20 replies to this topic

[rohollah]

Dear experts

As you can see in attached P&ID there are two conventional spring loaded safety valves on a steam turbine extraction line.

PSV-30476A with set pressure=34barg & PSV-30476B with set pressure=35.7barg

 The problem is that sometimes we are faced with premature opening of PSV-30476A while PI30454,PT30401  PT30403 show normal pressure which is 30barg.

We have checked all possibilities such as:  CDTP, process data, sizing calculation, spring and…

Despite everything is according to data sheet, problem is still exist.

I was wondering if you could find any reason for that.

Regards

Rohollah

Attached Files

•  PSV-30476A.pdf   268.32KB   51 downloads

Edited by rohollah, 26 January 2015 - 09:19 AM.

[fallah]

rohollah,

 

Is it possible uploading the data sheet of PSV-30476A? It might be a great help to find the reason of premature opening of the PSV.

[rohollah]

Dear fallah

Data sheet is attached.

Your assistance will be appreciated.

Regards

Rohollah

Attached Files

•  Data sheet.pdf   81.24KB   29 downloads

[ahmadikh]

 Rohollah,

 

Just a question! Can you please confirm that the protected lines and/or the PSVs are under ASME Sec I or VIII? The information on the datasheet specifies that they are under ASME sec VIII code as per the max. allowable overpressure. I just wonder if they are under section I or VIII.

 

Thank you

[amad]

rohollah,

 

beside data sheet, may be there was a mistake when the PSV was calibrated. We can call Inspection & Certification Company to ensure this or the vendor.

It is just IMO.
CMIIW

 

Thank you and regards

 

Isa

[rohollah]

Ahmadikh

As you mentioned ASME sec VIII is correct.

Rohollah

[rohollah]

Amad

 

We are quite sure about PSV calibration.

All tests have been done as per API520 & API527.

We are also sure about mentioned PI&PT calibration.

 

Rohollah

[Ajay S. Satpute]

Rohollah,

 

Please refer below link;

http://www.nationalb...geID=164&ID=379

 

Do we have a temperature correction factor value for these PSVs?

 

 

Regards.

 

Ajay S. Satpute

[rohollah]

Ajay

 

As I mentiond in post no1,  we have checked " CDTP".

According to manufacturer manual since this is a open bonnet PSV no temperature correction factor is needed.

 

Rohollah

[Ajay S. Satpute]

Rohollah,

 

Please refer post # 14 of below link;

http://www.cheresour...-test-pressure/

 

Let's wait for Fallah to come in the discussion and guide us.

 

 

Regards.

 

Ajay S. Satpute

[rohollah]

Ajay

 

Although temperature correction factor should be considered for T>121ºc (API 520 part II 3.4.2) some manufacturers don’t consider that for open bonnet PSVs.

Of course " Fallah"  is the one who can solve such problems.

I am really proud of him as a compatriot.

 

 

Rohollah

Edited by rohollah, 27 January 2015 - 05:12 AM.

[fallah]

Dear Rohollah,

 

Your attention is appreciated...I am really so...

 

Please specify the sizing scenario of PSV-30476A hasn't been specified in relevant data sheet.

 

If everything is ok, the only reason I would realize for such premature opening of PSV-30476A is high set pressure tolerance of mentioned PSV (may be higher than plus/minus 10%), then if so, if it would open at 30 barg, actually has been opened within allowable range. The set pressure tolerance hasn't been specified in the data sheet and I think the PSV is in ASME Section I category because contrary to ASME Section VIII in which the set pressure tolerance has been limited to +/- 3%, in ASME Section I there is no limitation for set pressure tolerance.

 

Please kindly recheck the matter and let's know the result, if any.

[rohollah]

Fallah

 

I rechecked it and as you can see in attached page set pressure tolerance shall be in accordance with ASME UG-126

Which is +/-3%

 

Rohollah

Attached Files

•  set pressure tolerance.pdf   91.92KB   31 downloads

Edited by rohollah, 27 January 2015 - 11:28 AM.

