A thermal relief valve exists for one situation: liquid shut in between two closed valves with no gas space to take up the growth as it warms. Superlok® relief valves are used for this duty on instrument and small-bore lines, and the logic fits in a paragraph. According to the August 2025 Process Safety Beacon from the Center for Chemical Process Safety, an AIChE publication and not a manufacturer one, liquid blocked in without a gas space or a relief device can rupture equipment. If you want the code vocabulary first, read our note on relief valve vs safety valve.

What Happens Between Two Closed Valves

A liquid looks incompressible, and for practical purposes it is. It still grows in volume as it warms. A 2009 paper presented at the CCPS Latin American conference makes the point directly: liquids that seem incompressible increase in specific volume with temperature, trapped liquids can see large pressure increases in high temperature service, and the daily swing in ambient temperature can be enough.

Chip Eskridge, whom the article identifies as chair of the ASME B31.3 Process Piping Code committee, set out the conditions in Chemical Processing in 2006. The concern applies only when a system is full of liquid, isolated and heated, and it arises because liquid is relatively incompressible and pipe is relatively rigid. For low-heat-flux cases such as solar heating, he gives an example: 2-in. Schedule 40 pipe with trapped water may reach about 1,200 psig with only a 30 °F rise. He also notes that the pressure is hard to predict, because seat leakage, shading, piping layout and gas pockets all change the result.

Heat source changes the sizing case. For high-heat-flux systems such as steam-traced or jacketed lines, Eskridge says the relief should be sized on vaporization of the trapped fluid, not on expansion. A small relief device that handles a slow solar rise is a different job from one that must pass vapor.

What the Incident Record Shows

The U.S. Chemical Safety Board investigated the April 2, 2019 release at KMCO. After a reactor charge finished, the charge pump piping was isolated by a check valve on one side and a block valve on the other. The isobutylene inside, in uninsulated piping, was cooled by outdoor air near 40 °F and later warmed by air approaching 59 °F and by solar radiation. The CSB concluded the most likely pressure source was liquid thermal expansion. The pump inlet piping, including the cast iron y-strainer that failed, lacked a pressure-relief device to protect it from that expansion. One worker died and at least 28 others were hurt.

The report cites API 521: install a pressure-relief device if liquid thermal expansion is a credible hazard. The CCPS paper adds a case on the same pattern at smaller scale, where two manual isolation valves were closed on a heat transfer oil zone with no relief between them, and heat from the neighbouring zone ruptured a flexible joint.

Where a Thermal Relief Valve Belongs in the Line

Start with the weakest component. Eskridge notes that B31.3 does not require a relieving device if the system can withstand the highest pressure that can develop, and that the weakest component usually governs: flanges in many systems, and block valves or process instruments on nonflanged ones. His factors that favour a device are low-boiling or hazardous fluid, large volume, outdoor runs in sunlight, long runs over 100 feet, and operation as an isolated batch system.

On instrument work the trapped sections are short but real: a liquid-filled sample line between two isolation points, an impulse line closed at both ends, or the cavity between the valves in a double block and bleed arrangement while the bleed is shut. A transmitter or gauge in that section is often the lowest-rated part. The SRVL series lists a cracking pressure range of 10 to 250 psig and a 300 psig maximum operating pressure with adjustable set points, so the set pressure can be placed under the lowest rating in the section. Check that rating against the pressure ratings page, and make sure the discharge goes somewhere safe.

Limits Worth Stating

The SRVL and SRVH series are instrumentation relief valves. They are not ASME code-stamped devices, and the product pages quote no relieving capacity, so none is claimed here. Whether a thermal relief device in a code-governed design must be certified is a decision for the design authority. The distinction between a gradual-lift relief valve and a pop-action safety valve is covered in the earlier note linked above. Body material is listed as 316 stainless steel on specific variants, a corrosion-resistant alloy, with grade otherwise confirmed per order.

The simplest protection is procedural. The Beacon advises not blocking in hoses or pipes filled with liquid unless there is a relief device, particularly where sun or heat can reach them. It also notes that this is easy to miss during troubleshooting, which is when valves get closed in a hurry. Where an installation must be blocked in as a matter of routine, a root valve vs isolation valve layout should show where each section ends, so the relief device is fitted to the section that can actually trap liquid.

Questions

Technical note: The sources establish that liquid trapped between closed valves can rupture equipment, with one worked pressure example and one investigated release. They do not give a general formula, a relieving capacity for the SRVL series, or the API 521 text, which we cite only as the CSB quotes it. Whether a thermal relief device must be code-certified depends on the governing design code and is not settled here. Grade and rating confirmed per order.

Trademark disclaimer: Superlok® is a registered trademark. AIChE, CCPS, ASME, API, the CSB and Chemical Processing are named only to identify the sources and standards cited and belong to their owners. Cross-references are made by form, fit, and function only.