In the world of process safety and piping design, Hydrostatic Relief Valves (HRVs)—often simply called thermal relief valves—are the unsung heroes that prevent piping ruptures during periods of inactivity.
While a standard Safety Relief Valve (SRV) protects against massive surges or process upsets, the HRV is specifically designed to handle the thermal expansion of trapped liquids.
Why They are Necessary
Liquids are essentially incompressible. When a liquid is trapped between two (2) closed valves (a “blocked-in” scenario) and is subjected to an external heat source, the pressure rises exponentially.
Common heat sources include:
- Solar Radiation: Sunlight hitting uninsulated outdoor piping.
- Heat Tracing: Steam or electric tracing is used to keep viscous fluids moving.
- Ambient Temperature Swings: Simple day-to-night temperature changes.
- Heat Exchangers: If the cold side is blocked in while the hot side is still flowing.
The Math of Danger: For many hydrocarbons, pressure can increase by 75 to 100 psi per 1°F rise in temperature. In a rigid piping system, it doesn’t take long to exceed the flange ratings or the pipe’s burst pressure.
Design and Operation
HRVs are typically small (usually 1/2″ x 1″ or 3/4″ x 1″ NPT). Because they only need to bleed off a tiny volume of liquid to cause a massive drop in pressure, they do not require the high flow capacities of primary process relief valves.
- Set Pressure: Usually set at or slightly below the Maximum Allowable Working Pressure (MAWP) of the piping system or the weakest component (like a flange or gasket).
- Trim: Often constructed with stainless steel internals to prevent “simmering” or corrosion that could cause the valve to stick shut.
- Discharge: These valves should discharge to a safe location—typically back into a storage tank, a closed flare header, or a dedicated collection system—rather than to the atmosphere, especially if the fluid is hazardous or flammable.
Standard Requirements
In your work with ASME B31.3 and B31.1, these valves are critical for compliance.
| Code Section | Context |
| ASME B31.3 (301.2) | Requires that the design of a piping system must account for the effects of thermal expansion of trapped fluids. |
| ASME Section VIII | While primarily for vessels, the principles for sizing thermal relief (API 520/521) are the industry standard for these valves. |
| NFPA 30 | Specifically mandates that piping systems for flammable liquids be provided with a means to relieve internal pressure caused by thermal expansion. |
Common Placement “Must-Haves”
- Loading Arms/Hoses: Any section of hose or pipe used for chemical transfer that can be valved off at both ends.
- Long Underground Lines: Despite being buried, temperature equalization can still cause expansion.
- Heat Exchanger Inlets/Outlets: Specifically on the cold side, where a bypass might be closed.
- Pumps: Between the suction/discharge valves and the pump casing.
Maintenance and Inspection (MI)
Because HRVs are small and often located in “dead legs” of the process, they are prone to fouling or plugging.
- Bench Testing: Like any PSV, it must be pulled and tested periodically to ensure it pops at the correct set pressure.
- Visual Inspection: Checking for “weeping” or salt/chemical buildup at the discharge point, which indicates the valve is not seating properly.
- Isolation: If a block valve is installed upstream of the HRV for maintenance, it must be car-sealed in the OPEN position.