the surge pressure through the valve while it is closing. The results are that the magnitude of the pressure surge is less than in the hypothetical case, and the pressure front is less steep.
At the upstream end of the line some pressure relief may occur through the pump and this would also serve to lessen the maximum pressure reached. If the effective closure time of the valve is several times greater than the pipeline period, pressure relief through the valve and the pump is extensive and a hazardous situation is unlikely to arise.
Downstream of the valve an analogous process is initiated w hen the valve closes, except that as the liquid is brought to rest there is a fall of pressure which travels downstream at the velocity of sound. However, the pressure drop is often relieved by gas evolution from the liquid so that serious results may not occur immediately, although the subsequent collapse of the gas bubbles may generate shock waves similar to those upstream of the valve.
Effective Closure Time of the Valve
In order to determine whether a serious pressure surge is likely to occur in a pipeline system the first step is to compare the time taken by the valve to close with the pipeline period.
The effective closure time, i.e. the period during which the rate of flow is in fact decreasing rapidly. is usually significantly less than the total time of movement of the valve spindle. It depends upon the design of the valve, which determines the relationship between valve port area and spindle position. Substantial flow reduction is usually achieved only during the closure of the last quarter or less of the valve port area.
If the effective valve closure time is less than, or equal to, the pipeline period, the system is liable to serious pressure surges. Surges of reduced, but still significant, strength can be expected when the effective valve closure time is greater than the pipeline period, but they become negligible when the effective valve closure period is several times greater than the pipeline period.
Derivation of Total Pressure in the System
In the normal type of ship/shore system handling petroleum liquids, where the shore tank communicates to the atmosphere, the maximum pressure applied across the pipe wall at any point during a pressure surge is the sum of the hydrostatic pressure, the output pressure of the pump at zero throughput and the surge pressure. The first two of these pressures are usually known.
If the effective valve closure time is less than or equal to the pipeline period, the value of the surge pressure used in determining the total pressure during the surge should be P. If it is somewhat greater than the pipeline period, a smaller value can be used in place of P and, as already indicated, the surge pressure becomes negligible if the effective valve closure time is several times greater than the pipeline period.
Overall System Design
In this Chapter the simple case of a single pipeline has been considered. In practice the design of a more complex system may need to be taken into account. For example, the combined effects of valves in parallel or in series have to be examined. In some cases the surge effect may be increased; this can occur with two lines in parallel if closure of the valve in one line increases the flow in the other line before this line in its turn is shut down. On the other hand correct operation of valves in series in a line can minimise surge pressure.
Transient pressures produce forces in the piping system which can result in large piping displacements, pipe rupture, support failure, and damage to machinery and other connected equipment. Therefore the structural response of the piping system to fluid induced loads resulting from fluid pressures and moments must be considered in the design. In addition restraints are usually required to avoid damage ensuing from large movements of the piping itself. An important consideration in the selection of the restraints is the fact that the piping often consists of long runs of straight pipe which will expand considerably under thermal loads. The restraints must both allow this thermal expansion and absorb the surge forces without overstressing the pipe.
REDUCTION OF PRESSURE SURGE HAZARD
If as a result of the calculations, it is found that the potential total pressure exceeds or is close to the strength of any part of the pipeline system it is advisable to obtain expert advice.
Where manually operated valves are used, good operating procedures should avoid pressure surge problems. It is important that a valve at the end of a long pipeline should not be closed suddenly against the flow; all changes in valve settings should be made slowly.
Where motorised valves are installed, several steps can be taken to alleviate the problem:
Reduce the linear flow rate, i.e. the rate of transfer of cargo, to a value which makes the likely surge pressure tolerable.
Increase the effective valve closure time. In very general terms total closure times should be of the order of 30 seconds, and preferably more. Valve closure rates should be steady and reproducible, although this may be difficult to achieve if spring return valves or actuators are needed to ensure that valves fail safe to the closed position. A more uniform reduction of flow may be achieved by careful attention to valve port design, or by the use of a valve actuator which gives a very slow rate of closure over, say, the final 15% of the port closure.
Use a pressure relief system, surge tanks or similar devices to absorb the effects of the surge sufficiently quickly.
Limitation of Flow Rate to Avoid the Risk of a Damaging Pressure Surge
In the operational context pipeline length and, very often, valve closure times are fixed and the only practical precaution against the consequences of an inadvertent rapid valve closure, e.g. during topping off, is to limit the linear flow rate of the oil to a maximum value tome. This flow rate is related to the maximum tolerable surge pressure P_max by the equation:

P_max = wav_max

If the internal diameter of the pipeline is d, the corresponding maximum tolerable volumetric flow rate Q_max is given by:

With sufficient accuracy,
a, the velocity of sound in petroleum, is 1300 metres/second
w, the density of oils, is 850 kilograms/cubic metre so that, approximately,
Q_max = 7·1 × 10^-7 d² P_max
where Q_max is in cubic metres/second, d in metres and P_max in Newtons/square metre.
In two alternative sets of units:
Q_max = 0.025d² P_max
where Q_max is in cubic metres/hour, d in metres and P_max in kilograms force/square metre;

where Q_max is in cubic metres/hour, d in inches and P_max in kilograms force/square centimetre. 

http://homezwork.com/-172609.htm