It depends on who you listen to.
- Many people say 5D is the rule.
- Many people say 10D is the rule.
- Many people are now saying that all pump discharges should have two check valves and these check valves should have 10D upstream of both the first Check Valve and the second Check Valve, plus 5D downstream of the second Check Valve before the Block Valve or elbow.
Now spend a little time looking at the different sized pumps (maybe 8" suction & 6" discharge and larger) we now find in the typical Mega-sized plants and see what the pump piping will look like, then go and sell it to the CEO of your current Client
- The consensus of past installations tells us minimum works and is the most cost effective when considering the cost of Real Estate inside the fence of a plant and the extra cost for pipe supports, stairs or ladders and platform to get over or around the obstructions.
Nothing is mandatory (Ps please don't use CAPITALs nex time it looks like you're SHOUTING!). Some pump vendors can advise it, especially for high flow high head units as it can disturb the flow in with excessive swirl. you need to check the pump mfrs installation instructions as if you don't follow it they could use that as an excuse if the pump doesn't work as well as they promised or there are things like vibration or seal problems.
I've not come across all these check valves, but if you're short of space on the inlet you can always put in a flow straightener - some fit between flanges (google ISO 5167 flow straightener), which are normally accepted as being the equivalent of the straight pipe run
My motto: Learn something new every day
Also: There's usually a good reason why everyone does it that way
I've seen the second check valve used with plunger pumps to dampen pulsations. They don't work as well as a real pulsation damper, and they tend to cost more, but they are occasionally used. Funny thing is that if they are an integer multiple of the pulsation wave length apart they tend to amplify the pulsation instead of dampening it. Other than an integer multiple they do OK. This is a huge problem on VFD plunger pumps since sometimes over the operating range you get into pulse amplification which can break stuff. I've found doubled check valves to be a problem far more often than they've been a solution.
For dynamic pumps the second check is a waste of money (it does nothing good or bad, just expensive. For non-pulsating PD pumps (like a progressing cavity pump), pulsation is rarely a problem and dual check valves are a waste of money.
