25 replies to this topic

[rohollah]

Dear experts

 

As you can see in attached P&ID there is a PSV-11151 on top of a natural gas drum. Mentioned PSV has been sized according to following conditions:

Q=17600kg/h

P1=6.55bar

M=16.23

Z=0.99

T=41°C

On the other hand maximum flow rate which can pass from PV-11101 is 27210 kg/h

My question is that, why max flow rate=27210 isn’t being considered for sizing mentioned PSV?

 

Regards

Rohollah 

Attached Files

•  PV-11101 DATA SHEET.PNG   99.67KB   9 downloads
•  PSV-11151 DATA SHEET.PNG   78.45KB   5 downloads
•  P&ID.PNG   41.18KB   11 downloads

[fallah]

 

My question is that, why max flow rate=27210 isn’t being considered for sizing mentioned PSV?

 

rohollah,

 

Mostly because the outlet overhead line from D-1101 can release at least around 10,000 kg/hr (minimum flow passing through the PV-11101 per its data sheet) in PSV relieving conditions due to PV wide open case...then PSV-11151 would relieve remaining flow i.e the difference of 27,000 kg/hr and 10,000 kg/hr as maximum flow due to PV wide open scenario...

[rohollah]

Dear Fallah

 

First of all thanks for your clarification.

What about blocked discharge?

what if we have both scenario at the same time which are Pv wide open and blocked discharge?

 

Regards

Rohollah

Edited by rohollah, 10 August 2015 - 11:29 AM.

[fallah]

Dear rohollah,

 

Per PSV data sheet the PV wide open case seems to be governing scenario and blocked outlet case didn't mentioned in PSV data sheet; then if even it would be credibible appears the normal flow incoming to D-1101 to be lower than 17,600 kg/hr as design flow for PSV sizing. Then the PSV would handle the blocked outlet case...

[rohollah]

Dear Fallah

 

Don't you think at blocked discharge and PV wide open condition the flow which incoming to D-1101 would be 27210 kg/h not 17600 kg/h?

[fallah]

Dear rohollah,

 

Simultaneous PV wide open and blocked outlet scenarios is considered as double jeopardy and can't be accepted per API 521...

[rohollah]

Dear Fallah

 

What exactly double jeopardy is?

If it is not accepted per API 521 so where should we find that?

To be honest D-1101 has experienced such double jeopardy during shut down, due to PV passing and  blocked discharge (there is a fail to close control valve at the downstream of mentioned drum). at that time drum pressure increased to 27bar.

 

Regards

Rohollah

[fallah]

 

What exactly double jeopardy is?

If it is not accepted per API 521 so where should we find that?

To be honest D-1101 has experienced such double jeopardy during shut down, due to PV passing and  blocked discharge (there is a fail to close control valve at the downstream of mentioned drum). at that time drum pressure increased to 27bar.

 

 

Dear rohollah,

 

Double jeopardy means supposing simultaneous happening of two unrelated events which per API 521 is considered as remote occurence...

 

The detail of shut down conditions which you mentioned isn't clear...if it will lead to downstream control valve to be closed it can be supposed upstream PV is in normal position...

 

 

[rohollah]

Dear Fallah

 

As soon as plant shut downed both inlet and outlet control valves should be closed.

Once downstream control valves closed while inlet PV remains open (due to mechanical problems) despite in such situation PSV should release the pressure, drum pressure increased.

 

Regards

Rohollah

[fallah]

 

As soon as plant shut downed both inlet and outlet control valves should be closed.

Once downstream control valves closed while inlet PV remains open (due to mechanical problems) despite in such situation PSV should release the pressure, drum pressure increased.

 

Dear rohollah,

 

Appears those control valves are also shut down valves (equipped with solenoid valves). Anyway, during shut down if inlet control valve can't be closed (but remain in normal position) while downstream one is closed; PSV on mentioned drum can release the incoming flow through the PV to prvent the drum to be overpressurized...

[rohollah]

Dear Fallah

 

I do believe with your point of view.

so in such case the only reason for over pressurizing D-1101 could be any problem in PSV isolating valves.

Am I right?

