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Surge Analysis

What is pressure surge?
Changes in pressure arise in pipelines when there is a change in fluid velocity. These changes are the result of events such as pump switching and valve operation.

The greater the rate of change of velocity, the greater the pressure wave travelling along a pipeline. The initial wave may be positive or negative, depending on the cause of the change, and the initial wave is followed by an equal and opposite reflected wave.

Standard operating procedures are designed to minimise these pressure waves. However, an unplanned pump trip or rapid valve operation will, on an unprotected system, cause much larger pressure waves than under normal operating conditions. This is known as pressure surge or waterhammer.

Why is pressure surge a problem?
Unless surge protection measures are taken, the life expectancy of the installation may be reduced or sudden failure may result. Small systems are just as vulnerable as large systems.

Sub-atmospheric pressures can cause pipe wall collapse or allow air or water ingress. If the pressure drops below the fluid vapour pressure, then vapour cavities will form and subsequently collapse, causing additional pressure surges.

High pressures may weaken and burst sections of pipe (some pipe materials are particularly sensitive to abrupt changes in pressure). In addition, pressure surge will increase the loads on pipe supports, especially at bends.

In some systems, pressure waves are reflected back at different times from different parts of the network. When these reflected waves meet, they can combine to cause pressures higher than the initial surge.

How can a system be analysed?
An experienced engineer can assess whether a simple system is likely to be vulnerable to pressure surge from inspection of the system layout and operating parameters.

Complex systems will require the use of network analysis software with a transient analysis capability. Systems with a simple layout but with unusual or complex operating procedures also come into the category of complex systems.

A transient analysis will predict the time history of pressures (and flows) throughout a system. From the results, an experienced engineer can determine whether surge protection is required, what form of surge protection is most practical, its design and where it should be sited.

When should an analysis be carried out?
A surge analysis needs to be carried out at the design stage of a vulnerable system (typically a pumping station or hydropower plant). Checks should also be carried out before the geometry or operation of a system is changed, as the existing surge protection might no longer be sufficient.

A variety of options for surge protection can be modelled, depending on the fluid as well as the system layout. The object is to control the transient effects within satisfactory limits, taking into account the constraints on the system.

How can pressure surge be controlled?
Solutions to the problem to may include one or more of the following: ·

  • selection of the appropriate type of valve ·
  • a change in operating procedures ·
  • a change in valve closure rates ·
  • provision of air valves ·
  • increase in pump inertia ·
  • provision of surge vessels or tanks ·
  • installation of surge protection valves

Some of these solutions will require periodic checks to maintain their effectiveness.

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