I am always amazed to hear people complain about the degree of our auditing. Most clients hire us for detailed auditing, but some see the PSM/RMP 3-year audits as a “check-the-box” exercise and do not appreciate our auditing skills and experience (LOL). Comments/Questions like… “Would OSHA really cite for that?” are commonplace. Let me be clear; our job is to find more than OSHA/EPA would ever find – not to reduce your compliance risks, but to ensure a safe process!
One critical item that is unfortunately not managed well is our last line of defense… relief valves. We sample a lot of Pressure Relieving Devices (PRDs) within a process to ensure they are of the right design, size, and set point. Today, most businesses have the proper type of PRD, and more and more are paying attention to their sizing scenarios. But one aspect still eludes many… weep holes and/or weather caps.
To state the fact… UG-136 Minimum Requirements for Pressure Relief Valves actually discusses “weep holes” and/or “cover or discharge piping” as being REQUIRED.
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UG-136 Minimum Requirements for Pressure Relief Valves … (8) If the design of a pressure relief valve is such that liquid can collect on the discharge side of the disk, except as permitted in (9) below, the valve shall be equipped (9) Pressure relief valves that cannot be equipped with a drain as required in (8) above because of design or application may be used provided: (a) the pressure relief valves are used only on gas service where there is neither liquid discharged from the valve nor liquid formed by condensation on the discharge side of the valve; and (b) the pressure relief valves are provided with a cover or discharge piping per UG-135(f) to prevent liquid or other contaminant from entering the discharge side of the valve; and (c) the pressure relief valve is marked FOR GAS SERVICE ONLY in addition to the requirements of UG-129.
UG-135 Installation … (f) Discharge lines from pressure relief devices shall be designed to facilitate drainage or shall be fitted with drains to prevent liquid from lodging in the discharge side |
So it is clear that a “weep hole” is REQUIRED on the discharge side of a PRD on an ASME pressure vessel. Notice that the “weep hole” and its drain line (if one is utilized) MUST be sized properly to ensure there is NO backpressure on the PRD.
If a weep hole is NOT a proper design for some reason, the code allows for a different design as long as three (3) conditions are met.
So when we audit a PRD system, we first look for a “weep hole”, and if there is not one, we look for the reasoning in the “design basis” explaining why a weep hole is not part of the design. We then examine the design to ensure that the three (3) requirements in item (9) are necessary for a lacking weep hole.
Note: Some industries attempt to design out the need for weep holes and/or weather caps. These design attempts typically violate the most fundamental PRD design… discharge must be UPWARDS and UNOBSTRUCTED.