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Posted by Tony Conner on May 23, 2002 at 06:25:55:
In Reply to: Re: Boiler feeding line Water hammering posted by Joe Lambert on May 23, 2002 at 04:33:32:
The biggest problem I've seen with check valves, is that they are routinely WAY oversized for the flow. However, I believe the ASME code calls for a check right at the boiler, and that it be the same size as the feedwater inlet to the boiler. However, the one back at the pump can be sized for the flow. It's often amazing just how small they need to be. Sizing the check at the pump off the discharge flange size usually results in a grossly oversized valve. I've installed an 1-1/2" check on a 4" pump discharge flange, and it was the correct size. I got a lot of looks, but it works just fine.
Check the info in Crane Technical Paper No. 410 - that's where I first found it. After reading the section on check valves, I finally realized why swing checks kept wearing out in all of the plants I'd worked in - they were way too big. The valve never opened all the way, but rather flaps up and down in the fluid flow like a flag in the wind. That's why the wear out at the shaft. You also get a much greater pressure drop through an oversized valve that doesn't open all the way, as compared to a smaller, correctly sized valve that gets pushed wide open by the fluid flow, and stays open. If you've got the Crane/ABZ piping program, it'll actually flash a warning on the screen that you've selected a check valve that's too big for the flow. (I don't know if the newer versions of the program still do that - mine's from the late '90s, but it's all based on the same formulas and data that have been around for 50 - 100 years, so it doesn't really matter. Except to newly graduated engineers that figure nothing of importance was developed or discovered until about 1980. I enjoy pulling out books from 1905 and showing them piping tables that have the same data as the new ones they're looking at.)
Anyway, checks are like control valves, a pretty good rule of thumb is that in a correctly sized line, the check will be one pipe size smaller.
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