Pressure reducing valve working in the oppsite flow direction

[waseem19]

All,

I've come across an existing PRV valve installed in a way that it is subject to flow in the reverse direction it should normally works in. I know that some PRVs come with a check valve feature so this shouldn't be a problem for them, my question is what happen to the valve if this feature is not their and the flow is reversed?

There is a good chance that this valve does have the check feature cause the operator hasn’t noticed the contrary (he should notice cause he want be getting any water on the other end otherwise)

Is it going to act in its fully open position? Should I only consider the valve minor losses if this is the case?

The PRV valve is on a 100mm pipe. I’ve only seen it on drawings and I have no idea who the manufacturer is and I have not got the technical data sheet for it, it was installed in 1999.

Regards

 

[ScottyUK]

I’ve only seen it on drawings

Is there the smallest possibility that the drawing is wrong, and the field installation is correct?

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[JimCasey]

A 100mm PRV >may< have an external sensing connection. If the external sensing connection is connected to the conventionally accepted downstream side of the valve then the valve will work nearly normally. There will likely be some offset due to the inverse sympathetic ratio being inverted, but your operators may not have noticed or just figured they have to set the valve a little off the desired set pressure to get proper function.

If the valve has intrernal sensing,and it is installed backward, then it will open whenever the upstream pressure is low, and close (probably slamming abruptly) whenever the upstream pressure exceeds the set pressure. If this is the case since the valve is spring-loaded open, you may never have reached the set pressure. Once it closes, you may find it necessary to vent the system upstream to get it to reset.

 

[Joesteam]

Some of these water PRV valves (Like Cla-Val, Watts, or OCV) come with a check feature as an option and some do not. No telling how much water may be flowing backwards, depends on exactly the valve used, the pressures on both sides of the valve, etc. If it is an issue, install a separate check valve

 

[BigInch]

JimCasey,

I'm not understanding that first paragraph. Can you tell me how a PRV would work if the sensor was installed "on the conventionally accepted downstream side of the valve". That description leads me to think it means the sensor is on the downstream side of the PRV, which would place the sensor on the vent line. If that were the case, the process piping or the vessel would not be protected, but the vent line would. Shouldn't your description say "upstream of the PRV"? Have I missed this entirely?





Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[chadl]

I believe waseem is talking about Pressure "reducing" valves, not pressure "relief" valves. ie. the sensing line is monitors the downstream pressure, and adjust the valve accordingly to maintain an output pressure.

Please correct me if I'm wrong.

 

[BigInch]

Must be.

In that case, as long as the "downstream" pressure is less than the set pressure, the valve is basically open and flow will go from hi P to lo, no matter which direction thats in. If pressure is higher than the set pressure, its closing.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[Ashereng]

Ah, I see the source of confusion.

waseem19 is using PRV for "pressure reducing valve". Instead, he should have said pressure regulator. PRV is usually a pressure relief valve.





"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[BigInch]

That's it!

I have seen "PRV" and "PSV" more or less interchangably, all for pressure R="relief" or S="safety" valves.

Regulator type valves seem to just get included into the broad "FCV"-"PCV" family => Flow or Pressure Control valves.

At least in the specs I've been using lately.

Going the Big Inch!

http://virtualpipeline.spaces.msn.com

 

[stanier]

PSV is also used for pressure sustaining valves!

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[waseem19]

All,

thanks for all the replies, I thought the title of my question was enough to avoid the confusion.

jimcasey,

if it externally sensed, you said it will work normally , but for which direction ? both direction of flow ?

if the valve is internally sensed and it work like you said then why some manufactures provide the check feature as an additional feature if their valves block the flow reversal naturally any way ?

In this part of the world PRV means pressure reducing and PSV means pressure sustaining !

This is a treated effluent pipeline if its make it clearer.

waseem.

 

[JimCasey]

I was assuming that PRV meant Pressure Reducing Valve. i.e.: Regulator.

A regulator works by being spring-loaded open, and using the downstream pressure against a diaphragm to compress the spring to close the valve. Controlled pressure lower than setpoint: valve opens. Controlled pressure too high: valve closes. If the valve is in backwards, but an external sensing line is still in the right place, it will still work-maybe not as well.

Regulators are unidirectional devices. 2-way flow >may< happen but it is not regulated in the reverse direction.

There ARE Back-pressure regulators that hold a controlled pressure upstream and vent any excess. These are much like a throttling safety-relief valve, but intended for continuous duty instead of upset protection. Internally they are the reverse of a Pressure Reducing Valve: Spring-loaded closed, use the UPSTREAM sensed pressure on the diaphragm to open the valve.

 

[chadl]

if the valve is internally sensed and it work like you said then why some manufactures provide the check feature as an additional feature if their valves block the flow reversal naturally any way ?

Are we talking about check valves in parallel with PRV or in series. In some instances a check valve is installed in parallel to allow reverse flow. An example I deal with from time to time is on water distribution systems. A pressure reducing (or is it regulating?) valve is utilized to reduce the pressure for a certain zone in the distribution system. In normall operation the upstream pressure is higher than the downstrema pressure and PRV maintains the downstream pressure at it's set point; however if something was to occure to cause the pressure upstream to drop lower than the downstream pressure, the regulating valve would close, but the check valve would allow water flow from the (normally) downstream to the (Normally) upstream side perhapse providing water if the system is experiencing heavy use or something.

PRV are kind of Finicky in nature, at least in my experience, check valves maybe installed even on internal valves, in series to ensure that no backflow occures if the valve fails to shut.

I checked Cla-Val website briefly, but did not see any info on pressure reducing valves that were factory equipped with a check valve. If you don't mind me asking, which particular valve were you looking at?

Chad

 

[Joesteam]

The Cla-Val does have an option for a check valve feature.
Look at:

http://www.cla-val.com/pdfs/E-90-01.pdf

The other manufacturers mentioned have the same option in these valves. This is a very simple and straightforward application. If he is getting back flow, and it is not acceptable, either get a pressure regulating valve with a check valve feature or install an additional check valve in series with the reducing valve.

 

[Ashereng]

I have seen "PRV" and "PSV" more or less interchangably, all for pressure R="relief" or S="safety" valves.

Regulator type valves seem to just get included into the broad "FCV"-"PCV" family => Flow or Pressure Control valves.

In the oil and gas industry, the definitions of the acronyms PRV and PSV are as BigInch stated.

Also for regulators, FCV and PCV as BigInch stated.

In other industry, as I am finding out now, it seems to be different.

I guess this is where the confusion originated.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[proinwv2]

Be aware that the downstream pressure rating of a PRV can be less than the inlet pressure rating.

Other than that the valve will not function as intended. Simply look carefully at the manufacturers cross section drawing to understand how they function. They really are simple. Then look at it in your reversed application.

Paul Ostand

http://www.ostand.com

 

[quark]

The pilot diaphragm type PRVs (pressure reducing valves) control the downstream pressure by sensing the downstream pressure and by activating a secondary diaphragm. The connection can be either internal or external. In either way, the valve malfunctions if installed in a reverse way.

Some of the valves, I used, have steam inlet below the disc and outlet port above the disc. In this case, the steam pressure supplements the spring load and the valve will be shut forever. However, these are for typical steam/gas application and I don't see any problem with liquid PRVs.

It is a general notation in India to refer reducing valves as PRVs (pressure regulating valves) and PRVs (pressure relief valves) as safety valves.