Pressure Surge

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PRESSURE SURGE

description

Pressure surge is a very dangerous phenomenon that can occur on board tankers during cargo operations. Right from pumping and piping design to operations the criteria should be to avoid pressure surges happening during normal cargo operations.

Transcript of Pressure Surge

Page 1: Pressure Surge

PRESSURE SURGE

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Introduction

A pressure surge is generated in a pipeline system when there is any change in the rate of flow of liquid in the line.

The surge can be dangerous if the change of flow rate is too rapid. Pressure surges are most likely to be created during cargo transfer

as a result of one of the following actions: • closure of an automatic emergency shutdown (ESD) valve; • rapid closure or opening of a manual or power-operated valve; • slamming shut of a non-return valve; or • starting or stopping of a pump. If the total pressure generated in the pipeline exceeds the strength

of any part of the pipeline system upstream of the valve which is closed, there may be a rupture leading to an extensive spillage.

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Introduction

There are similar risks if a valve is opened rapidly to fill a downstream system.

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Generation of Pressure Surge

The pressure at any point in the cargo transfer system while liquid is flowing under normal conditions has three components: the hydrostatic pressure; the vapour pressure of the product if the tank is closed, or atmospheric

pressure if the tank is open; the pressure generated by the pump, which is highest at the pump outlet

but falls steadily with distance along the line due to frictional losses. The first two pressure components are constant and will be referred

to as the static component.

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Generation of Pressure Surge

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Generation of Pressure Surge

Rapid closure of a valve superimposes an additional transient pressure.

This is due to the sudden conversion of the kinetic energy of the moving liquid into strain energy by compression of the fluid and stretching of the pipe.

The stop in the flow of liquid is propagated back along the pipeline at the speed of sound and as each part of the liquid comes to rest the pressure is increased.

It is this disturbance that is known as a pressure surge. The height of the surge depends on the density of the liquid, the

rate of its deceleration and the velocity of sound through it.

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Generation of Pressure Surge

The surge pressure is greatest if the valve closes instantaneously. The sequence of events after instantaneous valve closure is illustrated

in Diagrams II-VI in figure G.I, and is described below. Diagram II shows the development of the surge as the

valve shuts.

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Generation of Pressure Surge

Diagram III shows that upstream of the surge the liquid continues to move forward and still has the pressure distribution applied to it by the pump.

Downstream of the surge, the liquid is stationary and its pressure has been increased at all points by a constant amount.

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Generation of Pressure Surge

There continues to be a downstream pressure gradient behind the surge, but a continuous series of pressure adjustments takes place in this part of the pipeline to equalise the pressure throughout the stationary liquid.

These pressure adjustments also travel through the liquid at the speed of sound.

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Generation of Pressure Surge

When the surge reaches the pump (Diagram IV) the flow through the pump ceases.

The pressure at the pump outlet (leaving aside the static component) becomes approximately equal to the sum of the surge pressure and the output pressure of the pump at zero throughput.

The process of pressure equalisation continues downstream of the pump.

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Generation of Pressure Surge

If not relieved in any way, the pressure throughout the whole length of stationary liquid reaches approximately the sum of the surge pressure, the pump outlet pressure at zero throughput and the static component.

The final pressure adjustment to achieve this correlation leaves the pump as soon as the original surge arrives, and travels back to the valve at the speed of sound.

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Generation of Pressure Surge

The time required for the whole process is therefore 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 is known as the 'pipeline period'.

Therefore, if the valve closes instantaneously, the liquid upstream in the line experiences an abrupt increase in pressure followed by a slower (but still rapid) further increase until the pressure reaches approximately the sum of the surge pressure, the pump outlet pressure at zero throughput and the static component.

In practical circumstances the valve closure is not instantaneous and there is then some relief of the surge pressure through the valve while it is closing.

The pressure front is less steep and as a result the height of the pressure surge is less than if closure had been instantaneous.

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Generation of Pressure Surge

The pressure rise created will be less and the reflected pressure rise will be relieved through the partially open valve.

At the upstream end of the line some pressure relief occurs through the pump and this also serves to lessen the maximum pressure reached; however if a check valve is fitted it may aggravate the surge.

If the effective closure time of the valve is several times greater than the pipeline period then surge pressure alleviation may be significant.

Loss of pressure through the pump continues after the lapse of the time interval 2L/a until the pressure throughout the line between the pump and the closed valve is reduced to the pump output pressure at zero throughput.

This is the situation that would have resulted from a very slow closure of the valve, but the pressure surge would not then have occurred.

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Generation of Pressure Surge

However, the pressure drop may be relieved by vapour evolution from the liquid although the subsequent collapse of the vapour bubbles may generate shock waves; for example, if the downstream liquid flows up and down through the transfer arm or hose.

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Other Surge effects

The description above refers to the simple case of a single pipeline. In practical cases the design of a possibly complex system should

be taken into account like, the combined effects of valves in parallel or in series have to be examined.

In some cases the surge effect may be increased. For instance, with two lines in parallel the closure of a valve in one

line can increase the flow in the other line before this line in its turn is shut down.

However, correct operation of valves in series in a line can minimise surge pressure.

The maintenance and regular testing of valve closure times is an important safety procedure.