Thermal Relief Valves: Protecting Blocked-In Liquid Lines from Expansion

Liquids expand when heated, and they are almost incompressible. Trap a liquid in a section of pipe between two closed valves - a common situation during maintenance, unit shutdown, or product changeover - and then let it warm from sunlight, ambient heat, a fire, or a warm adjacent stream, and the trapped liquid tries to expand. With no vapour space to absorb the expansion, the pressure climbs extremely fast. A temperature rise of just a few degrees can generate hundreds of bar of pressure, enough to rupture a pipe, blow a flange gasket, or split heat-exchanger tubes. A thermal relief valve (TRV), also called a thermal expansion relief valve, is a small pressure-relief valve installed on such blocked-in sections to bleed off a tiny volume of liquid and collapse the over-pressure.

Why the Pressure Rise Is So Severe

The pressure generated by blocked-in liquid expansion depends on the liquid's coefficient of thermal expansion and its bulk modulus (its resistance to compression). Because the bulk modulus of most process liquids is very high, the pressure rise per degree of heating is dramatic - and it does not depend on how large the trapped volume is, only on the temperature change. This is why even a short, small-bore blocked-in section is dangerous, and why the fix - relieving a very small volume of liquid - is enough to bring the pressure back down.

Where Thermal Relief Valves Are Required

• Pipeline sections that can be isolated between two block valves (pigging sections, transfer lines).
• The cold (tube or shell) side of heat exchangers that can be blocked in while the hot side stays in service.
• Liquid-full vessels and long runs of pipe exposed to solar heating or fire.
• LPG, LNG, and cryogenic liquid lines, where trapped liquid warming to ambient expands violently.
• Cooling-water and glycol loops that can be valved off while a heat source remains.
• Any line downstream of a positive-displacement pump that can be dead-headed.

Sizing a Thermal Relief Valve

Unlike a process relief valve sized for a large vapour-generation or blocked-outlet case, a thermal relief valve for pure liquid expansion needs only to pass the small volumetric expansion rate of the trapped liquid as it heats. API 521 gives guidance on the thermal-expansion relief case. The required relieving rate is a function of the heat-input rate, the liquid's cubical expansion coefficient, its density, and its specific heat. Because that rate is small, thermal relief valves are typically small - often 3/4 inch by 1 inch - and use a modest orifice. The key sizing inputs are:

Setpoint and Discharge

The set pressure of a thermal relief valve is chosen below the maximum allowable working pressure (MAWP) of the weakest component in the blocked-in section - usually the pipe, a flange rating, or the exchanger tubes. Where the process operating pressure is high, the TRV is set a suitable margin above operating pressure but below MAWP. The discharge should route to a closed system (flare or relief header) for hazardous, flammable, or toxic liquids, and may discharge to a safe location only for clean, non-hazardous fluids like water. For LPG and hydrocarbon service, the TRV normally relieves into the flare or a lower-pressure part of the system, never to atmosphere.

Selection and Specification

1. 1Identify every section that can be blocked in with liquid and exposed to heat.
2. 2Determine the governing heat input case (solar, fire per API 521, or exchanger).
3. 3Calculate the thermal-expansion relieving rate and select a valve orifice to suit.
4. 4Set the pressure below the weakest component MAWP with appropriate margin.
5. 5Choose body and trim material for the liquid and temperature (LTCS or stainless for cold service).
6. 6Route the discharge to flare / header for hazardous fluids; document the case per API 521.
7. 7Specify the code stamp (ASME Section VIII UV, or applicable code) and EN 10204 3.1 certificates.

Vajra Industrial Solutions supplies thermal relief valves and small-orifice liquid-expansion relief valves sized to API 521 for pipeline, heat-exchanger, LPG, and cryogenic blocked-in service, with correct set pressures, code stamping, material selection for the fluid, and full certification.