[fallah]

Rohollah,

 

The page you attached relates to project specification regarding the PSV sizing and manufacturing which has to be followed by vendor. Are you certain the vendor hadn't deviate from that specification in "set pressure tolerance" standpoint? Have you adequate evidence(s) by which you would ensure the vendor had applied specified set pressure tolerances?

[rohollah]

Dear Fallah

 

As per attached calculation sheet, following cods have been considered for sizing mentioned PSV:

ASME sec I (PG-69.2)

ASME sec VIII (UG-131)

ISO 4126

So what set pressure tolerance has been considered ?

Rohollah

Attached Files

•  calculation sheet.pdf   60.37KB   27 downloads

[MTumack]

What is the flow arrangement of the actual equipment?

 

Is it possible that between the Pressure Transmitter and the PSV's you have a piping geometry that temporarily amps up the pressure in the line enough to start the spring opening up? Maybe Hydraulic shock from that Check valve XV-30402 being pneumatically forced closed? Most conventional style PSV's wont stop relieving until they get about 5-15% below the set point as the spring needs to be able to put out more force than the fluid relieving does in order to seal, but it needs to be able to open at the relieving pressure; mechanically you are getting less resistance from the spring once your seal has been cracked.

 

Other interesting question; Datasheet is time stamped in 2001, has this happened continuously since the unit was installed (presumably in late 2001 / early 2002) or is this a recent occurrence? If it has only been a recent occurrence, then I would suggest that after ~15 years of service maybe some sort of mechanical failure has come into play. Check your maintenance logs with respect to the PSV's. Maybe your spring is on the fritz, maybe your seals are worn out. Maybe one of your maintenance guys changed out a seal and accidentally tweaked the set point on the valve.

Edited by MTumack, 27 January 2015 - 01:57 PM.

[rohollah]

MTumack

 

As I mentioned, although we have replaced the spring problem is still exist.

 

Rohollah

[fallah]

Rohollah,

 

You didn't specify the sizing scenario of the PSV as i requested...

 

Anyway, considering overpressure value appears the governing code would be ASME Section VIII, but it's not clearly specified...

 

The blowdown percent of the PSV also not specified in relevant data sheet and calculation sheet...

 

I think, there might be a transient overpressure scenario during which when the PSV would open at 34 barg the pressure relieving is so fast while you are reading the PIs to record the relieving pressure (30 barg) the PSV is at the blowdown condition, let say around 30 barg at its inlet nozzle assuming the blowdown of around 10%...Then you might wrongly imagine that the PSV is opened at 30 barg while it really has been opened at 34 barg and quickly came down to blowdown condition due to fast relieving the overpressure of mentioned transient upset...

[rohollah]

Fallah

 

Sizing scenario is outlet blocked due to XV-30402 failure.

Blowdown= 5-7%

We are pretty sure that mentiond PSV reaches to full lift because of the pop action.on the other hand PTs have not registered pressure more than 30.9 barg.

Rohollah

Edited by rohollah, 28 January 2015 - 03:17 AM.

[fallah]

rohollah,

 

Let's go back to the main issue:

 

The situation is: while operating pressure is 31 barg and there is no blocked outlet due to XV-30402 failure and everything is ok regarding the PIs, PTs, calibration standpoints and PSV spring; the PSV-30476A with set pressure of 34 barg will be opened occasionally at 30 barg i.e. at a pressure around/lower than the operating pressure. As far as I know this situation is to be titled "Inadvertent Opening" or "Spurious Opening" of PSV rather than "Premature Opening" because premature opening means PSV opening at a pressure higher than maximum operating pressure but lower than the set pressure. Then in my opinion such PSV even with new spring can't be approved in mechanical integration standpoint or hasn't been inspected/calibrated by an authorized company and should be replaced by a new one.

[rohollah]

Fallah

 

First of all thanks for your clarification of defining problem.

Since there is no history for XV-30402 I am not quite sure whether it is open or close during the PSV opening. Actually the only suspicious thing would be the position of XV-30402.

I will tell you as soon as I find any relevant .

 

Rohollah

Edited by rohollah, 28 January 2015 - 05:39 AM.