We visit this topic again and again, the short answer is NO!
According to ASME Section VIII, Division 1 (UG-135) and API RP 520, Pressure Safety Valves (PSVs) must be installed in a vertical position with the spindle (stem) oriented upright. Installing a valve horizontally is considered a significant deviation from engineering standards and can lead to dangerous malfunctions.
Here is why vertical orientation is mandatory and the risks of a horizontal installation:
SAFTENG TIP: Check your specific Valve Data Sheet: If it doesn’t explicitly say “all-angle mounting,” assume vertical is the only safe and code-compliant option.
Most PSVs are “gravity-assisted” in their alignment. The internal trim (the disc and spindle) is designed to move up and down with minimal friction.
- Side-Loading: In a horizontal position, the weight of the disc and spindle presses against the side of the guide. This creates lateral friction that can cause the valve to stick or chatter.
- Set Pressure Drift: The friction caused by side-loading can change the actual pressure at which the valve opens, making it unpredictable and potentially higher than the nameplate set pressure.
After a PSV lifts to relieve pressure, it must snap shut (reseat) firmly.
- In a horizontal position, the disc may not align perfectly with the nozzle seat as it returns.
- Even a microscopic misalignment can lead to a simmering leak, which will eventually wire-draw the seating surfaces and destroy the valve’s ability to hold pressure.
If the valve is horizontal, the body can act as a “sump.” Condensate, scale, or debris can settle in the nozzle or the guiding area. Trapped liquid can cause corrosion or, in outdoor installations, freeze and physically prevent the valve from opening during an overpressure event.
Are there any exceptions?
There are very few cases where a horizontal installation is permissible:
- Manufacturer Certification: Some specific types of valves (usually small, spring-loaded thermal relief valves) are designed and certified by the manufacturer to be mounted in any orientation. However, this must be explicitly stated in the manufacturer’s documentation and the National Board certification.
- Pilot-Operated Valves: Certain pilot-operated valves (POSV) are less sensitive to orientation because the main valve is controlled by a pilot. However, even then, the pilot itself usually must be mounted vertically to ensure proper operation of its internal diaphragms and springs
If your piping layout makes a vertical installation difficult, you should use long-radius elbows or modify the piping to create a vertical “standpipe” for the PSV. Always ensure the inlet piping is as short and straight as possible to minimize pressure drop (<3% of set pressure).