››› Thermal Relief Overview
Thermal relief looks small, but the pressure rise can be fast
A blocked-in liquid section may contain only a small volume, but liquid expansion under heat input can create pressure above the protected piping or equipment design limit. The relief valve may have a low required flow, yet incorrect set pressure, unavailable discharge path or poor seat performance can still create a serious field problem.
For procurement, thermal relief should be described as a pressure protection case, not only as a small threaded valve. The RFQ should state the trapped volume, liquid, temperature range, design pressure, set pressure, discharge destination, back pressure and documentation requirements.
Important Engineering Note
Final valve sizing and installation must be confirmed by the project relief calculation, protected equipment datasheet, piping design pressure, applicable code, owner specification and authorized engineering review. This page is written for RFQ preparation and technical communication.
››› Thermal Relief Applications
Where thermal relief valves are commonly required
Thermal relief is commonly reviewed wherever liquid can be trapped and later heated. Typical locations include utility water loops, glycol systems, thermal oil systems, LNG transfer lines, heat exchanger sections, pump discharge lines, loading arms and compact skid piping.
01
Blocked-In Liquid Lines
Liquid trapped between two isolation valves can expand during temperature rise. Review trapped volume, temperature range, line rating, set pressure and discharge route.
02
Heat Exchangers & Jackets
Shell, tube, jacket or coil sections may be isolated while exposed to hot process fluid, steam, thermal oil or ambient heat.
03
Water & Glycol Loops
Closed chilled water, hot water and glycol loops may need thermal relief when maintenance isolation can trap liquid.
04
LNG & Cryogenic Lines
Blocked-in LNG can build pressure as heat leaks into the line and may flash during relief. Cryogenic material and cold discharge must be reviewed.
05
Thermal Oil Skids
Hot oil heaters, return headers, pumps and compact thermal fluid packages require review of oil properties, fire risk and return path.
06
Pump, Filter & Skid Piping
Small packaged systems can trap liquid between checks, filters, valves and instruments. Compact layout does not remove thermal relief need.
››› Relief Case Analysis
Thermal relief selection starts with the trapped volume and heat source
The governing case may be ambient heating, heat tracing, hot process fluid, steam-side leakage, blocked return, pump heat input or emergency shutdown isolation. The valve should be reviewed from the credible pressure rise scenario rather than normal flow.
01
Ambient Temperature Rise
Outdoor or unconditioned piping can warm after shutdown. Solar exposure and seasonal heat can be enough to raise pressure in a trapped liquid section.
02
Heat Tracing or Insulation
Heat tracing can continue after isolation. Insulation may slow heat loss and keep liquid expanding while the discharge path is not available.
03
Hot Utility or Process Side
Heat exchangers, jackets and coils can transfer heat into an isolated liquid side from steam, thermal oil or hot process fluid.
04
Pump Deadhead / Blocked Outlet
Pump heat input and blocked discharge may combine with thermal expansion. Pump curve and shutoff pressure should be checked.
05
Cryogenic Heat Leak
LNG or other cryogenic liquid trapped in piping can warm, expand and flash during pressure relief. Cold discharge route is critical.
06
Maintenance Isolation
A valve can be safely sized but unavailable if the drain, return, vent or thermal relief path is isolated during maintenance.
››› RFQ Data Checklist
What to prepare before asking for a thermal relief valve quotation
A useful RFQ should let the supplier understand both the pressure boundary and the thermal expansion case. If the trapped volume or temperature rise is not known, send the piping sketch, equipment drawing or datasheet first so the missing data can be identified.
- Protected equipment, piping section, line size and pressure rating.
- Liquid type, density or composition, viscosity if relevant, and phase condition.
- Trapped volume or pipe length between isolation points.
- Minimum and maximum temperature, heat tracing or heat source condition.
- Design pressure, MAWP, set pressure and allowable accumulation basis.
- Discharge destination: safe drain, return tank, expansion vessel, flare, BOG header or closed collection.
- Back pressure, downstream isolation risk and whether discharge path remains open.
