Yes, we still need to consider drainage at the low point of our discharge piping, even with a “rain cap”. While a rain cap is designed to deflect direct vertical precipitation from entering the discharge lines/piping, it is not a hermetic seal. Over time, moisture can still enter the discharge piping through condensation, wind-driven rain, or melting frost/snow.
Looks look at ASME Section VIII for what is required at a minimum:
NOTE: I also discuss API 520, Part II in this post.
UG-135 INSTALLATION
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(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 of the pressure relief device, and such lines shall lead to a safe place of discharge. The size of the discharge lines shall be such that any pressure that may exist or develop will not reduce the relieving capacity of the pressure relief devices below that required to properly protect the vessel, or adversely affect the proper operation of the pressure relief devices. [See UG-136(a)(8) and Nonmandatory Appendix M.]
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(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 with a drain at the lowest point where liquid can collect (for installation, see UG-135).
(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.
Why Drainage Matters
The primary purpose of a drain hole (often called a “weep hole”) at the low point of a Pressure Safety Valve (PSV) discharge is to prevent the accumulation of liquid in the piping. Any liquid trapped in the discharge pipe creates a column of fluid. This exerts “backpressure” against the PSV disk. For many valve designs, this additional pressure directly interferes with the valve’s opening setpoint, potentially preventing it from opening at the required pressure. Standing liquid can also accelerate corrosion in the discharge piping or freeze in cold climates, which could completely plug the vent line and render the PSV inoperable.
If you are designing or evaluating your installation per API RP 520 Part II, keep these points in mind:
- The weep hole is to be located at the absolute lowest point of the discharge elbow or the lowest vertical run of the pipe where liquid could pool.
- The hole size should be kept small, typically 3–6 mm or 0.125–0.25 inch.
- Orientate the weep hole drain away from relief valve , structural steel, and operating area
- Depending on the location, a screen or mesh may be needed to prevent insects or debris from entering the vent piping.
- Ensure the location does not violate HAZLOC classification requirements if the discharge contains flammable gases/vapors
Before finalizing your design, it is best practice to review your specific site safety standards and the original manufacturer’s installation manual for your specific valve type to ensure compliance with your facility’s mechanical integrity program.