Been watching this discusssion from the sidelines, and I want to agree with Erik
- if a check valve is doing it's proper (ideal) function, it would close at the
moment of zero flow/zero pressure differential. Then, it would be a node in the
pipeline, with the valve defining a zero flow point. My question is, how fast
can the check operate? You have cases where the valve is lightly spring loaded,
so that it has to have a positive differential to open, and will try to close as
it approaches zero differential/flow. On the other extreme, you may have a
dampened check, which will close slowly, and can allow reverse flow - you will
get a hammer effect there!
On a fast acting valve creating water hammer, the example of closing a water valve quickly isn't a proper analogy, because actuating (closing) a valve with some flow through it is not the same as closing with no flow. Sort of like the difference between driving your car into a brick wall at 30 MPH, versus building the brick wall in front of your bumper while parked (zero MPH). In which case will you have more of an impact/rebound?
George McKinney
erik.scheir et al.
excellent and interestingly informative discussion.
perhaps if you sent Paul he could post the charts of
difference between nozzle check and wafer. I'm curious.
the Joukowski formula is DP = Fluid density*Fluid velocity*speed_of_sound
This pressure wave = dP should be less than Ps-Pv to avoid cavitation. Similarly the pressure rise will be positive upstream of a closed valve and negative downstream of an open valve.
The idea is to calculate the time of travel of the wave (at the speed of
sound) in a fluid of given density.
Time to travel (of the pressure wave) Less than an empirically measured
result this tells us water hammer will occur. (typically 10sec.)
ie. a faster closing valve is waht causes water hammer.. try it at home.
to cure try to slow down the valve so it cant be clsoed in less than 10sec. al
-----Original Message-----
From: Sajit Viswan [mailto:sviswan@tebodin.co.om]
Sent: Tuesday, November 02, 2004 10:02 PM
To: <a href="/group/PipingDesign/post?postID=e_MTw125KmouKLKUDmRo04UVjn25PNKoQMUfEN_9bOWlOqFxaMrpgUJ7MFo6WKCwjKQO8ixhL8tnAZS-CHEex3G_YA">PipingDesign@yahoogroups.com</a>
Subject: RE: [PipingDesign] Question on Wafer NRV installation
Pankaj,
I do not know the Joukowski's law.
However I am more inclined to believe that a faster closing valve will limit the water hammer effect from the reverse flow of the liquid in a pump discharge piping. I have read this in more than 1 valve manufacturers literature.
Can you explain what the Joukowski's law say, for a lay man's understanding.
Sajit
-----Original Message-----
From: <a href="/group/PipingDesign/post?postID=einJeNvlGHTsbHaQiAXw46JejsfMQIevSTXIkEnSb7fQCtXv6qN1RIVx5XqlvA8xTMmDgZecctwifSViONLv">Pankaj.Mandal@fluor.com</a> [mailto:<a href="/group/PipingDesign/post?postID=einJeNvlGHTsbHaQiAXw46JejsfMQIevSTXIkEnSb7fQCtXv6qN1RIVx5XqlvA8xTMmDgZecctwifSViONLv">Pankaj.Mandal@fluor.com</a>]
Sent: Wednesday, November 03, 2004 8:11 AM
To: <a href="/group/PipingDesign/post?postID=e_MTw125KmouKLKUDmRo04UVjn25PNKoQMUfEN_9bOWlOqFxaMrpgUJ7MFo6WKCwjKQO8ixhL8tnAZS-CHEex3G_YA">PipingDesign@yahoogroups.com</a>
Subject: RE: [PipingDesign] Question on Wafer NRV installation
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=e_MTw125KmouKLKUDmRo04UVjn25PNKoQMUfEN_9bOWlOqFxaMrpgUJ7MFo6WKCwjKQO8ixhL8tnAZS-CHEex3G_YA">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=e_MTw125KmouKLKUDmRo04UVjn25PNKoQMUfEN_9bOWlOqFxaMrpgUJ7MFo6WKCwjKQO8ixhL8tnAZS-CHEex3G_YA">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=e_MTw125KmouKLKUDmRo04UVjn25PNKoQMUfEN_9bOWlOqFxaMrpgUJ7MFo6WKCwjKQO8ixhL8tnAZS-CHEex3G_YA">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 Wed Nov 03 11:13:00 2004
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