5 replies to this topic

[Fakhri]

Dear All,

Refer to the previous discussion which as a new member I can't comment on it. 

 

http://www.cheresour...ep_filter_set=*

 

It was discussed and clearly explained that the relief load for PSV at 2 or 3 phase separator shall take the full two phase flow of the inlet. The explanation was that when the liquid outlet is blocked, the level will raise and subsequently the pressure will also raise, the LZHH can give protection while the PZHH can't give protection due fast pressure raise.

 

Is there any one has checked that the separator pressure raise so fast after initiating PZHH considering gas and liquid outlet blocked that the pressure is raise is faster than SDV closing time?

 

Or can we use justification that LZHH and PZHH are not independent protection layers since both are connected to the same SIS.

 

Please share your thoughts.

 

 

Best regards,

 

 

Fakhri 

 

[paulhorth]

Fakhri,

Since no-one has responded to your question, I will make a comment. I have not been able to open the link you provided to read the previous thread, so I am not clear on your question.

I think you are asking why the PZHH and the LZHH are not counted as two independent levels of protection, which would make a relief valve unneccessary (for blocked outlet). If this is your question, then your own answer is correct:

 

Or can we use justification that LZHH and PZHH are not independent protection layers since both are connected to the same SIS.

That is right, there may be two instruments but there is only one valve, on the inlet, so there is only one level of protection, hence a PSV is required. To eliminate the relief case you would need a second independent means of shutting off the inflow,a second valve, with the two valves actuated by either or both instruments. You may need duplicate instruments to achieve the required integrity (SIL 3), that would require a study.

I think the reaction time is not the reason why the two instruments do not provide the required integrity.

 

Comments from other readers would be valuable.

 

Paul

[fallah]

Fakhri,

 

As Paul mentioned the link you provided can't be opened. Anyway in general, per ASME and API a SIS with high integrity level can be considered as an alternative to pressure relieving device by deletion of over pressure cause itself. Then the SIS you briefly described, should be precisely analysed if it's intended to be as an alternative to the PSV for mentioned separator...

[Fakhri]

Thank you Paul and Fallah,

Yes, the problem statement is "do we need to size two phase relief valve since we already LZHH and PZHH in an inlet separator?"

 

Yes, I can understand we can not take credit if both LZHH and PZHH has the same final element which is the inlet SDV of the inlet separator. But normally for an inlet separator, both LZHH and PZHH will cause PSD which also close the riser SDV in a CPP or all wells in a WHP. 

 

Appreciate your thoughts on this,

 

 

Best regards,

 

 

Fakhri

[paulhorth]

Fakhri,

 

But normally for an inlet separator, both LZHH and PZHH will cause PSD which also close the riser SDV in a CPP or all wells in a WHP.

You are right that the two separate means of closing the flow do provide two independent levels of protection against overpressure. However, I am not sure that they will provide the required integrity to eliminate the PSV(10 ^-3 failures per year). For this, I think the shutdown actions will have to be implemented through a dedicated logic solver (separate from the plant ESD system), and also the actions will have to be regularly tested. You will need to do an integrity study or consult with an expert in this subject (which I am definitely not),

 

In the case of multiple wellheads, I believe that one oil company at least specifiess that a PSV would still be required, sized for the capacity of ONE well, to allow for the possibility of failure of one wellhead valve to close.

 

Paul

[Fakhri]

Paul,

 

Thank you for your clear explanation...

 

The requirement of separate logic solver is also a common debate in our company....I agree we can leave this issue to instrument engineers who will verify the SIL rating of the safety devices..

 

Regarding one PSV for multiple wells, this is also SIL study issues. I know from one of oil company, they consider shutdown system (PZHH,WV, SSV) and PSV for obne wells as a so called one hybrid protection. Shutdown system and PSV are not considered as two layer protections..

 

Best regards,

 

Fakhir

 

 

Best regards,

 

 

Fakhri