Pressure rises when heat expands a blocked-in fluid faster than the system can absorb or discharge the added volume. With liquid service, a small temperature increase can push pressure past the piping or exchanger limit because the liquid is nearly incompressible. With fire exposure, sustained external heat can generate vapor or expand fluid at a rate that requires a much larger relieving capacity. This is heat, not logic: valve selection starts with heat input, fluid response, allowable pressure, and exposure time.
Wrong fixes and their failure modes
| Attempted fix | Why it fails | Required correction |
|---|---|---|
| Use the normal process PSV for every thermal case | The existing valve may have the wrong set pressure, liquid capacity, inlet arrangement, or discharge destination for a blocked-in segment. | Evaluate the trapped volume as a separate overpressure scenario, then confirm the installed PSV can cover it. |
| Assume the fire case is covered because a PSV already protects the vessel | A normal process contingency and external fire can have different heat inputs, relieving phases, allowable accumulation, and required capacities. | Calculate fire relief independently and compare its requirements with the installed valve certification and datasheet. |
| Install a standard small thermal valve without calculating capacity |
1/2 in × 1 in and 3/4 in × 1 in are commonly cited connection sizes, not proof of adequate capacity. |
Calculate thermal expansion flow and select an orifice and valve certified for the required fluid service. |
| Apply one volume threshold to all blocked-in lines | A screening value such as 500 L does not account for fluid expansivity, heating rate, pipe compliance, allowable pressure, or governing rules. |
Use the project relief philosophy and governing code, supported by a calculation for each credible trapped-liquid case. |
| Ignore a line because it operates below ambient temperature | Ambient air, sunlight, steam tracing, or electric tracing can warm the isolated liquid after the valves close. | Use the maximum credible metal and fluid temperature after isolation, not only the normal operating temperature. |
Blocked-liquid pressure physics
A thermal relief valve protects a liquid-filled volume trapped between closed valves or other isolation points. Common locations include transfer lines, oil pipelines, exchanger tube sides, and piping exposed to sunlight or heat tracing. When the liquid warms, it tries to occupy more volume. The closed piping resists that expansion, converting the thermal input into pressure.
For an initial screening calculation, the unconstrained expansion flow is:
Qthermal ≈ V × β × dT/dt
where Qthermal is volumetric relief demand, V is trapped liquid volume, β is the liquid volumetric expansion coefficient at the applicable conditions, and dT/dt is the credible heating rate. Refine the calculation for pipe thermal expansion, system compliance, fluid entering or leaving the boundary, and property variation with temperature.
A pressure-rise estimate for a fully constrained liquid can be expressed as:
ΔP ≈ β × ΔT / (κfluid + Csystem)
Here, κfluid is fluid compressibility and Csystem represents the pressure compliance of the pipe, exchanger, gaskets, and connected volume. Both denominator terms have inverse-pressure units, so the result is pressure. Read fluid properties from the approved process-property source and derive system compliance from the mechanical design rather than treating the pipe as perfectly rigid.
A field heuristic sometimes quoted is a pressure increase of up to 100 psi for a 1°C temperature rise. That number is not a universal conversion: the actual rise changes with the liquid, vapor or gas pockets, pipe flexibility, initial pressure, and temperature. Use the equations and property data for the installed system.
Thermal-relief scenario boundaries
The number that matters is the liquid volume that remains hydraulically trapped after the actual valve lineup, check-valve action, and equipment isolation. A piping drawing alone may hide check valves, spectacle blinds, control valves that fail closed, or operating procedures that create a temporary blocked segment.
- Mark every credible isolation point and define each trapped boundary.
- Determine whether the boundary remains liquid-full over the complete temperature range. A vapor pocket changes the pressure response but does not automatically remove the overpressure scenario.
- Record the initial pressure and fluid temperature at the moment of isolation.
- Identify ambient heating, solar exposure, steam tracing, electric tracing, exchanger heat transfer, and any hotter connected equipment.
- Read the maximum allowable pressure from the piping or equipment documentation and identify the weakest protected component.
- Calculate thermal expansion rate and resulting relief demand using approved fluid properties.
- Check the governing project and jurisdictional requirements before deciding whether a relief device is required.
One cited screening practice calls for thermal relief when blocked-in liquid volume exceeds 500 L and the environment is warmer than the operating fluid—for example, liquid at 25°C exposed to 35°C ambient. That is not a general exemption for smaller volumes. A small, rigid segment containing a high-expansion liquid can still exceed its pressure limit, while a larger system may have expansion capacity or another open relief path. Heat tracing is a direct thermal source and requires evaluation regardless of ordinary weather conditions.
External-fire relief physics
Fire relief addresses external heat from a fire incident acting on a vessel, exchanger, tank, or other pressure-containing equipment. The heat can warm liquid, boil or flash liquid into vapor, expand an existing vapor space, or produce a mixed-phase relieving condition. Unlike the small displacement commonly associated with trapped-liquid expansion, fire exposure can impose sustained vapor-generation demand.
Calculate the fire case from the governing fire-relief method using the equipment geometry, credible exposed or wetted area, insulation or fireproofing credit permitted by the design basis, fluid properties, relieving pressure, and discharge backpressure. Obtain each input from the vessel drawing, process simulation, insulation specification, relief study, and valve datasheet. A spring-loaded or pilot-operated PSV may be applied only after checking its certification, pressure limits, backpressure behavior, and suitability for the calculated relieving phase.
