15 replies to this topic

[Rach]

I am sizing a PSV for vapor relief discharging into atmosphere.. When sizing the psv discharge line to meet the 10% backpressure rule for conventional valves, what is the pressure that we consider at the psv outlet ? is it relieving pressure ( set pressure plus accumulation ) or do we back calculate by adding the superimposed pressure P1 ( atmospheric in this case ) and builtup back pressure P2..( P1+P2 ) and use that as the flowing /operating pressure in the discharge line ? The fluid being vapor phase, the operating pressure in the line is a critical input to size it...
Request a clarification..
Thanks
Rach

[fallah]

Rach'

In PSV sizing stage it is enough to specify that superimposed back pressure is ATM.Build up back pressure should be less than 10% of set pressure and later in PSV discharge line sizing it should be taken into account.

Fallah

[Rach]

Thanks Fallah for the quick response..

I'll make my query more specific...the PSV sizing is already done...and now I am doing the discharge line sizing , where i am trying to limit the builtup back pressure to 10% of the set pressure...Since i am sizing a vapor line , I need to put in the operating pressure in that line..I have the understanding that the operating pressure in the discharge line should be actually the PSV relieving pressure ( say right at the psv outlet, or at the start of the discharge line ).

But some sources say that the operating pressure in the outlet line is to be back calculated by adding superimposed pressure ( atmospheric in my case ) and built up back pressure ( say SP + BP ). .I feel, that SP + BP is actually the total back pressure acting at the outlet of the PSV. BUt the flowing / operating pressure at the beginning of outlet line should be the PSV relieving pressure ( set pressure + accumulation ), which will over come the total back pressure and provide the positive driving force to the relieving vapor.

The psv is set at 7 kg/cm2g, so the relieving pressure for fire case is 8.47 kg/cm2g. The total back pressure is just around 0.3 kg/cm2g...while sizing the outlet line, if I use 8.47 as the oper. pressure in the line ( in the line sizing spreadhseet ) I get a very small line size, say 1 ", that suffices for the 10% back pressure rule. If I take 0.3 as the operating pressue in the line, I need a much higher line size to take care of the 10 % rule..say , 6 ".

So that is my question..Do I take the psv relieving pressure as the operating / flowing pressure in the discharge line..or do i take the total back pressure as the operating pressure in the line...( whenever i say discharge line , I mean a point right in the beginning of the discharge line )...

Rach'

In PSV sizing stage it is enough to specify that superimposed back pressure is ATM.Build up back pressure should be less than 10% of set pressure and later in PSV discharge line sizing it should be taken into account.

Fallah

[katmar]

You cannot assume that the pressure in the discharge line is the same as the set pressure of the valve. If you had 8.47 kg.f/cm²g pressure on the upstream and downstream sides of your PSV then you would have no differential pressure across your PSV and therefore no flow. My understanding is that you should limit the downstream pressure to 10% of the relieving pressure, or 0.847 kg.f/cm²g in this case.

[Rach]

I agree, there has to be a differential across the psv for the flow to happen...
So just before the psv opens, we have the relieving presssure upstream of psv and the superimposed pressure downstream of the psv ( atmospheric in my case ) ... Now when the psv opens, the fluid should start relieving with the relief pressure ..which has to overcome that builtup back pressure or frictional losses ( which we r trying to limit to 10% of set pressure ), right ?
So the built up back pressure ( 0.847 kg/cm2g) is actually the back pressure that the relieving fluid is facing and has to overcome..it is not the driving pressure of the flowing fluid....in fact, that 0.847 kg/cm2g is the total pressure drop in the fluid's flowing pressure, by the time it flows out of the discharge line into the atmosphere....
So I should think that when the psv opens and the relieving fluid flows from point A to point B ( psv outlet to atmosphere in this case ), the flowing pressure of the fluid at A should be the relieving pressure..and at B ,it should be 8.47-0.847 kg/cm2g..Is that wrong ?

Now if I take the differential across the valve as 8.47-0.847, like you say, it would mean that the psv kills almost 7.6 kg/cm2g pressure and lets out the relieving fluid at the required 0.847 kg/cm2g..
Then if I have another psv set at 37 kg/cm2g, and the builtup back pressure is limited to 3.7 kg/cm2g, would it mean that here, the psv is killing 33.3 kg/cm2g ? .
Is that how it works ?

