Relief Valve exemption on a PV ?

[NovaStark]

Hi All,

I currently doing some inspection on some small pressure vessels that store air for the purposes of failing open/close control valves. So these are low pressure (less than 200 psig) and low temperature (near ambient temp) - they're also less than 5 ft high. While doing visual inspection, I noticed that some of the vessels have a relief valve whilst others don't. Looking up the drawings on the ones that don't, the drawing crossed out all the vessel connections which included one for a Relief Valve and it has an ASME stamp.

I looked up from the code and saw this:

(a) Other than unfired steam boilers, all pressure vessels within the scope of this Division, irrespective of size
or pressure, shall be provided with overpressure protection in accordance with the requirements of UG-125
through UG-138, or with overpressure protection by system design in accordance with the requirements of
UG-140, or a combination of the two.

Now all there is to the vessel is a tubing carrying air to it. That's it. No rupture disk or anything pressure relieving.

UG-140 OVERPRESSURE PROTECTION BY
SYSTEM DESIGN
(a) A pressure vessel does not require a pressure relief
device if the pressure is self‐limiting (e.g., the maximum
discharge pressure of a pump or compressor), and this
pressure is less than or equal to the MAWP of the vessel
at the coincident temperature and the following conditions are met

Now since it is from an Instrument air compressor whose discharge pressure is less than the MAWP (max is about 120 psig on a good day) would this lack of relief valve be allowed ? (there would be at least one RV on the compressor discharge line).

The reason I find this odd is that if it were by system design then none of the vessels would have RVs yet the majority of them do.

 

[EmmanuelTop]

The last statement is only partially true. For plants handling flammable fluids, one of the contributors to overpressure scenarios is fire. That doesn't sound likely in this particular case (instrument air area), but we don't know how fire zones are defined in that facility (if at all).

In many instances, local regulations require pressure relief valve to be installed on a pressure vessel, irrespective of the fact that there are no credible causes for process overpressure.


Dejan IVANOVIC
Process Engineer, MSChE

 

[NovaStark]

I don't think fire zones are identified. I believe I'd need to look up the local laws.

7. The safety valve of every air pressure container shall be
directly connected to the container with no stop valve or cut off
between the safety valve and the container:
Provided that where the safety valve is fitted to the pipe
connecting the compressor to the container, a stop valve may be
fitted between the safety valve and the container if the container is
fitted with a safety bursting disc which will burst at a pressure not
exceeding ten per cent more than the safe working pressure of the
container. Every such disc shall be so designed and constructed as
to prevent the scattering of fragments when it bursts.

Well I found this in the local law which we are "supposed" to follow.

 

[chicopee]

Reading the second highlighted paragraph on UG 140, the very last line stating"...and the following conditions are met:" makes me think that there is an unsuing condition stating as long as the vessel has no isolation valves downstream and is part of another storage configuration fitted with a properly sized PRV.

 

[chicopee]

Conversely has no isolation valve upstream and is part of additional storage configuration (upstream of the tank in question) fitted with properly sized PRV's

 

[NovaStark]

In the case of the PV, it only has one instrument air tubing in and one out to the control valves.

Apart from that, there is one RV on the entire instrument air header right off the discharge of the compressor.

 

[davefitz]

If the PV can be blocked in and it contains a liquid , then it will need at least a thermal relief valve . If it contains air and cannot be overpressured by the compressor's dead head capacity, the UG-140 may be applied. Section VIII also has another provision that would allow avoidance of the relief valve if the client certifies that the PV is directly vented to atmosphere and agrees that no relief is required, as with some flash tanks.

"...when logic, and proportion, have fallen, sloppy dead..." Grace Slick

 

[NovaStark]

If the PV can be blocked in and it contains a liquid , then it will need at least a thermal relief valve . If it contains air and cannot be overpressured by the compressor's dead head capacity, the UG-140 may be applied. Section VIII also has another provision that would allow avoidance of the relief valve if the client certifies that the PV is directly vented to atmosphere and agrees that no relief is required, as with some flash tanks.

I don't think these PVs fit any of the exemption criteria and some of these are located in close proximity to both low and medium pressure steam piping so if there is a leak in the area, I can see there being a possible overpressure scenario assuming there is steam directly hitting the air receivers.


The IA compressor in my case would pressure a large air receiver which has an RV on it which then feeds the header and then to the PVs in question. So UG-140 here may work however I can't understand why the majority of PVs would have the RV unless that is just based on the manufacturer.

 

[GenB]

All vessels shall have a safety releaf device. You can install a main safety R.V. after the Pressure regulator to protect as the Code implies.

General Blr. CA,USA

 

[NovaStark]

Thanks, that is along what I was thinking.