29 replies to this topic

[stu]

Dear All,
Will you please explain me What is Lock open condition & lock close condition in valves.
Please explain in which situation we use these scenario.
Thanks in advance.
Stu

[djack77494]

Stu,
For very critical services to avoid poor operator choices or inadvertant operation of important safety related valving, some facilities will Lock Open and Lock Closed certain valves that should not ordinarily be in the alternate position. An example might be at the inlet to a PSV. The inlet and outlet valves could be locked open so that the operation of the PSV is not erroneously defeated. The valves could still be closed, but administrative procedures should be in place to ensure that the unit's safety was not compromised by this unusual valve operation.
Doug

[Qalander (Chem)]

Dear stu, Although Doug's suggestion is comprehensive reply, but I may briefly add that 'Motive/ Objective' is

To ensure safer process condition prevails and 'Halt/Hinder' any unsafe condition possibility.
Best regards
Qalander

[PME]

Yes u r right . But in some cases of refinery I have seen that some PSV inlets are LO & LC while PSV outlets are LO. Why this is so for inlets?

Regards
PME ( Refinery )

[Qalander (Chem)]

Dear PME Hello/Good Morning,

Probably you could not get the true message behind somehow.

The situation referred could be a case of two parallel PSV's in services with having one at a time in locked open(Car seal open) and the other one in locked close(car seal closed) position.

As such the matter referred is no surprise I understand.
However w.r.t. specific process location/requirements the respective PFD or P&ID might have to be shared for accurate discussion/estimating judgment, you should appreciate; accordingly more satisfying reply.
Best regards
Qalander

[djack77494]

I think Qalander has correctly identified the situation when you may see two sets of valves - one labelled "Lock Open" while the other is labelled "Lock Closed". I find it hard to justify such an arrangement and do not agree that any valve around a PSV should be locked closed. There is no reason to lock the valve closed. Open, yes, since it is to ensure the safety of the system against inadvertant unwise valve operation. But, there is no hazard introduced if the normally closed valve is not locked. If it is "inadvertantly" opened, then you merely generate a situation where you have two parallel relief valves. This is not a problem, and so does not need to be mitigated by the obtrusive step of locking the valve.

On a separate note-
Unlike some others, it has been my experience that operators very much hate locked valves and would greatly prefer car sealed valves instead. Locking a valve means that any operation is made quite onerous. It may prevent timely operation of the valve in an emergency or unforeseen situation. It also suggests that the operators cannot be trusted to not foolishly or maliciously operate sealed valves against their purpose. Some don't like the implication, and I don't blame them.

[JoeWong]

PME:

Avoid PSV chattering...

[djack77494]

JoeWong:

I don't understand how this could be valid. For an installed but out-of-service PSV, are you proposing that the inlet isolation valve should be Locked Closed? What is the position of the outlet isolation valve? Why is this in any way superior to merely having "normally closed/open" valves? It's very hard for me to imagine a valid reason to even car seal these valves, let alone locking them. I'm accustomed to reserving the use of car seals and locks to avoiding potentially hazardous/dangerous situations and not for routine operations.

[JoeWong]

Doug,
I am sure you know about this phenomenon... Let me help you to recall...

A system is protected with 2 x 100% PSV with identical set pressure.
If both PSVs inlet are open to protected system, in the event pressure built-up to set pressure, both PSVs will open. However, in real world. Non-ideal spring force, friction , etc would cause 1 x PSV open slightly faster than the other. This subsequently lead to both PSV's spring seeing different pressure on disc. One might on opening cycle and the other on blowdown cycle...The PSV on opening cycle will experience less force while the PSV on blowdown cycle will experience more force due to flow. They will change in cycle...Both PSV will fighting each and other...the is another type of PSV chattering phenomenon...

To avoid above chattering phenomenon,
...Inlet isolation valve of the PSV on DUTY will be on LOCKED (CAR SEAL) OPEN position
...Inlet isolation valve of the PSV on SPARE will be on LOCKED (CAR SEAL) CLOSE position

Apart from chattering phenomenon, SPARE PSV inlet isolation in CLOSE position is to minimize / avoid SPARE PSV passing.

...Outlet isolation valve of DUTY PSV will be LOCKED (CAR SEAL) OPEN position to ensure relief flow path.
...Outlet isolation valve of SPARE PSV will be LOCKED (CAR SEAL) OPEN position to avoid passing (in case inlet isolation as well as PSV passing), pressure built-up, condensation and finally corrosion of the section between SPARE PSV outlet and isolation valve.

Hope i don't miss any points...

[Qalander (Chem)]

Dears Hello/Good morning,

I have witnessed use of similar stratagy for parallel identical Pressure relief Valves on LPG bullets

provided at 1X2 (single common upstream inlet and separate PSV inlets& outlets)

connected through a selective positioning valve

thus permitting only one PSV circuit in service at any time(thus permitting positive isolation for operation/maintenance needs)

both outlet always open to flare piping

Hope this adds to the discussion positively.
Regards
Qalander

[djack77494]

Qalander and JoeWong,
Thank you for your replies. I am familiar with the situations that both of you reference. Regarding PSV protection for a horizontal vessel holding liquified gas (bullet), I have seen a four valve (=4 PSV's) assembly mounted on an anhydrous ammonia storage bullet. The assembly was constructed so that three of the valves were actively protecting the bullet, while the fourth was isolated and accessible for maintenance. I have also seen dual position valves mounted upstream of two PSV's. These were designed such that one PSV was always in service while the other was isolated. Using these devices, it is impossible to isolate the process from all PSV's (unless other extraordinary means are used). I fully accept the value of these assemblies.

