High BP Liquid - PSV vs Fusible Link & SIL certification issues

[Cagliostro84]

Good afternoon,
I'm analyzing a case of protection from overpressure due to external fire, for a vessel cointaining high-boiling point liquid. PSV does not prevent vessel to collapse due to overheating, as set pressure would be reached only for temperatures higher than maximum allowable for the material (carbon steel).

So I'm evaluating to install a fail-to-open thermal fusible link valve in the bottom head of the vessel, to remove the inventory from the vessel when external fire starts and safely discharging the liquid to a collecting system when its temperature is still near ambient.

Valve body is certified as SIL-3, while the fusible link cannot be certified, and this is creating some concern about the reliability of the system.
I'm not an expert on SIL certification, but as far as I know SIL certificates are relevant only to instrumented system, not to mechanical ones (e.g. PSV, rupture disk, etc. are SIL certified?).

Does anybody have some experience on similar issues?

Thank you!

 

[The Obturator]

I can't comment on your fusible plug issue, but I can suggest that the PSV installed be also used as a "Dump Valve". Ie., A PSV design that has the ability to open below the set pressure. This could be accomplished in several ways such as a spring operated PSV with actuator or even air or electric motor. Both would require some form of controller. Valve size would also need to satisfy both scenarios.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Cagliostro84]

Thank you very much for replying.
In my understanding dump valve you are talking about requires some activation, so in this case it would work similarly to a depressurization valve (with the difference that this last would not reclose), is my understanding correct?

On the other hand I'm looking for something purely mechanical, without need of pneumatic or electrical connections.
Moreover I cannot lower set pressure too much as it would enter in vessel operating range.

What about SIL or SIL equivalent certification for safety valve?

Thank you!

 

[The Obturator]

Your understanding is correct. Except that this type of SRV would still offer protection at the set pressure point (fire case), but would actually be able to be opened remotely at any point before the set pressure using an auxiliary device. The manufacturer would most probably need to redesign the spindle since it is basically designed to lift the valve at a minimum pressure of at least 75% of set pressure when using a lifting lever. You can also construct a pilot operated PSV with say a solenoid to open the main valve remotely. I've come across similar applications in the past where valuable process needed to be saved, mainly ARCO/Lyondell PO Plant.

Dependant on code/spec/local law, you may want to consider the full actuator benefits. This is where you could have operating pressure as high as 98% of set by having a supplementary load on the valve seats. These then become CSPRS Controlled Safety Pressure Protection Systems an are in ISO-4126.

Hope these ideas help.

SIL for SRV - Not generally applicable to SRV, but you would need to talk to the manufacturer on that. It may be so that it applies to the actuation part and that maybe the whole unit would require a sign off.

Then there are HIPPS but I don't have enough knowledge to comment on that.

Per ISO, only the term Safety Valve is used regardless of application or design.

 

[The Obturator]

Here's an Air Motor example with short write up (for a spring operated SRV).


Per ISO, only the term Safety Valve is used regardless of application or design.

 

[Cagliostro84]

Your ideas are very helpful, however for my needs I'll look to look for mechanical only items, taking into account the alternative you are suggesting
Thank you!

 

[gguliye]

Dear Sir,
I think u can make a long pipe at the inlet of the PSV in order to reduce the fluid temperature PSV is experiencing and u may choose pilot operated valve with remote sense option