TSV and PSV selection starts at the pressure source. Trace the protected fluid from the blocked boundary or pressure-generating event, through the valve inlet, across the valve, and into the disposal system. A thermal relief valve handles one narrow overpressure case: expansion of trapped liquid as its temperature rises. A pressure safety valve protects against the governing overpressure cases assigned to it and may operate in liquid, vapor, gas, or mixed service. The service case and relieving fluid state—not the acronym—control set pressure, valve construction, and sizing.
Where does the pressure originate and where does it stop?
Layer one first. Mark the physical boundaries of the protected system before selecting a valve. A liquid-filled line becomes blocked in when closed valves, check valves, blinds, or isolated equipment prevent expansion from moving into a connected volume. Heating then expands the liquid; because the trapped volume is nearly fixed, a small temperature rise can produce a large pressure increase.
| Reading or observation | Outcome | Next check |
|---|---|---|
| Valve lineup and check-valve direction | Liquid can become trapped between boundaries | Identify every credible heat source |
| Connected vapor space or expansion volume | Expansion may be absorbed before pressure reaches the limit | Calculate available expansion volume and maximum temperature |
| Pressure at the equipment versus the relief-valve inlet | A difference indicates restriction, elevation effect, or a blocked path | Inspect and calculate the inlet path |
| Pressure at the valve outlet | Backpressure can change capacity and valve operation | Evaluate the discharge system at relieving flow |
Follow the pressure path in both directions. The inlet must connect continuously to every component being protected, and the outlet must remain available under the same event. A valve cannot protect equipment isolated from its inlet or discharge through a closed or obstructed route.
Is thermal expansion the governing case?
A TSV is an application label for a relief valve assigned to trapped-liquid thermal expansion. It is not a universal construction class. A PSV is a broader pressure-protection designation and is not limited to gas service. Terminology varies among plants and governing codes, so define the service case and fluid phase on the datasheet instead of selecting from the abbreviation alone.
| Condition | Thermal-expansion case | Broader PSV case |
|---|---|---|
| Protected inventory | Normally blocked-in liquid | Liquid, vapor, gas, or a combination |
| Pressure source | Liquid heating and volumetric expansion | Any identified credible overpressure source |
| Typical required flow | The expansion rate that must leave to hold pressure below the permitted limit | The load generated by the controlling scenario |
| Key inputs | Heat input, liquid properties, trapped volume, temperature range, and available expansion volume | Scenario-specific mass or volumetric generation rate, fluid state, inlet condition, and outlet pressure |
| Decision | Use liquid relief sizing if the fluid remains liquid at the valve inlet | Use the sizing method matching the actual relieving phase |
If the protected system can receive pressure from a pump, compressor, high-pressure connection, control-valve failure, or another source, evaluate that case separately. Do not classify a case as thermal merely because temperature changes during the event. Select the largest required area among the credible cases after applying the correct phase-specific method to each.
What reading determines the set pressure?
Read the allowable pressure of every protected component at the applicable temperature. The lowest governing limit along the connected pressure path becomes the constraint. Normal operating pressure does not determine set pressure by itself; it only establishes the operating margin needed to avoid leakage, chatter, or nuisance lifting.
| Setting input | What it controls | Decision |
|---|---|---|
| Allowable pressure at relieving temperature | Maximum permissible set-pressure basis for each protected item | Use the lowest applicable equipment limit, subject to the governing code |
| Normal and maximum operating pressure | Margin below the valve set point | Resolve operating excursions before finalizing the set pressure |
| Permitted accumulation | Pressure allowed during discharge | Read the value from the governing code and equipment design basis |
| Expected outlet pressure | Backpressure imposed on the valve | Select a valve design compatible with the calculated backpressure |
| Static liquid head | Difference between equipment pressure and valve-inlet pressure | Include elevation in the pressure-path calculation |
For a thermal case, setting the valve above the normal hot operating pressure does not prove that the protected equipment remains within its permitted pressure. Compare the selected set pressure and allowable accumulation with the lowest-rated component at the coincident temperature.
