relief devices on vessels containing gases 3

[abettoni]

Traditionally, relief devices on vessels containing gases are sized for fire based on the API 521. Unfortunately the rationale of the sizing equations is not clear.

Is there any available reference on how to use API 521 equations, or are there any valuable methods that may be used as an alternative? I know that some companies suggest that for such scenario a relief device is useless, and a pilot operated valve (or other depressuring systems) should be used instead. I have heard of criterias such as sizing the relief valve in order to reduce pressure by 50% in a certain time (e.g. 5 minutes) - does anyone have any information on these criteria?

 

[butelja]

Beware that if there is no liquid in the vessel, then the metal can rapidly be heated above its failure temperature.

 

[TD2K]

Butelja's comment is the basis that some companies will not use PSVs to protect a vessel in vapor service. The fire will heat the metal to the point that it can no longer withhold the internal pressure (since the PSV will only keep the pressure at or less than 110% of set pressure) and the vessel fails.

The criterial I've seen out of API is that a blowdown system must reduce the pressure in the vessel within 15 minutes to the lessor of either: 50% of design or 100 psig.

 

[rusman]

In sizing and selection of relief valve, you can refer to API RP 520 Part 1, and in the selection of the flange orifice relief valve, you can refer to API Std 526.

The PSV is required to be installed on pressure vessel as per ASME Sect VIII Code or BS 5500 code. Thus we have to install PSV as a pressure relieving device (secondary protection) and High high pressure switch as primary protection - refer API RP 14C.

PSV for fire case shall have accumulation of 21%, refer to API RP 520 Part 1 about this terms and terminology.

If there is no liquid presence in the vessel, thus the PSV must be designed based on unwetted surface area, ie gas expansion due to fire-refer API RP 520 Part 1 for the formulae. If there is a liquid, one must determine and calculate the boiling and vaporisation of the liquid. API RP 520 Part 1 latest release does give procedure in determining and sizing the PSV for this application. If I'm not mistaken, it is release 2000.

Pilot operated is normally used when:-
1. The operating pressure is close to the PSV set pressure.
2. The back pressure of PSV is more than 40 % of the PSV set pressure. If the back pressure is between 10 to 40 %, a balance bellow safety valve is used. Conventional PSV is used if the backpressure is less than 10%.

Hope this help

Thanks
rusman
process

 

[saidsaz]

if the container qualifies as vessel that can be isolated as per ASME then a PSV is a MUST.

for the gas vessels or vessels which store high boiling liquieds (above deign temp of vessel )a secondary aid like the blow down system (with API 15 minutes criteria ) or a sprinkle system should be employed,,

cheers

 

[grt12]

This is always a problematic area. Whenever I have tried to size a PSV for a gas filled vessel I have found that the relief temperature is way above the metal failure temperature and therefore a PSV would do no good whatsoever. However, the codes demand a PSV for a pressure vessel even though it will not protect the vessel in this instance. The usual route is to put a PSV on to satisfy the codes but also have a blowdown valve that will reduce the pressure to 50% of design pressure or 7 barg in 15 minutes to reduce the stress on the vessel wall and hopefully save the vessel.

 

[br549]

I have also used a PSV and a rupture disc in combination
Design the PSV for process upsets and the rupture disc for fire case since the disk is both temperature and pressure sensitve.

 

[JDN]

Unwetted Vessel Fire Size

Does anyone know how to take credit for insulation on a vessel when considering FIRE sizing of the unwetted surface. API-521 is quite clear that for a wetted vessel with insulation, some heat absorption credit can be taken provided the insulation can withstand the temp etc. API-521 also states that in the sizing of the required PSV orifice area for a unwetted vessel, credit has NOT been considered, but credit CAN be taken according to the environmental table etc. How do you apply the environmental factor?

 

[Guidoo]

JDN,

Why didn't you start a new threat?

The text in API RP 521 section 3.15.2.1 is indeed not very clear. I would advise you to use the method explained in

http://www.cepmagazine.org/pdf/080234.pdf

. The article is about vessels filled with supercritical fluids, but the method also works for gas filled vessels. As you can see, in equation (3) an environmental factor of 0.3 was used, corresponding to insulation with conductance value of 4 BTU/hr/ft2/°F.