Regards...

You cannot assume that the pressure in the discharge line is the same as the set pressure of the valve. If you had 8.47 kg.f/cm²g pressure on the upstream and downstream sides of your PSV then you would have no differential pressure across your PSV and therefore no flow. My understanding is that you should limit the downstream pressure to 10% of the relieving pressure, or 0.847 kg.f/cm²g in this case.

[breizh]

Rach ,
Take a look at this paper , it should help

Breizh

[paulhorth]

Rach,

You had it right in your last paragraph. It's very simple.
Pressure UPSTREAM of the valve is the relieving pressure which is the set pressure, plus 10% accumulation when the valve is open and flowing. The pressure DOWNSTREAM of the valve is of course the BACK PRESSURE, made up of the superimposed (at the end of the tailpipe)and built-up (friction in the tailpipe) back pressure. The built-up back pressure must be no more than 10% of the set pressure. The difference between the upstream and downstream pressure is dissipated in the valve, with a lot of noise to prove it.

Think about the force balance on a spring loaded valve. The spring force plus superimposed pressure acting to close the valve, is balanced by the upstream pressure acting to open it. The valve will open when the upstream pressure is just higher than (spring force plus superimposed). When the valve is fully open, the 10% extra pressure upstream is balanced by the 10% built-up back pressure on the downstream side. If the back pressure exceeds the accumulation of 10%, the back pressure can partially close the valve and restrict the flow.

The upstream pressure is never seen downstream - which is the idea that you first mentioned.

Paul

[Rach]

Thank you everybody for helping me out in this..

Paul, thanks a ton for understanding my specific query and resolving it ..you couldnt have made it clearer...Thanks again.

Regards
Rach

Rach,

You had it right in your last paragraph. It's very simple.
Pressure UPSTREAM of the valve is the relieving pressure which is the set pressure, plus 10% accumulation when the valve is open and flowing. The pressure DOWNSTREAM of the valve is of course the BACK PRESSURE, made up of the superimposed (at the end of the tailpipe)and built-up (friction in the tailpipe) back pressure. The built-up back pressure must be no more than 10% of the set pressure. The difference between the upstream and downstream pressure is dissipated in the valve, with a lot of noise to prove it.

Think about the force balance on a spring loaded valve. The spring force plus superimposed pressure acting to close the valve, is balanced by the upstream pressure acting to open it. The valve will open when the upstream pressure is just higher than (spring force plus superimposed). When the valve is fully open, the 10% extra pressure upstream is balanced by the 10% built-up back pressure on the downstream side. If the back pressure exceeds the accumulation of 10%, the back pressure can partially close the valve and restrict the flow.

The upstream pressure is never seen downstream - which is the idea that you first mentioned.

Paul

[kkala]

Superimposed & built up back pressures have been often an issue in relevant forums. Following notes may be useful, concerning local present practice. Concepts and definitions may be somehow different elsewhere, but this is judged simple.
Built-up back pressure is the pressure drop from PSV to the header (e.g. flare header). Definition may sound a bit inconsistent, let us say it is the back pressure in case that PSV line would discharge to atmosphere.
Superimposed back pressure is the operating pressure of this header (at the connection point of PSV discharge line). It is 0 Barg if PRV discharges to atmosphere.
Above pressures are local and take max values when flows get maximum, without distinction between constant and variable flow.
Total back pressure is the sum of these two pressures above.
If PSV is of conventional type, max total back pressure (gauge) should be lower than 10% of set pressure (gauge), and discharge line should be sized accordingly. Otherwise a bellows type of PSV is selected.
This practice seems practical and also applied e.g. by robsalv, http://www.eng-tips....d=223304&page=6.
See http://www.leser.com...ck-Pressure.pdf for a slightly different concept, where the constant part of superimposed back pressure is taken into account.
However this thread discusses a PRV case discharging to atmosphere (total back pressure = built up back pressure), so this post does not affect validity of previous statements. It is for general consideration.

Edited by kkala, 20 March 2011 - 08:43 AM.

