Pressure Relief for Pressure Vessels 1

[dhoxworth]

Application
Cylindrical vessel (reactor) that is vented (off-gas) and has UF6, N2, H2, and H20 each entering at pressures around 20-30 psig. The vessel has a design pressure of 29 psia.

Accident Scenario
The scenario is that the off-gas vent line plugs, pressurizing the vessel above its MAWP (which we believe from experience is an incredible accident scenario).

Questions
1. What defines whether something must be a pressure vessel or not?
2. If it is a pressure vessel, will a pressure relief device be required?
3. What are the ways to avoid having to use a pressure relief device?

 

[jte]

dhoxworth-

A lot of your answer depends on where your reactor is installed, whose jurisdiction it falls under, and what code the vessel was built to. With a design pressure of 29 psia it would seem that it was intentionally designed to roughly 14 psig which nominally puts the design pressure out of the scope of ASME VIII - provided that you define "MAWP" as "design pressure." If you have inlet fluids at nearly twice the MAWP of the vessel, however, and the outlet is capable of being plugged or has ANY valve on it, car-sealed open etc. or not, then it is possible that on a bad day the line will plug or some operator has a bad day and closes the valve. Now you have a container at twice its MAWP with no relief. Going to court and claiming that you didn't think that anyone would close the valve ain't gonna get the jury on your side. Especially if you have a nasty spill which results in a public impact or employee injuries or fatalities.

If it is within the scope of ASME VIII in a jurisdiction which requires it (being on federal property may be a solution but I'm not familiar with the applicable CFR's) then you must have a relief device. Unless you invoke Code Case 2211. But you need to invoke 2211 in a jurisdiction which allows it and before the vessel leaves the shop. So if you have an existing vessel, then 2211 doesn't give you a way out.


jt

 

[davefitz]

Per ASME sect VIII div 1, the tank is not a pressure vessel within the jurisdication of sect VIII if the design pressure is below 15 psig, per paragraph U-1(c)(2)h. Other parts of the Introduction to this code define other components not considered to be pressure vessels witin the Code jurisdiction. However, you could still get it stamped with the Code U stamp if you follow all other rules as if it were within the jurisdiction.

For the case described, it would be useful to determine the cost to provide some form of overpressure protection on the tank. A bursting disc on the tank or inlet line might be one option, but it is also possible that a tank built to a "design pressure" of only 14 psig may actually be sutiable for much higher pressures becaseu fabrication requirements often demand a minimum wall thickness for bending, welding , corrosion and structural attachments much in excess of simple pressure retention.

 

[bvi]

If you have an expectation that the vent line may plug, it is an ideal case for Code Case 2211, Overpressure Protection by System Design. Note that we have used this Code Case to remove relief valves from existing installations.

 

[jte]

bvi-

I'm curious as to how you used 2211 to remove relief valves from existing equipment. In particular, it would seem to be difficult to comply with part (e) of the Reply which states that "This Case number shall be shown on the Manufacturer's Data Report..."

jt

 

[bvi]

First, remember that Section VIII, Div 1 is for new construction. For post construction, one should look to codes such as NBIC and API 510.

With respect to the specific question, it is a matter of gaining acceptance from the jurisdiction (in those states with pressure vessel laws). And following CC 2211 facilitates the process, is shows a generally accepted good practice.