PSV without calculation 1

[MortenA]

For a TZV usually a calculation would not be required since the relief rate is low.

What about other applications of relief valves where over pressure is not the issue but e.g. pump protection is? Take a chemical injection pump. The pump might not be able to over pressure the piping system but would fail if blocked in. For this reason a relief valve is installed returning the liquid to the tank from where the pump is fed - would you request a calculation note here - or would you accept the vendor design given that he has a performance guarantee (but this does not cover lost revenue i guess)?

Best regards, Morten

 

[TD2K]

What do you mean by "fail"?

If the pump can not overpressure the piping and would basically "stall", then I would accept a calculation note. If by fail you mean a loss of containment due to failure of the pump, then I would want a relief valve sized to prevent that.

 

[EmmanuelTop]

The companies I worked for require all PRV devices to be included in the sizing report. I think this is very legitimate policy/request.

Technically speaking, there is absolutely no difference between the PRV on the metering pump skid and the PRV on the vendor supplied Propane refrigeration package. I'm very much sure we all agree that relief rate determination and sizing of the relief device would be requested for this package. There are no reasons to exclude the same requirements for the chemical injection package, just because the (estimated) consequences of a failure to provide adequate relief are deemed less than in the Propane case. If we start looking at the PRV's this way, we may end up not having calculation notes for half of PRV's in the plant/project.

I think you should request the calculation note. If they have it, they will send it in a minute or two. If they don't have it, then you did a very good thing by asking for it.

Dejan IVANOVIC
Process Engineer, MSChE

 

[e43u8]

MortenA,

You should ask vendor to provide calculation sheet of mentioned PRV sizing to handle the full recycle flow back to the suction drum in blocked discharge condition. This isn't comparable with TSV sizing for thermal expansion case in blocked-in liquid for which no sizing detail is normally being provided in most cases due to very small relief load...

 

[georgeverghese]

Where there is a risk of loss of containment due to overpressure on a Code designed equipment, a Code compliant safety relief device is required.

A Code relief device is not mandatory for mechanical failure which does not result in loss of containment.

Obviously, for a backpressure relief valve installed on the discharge of a CI pump(which is installed for mechanical asset integrity / operational flexibility reasons), one would expect to see the calcs on this anyway in the vendor handover documentation. This calc may need to be checked anyway to confirm that the overpressure required to achieve full recycle flow is within the discharge line normal operating pressure envelope, else a PRV / PCV external to the pump would be in order.

 

[georgeverghese]

Typical CI pumps are PD pumps, for which some makes have built in relief valves. These are not code constructed PSVs', and an external code designed PSV should be installed ( in addition to the external recycle PRV if the internal one is not suitable)to maintain pressures within the process design limit.