10 replies to this topic

[Sara Owens]

Team,

I have a hazop item that i find very hard to resolve.

I have an emergency depressurization valve being stuck close. the recommendation is to  determine if the emergency depressurization rate can be covered by the existing relief valve. the existing relief valves are sized for fire. given the EDP rate, the relief valves will not be able to take care of the high rate. Is there any way to resolved this? has anyone ever seen a HAZOP done on a EDP valve being stuck close? the EDP valve is already my safeguard, i don't  see how a HAZOP can fail a safeguard. anyhow i need some ideas on how to close this hazop. i don't want to buy any more relief valves. 

[Zauberberg]

Emergency depressurization is a different topic from pressure relief. Depressurization removes the inventory from process systems whilst pressure relief just prevents from system overpressure yet maintaining inventory, so you cannot compare the two or interchange their basic functions.

 

If you need a certain reliability of the depressurization loop (including the EDV), you can achieve that with appropriate SIL rating of the entire loop.

[fallah]

 

Hi Sara,

 

Agreed with Dejan's viewpoint on your query.

 

Besides, an EDP system comprising depressurization valve is an additional layer of protection. It's not mandated by code, hence cannot be a substitute for the pressure relief device mandated by code.The matter should be evaluated in a, at least semi quantitative, risk assessment method such as LOPA study to check the adequacy of existing layers of protection and it's obvious during such assessment, the depressurization valve stuck close would come into play as a negative feedback for relevant protection layer.

[SyedAhmed]

In case of Fire the voting system activate the BDV. However if the HAZOP study considering the contingency of fire than the another contingency should n't be consider.

 

As per my understanding at a time only one occurrence should be consider.

[fallah]

In case of Fire the voting system activate the BDV. However if the HAZOP study considering the contingency of fire than the another contingency should n't be consider.

 

As per my understanding at a time only one occurrence should be consider.

 

You mean it's a double jeopardy...

 

If so, i don't think to be; because it's a case in which an instrument wouldn't respond to a request when needed to be activated, besides it might the valve already being stuck close and the malfunction recognized when needed to be opened...

[Zauberberg]

BDV stuck closed is a hidden failure (because you won't see it is stuck until there is a demand for it to go to open position). This means it could have failed anytime before the initiating event for BDV opening, and hence it is not a double jeopardy case.

[SyedAhmed]

 

In case of Fire the voting system activate the BDV. However if the HAZOP study considering the contingency of fire than the another contingency should n't be consider.

 

As per my understanding at a time only one occurrence should be consider.

 

You mean it's a double jeopardy...

 

If so, i don't think to be; because it's a case in which an instrument wouldn't respond to a request when needed to be activated, besides it might the valve already being stuck close and the malfunction recognized when needed to be opened...

 

 

Thanks for your response. its a learning point for me. But as you mention that Instrument response failure when needed to be activate.

 

This is a double Jeopardy like fire and BDV failure.

 

Mostly the scenarios which consider is the instrument air failure (ESD system failure), in that case BDV should be Fail open position but we can't take double Jeopardy that instrument air failure and BDV stuck close. If we will consider like this than in this way we have to consider the failure of other emergency valve failure cost will increase. 

 

If we consider ESD system failure is a cause in which power failure, instrument air failures are consider but if we consider BDV failure in the ESD system failure than we have to consider other ESD system failure which will increase the cost and complex control system.

 

Thanks

[Zauberberg]

Double jeopardy refers to any scenario where two unrelated events occur simultaneously. BDV stuck-open could have occurred in any point in time, therefore it goes out of the equation for considering double jeopardy. It is like determining if a hole on the road and sudden failure of your brakes are double jeopardy - they are not because the hole could have been there sitting on the road since long time ago. You are comparing apples and oranges in your example.

[Sharma Varun]

As rightly put by both Dejon & Naser, the depressurization valve shall be selected based on risk assessment & SIL Study. 

To comment on HAZOP recommendation, please share the details of HAZOP sheet,

Have you ignored any of the operating relief scenarios based on this EDV? Remember ESD valves (Instrumenattion say EDP valve) are pen-ultimate safeguards & relief valves are ultimate safeguards.

Please confirm if the deviation in HAZOP is external fire, because in that case PSV sized for fire shall suffice as safeguards.

Edited by Sharma Varun, 04 December 2018 - 12:34 AM.

[shan]

Double Jeopardy is defined as considering two unrelated causes of overpressure simultaneous occurrence. 

BDV malfunction is a latent failure as defined in API 521 (2014) 4.2.4, which is an existing condition and not a cause of overpressure. 

[MF003]

Team,

I have a hazop item that i find very hard to resolve.

I have an emergency depressurization valve being stuck close. the recommendation is to  determine if the emergency depressurization rate can be covered by the existing relief valve. the existing relief valves are sized for fire. given the EDP rate, the relief valves will not be able to take care of the high rate. Is there any way to resolved this? has anyone ever seen a HAZOP done on a EDP valve being stuck close? the EDP valve is already my safeguard, i don't  see how a HAZOP can fail a safeguard. anyhow i need some ideas on how to close this hazop. i don't want to buy any more relief valves. 

 

dear, buying any more PSV will not help you.i think, emergency relief valve have different pupose and it is not a susitute of PSV. it is for higt temerature protection during fire and protect the equipment on failue dut to high temp. due to fire. because PSV will open when it will reach to its set press. but heating is continued due to external fire and before reaching psv set press. metal fails. this metal failre is protected by depressurization valve. , i think not for high pressure protection. depressureization valve covers a loop where as PSV genrally protect equipment. In a loop they i genrally manu PSVs nut one depressuration valve. so if u r assing that emergency depress. stuck close second option is deluge like in tak farms. but prving deluge is anoth very costly option.it will be better if you provide on more EDV in the same loop  (with ins logic that if ons EDV is not open in 5 sec then othe will take care like alfa and bravo) and keep both fail to open.