16 replies to this topic

[rohollah]

Dear experts:

 

What if a flow higher than the required flow rate is applied to a conventional, spring-loaded PSV?

 

In other words, as per equation 3.8 from API 520 part 1, 7^th edition, there is a reverse relation between flow rate and pressure.

 

My question is that, can it be expected that a PSV open in lower pressure than its set pressure due to higher flow rate?

 

Your assistance will be appreciated.

Regards

Rohollah

[fallah]

rohollah,

 

A conventional PSV opens at the set pressure if there wouldn't be any concern regarding the value of the back pressure. Then if the relief load in full open condition is going to be higher than PSV rated capacity, the upstream vessel will be pressurized higher than prespecified relieving pressure i.e. 110% of the set pressure (121% of the set pressure for fire case). This is inline with the equation you mentioned, because due to fixed PSV orifice size higher relief load will result in higher upstream relieving pressure.

Edited by fallah, 15 October 2013 - 02:51 AM.

[rohollah]

Dear Fallah

First of all thank you for your valuable comments.

Let me clarify my question by following example.

As you can see in attached drawing, there is a conventional  PSV-30476/A which is located on extraction pipe of a steam turbine.

Although  mentioned psv has been set correctly in 34bar it opens in 30bar during start up or shut down.

So my question is that ,since during shut down or start up we have higher flow rate, dose it has something to do with problem or not?

On the other word since mentioned  psv is designed for max flow rate=225000kg/h what if higher flow apply to this psv?

Regards

Rohollah

Edited by rohollah, 05 July 2014 - 02:46 AM.

[fallah]

Dear Rohollah,

 

Nothing attached...

[rohollah]

please find attached files

Attached Files

•  Data sheet1.pdf   154.43KB   23 downloads
•  P&&ID.pdf   268.46KB   33 downloads

[fallah]

Dear Rohollah,

 

Actually they are two PSVs have been installed on the extraction pipe of the steam turbine and have been set in staggered situation; one set at 34 barg and another at 35.7 barg i.e. equal to 1.05*34.

Based on the info you provided, i think the problem you are faced with has nothing to do with the flow rate because when a PSV hasn't still be opened there is no flow rate passing through it and after its opening if the relief load to be higher than the rated value it may pass higher flow rate only in expense of the higher relieving pressure than the design value.

 

I guess the problem due to premature opening of the PSVs may be related to unstable flow pattern would be created for the PSVs haven't properly been installed on the pipelines in which a pressure spike can open the PSV below relevant set pressure. The installation of such PSVs should be performed so that avoiding mentioned unstable flow pattern....

 

Having the isometric drawings of the PSVs inlet lines and the line where they are installed may help submitting better assistance...

[rohollah]

Dear Fallah

 

I was wondering if you could tell me how unstable flow pattern results in premature opening?

 

Regards

Rohollah

[fallah]

Dear Rohollah,

 

As i mentioned i guess about the reason of the premature opening of the PSV that to be unstable flow pattern due to installation of PSVs on the line. Actually as API 520 Part II mentioned if a PSV not to be installed properly on a line, it will lead to unstable flow pattern might cause the PSV opens earlier or later than when should be opened and may be subject to chattering. Obviously, to make a reasonable conclusion for such premature opening, the turbine system and associated piping should be analysed in operational standpoint and PSV response to over pressure should be investigated in installation location point of view...

[rohollah]

Dear Fallah

I checked isometric drawings and I should say that everything has been installed according to drawings. The only difference which can be mentioned is about lack of 10" pipe between reducer and weldolet. As you can see in attached drawing, there should be a 10" pipe between reducer and weldolet. On the other hand reducer has been connected to the weldolet directly and without mentioned pipe.

As I mentioned before we are faced with this problem only during start up or shut down. that is why I do believe with your point of view that unstable flow might be the main reason.

So in such situation what would be the best and the most applicable way to solve this problem?