- Material, connection, seat tightness, certificate and testing requirements.
››› Service Data Matrix
Thermal relief valve data matrix
Use the following matrix to convert a general thermal relief request into RFQ-ready engineering data.
Water / Glycol Loop
Medium: water, chilled water or glycol mixture. Common cause: blocked-in liquid heating. Key checks: trapped volume, temperature rise, glycol concentration, drain or expansion tank route.
Thermal Oil System
Medium: heat transfer oil or hot hydrocarbon liquid. Common cause: pump deadhead, blocked outlet or hot oil expansion. Key checks: oil properties, return pressure, fire risk and leakage control.
LNG Transfer Line
Medium: LNG, cold vapor or flashing liquid. Common cause: heat leak into isolated cryogenic line. Key checks: cold material, flashing, icing, BOG header or tank return.
Heat Exchanger Section
Medium: liquid trapped in tube side, shell side, jacket or coil. Common cause: heat transfer from steam, thermal oil or hot process side. Key checks: low-side MAWP and tube rupture assumptions.
Pump / Filter Skid
Medium: process liquid, utility water or chemical solution. Common cause: isolation around pumps, strainers, filters or check valves. Key checks: compact piping and maintenance isolation.
Loading / Transfer Line
Medium: LPG, LNG, solvent, hydrocarbon or chemical liquid. Common cause: blocked transfer, ESD isolation or hose isolation. Key checks: safe discharge and operator exposure.
››› Selection Framework
How to specify a thermal relief valve correctly
Thermal relief valve selection should connect the protected pressure boundary, the temperature rise scenario, the required liquid capacity and the discharge philosophy. Small valve size does not mean small engineering risk.
A thermal relief valve may need tight shutoff, corrosion-resistant trim, cryogenic material, high-temperature compatibility, stable liquid opening behavior, or a discharge route that remains available during maintenance.
- Confirm protected pressure boundary and weakest design pressure.
- Define trapped volume and whether any section can be isolated.
- Use the credible maximum temperature or heat input case.
- Check liquid expansion, relieving capacity and phase behavior.
- Review valve configuration, material, seat and connection standard.
- Confirm that discharge routing and back pressure are acceptable.
››› Installation & Discharge
Thermal relief depends on an available discharge path
Thermal relief valves are often installed on small piping branches, but installation still requires review of inlet restriction, outlet blockage, drainability, maintenance isolation, freezing, back pressure, leakage visibility and safe discharge location.
For cryogenic or hot liquid service, discharge routing must protect personnel from cold plume, flashing liquid, scalding, fire risk, toxic release or unsafe return flow. A blocked or isolated discharge line can defeat the relief function.
Field installation checks
- Install close to the trapped liquid section with short, clear inlet path.
- Avoid isolation valves that can block the relief path unless locked/open and controlled by procedure.
- Route discharge to safe drain, collection, return or approved vent system.
- Check freezing, plugging, fouling, corrosion and leakage management.
- Confirm set pressure and material compatibility before commissioning.
››› Standards & Documents
Documents to confirm before ordering
Thermal relief requirements may be governed by project specifications, pressure equipment codes, process safety studies, piping design rules and owner relief philosophy. Before purchasing, confirm whether the project requires ISO, API, ASME, EN, GB or local pressure equipment documentation.
Technical Datasheet
Tag number, protected equipment, set pressure, connection, material, seat requirement and discharge philosophy.
Sizing / Capacity Basis
Trapped volume, temperature rise, liquid expansion, governing relief case and required relieving capacity.
Test & Certificate Package
Set pressure calibration, pressure test, seat tightness test, material certificate and project-required inspection record.
››› Minimum RFQ Data
Thermal relief RFQ checklist
Include these fields in the message or attach a datasheet. Missing information can be clarified later, but these points prevent unsafe assumptions.
