Check Valve Flutter 1

[nalinds]

Hi

We have installed two vacuum pumps and refurbished all pipework in a vacuum pumping station. The non-return valves are dual-flap wafer type valves. The flaps of these valves vibrate/flutter when the pumps are running; but this does not happen always, only sometimes.

Any ideas what causing this?

 

[BigInch]

dual flaps, one extra to make sure it flutters as much as possible.

Learn from the mistakes of others. You don't have time to make them all yourself.

 

[btrueblood]

Lack of damping.

 

[bimr]

Would you add some further details on this application. Is this wastewater?

 

[LittleInch]

you clearly have a fluctuating flow. At lower velocities, these valves can chatter - all depends on the actual velocity of the fluid going through it and probably density also as they tend to be spring loaded. Might not be the best type of NRV for your duty.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[nalinds]

Thanks for the info.

I'll give some more details:

-The vacuum pumps are Busch, in a duty standby arrangement, running at nominal speed 1600 m^3/hr.

- Pumps are connected to a vacuum receival tank via. a liquid separator.

-It is domestic sewage that collects to the vacuum receival tank. but it doesn't/shouldn't go through the pumps.

- Fluttering is random. occurs once/twice a day only. Happens in both pumps.

- We have installed these vacuum pumps with same type of NRVs, but of smaller sizes, in a few other pumping stations; but there is no fluttering observable.

- The valve TDS says this:

"Gulf Wafer Check can be installed in both horizontal and vertical piping systems. (Contact the factory for vertical DOWN installations.) The accurate sizing of check valves is crucial to ensure an acceptable pressure drop and a long service life. The pressure drop of the Gulf Wafer Check can be calculated, for liquid or gas, using the charts and calculations shown on page 3. It is a known fact that all check valve discs flutter. The higher the velocity and turbulence of the media (liquid or gas) moving through the piping system, the greater the flutter and wear. Once again, the Gulf Valve technological know-how has met the challenge. Gulf Wafer Check is designed to provide long service life if the velocity is kept within the flow rate ranges shown in table, and if a minimum of 5 pipe diameters is maintained downstream from a likely cause of turbulence (i.e., pump discharge, reducers, or elbows)."

 

[btrueblood]

So, did you follow those guidelines? Is there a transient condition that could cause an increase in the mass flow/velocity (sudden flush storm at halftime on home-game weekends for the local team)? Can you go up in pipe size to get the valve down below the critical velocity range, even in these transient conditions?

In a past life, I had more trouble with check valve "chatter" at very low flow rates in a vacuum system; are you sure you have flutter (high flow velocity) and not chatter (valve rising off seat just enough to relieve pressure differential, then falling back)?

 

[bimr]

Your comment points to the check valve being slightly oversized.

"We have installed these vacuum pumps with same type of NRVs, but of smaller sizes, in a few other pumping stations; but there is no fluttering observable."

An oversized check valve is often indicated by continuous valve chatter, which is the repeated opening and closing of the valve that occurs when the valve is only partially open. It is caused by the fact that when the valve opens, there is a drop in the upstream pressure; if this pressure drop means that the differential pressure across the valve falls below the required opening pressure, the valve will slam shut. As soon as the valve shuts, the pressure begins to build up again, and so the valve opens and the cycle is repeated.

Oversizing can usually be rectified by selecting a smaller valve, but it should be noted that this will increase the pressure drop across the valve for any one flow. If this is not acceptable, it may be possible to overcome the effects of chatter by reducing the closing force on the disc. This can be done either by using a standard spring instead of a heavy-duty one, or by removing the spring altogether. Another alternative is to use a soft seat; this does not prevent the chatter but rather, reduces the noise. Care must be taken however, as this may cause excessive wear on the seat.