[kkala]

The psv is set at 7 kg/cm2g, so the relieving pressure for fire case is 8.47 kg/cm2g. The total back pressure is just around 0.3 kg/cm2g...while sizing the outlet line, if I use 8.47 as the oper. pressure in the line ( in the line sizing spreadhseet ) I get a very small line size, say 1 ", that suffices for the 10% back pressure rule. If I take 0.3 as the operating pressue in the line, I need a much higher line size to take care of the 10 % rule..say , 6 ".
So that is my question..Do I take the psv relieving pressure as the operating / flowing pressure in the discharge line..or do i take the total back pressure as the operating pressure in the line...( whenever i say discharge line , I mean a point right in the beginning of the discharge line )...

The posts have lead to accepting 0.3 kg/cm2 g as operating pressure in the discharge line of PSV (that is total back pressure), which is right.
This could be also justified by the fact that there is choked flow through PSV, since ratio of absolute upstream to downstream pressure is much higher than 2. This is general for (conventional) PSVs applied on gases.
Under chocked flow conditions, flow rate is determined by upstream pressure only; downstream pressure is determined (for this flow rate) by the pressure at the end of discharge pipe (here atmospheric) plus pressure drop from end to PSV (built up back pressure). Pressure downstream PSV should be back calculated, indeed. So there is a pressure discontinuity before and after PSV.
This is generally valid for valves, orifices, etc handling gases under chocked flow. One cannot specify a ΔP thru them on basis of velocity head; this is possible only in case that flow is not choked. In case of chocked flow downstream pressure is determined only by downstream piping and outlet conditions. ΔP is the difference between upstream and downstream pressure, but in data sheets we specify these two pressures separately (irrespectively of the kind of flow, choked or not).

Edited by kkala, 25 March 2011 - 12:30 PM.

[demank]

One more condition for discharge line of PSV. I usually add two criteria for it.
1. Rhov2, should be less than 150,000. When rho in kg/m3 and v in m/s.
2. Mach no. Should be less than 0.7

In norsok, rhov2 criteria is 200,000. These criteria may be different from one company to others.

[Kunal_DNV]

Hi Demanm,

Can you please reply to my post in the link below?

http://www.cheresour...m-gas-velocity/

[eastorca]

Dear Rach,

For conventional PSV, total back pressure/set pressure must less than 10% to ensure PSV capacity during its operation. Thus, try a pipe size for discharge line, then calculate the pressure drop during PSV operation. Total back pressure is sum of outlet pressure (ATM) and pressure drop on discharge line. Calculate ratio of Total back pressure/set pressure of PSV. If it is less than 10% -> OK. If not, try a new size of discharge line.

By the way,
Check the dynamic momentum (rhov2). Should be < 25 000kg/m/s2
Check the match number. Should be < 0.7 M
Regards

I am sizing a PSV for vapor relief discharging into atmosphere.. When sizing the psv discharge line to meet the 10% backpressure rule for conventional valves, what is the pressure that we consider at the psv outlet ? is it relieving pressure ( set pressure plus accumulation ) or do we back calculate by adding the superimposed pressure P1 ( atmospheric in this case ) and builtup back pressure P2..( P1+P2 ) and use that as the flowing /operating pressure in the discharge line ? The fluid being vapor phase, the operating pressure in the line is a critical input to size it...
Request a clarification..
Thanks
Rach

Edited by eastorca, 20 September 2011 - 09:07 PM.

[mm217]

Thanks every body. That was very useful topic. but i'm so please if anyone cleared that how i should fill the data sheet, before sizing the PSV. I mean is it better to say:
superimposed pressure:By vendor;
Build-up: By vendor
Total Back Pressure: ATM

Thanks a lot

Edited by mm217, 18 December 2011 - 03:36 AM.

[fallah]

I mean is it better to say:
superimposed pressure:By vendor;
Build-up: By vendor
Total Back Pressure: ATM

mm217,

All above information should be submitted by the customer and build-up BP to be checked after PSV sizing and selection.

Fallah

[mm217]

I mean is it better to say:
superimposed pressure:By vendor;
Build-up: By vendor
Total Back Pressure: ATM

mm217,

All above information should be submitted by the customer and build-up BP to be checked after PSV sizing and selection.

Fallah

Fallah;

Thanks a lot. That was very kind of you to answer.