RE: Question on Wafer NRV installation

From: <Steve>
Date: Tue Nov 02 2004 - 06:29:00 EST


Hello Erik

in my opinion, it is unfair to blame a check valve for causing water hammer. The check valve merely closes under reverse flow, or, if you are lucky, at zero flow. That is what it is designed to do. It is the pressure of the water after the check valve has (nearly)closed that causes the damage. How can you blame the check valve for that? In my opinion, check valves work just fine. It is just that they also double as surge (water hammer) control devices; often without the "designer" even being aware of it.
For example, an undersized spring wafer check in a low pump inertia/high lift situation will close almost immediately after its pump stops. There will be no appreciable flow reversal so the water column will just "sit down" and the wafer will see about twice (conservatively) the normal operating head. The problem is that the return springs on the wafers are probably made of shit metal amd will set so reverse flow is the only available force that will close the wafers. So in new condition the wafer check valve works fine, but in worn condition (no springs) reverse flow must be allowed for in the design.
Now take the humble swing check valve. It has no springs, requires reverse flow to close, costs a fortune, and is sold by the thousand. It also has a spindle.
The worst thing you can do with a swing check is to oversize it. That means the disc is hanging, swinging in the breeze and wearing the spindle. Size them tight so the disc is banged hard against the stops. If you are worried about opex, do the sums; I have yet to see, this side of Mars, where a large check valve is economically justified. However, you can still overdo it. Many pumps have screamingly high discharge velocities; upwards of 7 m/s to keep the(casing) cost down. I try to limit the velocity that a check valve sees to around 4 m/s depending on spec and pennies; preferably around 2.5-3 m/s, but higher with a "smooth" design. Have yet to see a tilting disc design that pays its way, but I am used to cheap power costs, so will have to mend my ways as the oilwells dry up.

So Erik, if the function of a (wafer) check valve is to prevent reverse flow, and you recommend not using them, then you should see if you can suggest an alternative way of arresting reverse flow. Buggered if I know,but I hate the things. Check valves,that is.

Cheers

Steve

-----Original Message-----
From: Erik Scheir [mailto:erik.scheir@pandora.be] Sent: Tuesday, November 02, 2004 10:20 PM To: <a href="/group/PipingDesign/post?postID=AiL-Ozc3c2L28JYZVKhSeSyR5-SLDphOvW72wNYdb6kIyxhIbIbpLnxarLyDJm9oCcP_OHjRMw6yvYQKCjrsPvEq1rNuqw">PipingDesign@yahoogroups.com</a> Subject: RE: [PipingDesign] Question on Wafer NRV installation

Hi Robin,

It is much better not to install Wafer Check Valves at all. They cause often waterhammer when there is a pumptrip.The reason for that is that they have a bad dynamic characteristic,

Best Regards,

  Erik

Van: Robin Badcock [mailto:rbadcock@vision.net.au] Verzonden: zaterdag 30 oktober 2004 9:42 Aan: <a href="/group/PipingDesign/post?postID=AiL-Ozc3c2L28JYZVKhSeSyR5-SLDphOvW72wNYdb6kIyxhIbIbpLnxarLyDJm9oCcP_OHjRMw6yvYQKCjrsPvEq1rNuqw">PipingDesign@yahoogroups.com</a> Onderwerp: [PipingDesign] Question on Wafer NRV installation

Hi Paul,

Looks like I must have some how upset our 900 or so members as I had recently posted a query with respect to correct wafer check valve installation and received no replies, or did it go off into cyber space.

Perhaps the query was inappropriate?

Cheers,

Robin

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