[fallah]

 

so in such case the only reason for over pressurizing D-1101 could be any problem in PSV isolating valves.

Am I right?

 

Dear rohollah,

 

If the PSV isolation valves to be locked open, there would be no safety issue regarding over pressure of D-1101 due to mentioned valves malfunction...

[rohollah]

Dear Fallah

 

Many thanks for your help.

As usual your comments were useful.

 

Regards

Rohollah

[rohollah]

Dear Fallah

 

As I mentioned in post no #7 drum pressure reached to 27 barg while its design pressure is 5.5 barg.

Despite drum pressure exceeded to more than MAWP and MAAP no defect has found on it.

My question is that:

Can we steel thrust on mentioned drum?

 

Regards

Rohollah

[fallah]

Dear rohollah,

 

What is the MAWP value of D-1101? Obviously, the accumulated pressure of D-1101 in relieving conditions can't be exceeded than 1.1 times of MAWP (or 1.21 times of MAWP in fire case)...

[rohollah]

Dear Fallah

 

Design pressure=MAWP= 5.5 barg

In terms of MAAP limitation you are absolutely right. but what I mean is relating to material mechanical properties.

 

Regards

Rohollah

[fallah]

Dear rohollah,

 

If MAWP of the drum is 5.5 barg and might be subject to 27 barg, it can be supposed the drum would fail before reaching to that high pressure in material strength standpoint... 

[rohollah]

Dear Fallah

 

As I mentioned in post 7 drum pressure reached to 27 barg and did not fail.

Regardless to MAAP limitations I just want to be sure whether D-1101 is steel safe or not?

Shell material: A516 gr70 (yield strength= 38ksi , tensile strength= 70ksi)

Inside diameter= 1200mm

Thickness= 8mm

stress=pr/t

If we consider =38ksi then p=34.5barg

If we consider =70ksi then p=63.48 barg (rupture pressure)

Above equations show that in 27bar drum wouldn’t fail.

 

Regards

Rohollah

Edited by rohollah, 14 August 2015 - 05:19 AM.

[fallah]

 

As I mentioned in post 7 drum pressure reached to 27 barg and did not fail.

Regardless to MAAP limitations I just want to be sure whether D-1101 is steel safe or not?

Shell material: A516 gr70 (yield strength= 38ksi , tensile strength= 70ksi)

Inside diameter= 1200mm

Thickness= 8mm

=pr/t

If we consider =38ksi then p=34.5barg

If we consider =70ksi then p=63.48 barg (rupture pressure)

Above equations show that in 27bar drum wouldn’t fail.

 

Dear rohollah,

 

If the drum is to be  designed per ASME  Sec. VIII or another credible code, you should use allowable stress for thickness checking not yield or tensile strength...and yes, in theoretical calculation standpoint it might you would conclude that the drum don't fail well above its MAWP...

[Bobby Strain]

rohollah,

   You got lucky. You best find what went wrong and make corrective modifications so that it doesn't happen again.

 

Bobby

[rohollah]

Dear Fallah

 

With regard to attached data sheet do you think calculated orifice area for PSV-11151 is correct?

 

Regards

Rohollah

[fallah]

 

With regard to attached data sheet do you think calculated orifice area for PSV-11151 is correct?

 

 

Dear Rohollah,

 

At a glance appears the required orifice area, based on the data sheet info, has been rightly calculated; but to be sure completely you can apply relevant sizing equation available in API 520 part I...
 

[rohollah]

Dear fallah

 

According to API520 Eq3.2 and data sheet information, calculated orifice area would be 33cm2 not 49.958cm2.

As I mentioned in post no#1 despite of the fact that the max inlet flow rate to D-1101 is 27210KG/h flow rate 17600kg/h has considered for psv sizing.

regarding to calculated orifice area It seems as if max flow rate is 27210kg/h and maybe 17600kg/h is just a mistake?

 

Regards

rohollah

[fallah]

Dear Rohollah,

 

To clarify this confusion, at first you need to specify sizing case of PSV-11151...

[rohollah]

Dear Fallah

 

As per data sheet psv-11151 has been sized for pv-11101 fully open case.

 

Regards

Rohollah