Thursday PSV Challenges 1

[ProcessRookie]

Good Afternoon Good Sirs and Madams of the PSV world.

I have two questions relating to relief design to ask the forum.

1) For vessels operating on high fouling service (for example, coke in say a coker or a cracker) can anyone recommend some design techniques to prevent fouling on the PSV inlet line? My concern is that the line either fouls to a slight degree but enough to make inlet line losses >3% or to an extreme degree where the ability to relieve is compromised.

2) For low pressure storage tanks on corrosive service, is there any way to prevent corrosion of the breather valves? Obviously specifying in stainless steel would help but anything else?

The scientist describes what is; the engineer creates what never was.

 

[The Obturator]

Right! Let's throw in some comments regarding potential fouling of medium at SRV inlet. No comment to part 2. of you question btw.

Without knowing the actual medium, process conditions and other specifics consider;-

i. Bursting Disc.
ii. Heated inlet (jacketed/tracing)and/or steam purge for nozzle cleaning.
iii. Possibly Pilot Operated Valve with remote inlet sensing AND chemical barrier.
iv. Special design flush seated SRV - seat is actually the vessel.
v. CSPRS.
vi. A combination of the above noting local code & regulations.

The important thing here, is to ensure the fluid is of a viscosity to be able to flow at the time of relief. Also pay regard to any solids in the fluid.


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

 

[don1980]

In a known fouling services, relief devices are generally not going to provide the safety/reliability/operability that is needed. In some cases one can possibly achieve the necessary integrity with frequent inspections/cleaning, but in other cases (most cases) that's just not realistic. When relief protection is needed, it goes without saying that that protection needs to be a reliable, and for known fouling services, common relief devices usually just can't provide that. In discussing this, it's important to distinguish between what is required (legal, code, corporate, etc.) and what is needed, because those two things are not always the same. Good risk managers (and all relief designers are risk managers whether they recognize it or not) know that meeting the requirements doesn't always mean that the risks are safely managed, and plugging/fouling services are good examples. The requirements in the codes and standards are met by simply installing a relief device, even when there's a high risk that that relief device will become plugged. That's not good enough, so what do we do?

This problem is recognized by ASME and PED. They both allow the use of high integrity instrumentation as a fully compliant alternative to common relief devices. In ASME jurisdictions, these instrumented pressure protection systems are referred to as HIPS - High pressure protection System (aka HIPPS -High Integrity Pressure Protection System). In PED, they're known as SRMCR - Safety Related Measurement, Control and Regulation systems. ASME authorizes HIPS in paragraph UG140(b). PED authorizes SRMCR by listing it in the definition of safety accessory (Chap 1, Article 2, paragraph 4).

Practical guidance for designing and implementing HIPS (and SRMCR) is found in API 521 Appendix E. Application guidance for SRMCR is found in EN 764-7, but it's not as detailed as that in API 521 Appendix E. The instrumentation can be designed to either shut off the source of the pressure (e.g. shut off the steam to a reboiler, or shut off the burners in a coker), or it can be designed to open a relief path.

 

[georgeverghese]

1) How about applying a constant purge of steam or fuel gas into the PSV inlet line?

 

[The Obturator]

I fully endorse what don1980 states. The system described is noted in my suggestions as v. CSPRS Controlled Safety Pressure Relief System. There is an ISO standard for this issued as ISO-4126 part 5.

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