Check valve dynamic characteristics 2

[gguliye]

Hi,
I am reading a book regarding the check valves and it says to prevent water hammer:
If the valve does not close instantaneously but within the time of a
pressure-wave round trip of 2L/a, where L is the length of the pipeline, the
first returning pressure wave cannot cancel the last outgoing pressure wave,
and the pressure rise is the same as if the valve were closed instantaneously.
This speed of closure is said to be rapid.



so what i understood, we have to be really careful with the closing time. how to calculate the check valve closing time? is it liquid dependent or manufacturer has to give me the number?
regards,

 

[RVAmeche]

The closing time will depend greatly on the type of check valve and the internals (such as springs) used. I would assume the manufacturer would provide something to estimate time, and it may be liquid dependent.

 

[gguliye]

Thx
Let's see what others think too

 

[Compositepro]

Spring loaded valves will rarely have a water hammer problem because the the valve closes before flow actually reverses. They will have a higher pressure drop during normal flow.

 

[gguliye]

Thank you very much for your info

 

[RVAmeche]

Hence the name "silent" check valves

 

[gerhardl]

The text you refers to is not a description that precisely covers all the multitude of spring loaded check valves.

The text probably refers to a standard spring loaded check valve.

Soft closing check valves as a more advanced check valve group is not closing rapidly, as the closing movement for this group is not rapid or sudden. The soft closing valve opens according to increasing flow and closes by lower flow, down to being already almost closed when the flow stops.

For standard check-valves a too rapid closing will often give chattering and water hammer if the valve is too large for the actual flow. (As one example)

The best when calculating a check valve is to dimension by the actual flow (highest, medium and low). If high variation a higher priced soft-closing valve could be a better choice.

The text in itself is not wrong but limited to given cases.