Regards

Rohollah

Attached Files

•  Untitled2.pdf   248.59KB   16 downloads

Edited by rohollah, 06 July 2014 - 03:50 AM.

[fallah]

Dear Rohollah,

 

I am investigating about the symptom of early opening of the PSVs you described and need clarification on some points as follows:

 

- In relevant data sheet you did attach in earlier post there isn't any info about the CDTP correction to check if correct set pressure has been selected...Of course as per item 32 of  the data sheet appears there had considered no correction in this regard...

 

- The operating pressure (31 barg) is too close to set pressure (34 barg); I want to know about the reason...

 

- The PSV type appears to be conventional; please confirm...

[rohollah]

Dear fallah

1-Although operating temperature is 344°c, CDTP=34barg is given on the nameplate .according to vendor operating manual no temperature correction factor is needed for this type (open bonnet) of psvs.

2-I do believe that operating pressure (31barg) is too close to set pressure. Since real operating pressure is 30barg, that is why it submits the equation figure1 of  API 520 part I.

I would like to add that during normal operation we don’t have any problem with mentioned psv.

3-this is a conventional type.

Your assistance will be appreciated.

Regards

Rohollah

[fallah]

Dear Rohollah,

 

As you did mention everything in normal operation is OK, then a question will be arised for transient conditions such as start up and shut down in which there would be pressure variation at PSVs inlet: How did you recognize there is a premature opening of PSV at 30 barg rather than 34 barg during such transient conditions?

Edited by fallah, 06 July 2014 - 02:12 PM.

[rohollah]

Dear Fallah

As you can see in attached P&ID there are two pressure indicators (PI-30452 & PI-30401).both mentioned indicators show 30barg during start up or shut down.

On the other hand due to piping conditions (because of distance and other branches) I am not sure if inlet pressure of PSV is equal to mentioned PI or not, that is why we have just installed another PI as close as possible to PSV in order to monitor the pressure during transient conditions. In normal operation new PI shows the same pressure. But we are waiting to compare it during shut down or starts up which haven’t happened since the new PI has been installed.

Regards

Rohollah

Edited by rohollah, 07 July 2014 - 12:13 AM.

[fallah]

Dear Rohollah,

 

Ok, thanks; then please specify:

 

- Which PSV among A and B is subject to premature opening at 30 barg (estimated value)...

- Credible scenarios for PSV-30476A/B and their sizing scenario...

[rohollah]

Dear Fallah

Although I don’t like shut down ,I am so enthusiastic about  comparing  the new PI. And of course I will tell you the result.

Since PSV-30476/A has lower set pressure then it opens during transient conditions. This is suspicious of premature opening.

In terms of sizing scenario I am not pretty sure, but I think these psvs have been installed to protect turbine due to any overpressure in extraction line.(as you can see in P&ID there is a ESD on PT-30401).

I am so proud of you(hamvatan)

Regards

Rohollah

Edited by rohollah, 07 July 2014 - 03:27 AM.

[fallah]

Dear Rohollah,

 

I am also proud of you and glad if I can provide any assistance regarding your query...

 

Thanks in advance if I will be informed about the result of comparing two different PIs's indications during shut down, but:

 

I think at first it should be checked if such PIs is recently calibrated and the calibration certificate is in valid time duration or not..., and one more thing: I think if PI-30452 which has been installed at pressure source shows a pressure of 30 barg, the pressure will be indicated at the same time by PI-30401 located at downstream of the pressure source cannot certainly be higher than 30 barg, am I right?

 

On the other hand, it is worth to mention that care should be taken in simultaneous reading the pressure from two PIs and subsequent comparing, because as per relevant PID the PI-30452 is located locally in the field while PI-30401 is remote one and is installed in control room...

 

Bottom line, it's better to be certain about the PSV set point accuracy by any means/procedure might available in relevant operating/testing manual...

[rohollah]

Dear Fallah

You are absolutely right, both PI-30452 & PI-30401 should indicate same pressure.

acthally I made a mistake I will attach new drawing in order for clarification.