Which fluid state reaches the valve inlet?
Measure or calculate pressure, temperature, and composition at the relief-valve inlet for the controlling case. These conditions decide whether the sizing calculation is for liquid, vapor or gas, or two-phase flow. A line that contains liquid during normal operation can flash as pressure falls through the valve; that behavior must be represented by the applicable method rather than forced into a single-phase liquid calculation.
For blocked-in thermal expansion, establish whether heating keeps the inventory subcooled, produces vapor, or causes phase separation. Then inspect the physical inlet: line size, length, fittings, elevation, isolation-valve position, and deposits. Excessive inlet loss can make the valve cycle because pressure falls at the valve as soon as flow begins, then rebuilds after it closes. Outlet restriction can reduce usable differential pressure and alter opening or closing behavior.
Material compatibility also belongs in this branch. Check the wetted materials, seals, temperature range, corrosion tendency, viscosity, and likelihood of plugging. A small thermal-relief path is particularly vulnerable to solidification, debris, and inadvertently closed isolation valves.
Does API 520 use one sizing procedure for TSV and PSV service?
API 520 provides relief-valve sizing methods, but TSV and PSV labels do not select one common equation. First calculate the required relieving load from the overpressure scenario. Then apply the method matching the fluid condition at the valve inlet and through the valve.
| Sizing stage | Thermal liquid relief | Other relief cases |
|---|---|---|
| Required load | Derive from liquid expansion caused by the maximum credible heating condition | Derive from the specific pressure-generating event |
| Fluid method | Liquid method when the stream remains liquid | Liquid, vapor or gas, or two-phase method as applicable |
| Pressure basis | Valve-inlet relieving pressure and outlet backpressure | The same locations, evaluated for the controlling event |
| Selected area | Area meeting the calculated thermal load | Largest required area from all assigned cases |
Do not size a TSV solely by choosing a small catalog orifice. Document the heat source, expansion calculation, relieving properties, pressure losses, backpressure, selected area, and discharge destination. If another credible case demands more area, that case governs even when the valve is informally called a TSV.
How should the resolving branch be implemented and tested?
- Draw the protected pressure boundary and mark every valve, check valve, blind, restriction, elevation change, and outlet destination.
- List credible pressure sources. Separate trapped-liquid heating from pump, compressor, high-pressure connection, and other process cases.
- Read the allowable pressure and temperature basis for each connected component. Select the governing set-pressure basis under the applicable code.
- Calculate the required relieving load for each case. For thermal expansion, use the maximum credible heating condition and the liquid properties over the relieving temperature range.
- Determine the phase at the relief-valve inlet and apply the corresponding API 520 sizing method. Evaluate flashing or two-phase behavior where applicable.
- Calculate inlet loss and outlet backpressure at relieving flow. Select construction and materials compatible with those conditions and the process fluid.
- Install the valve with an unobstructed inlet and a discharge route suitable for the released fluid. Lock or administratively control required isolation-valve positions.
- Verify the certified set pressure through the approved test method, confirm the installed flow path against the drawing, and record the final valve and system configuration.
FAQ
Can I use a PSV for trapped-liquid thermal expansion?
Yes. A pressure relief valve may protect a blocked-in liquid volume when its construction, set pressure, liquid capacity, materials, inlet losses, and backpressure suit the calculated thermal-expansion case.
Does a TSV always use a different API 520 sizing equation?
No. The relieving phase selects the method. Use the liquid method when the thermal-relief stream remains liquid; evaluate flashing or two-phase behavior when pressure reduction changes its state.
Does checking the valve set pressure complete verification?
No. Confirm the set pressure, trace the open inlet and outlet paths, verify required isolation-valve positions, compare the installed valve with the sizing record, and document the final configuration as the last verification step.