i have problem of water hammer in pump station, and i want to know how to size an air vessel (analytical approach)?
should i always also use a relief valve ?
(this is the first time i work on this subject)
You don't always need a relief valve, depending on what kind of surge control mechanism you are planning to build. Then again you don't always need a vessel either.
I'd like to explain more, but as you can see it is a bit too complicated to explain all of how you might want to do this it in an internet help context. You need to get a hydraulics engineer over there to help you.
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You normally start by doing transient analysis on the system to see how much volume you need to absorb to reduce the surge pressure. Then discuss with a vendor.
Usually much better to solve the root cause instead of dealing with the symtoms.
Do some searching here for surge, water hammer, things like this