SURGE PRESSURE or WATER HAMMER
Surge or water hammer, as it is commonly known is the result of a sudden change in liquid velocity. Water hammer usually occurs when a transfer system is quickly started, stopped or is forced to make a rapid change in direction. Any of these events can lead to catastrophic system component failure.
The objective for a water hammer investigation should be to suppress transient pressures to acceptable limit and avoid the following:
Pipeline bursts and leaks.
- Positive water hammer pressures should be limited to the pipe acceptable working pressure. It is not uncommon in plastic pipes, such as polyethylene to add 25% to the allowable working pressure because these materials are stronger and stiffer under rapid loading rates (e.g. transient loading conditions) than they are under lower loading rates.
- Pipelines can fail by buckling resulting from excessive vacuum during transient conditions in the case of thin walled large diameter steel pipe, low pressure rating plastic pipe and plastic pipes exposed to high temperatures.
- Cement lining in steel pipes has spelled off the pipeline in situations where the pipeline is subjected to vacuum conditions accompanied by large pressure fluctuations. The exposed metal surface corrodes resulting in accelerated pipeline failure.
- Asbestos Cement rubber ring joints have failed from vacuum pressures resulting from pump stoppages. The vacuum pressures have allowed air to enter the pipeline via the rubber ring joints and the joints have failed with time exposure.
· This may occur due to the violent movement of mechanical parts. Examples of these are check valves slamming shut following pump stoppages at multiple pump stations and the sudden closure of large orifice air valves when filling pipelines.
The magnitude of a pressure change under a transient condition is expressed by Joukowsky’s law for instantaneous valve closure:
△ h(max) = a Vo
where △ h(max) = change in pressure in meter head
a = celerity or the speed of the surge wave
through the liquid in the pipeline in m/s
Vo = change in velocity m/s
g = acceleration due to gravity as 9.8 m/s²
The formula provides an initial first step guide as to what likely pressures could be developed during a transient condition. It also demonstrates the impact of different pipe materials.Steel pipe has celerity of 1000 m/s compared to 250 m/s for polyethylene pipe.
The sudden closing of a valve with a pipe flow velocity of 1.0 m/s would generate a pressure change of 100 m head in the steel pipe compared to 25 m head in the polyethylene.
The use of a water hammer program is recommended for determining likely pressure surges even in simple piping system. There are many factors that influence transient pressure surges, they include:
· Pipeline profile (particularly high points)
· Pipeline anchorage
· Type of pipe material (and presence of linings)
· Location of storage’s
· Type of check valves (some are vulnerable to valve slamming)
· Pump performance curves and operating speeds
· Rotational moment of inertia’s for pump/motor assemblies
· Pump station configurations
· Location and type of air valves
· Protection devices installed
· Configuration of piping network
· Valve types, sizes and their opening/closing speed
A very effective method to visually demonstrate water hammer in a system is to use animation enhancement. The animation program shows a time-based simulation of the changing hydraulic levels and velocity profiles changing along the pipeline profile during a transient condition. It shows the interaction between the traveling pressure surge wave, changing hydraulic level and velocity profiles and the impact of boundary conditions such as storage tanks, check valve, pumps, changes in pipe diameter, etc. The program is particularly useful when demonstrating water hammer to personnel with limited experience in the subject.
Every water supply system is unique in relation to water hammer effects. The most effective solution to a potential water hammer problem may be a single or combination of protection devices. The relative merits of various devices should be compared and the best solution evaluated during the design phase of a new project.
A number of commonly used protection devices are described:
An effective device attached to pumps for generally shorter pipeline lengths. They help to dampen surges by slowly decelerating the pump speed on pump stoppage.
A pressure vessel containing air and water. It is a very effective device for controlling both positive and negative pressure surges and is often used as a last resort because of high capital costs.
One Way Surge Tower
An open ended device which is connected to the pipeline by a check valve. It allows water to enter the pipeline when the pipeline is subjected to vacuum pressures.
Non Return Valves
These are often used on steeply rising pumping mains. They help to prevent the pipeline length of water falling back on the pump’s check valve following pump stoppage.
Can be used in low pumping head systems where the height of the standpipe does not become excessive. They are often used on gravity systems.
They are often fitted on pump discharges. They are opened and closed slowly to minimize water during pump stoppage and startup. They are not effective during a sudden pump stoppage.
Surge Anticipator Valves
They are fitted to a pump delivery. They are hydraulically activated control valves which open when a pump stops and start to close as the pressure starts to build up when down surge reaches the pump. The slow closure of the valve minimizes water hammer pressures.
The difficulty in understanding the process of water hammer has given rise to certain misconceptions:
1. pipeline velocities must be low to reduce the effects of water hammer
2. surges only occur in long pipelines
3. the noise and knock in a system is an indicator of the magnitude of a surge
4. surge protection devices should be installed on an experimental basis because it is not possible to accurately determine the magnitude of surges
5. the use of surge protection devices are not economically viable
6. pressures in a system must be high before dangerous surges can occur
A water hammer investigation should be an integral part during the design phase for a new project, and if potential water hammer problems are identified, then the most effective selection of protection devices should be installed for that system.