What does UG-135(b)(1) mean?

[jegs]

We need to install an ASME pressure relief valve on the inlet line to a chromatography column to protect the column from the feed pump. The installation needs to be sanitary and rupture disks are not preferred. The calculations show a 1/2" valve meets relief requirements. The line is 1/2" tubing. We have been unable to source a sanitary/clean PSV with a 1/2" tri-clamp connection. Can we use a larger PSV and comply with UG-135(b)(1)? One with a 1/2" orifice but a 1" connection? UG-135(b)(1) says "The opening though all pipe, fittings, and nonreclosing pressure relief devices (if installed) between a pressure vessel and its pressure relief valve shall have at least the area of the pressure relief valve inlet. I am being told increasing to a larger valve violates the code. This makes no sense to me as we would just be increasing relieving capacity because we can not source the valve required. Does the second sentence in UG-135(b)(1) (which we satisfyin my case) trump the first? Is the code intent really to ensure an installation without flow restriction to the relief device based on what valve size you need versus requiring users to match the line size exactly? Is a slightly bigger valve size than what is required really not allowed? See link to valve we planned to use:

http://www.leser.com/valve-4834.7708.aspx

 

[Duwe6]

Oversize valves are allowed. ASME just wants to prevent "BOOM's". However more is sometimes worse for the Operators. When a PRV is greatly oversized, it usuallu will not reclose and fully reseat. It will 'simmer' for the rest of that run. This is not unusual in properly sized PRV's, as they are positive OPENING items, and may or may not fully close after a relieving event. SIDENOTE: That was the root mechanical problem that led to the 3-Mile Island fiasco.

Since you have to go big, make sure that Operator training includes a section about not using PRV's for a upper-limit control point. It probably will 'simmer' thru the rest of your batch. Then it will have to have the seat wiped -- required to be performed at a facility that holds a "V" stamp. The cleaning requires breaking the seal wires to access the disk & seat.