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Lekoso (Mechanical) (OP)
2 Sep 09 5:21
Good People,

I need another one.

Could anyone explain what benefit could be derived from instaaling a "orifice plate" downstream of a Pressure Relief Valve servicing a Centrifugal Pump?

Regards

'Lekan
Helpful Member!  BigInch (Petroleum)
2 Sep 09 6:40
Increases the system curve, thereby increasing the required head and pressure to move any given flowrate.  Increases the pressure drop and thereby reduces the pressure downstream, until you try to start or stop the pump when it will run temporarily at or near full shutoff head, possibly opening the relief valve until flow increases and the discharge head drops.  

If the pump has the wrong curve for the system, it might help keep the operating point somewhere within the pump's efficiency range, but otherwise it will be mostly a benefit to the power company.

**********************
"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/

rmw (Mechanical)
7 Sep 09 17:06
Big Inch,

Good answer, but I believe that the OP states that the orifice plate is down stream of the pressure relief valve, not the pump or did I read it wrong?

It may be that the relieving capacity of the pressure relief is such that its flow needs to be reduced.

rmw
Lekoso (Mechanical) (OP)
7 Sep 09 17:11
Yes,

The orifice plate is at the relieve valve downstream, not upstream as Big Inch assumes.
HEC (Mechanical)
10 Sep 09 0:10
what is downstream of the orifice plate, what is the function of the system? once we know these things we can give a sensible response for why the orifice plate is there.

Mark Hutton


 

BigInch (Petroleum)
10 Sep 09 1:24
I know where the relief valve is.  The OP assumes there is some advantage.  I don't.  The effects are as I said, specifically the relief valve is held closer to its opening pressure, so at low flows where the pump curve is high, it might tend to open sooner than without the orifice plate.  Without knowing more, that's all I can say.  Perhaps the orifice plate is needed to reduce downstream pressures or the flows for some reason, but frankly with the info given, I can't tell you why its even there at all.

**********************
"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/

Artisi (Mechanical)
10 Sep 09 2:08
The thinking behind the setup (that's if there was any) would help in giving advice.
rmw (Mechanical)
12 Sep 09 14:58
I think that the relief valve is going to open at its set pressure regardless of whether or not the orifice plate is there or not.  What I think the orifice plate does is to throttle back on the relief valve flow to prevent the pump from running too far out on its curve due to the relief valve flow.  Without knowing particulars, it is impossible to tell how much impact the pressure relief valve has on the overall pump flow.

Or it reduces the relief valve flow once the valve opens at its set pressure to reduce the sudden impact of the relief valve opening on the driver.

Lekoso is being rather sparse with the information.

rmw
BigInch (Petroleum)
13 Sep 09 7:49
Of course reducing flow at any given head is the net effect, but with the orifice plate, the pump runs closer to a given relief valve set pressure and a much smaller transient pressure might open it.  Whereas, if the orifice plate was not included, there would be a more significant margin between running pressure and set pressure.  Fact is that the orifice plate increases the friction component of the system curve; does nothing to the pump curve.  Therefore the head discharged by the pump at any flow, other than zero, is increased.

Therefore, since the pressure is increased at any given flowrate, the relief valve set pressure must be raised for any given flowrate, as opposed to what it could have been before installation of an orifice plate.  Hence, now at any given flowrate, since the inlet pressure to the relief valve must be raised, the flow across the relief valve for any system flow would also be increased over what it would have been without an orifice plate.  Result is more outflow into the relief system and less flow to downstream piping than without an orifice plate.

 

**********************
"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/

SNORGY (Mechanical)
13 Sep 09 11:01
I think, of all answers given, rmw's most recent one comes closest to what I would believe.  However, if the PSV opens and the pump responds by immediately wanting to move outward (well right of BEP) on its characteristic, the orifice restriction probably serves more to dampen the transient than protect against pump run-out, especially if the characteristic is somewhat flat to allow wide swings in flow rate over a narrow change in head.  It also might help control PSV chatter.

What then becomes difficult for me to rationalize is, if the characteristic is somewhat flat left of BEP, then is the PSV required at all?  What is the rise to shut-off relative to head at BEP?  For pump overpressure protection, the PSV would make more sense to me if the characteristic is steep rather than flat.  If such is the case, then this might be a rudimentary attempt at minimum flow recycle.

Therefore, I believe every answer given has merit, but a P&ID with set points and pump curves would be helpful towards providing more definitive comments.

Regards,

SNORGY.