Project documentation may distinguish a normal or liquid-service case evaluated at 10% overpressure from a fire case evaluated at 21% overpressure. Confirm those values against the governing code edition and local requirements before using them. API RP 520 Part 1 and API 521 are documents to check for sizing and fire-scenario methodology; ASME Section VIII applies when the protected equipment and jurisdiction place the vessel within its scope.
Thermal-versus-fire decision basis
| Quantity or limit | Thermal case | Fire case | Where to read or derive it |
|---|---|---|---|
| Protected inventory | Normally blocked-in liquid | Liquid, vapor, or mixed inventory | Piping drawings, vessel drawings, valve lineup, operating procedure |
| Heat source | Ambient warming, sunlight, tracing, or heat transfer | External fire exposure | Process design basis, tracing specification, fire-scenario study |
| Relief demand | Primarily thermal liquid expansion flow | Heating, expansion, vapor generation, or phase change | Approved calculation method and fluid-property source |
| Cited overpressure basis | 10% appears in liquid-service practice | 21% appears in fire-service practice | Governing code edition, jurisdiction, project relief philosophy |
| Cited screening threshold |
500 L appears in one design practice |
Not applicable | Owner specification; replace screening with a calculation where required |
| Typical valve behavior | Small, intermittent discharge or a brief “burp” may relieve expansion | Sustained flow may be required during heat exposure | Scenario calculation and certified valve capacity data |
| Connection examples |
1/2 in × 1 in or 3/4 in × 1 in are cited examples |
No fixed connection size | Selected valve datasheet and capacity calculation |
A fire PSV may be additional to a valve selected for ordinary vessel overpressure, but separate hardware is not automatically required. One valve may cover multiple scenarios when its set pressure, certified capacity, phase rating, inlet and outlet hydraulics, materials, and discharge system satisfy every controlling case. Document the controlling case rather than adding relief capacities from mutually exclusive events.
Valve selection and discharge routing
- Set the valve pressure basis from the protected component limit and applicable rules. Include static head and operating-pressure variation where they affect margin.
- Select liquid, vapor, or mixed-phase service from the calculated relieving condition. Valve capacity data must match the service being evaluated.
- Compare required flow with certified capacity at the calculated relieving pressure and backpressure. Nominal connection size alone is not a capacity check.
- Calculate inlet and outlet hydraulic losses using the installed pipe sizes, lengths, fittings, elevation, and destination pressure. Excessive loss can produce instability, reduced capacity, or repeated cycling.
- Route thermal liquid to an approved lower-pressure system or storage destination where practical. Returning oil to a storage tank is one recognized arrangement, provided the receiver pressure and composition remain compatible.
- Check whether closed downstream valves, check valves, or operating lineups can isolate the relief outlet. The discharge path must remain available whenever the protected volume can be blocked in.
- Specify materials and trim for the fluid, temperature range, corrosion mechanism, and required tightness. Record whether the selected device carries the certification required by the governing equipment code.
A thermal valve that repeatedly opens is reporting recurring heat input or an operating lineup that traps liquid. Confirm that the discharge is genuine thermal expansion rather than valve leakage, unstable inlet pressure, flashing across the seat, or downstream backpressure. For fire service, examine the complete relief network because simultaneous credible loads and header pressure can control valve performance.
Calculation and installation verification
Verification ties the scenario, calculation, valve, and installed piping into one pressure boundary. Review the final installation against the approved relief-device datasheet rather than accepting the tag number as proof.
- Walk down the isolation boundary and compare valve positions, check-valve directions, blinds, bypasses, and tracing with the drawings.
- Recalculate trapped volume from installed pipe and equipment geometry. Include exchanger channels and branches that remain connected.
- Verify the initial and maximum credible temperatures, heating rate, expansion coefficient, compressibility, and allowable pressure in their stated units.
- Confirm that the selected set pressure protects the lowest-rated component and that the assumed overpressure basis matches the governing rules.
- Match required capacity to the certified valve capacity for the actual fluid phase, relieving conditions, and backpressure.
- Confirm the inlet and discharge paths remain open in every operating state that creates the trapped volume.
- Record the protected boundary, scenario, calculation revision, valve identification, discharge destination, and inspection or test requirement in the relief-device register.
Changes to tracing, insulation, fluid composition, operating temperature, isolation procedures, or discharge headers require another scenario review. A passing bench test confirms valve operation at test conditions; it does not validate the process sizing or installed piping hydraulics.
Frequently asked questions
Can I use one PSV for thermal relief and fire relief?
Yes, when the valve’s set pressure, certified liquid or vapor capacity, backpressure limits, materials, and installed piping satisfy each scenario. Calculate thermal expansion and external fire independently, then select the controlling case.
Does every blocked-in liquid line need a thermal relief valve?
No single volume rule answers that question. Evaluate the trapped volume, fluid expansion coefficient, credible heating rate, system compliance, allowable pressure, and any open expansion path; treat 500 L only as a project-specific screening value where the governing specification adopts it.
Can I select a 1/2 in × 1 in thermal relief valve without a calculation?
No; that connection is an example, not a verified capacity. Stop when the fluid phase, heat input, allowable pressure, code basis, or discharge destination cannot be resolved from controlled documents. Escalate the scenario to the valve manufacturer’s official technical support and the responsible pressure-relief engineer before approving the installation.