The effect of valve closure speed on surge pressures
Control valves play a major role in controlling surge (water-hammer) pressures in most pumping and pipeline systems. This can vary from pump control valves which ensure gradual starting and stopping of pumps, to relief valves which relieve surge pressures out of a pipeline; surge anticipating valves do the same thing.
Relief valves basically take high pressures out of the pipeline so they don’t cause any damages, but they do their job after a surge event, when the high pressure “spike” has already occurred. Pump control valves, on the other hand, are there to prevent that high-pressure spike in the first place. They are classified as preventative valves, whereas relief valves act after the event.
End of line valves, such as reservoir level control valves, play a major role in causing surge pressures if they close too fast. Now, the question arises as to what is too fast and what is the right speed of closure (or opening). This is all determined by the pipeline “period” defined by the formula:
Tr = 2L/a where
L = Pipe length
a = Wave velocity (approx. 1000 m/s)
Pressure change by closure of a valve is determined by the formula:
dP = 0.070 dV. l/t where
dV = Change in velocity
L = Pipe length
t = Time of valve closure
What this means in practice is that the faster a valve is closed and the shorter the length of the pipe, the higher the pressure rise in the pipeline.
Speed of valve closure can be controlled by various factors such as:
- Gearbox on a butterfly or gate valve, to physically slow the closure of these valves when operated manually.
- Flow characteristic of the chosen valve. Gate valves and ball valves are known to only start affecting the flow rate after 70% of closure (30% from closed position). So these valves can be operated faster in the initial phase and slower in the last phase of closure.
Speed of closure of self actuated reservoir level control valves are always difficult to regulate properly. This is normally achieved through setting needle valves on the pilot control circuit. The problem is that once a needle valve has been set to ensure slow closure, say 5 minutes, it is left to get on with its job. After a short period of operation these needle valves are easily clogged with debris or scale build-up, resulting in failure of the valve to close at all. Maintenance staff then adjust the setting to a faster mode, which results in faster closure and water-hammer pressures in excess of pipe ratings.
Ultra Control Valves has developed a system of speed control, which allows a self-cleaning, regulating system to set the closure speed to a finely tuned limit, and which is sustainable and repeatable over long periods.
The topic of valve closure speed, which has a large influence on pressure generation in a pipeline system, is often ignored. Many factors influence the closure speed of valves, not least of which are the flow characteristics of the valve, which can often be adjusted (or manipulated) by adding features such as V-notches to gate valves or ball valves.
The theoretical pressure impact of such modifications are well covered in a paper written by Dr Don Wood of Kentucky University which you can read on Don Wood’s blog or download here: Using Control Valves for Surge Protection