BigInch (Petroleum)
14 Sep 09 2:52
An orifice plate will not dampen a transient.  How could it possibly.  It just sits there.  No moving parts; no hydraulic capacitance.  The relief valve however will eliminate all pressure transients above its set point, assuming it acts fast enough.

The psv is needed no matter what characteristics the pump has, presuming there is some component of a lesser allowable than the total normal+transient pressures somewhere in the system.

I'd say it increases any psv chatter, since the system curve would increase more quickly at any given flowrate, thereby forcing the psv to close quicker, but doing nothing else to reduce the pressure, the psv would quickly open again.
 
 

**********************
"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/

SNORGY (Mechanical)
14 Sep 09 10:17
Perhaps a misnomor...

By "transient", I meant the propensity to somewhat rapidly experience an increase in flow from Q1 to Q2 as a result of the relief valve opening.  The extra restriction provided by the orifice downstream of the PSV (which I interpret to mean as installed in the PSV discharge piping) would tend to reduce the flow achievable by the fluid in this path, thereby serving to reduce the magnitude of the change from Q1 to Q2.  Perhaps the term "transient" has been used loosely, since it has a very different meaning if one is concerned about something like fluid hammer.  All that was meant by "transient" in this instance was "short duration event other than steady state".

"Change in flow rate" would have been a better choice of words.

As for orifice plates not being able to damp out a transient, I do know that when people have trouble with the magnitudes of pulsations and / or undersized pulsation dampeners on PD (plunger) pumps, what is sometimes done is to install restriction orifice plates under and / or adjacent to the pulsation dampeners in the system piping.  For whatever reason, that sometimes seems to work in reducing pulsations; perhaps, though, what is also being changed are the system frequency characteristics.  So, it may well be true that the orifice plate will not do much to reduce the magnitude of a pressure wave or spike.

Regards,

SNORGY.

BigInch (Petroleum)
14 Sep 09 14:45
OK, you mean an "excursion from steady state".

If an orifice plate is "small enough", it could tend to use the capacitance of the adjacent piping as a damper, but I would think that it would be a relatively small diameter orifice that would be needed to do so.

**********************
"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/

SNORGY (Mechanical)
14 Sep 09 15:21
Yes - and you are quite correct in that the orifice by itself will add nothing in the way of hydraulic capacitance.

Again, I need to choose words more carefully when industry-accepted terminology differs from more generic descriptions of words / terms.

I think we (where I work) have been doing too much "crude and close enough" stuff for too long, and it is having an erosive effect on technical correctness.

Regards,

SNORGY.

helmuthscholtz (Mechanical)
15 Oct 09 9:20
Hi i have the same problem help please
what is the use of the oriffic eplate
 
TenPenny (Mechanical)
15 Oct 09 9:43
I would expect that the orifice plate is meant to give what is in effect a 2 stage pressure drop when the relief valve opens (stage 1 is across the relief valve, stage 2 across the orifice plate), one would guess that perhaps this is hot water or other flashable liquid.

In boiler feed applications, many of the automatic recirc valves are fitted with back pressure regulators or fixed orifice plates for this reason.

But this is dragged up from the cobwebs of my mind, and is subject to being completely wrong.  This opinion is offered with a 100% money-back guarantee.
helmuthscholtz (Mechanical)
15 Oct 09 9:55
okey then. here is the problem. It is for jet a1 and avgas pump station.we want a drum filling area. the problem is the we have to reduce the pressure and flowarate from 60 m3/hr to 4.2m3/hr. can one run the pump at this flowrate of 60 waht will happen to the excess fuel if only have 1/2 inch relief valve between pump oulet and the oricife plate. my question is will be okey to run the pump at 60 m3/h without damage to pipr work. should nt you need anditional by-pass.


 
BigInch (Petroleum)
15 Oct 09 13:22
You can't usually run a fixed centrifugal speed pump at less than 10% rated flow.  Overheating will most definitely occur within 5 to 10 minutes.  You could try to recirculate the other 56 m3/h, but you might have to go all the way back to the tank to prevent overheating the recirculating fluid.  If you can recirculate that extra 56 m3/h using very very low power, maybe you can do it.

**********************
"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/

helmuthscholtz (Mechanical)
16 Oct 09 3:28
thanks big my point too. I did put in by-pass to the tank
the tank but the engineer that has to approved it say that the orrficep plate will reduce the pressure but what about that excess flow. the relief valve is only 1/2 inch. i dont understand his logic behind this.there will be heat built up built and pressure. and the pmp canot shut off and on this is not good for the pump.   
BigInch (Petroleum)
16 Oct 09 5:21
We are starting to hijack this thread.
Can you start a new thread for this topic.

**********************
"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/

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