Relief valves 2

[Ferex]

Hi everybody, I found that in most cases these kind of valves have an outlet with littler flange rating and bigger diameter than the inlet, I ask you why. Maybe the rating is for the pressure reduction but I really can't explain the bigger diameter.
Many thanks

 

[BigInch]

With the pressure reduction during product release through the valve, the product can expand greatly (esp. if a gas) and it is also usually desireable to slow the velocity down, expecially if relieved to atmosphere (preventing jet blast noise), thus a larger diameter is necessary.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25% to 50% of the total electrical energy usage in certain industrial facilities." - DOE statistic (Note: Make that 99.99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[msquared48]

Larger diameter = less head loss.

Mike McCann
MMC Engineering

 

[insult2injury]

Also.....for backpressure and blowback concerns

I2I

 

[DSB123]

Ferex,
Remember the Fluids and Thermodynamic courses you have undertaken as a chemical engineer and the answer stares you in the face. The fundamental requirement of a relief valve is to releive pressure. In doing so the pressure at the outlet of the valve is less than the pressure at the inlet and if we consider the event to adiabatic then using P1V1=P2V2 the volume must increase hence the requirement for a larger outlet. (i.e. to get the stuff out more easily!!)

 

[davefitz]

A pressure relief valve installed on a pressure vessel that contains a gas is nearly always operating with acoustically choked flow across the valve's seat upon opening . Its rating is related to the speed of sound at lifting conditions and the open area across the valve seat. In order for that rating to be correct , it is essential that none of the downstream systems ( piping, silencers, etc) form a restriction that would cause additional choking and derate the releif capacity.

Types of downstream choking that would derate the valve include frictional choking ( fL/d of exhaust piping), and acoustic choking ( flow area together with oblique shock waves that occur at elbows). To avoiod this latter choke effect, the outlet el needs to be a couple of pipe sizes greater than the valve seat .

 

[Ferex]

Great answers, many thanks

 

[SMIAH]

Related to this subject:

Can anyone tell me when putting the outflow from the Pressure relief valve back into the inlet of the pump is acceptable?

Thanks

 

[JLSeagull]

Some types of pumps may include a relief valve piped on the discharge with the relief valve pressure to the suction of that pump.

 

[SMIAH]

Is that acceptable? in most case.

Thanks

 

[davefitz]

smiah:
In some cases, such a recirc relief valve resullts in high fluid temperature and overheat of the pump.

 

[SMIAH]

For a Fire protection surpression purpose (water) : it should be acceptable ?
Thanks alot

 

[zdas04]

What you've described is a control function not a safety function. You as designer decide what control functions are installed and how they operate. I wouldn't ever use a standard PSV for this control function because they often leak around the stem and it is pretty common for them to fail to completely reseat. There are any number of control valves designed for recirc loops that would be more appropriate.

David

 

[JJPellin]

We commonly install PSV's with the outlet spilling back to the pump suction. But only in a special set of circumstances. We use this set-up on positive displacement pumps (usually screw pumps) to protect against a pipe or pump rupture if someone were to start the pump against a blocked discharge valve. It is not used for pressure control. We relieve to suction because we have no where else to go with a heavy product (typically asphalt). We don't want to relieve to atmosphere (big mess).

Johnny Pellin

 

[Zapster]

JJPellin, is correct in that PSV are not intended for pump discharge pressure control.

I have seen folks try to use PSV (pressure safety valves) in place of PRV (pressure reducing valves) for recirculation with bad results. PSVs are not intended for throttling applications. PSVs typically open too quickly and slam shut as well resulting in chatter that creates valve seat damage.

 

[BigInch]

Guys, the OP's question had nothing to do with using PSVs on pumps.

Please start a new thread, if somebody wants to continue this.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[BillPSU]

There is an issue if you reciculate the fluid directly with the pump. How do you get the air out at startup? We have had such an issue where the oil reservoir was used like a brake fluid reservoir. The oil never went through the the tank and the air in the system was causing noise in the hydraulic system. We did not see this problem until we started using pilot pressure operated controls. They eventually redesigned the hydraulic system to have the oil pass through the reservoir and get the air out. Bypassing the reservoir will also allow the heat issue mentioned previously but also any contamination coming from the system will be directed into the pump. I would not recommend the suggested plumbing arrangement as most hydraulic systems have already been compromised on the size of the reservoir and filtration only occurs on the return to tank.

 

[zdas04]

BigInch,
SMIH dragged us into the weeds, the rest of us are just following. I can't believe that none of us caught the first off-topic post, but we didn't seem to.

David

 

[BigInch]

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[gerhardl]

OK. Back to basic question.

The solution you describe is most commonly found on the market for the most common gas/liquid two phase (and single phase gas) used as heat and power carrying fluid: water steam with its condensate water.

Steam will as other gases expand when exposed to lower pressure, and likewise might condensate flash to steam of greater volume if released at lower pressure.