Erik,
A small query:
".........a nozzle check valve closes much faster then a duo-check
valve......"
I my opinion, a fast closing valve will induce higher dynamic shock load. You can have a quick reference to Joukowski's Law for details.
Though I have never used a nozzle type check valve and I hence not aware of the characteristics, I have seem many successful installation of wafer type check valves without major problems with respect to operations.
Regards,
Pankaj.
"Erik Scheir" <erik.scheir@pandora.be>
11/02/04 11:11 PM
Please respond to PipingDesign
To: <PipingDesign@yahoogroups.com>
cc:
Subject: RE: [PipingDesign] Question on Wafer NRV installation
.
Hello Steve,
I have a lot of (bad) experience with wafer check valves with respect to waterhammer.
It is not that I blame a check valve (in general) to cause waterhammer,
but
some do and all depends ont the dynamic characteristic of the valve.
I always use Nozzle Check valves and I can show some graphs to explain the
difference between a nozzle check valve and a wafer check valve but I
don't
know were I can send those graphs.
All check valves closes under reverse flow but a nozzle check valve
closes
much faster then a duo-check valve so the reverse velocity is much lower
and
so is the pressure rise.
The ideal check valve is a diode but ideal doesn't exist,
Cheers,
Erik
Van: Steve McKenzie [mailto:mechproj@xtra.co.nz] Verzonden: dinsdag 2 november 2004 12:30 Aan: <a href="/group/PipingDesign/post?postID=vt1huCdm3BM2HyIX6A3hdlDuer24WxLW6UTJvdbDXWIK7ZIdMMjBv6x5irvrqe5SWt2pd94trp_3s_Zs851uX9IkpLcSYA">PipingDesign@yahoogroups.com</a> Onderwerp: RE: [PipingDesign] Question on Wafer NRV installation
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=vt1huCdm3BM2HyIX6A3hdlDuer24WxLW6UTJvdbDXWIK7ZIdMMjBv6x5irvrqe5SWt2pd94trp_3s_Zs851uX9IkpLcSYA">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=vt1huCdm3BM2HyIX6A3hdlDuer24WxLW6UTJvdbDXWIK7ZIdMMjBv6x5irvrqe5SWt2pd94trp_3s_Zs851uX9IkpLcSYA">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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[Non-text portions of this message have been removed] Received on Tue Nov 02 23:10:00 2004
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