Silo PSVs

[jtk921]

A plant is constructed with silos to store the finished dry resin product. The silos are 8900 cu. ft. capacity, SA516 gr 70 material, design pressure 3.6 psi, static head 24 psi. The silo was constructed per API 620, and is ASME code-stamped. Material is to be pneumatically conveyed to the silos and a dust collector is provided on top of each that vents to the atmosphere. However, no relief valves are provided in the design for the silo. My question is, is a PSV or PVSV still required? Or maybe just good safe practice? I have worked in PVC, and all silos had at least pressure relief (some also vacuum relief) on a separate nozzle in addition to the dust collectors, even if the dust collectors had high differential pressure alarms. It would seem to me dP alarms are only there to protect the filter.

 

[pleckner]

You say the design pressure is 3.6 psi...psi what? psia? psig?

A design pressure of 3.6 psig is well below that required for an ASME stamp; minimum being anything greater than 15.0 psig.

If this is indeed code stamped then you must provide an ASME certified relief device and it doesn't have to be a valve, it could be a rupture disk. AND, how is it that is is constructed to API 620? To get a code stamp the construction has to meet ASME criteria.

ASME does not require vacuum protection but API 620 does, API also requires over pressure protection but the type of device is not ASME certified. Indeed, if the set pressure of the relief device is less than 15 psig, it will not be ASME certified.

I would hope the dust collector has an explosion panel.

 

[jtk921]

Pleckner, thanks for your help. I reviewed the design and stress calculation sheet again, and I am confused. The nameplate MAWP is 3.6 PSI (all pressure values given are in 'PSI'; I assume that means psig), but further down in the calculations it states that the design pressure is 3.6 PSI but the component lowest MAWP (conical reducer) is 30 PSI, which would make it a pressure vessel.

BTW, there is a dedicated conveying blower for this system which has a safety relief at 12 psig. Experiments show that this material does not propagate dust explosions. The dust collector does not have an explosion panel.

So if the MAWP is truly 30 psig, then I am understanding that API 620 requires both overpressure and vacuum protection, correct? Can you provide the section that states that?

 

[pleckner]

If the design pressure or MAWP is 30 psig, then this does not fall within API 620 but ASME Section VIII, Div. 1.

But the design pressure (or MAWP) is taken at the top of the vessel and not at the bottom so that will govern what type of vessel this is.

 

[jtk921]

Either way, API 620 or ASME Section 8, it still requires a relief valve - correct?

 

[pleckner]

Yes and for vacuum protection. For lower pressures, investigate conservation/breather vents from vendors such as ProtectoSeal and Groth.