A pressure surge is generated in a pipeline system when there is a change in the rate of flow of liquid in a line. The surge pressure can be dangerously high if the change of flow rate is too rapid. Surge pressures are most likely to be created during product transfer as a result of one of the following occurrences: -

1. Rapid closing or opening of a manually or power operated valve.

2. Rapid closure or opening of a non-return valve.

3. Slamming shut of a non-return valve.

4. Starting or stopping of a pump.

5. Rapid discharge of a freed blockage.

If the total pressure generated in the pipeline exceeds the strength of any part of the pipeline system upstream of the valve there may be a rupture leading to an extensive spillage. There are similar risks if a valve to fill a downstream pipeline system is opened too rapidly.

So far the simple case of a single pipeline has been considered. In practical cases the design of a possible complex system should 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.

Correct operation of valves in series in a line can minimize surge pressure. Valves, which are maintaining a pressure difference, can create surge pressures if opened too quickly. The pressure released can cause high liquid velocities and when the downstream line becomes liquid full, the sudden arrest of the flow can set up a very high surge pressure. A similar situation can occur when pumps are started.

In the case of manually operated valves, high pressure hosegood operating procedure should avoid surge problems. It is important that a valve at the end of a pipeline should not be closed quickly against full flow and all changes in valve settings should be made slowly.

 

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