5 replies to this topic

Hi

is there any recommendation for sizing a breather valve (vacuum+pressure relief) that can handle liquid relief from tube rupture where the liquid relief is the sizing case?

I am working on sizing selection of a vacuum relief valve.
there are two scenario:
1- blocked outlet with 2000 SCFH air
2- Tube rupture with 300 gpm of water.

the set pressure is 4 psig (API-2000 should be used). for the blocked outlet case, 2" relief valve will be ablle to handle it but I could not find any sources for sizing selection of a breather valve that can handle liquid relief.

[djack77494]

omaass,
I find your question very confusing. A vacuum relief valve protects equipment against developing a vacuum, or negative pressure. It offers no protection against overpressure (positive pressure). Vacuum relief valves typically open to the atmosphere to admit AIR into the equipment being protected from vacuum. There should not be any liquids passing through the vacuum relief valve, so it makes no sense to talk about a "liquid relief case". From what I could gather in your posting, you want a Pressure Relief Valve, and not a Vacuum Relief Valve. Your setpoint is very low, so your situation does not sound typical. You should seek help from a supplier of relief valves.
Doug

[proinwv]

I agree with djack77494.

If you overpressure, overfill the tank, the vac valve will BLOCK flow as it is designed to operate in the opposite direction.

The pressure relief will pass liquid but it is not usually sized for that. Also, there are emergency pressure reliefs that are more appropriate for that but most vent to atmostphere and that might not be acceptable for you. Is it?

Further, if you vent liquid thru a pipe-away pressure vent there will be a, possibly very significant, pressure drop from the piping that will be seen as additional backpressure in the tank, above the vent set pressure. Further, the pressure developed in the venting lines might rupture the vent line if it were designed for gas venting,

BE CAREFUL!!!!

Thank you Paul & Doug

Sorry for the confusion, the valve is a Breather valve that has vacuum and pressure relief.

[pleckner]

Let's not be concerned about the vacuum part of the issue since a liquid filled relief has nothing to do with vacuum protection.

To size the outbreathing rate, we convert the maximum inflow of a liquid into the equivalent gas flow rate. Based on your input of 300 gpm, this translates simply into about 2400 cfm (300/7.481*60). By the way, the calculation is actually cfm but the conversion to SCFH is usually assumed to be almost 1:1. thats why API 2000 puts everything in terms of SCFH. If there were significant temperature differences between the vapor space and the standard conditions (60F), you might consider adjusting the flow; but I degress.

This is the amount of outbreathing your pressure vent must be able to pass. Note that when dealing with low pressure tanks, we also consider simultaneous thermal outbreathing, which will increase this value somewhat.

Since your breather is sized for 2000 scfh, you are protected. But now for the liquid fill scenario. It is customary not to worry about this since the breather should be more than large enough to handle the liquid flow; afterall, you based your vent rate on this liquid flow to begin with. These valves, mechanically, are typically just pallets kept closed by either a spring or weight(s). The breather doesn't really care about the fluid, just the pressure. The only thing to really be concerned about is the static head you are putting on the vessel since water will provide one and vapor does not. AND, we cannot discount the problems brought up by "proinwv" below.