My point was essentially regarding the use of LOCKING or even car sealing one of the PSV's closed. Why? Why not just "normally closed"? In my mind locking and sealing are extraordinary measures taken when potential catastrophe may result from errors. I see nothing to justify locking closed an isolation valve on a PSV inlet; locking open, yes, but not locking closed.

[JoeWong]

Doug,

Locked close the spare PSV inlet isolation valve is that the consequence of damaged PSV is just too high till it is justfiable to provide lock/seal on the inlet isolation of PSV.

In the even some operator inadvertently open the isolation valve at the spare PSV inlet. The valve can left open without notice. Overpressure of the system lead to PSVs seat damage and continuous fluid leaks. The plant is forced to shutdown and depressurise due to this leak. Repair or replace of PSV plus production loss is just too high. Providing a lock / seal is justifiable. This is from cost perspective.

Small PSV i.e. D type chattering would have minor vibration. Large PSV chattering not only lead to seat damage, severe vibration lead to mechanical damage (which is difficult to be studied) could occur due to large PSV chattering and might lead to catastrophe. The consequence of catastrophe is just too high and providing a lock / seal is justifiable again from safety perspective.

Some may say that only those large spare PSV inlet valve locked/car sealed close, save cost on small spare PSV. Well... how small is small ? Studies required to analyse each and everyone PSV. Studies could be higher than cost of installing the lock/seal.

Some may say that you can not have operator error plus overpressure occur at the same time. It is double jeopardy. Well... two events may not occur at the same time but they may occur one follow by the other. It is not double jeopardy but likelihood of the two events to occur and it consequence.

Some may say that my operator is well trained and they are not suppose to touch those identified critical valves i.e PSV inlet valve. If the argument stands, why do we need to locked/car sealed open the duty PSV ? Both possibly lead to severe consequences...

So, locking and sealing are not really extraordinary measures taken when you analyse from risk, consequence and cost together. Hope this discussion sufficient to justify a lock or seal for a spare PSV.

[Qalander (Chem)]

Dear All,

Definitely a highly educating/interesting discussion.
Best regards
Qalander

[djack77494]

OK, arguments accepted. I don't think I'll be specifying any locked or car seal closed isolation valves upstream of PSV I design, but I can accept the reasoning for doing so.

[JoeWong]

Huh... finally you accept my arguments...

[fallah]

Another point in this regard:
As per conclusion extracted from above discussions we should consider LC or CSC for manual isolation valves usually installed in by-pass line of PSVs,while in many cases we observe only NC status for them.Would you please explain about this point?
Regards

[JoeWong]

fallah:

Spare PSV line instead "bypass line of PSVs"

[fallah]

For fire case the spare PSV stored in warehouse,so it should be considered bypass line.Indeed ,we could observe many configurations with two (or more) PSVs+one bypass e.g. in Gas Treating Plants.
Regards

[Qalander (Chem)]

Dear faallah,

It is not getting swallowed by me that in emergency conditions what kind of bypass system would be capable of substituting Relief Valve needs and what about the Human intervention Pros & cons?

Best Regards
Qalander

[fallah]

Dear Qalander
Please note to previous discussions.I don't mean bypass would be considered for substituting PSV but have been included in many configurations of PSV sets.Bypass lines included isolation valves and these valves (NC) could be opened inadvertently by operators, if not being LC or CSC.
Regards

[Qalander (Chem)]

Dear Fallah,

Thanks for clrafication; I become suspicious if Process safety is in Jeopardy. Moreover I missed earlier info momentarily.

Regards

Qalander

[JoeWong]

Any safety problem if NC ?

[fallah]

JoeWong:

If mechanical strength of bypass line against vibration due to sudden relief had been considered safety problem may not being considerable issue,but due to inadvertently opening, there may be process problems in the vessel/drum/...on which PSV set installed and also in downstream of it.Unreasonable loss of vessel/drum...containing fluid and decreasing its operating pressure and... are some process issues we can point to them.
Regards

[JoeWong]

The so call "PSV bypass" line shall be designed for the full flow...otherwise how shall your operator manage it ?

I would like to make definition clear. "PSV bypass". Is it really a PSV bypass ? Are we really bypassing the PSV. No. We shall not bypass the PSV. This is NOT allowed. In fact, it serve the purpose of manual blowdown.

there may be process problems in the vessel/drum/...on which PSV set installed and also in downstream of it.

What are the problem ? If you have, i would agree.

Unreasonable loss of vessel/drum...containing fluid and decreasing its operating pressure...and... "

This is inventory loss instead of safety related issue. One operator may decide to LC/CSC, while the other may not.

[fallah]

If in normal operation the valve in PSV bypass line inadvertently left open (fully or partially due to not being LC/CSC),what would happen in the case of relevant unit depressurization or shutdown?Don't we have any backflow from the flare system in the case of unit shutdown?Do we have a successful depressurization while the unit connected to the flare system in another point by left open valve?
Regards