Find the Real Maximum Pressure Of Vessels

Always consider static head when assessing pressure vessels.

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The first common misconception often encountered is that a vessel's rating allows operation at the design pressure at the pressure measurement point. This plant mythology is false. The ASME code doesn't specify the location of required pressure measurement points. The code specifies that the system pressure plus the coincident liquid head must be below the vessel design limit at all points on the vessel. For our example, this limit is the lower edge of the feed nozzle at the bottom of the vessel.

The feed nozzle centerline is at 3 ft. 3 in. above grade. The lower edge of the nozzle is 9 in. lower, at 2 ft. 6 in. The PRV currently is located in a piperack downstream of the vessel (Option 1). The PRV inlet is at 12 ft. 0 in. At an operating density of 48.8 lb/ ft3 this gives 5 psi of static head (rounded up). The maximum PRV relief setting is 161 psig (165 psig design limit minus 4 psi static head).

A proposal "to take full advantage of the vessel design pressure" would move the PRV to the Option 2 location. This idea stems from the thought that the vessel's top seam defines the design pressure. This idea is wrong. It's also very curious. Even if you believe that the top seam defines the design pressure, why move the PRV? Just reset it to account for the correct static head.

What must happen if the PRV moves to the Option 2 location? The elevation of the pipe is 67 ft. 4 in. and the PRV inlet is at 69 ft. 0 in. At an operating density of 48.8 lbs/ft3 this gives 23 psi of liquid head (rounded up again). The maximum PRV release setting is 142 (165 psig design limit minus 23 psi static head).

The key point is that pressure relief devices must protect all points of the vessel from exceeding design pressure. Moving PRVs doesn't change vessel design pressures. Always go back to the U-1 forms when you must verify design pressure. Other documentation, while convenient and helpful, remains secondary to the U-1 forms and related vessel code stamps.

ANDREW SLOLEY is a Chemical Processing contributing editor. You can e-mail him at ASloley@putman.net

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