Air pressure vessel safety valve sizing

[Guideon]

Hi everyone,

I have to determine the size of safety valve for an air pressure vessel. I found in ASME and BS 5500 a procedure to calculate the size of safety valve, which relates the mass flow capacity to the: pressure, temprature, discharge area, s.v coefficient, and air parameters. This can be helpfull if the flow capacity is known, so you can get the discharge area (valve size). But if the flow capacity is unknown (such in case manufacturing perss. vessels without compression units).
The next question is there any relation between the discharge capacity of the s.v to the compression unit flow capacity? I suppose yes, but is it equal/higher? how much?

Thank you,
Guideon

 

[TD2K]

Where does the compressed air to this vessel come from? What limits the pressure from that source? What is the design or MAWP of the vessel you are trying to protect?

Let's say you have an air compressor that has discharge pressure safety valves set at 150 psig. If your vessel is designed for 200 psig, the compressor can't overpressure your vessel because its design pressure is higher than the maximum source pressure. If your vessel was designed though for 100 psig, then the safety valve would need to relieve the capacity from the compressor at a discharge pressure of 110 psig (assuming 10% overpressure allowance applicable in this case).

If the MAWP of your vessel is greater than any of the source supplies then your only real relief case is fire. If the air is dry, then I'd likely look at a nominal 3/4" relief valve since a relief valve provides little protection against a fire case for a dry system. The fire will heat up the air which will expand and be relieved through the relief valve. However, as the vessel wall gets hotter and hotter, its strength starts to decrease. The relief pressure keeps the pressure in the vessel at about 100 psig so sooner or later a failure of the vessel is quite possible.

 

[Guideon]

Thank you TD2K

I'll try to make myself more clear..
Suppose I am a pressure vessels manufacturer (just the vessels without the compression units), I must drill a hole into the vessel and weld an openning for the safety valve! Then, what should be the size of that hole/openning?
Remember:
In such case you don't have the flow capacity of pressure source.
The safety valve should be operated in case of uncontrolled pressure rise (such pressostat failure/malfunction/fire).

Also, I would be glad if you can describe the process of choosing the apropriate s.v. for a vessel (what are the conciderations, parameters you concider?)

Guideon

 

[TD2K]

As the manufacturer you really have no way of knowing what size connection for a relief valve is required just as you don't really know what size inlet or outlet nozzle is required.

The client needs to tell you they need a 3", 6", 12", etc connection because they are the people that know (or better know) what the relief cases are and the corresponding nozzle required for the PSV.

You could provide a nozzle the same size as the inlet nozzle. That's 'likely' a reasonable worse case size with the client needing to confirm it. Or you provide a nominal 2" nozzle with the client again needing to confirm it.

It's similar to asking 'what size should the level connections be'? Different clients have different requirements. These used to be small, usually on the order of 3/4". Today, I see 1 1/2" as the usual 'minimum' with some clients specifying up to 3" for connections in dirty services or where they are going to install diaphram seals. Some clients will use an external standpipe and connect the level transmitter and level gauge to that, others will not allow it and want each device to have their own nozzles. There is no way of knowing.

 

[Guideon]

Thank you TD2K

I understand what you say, in other words you say that calculating the size of the p.s.v must be based on the compression unit to be installed on, especially the inlet flow capacity.
If so, practically, the flow capacity of the p.s.v should be equal (at least) or greater than the inlet flow into the vessel. Are there any directives\rules to the relationship between the inlet and the discharge flow capacity.

Thank you,
Guideon