Protected equipment
Line size / trapped volume
Liquid / phase
Design pressure / MAWP
Set pressure
Temperature range
Heat source
Required capacity
Back pressure
Discharge route
Material / seat
Documents
››› Selection Errors
Common thermal relief valve selection mistakes
Selecting only by pipe size
Pipe size confirms mechanical fit, but the valve must also match set pressure, liquid relief capacity, material, seat tightness, back pressure and discharge route.
Missing small trapped volumes
Small trapped volumes can still produce rapid pressure rise when heated. Every isolated liquid section should be checked, especially around check valves, heat exchangers, filters and maintenance bypasses.
Blocking the discharge path
A relief valve cannot protect the system if its discharge route is isolated, plugged, frozen, undersized or routed to an unsafe location.
Ignoring flashing or cryogenic behavior
LNG, LPG, hot water and some solvents can flash during relief. Discharge routing, material and capacity review should consider phase change.
Treating thermal relief as a generic accessory
Thermal relief is part of the pressure protection system. It should be documented, tagged, tested and maintained according to project requirements.
››› Related Engineering Resources
Continue your thermal relief valve selection review
These pages help move from thermal relief application requirements to detailed valve type selection, sizing, installation and RFQ preparation.
Safety Valve Selection Guide
Step-by-step guidance for selecting safety valves by medium, pressure, capacity, material and installation condition.
Safety Valve Sizing
Review required capacity, set pressure, back pressure and sizing inputs before RFQ.
Water Service Safety Valves
For water, chilled water, glycol loops, pump discharge and closed-loop thermal relief.
LNG Safety Valves
For blocked-in LNG, cryogenic transfer lines, flashing flow and cold discharge review.
Heat Exchanger Safety Valves
For tube rupture, thermal expansion, steam failure and exchanger pressure protection.
Ask an Engineer
Send datasheets, piping sketches, trapped volume data or working conditions for review.
››› FAQ
Thermal relief valve FAQ
When is a thermal relief valve needed?
A thermal relief valve should be reviewed when liquid can be trapped between closed valves or equipment boundaries and later heated by ambient temperature, heat tracing, hot process fluid, steam, thermal oil, solar exposure or warm-up cycles.
Is thermal relief sizing always small?
The required flow is often small compared with fire or pump relief, but the pressure rise can be rapid. Sizing should still be confirmed from trapped volume, liquid expansion, temperature rise, set pressure and discharge conditions.
Can I use a standard relief valve for thermal relief?
Not automatically. The valve should match the liquid service, set pressure, material, seat tightness, connection, back pressure and discharge route. Cryogenic, corrosive or high-temperature service may require special material or design review.
What data should I send for a thermal relief RFQ?
Send protected equipment, line size, trapped volume, liquid, temperature range, design pressure, set pressure, heat source, discharge route, back pressure, material requirement, connection and documents required.
Where should thermal relief discharge go?
Discharge may go to a safe drain, closed collection, return tank, expansion tank, BOG header, flare, vent or other approved safe destination depending on the fluid and project relief philosophy.
Why can blocked-in LNG require special review?
LNG can build pressure as heat leaks into an isolated line and may flash during relief. Cryogenic material, cold discharge, icing, seat tightness and safe venting should be reviewed together.
Prepare a complete thermal relief datasheet before quotation
Send the protected equipment datasheet, piping sketch, trapped volume, design pressure, set pressure, liquid, temperature range, heat source, discharge route, back pressure, material requirement and required documents. ZOBAI can review the information and confirm the next RFQ step.
››› Featured Series
Featured safety valve series
Explore selected ZOBAI safety valve series that may be reviewed for thermal relief, cryogenic service, high-temperature liquid relief, back pressure conditions and sanitary utility systems.
ZBA46 Series Pilot Operated Safety Valves
For higher-pressure service requiring tighter sealing and stable set pressure performance.
ZBWA42 Series Bellows Balanced Safety Valves
For back pressure applications where stable performance is important.
ZBA68 Series High Temperature Steam Safety Valves
For boilers, steam lines, hot water and elevated-temperature systems.
SVL488 Series Sanitary Safety Valves
For hygienic process lines, clean utilities and cleanable pressure protection.