How to Select Ball Valve Pressure Class at Elevated Temperature | ASME B16.34 Derating, Design Conditions

Class 150 does not mean 150 psi, and Class 300 does not mean 300 psi. The class number identifies a pressure-rating group. The allowable working pressure changes with material and temperature.

ASME B16.34 covers pressure-temperature ratings, materials, dimensions, tolerances, examination, testing, and marking for covered flanged, threaded, welding-end, wafer, and flangeless valves.^[1]

A correct selection needs three separate checks:

• Shell rating: Can the body, closure, and other pressure-containing parts hold the internal pressure at the design metal temperature?
• Seat rating: Can the seats seal against the maximum pressure difference across the closed valve?
• Complete-valve rating: Can the packing, body seals, stem, bolting, end connections, and actuator work under the same conditions?

A valve may pass the ASME B16.34 shell check but still be unsuitable because its seat, seal, or actuator limit is lower.

Know the Units

Use one pressure unit and one temperature unit throughout the calculation. Do not mix gauge pressure with absolute pressure.

NIST lists 1 psi as 6,894.757 Pa, which gives approximately 14.504 psi per bar.^[2]

Common pressure labels include:

• bar(g) or psig: Pressure above local atmospheric pressure
• bar(a) or psia: Pressure measured from absolute vacuum
• Differential pressure: Pressure difference between the two sides of the closed valve

For example, 10 bar(a) is not the same as 10 bar(g). Near normal atmospheric pressure, 10 bar(a) is approximately 9 bar(g).

Set the Design Conditions

Do not select Class 150, 300, or 600 from normal operating pressure alone. List every pressure and metal-temperature combination the valve may experience.

ASME B31.3 contains requirements for process piping used in refineries, chemical plants, hydrogen facilities, pharmaceutical plants, semiconductor plants, cryogenic plants, and related processing facilities.^[3]

For steam applications, also check steam state, tracing temperature, cleaning temperature, seat material, packing, and bolting. See this guide to sizing a ball valve for steam service.

Record Time and Frequency

Temperature alone does not describe the full service. Record how long the valve stays at each temperature and how often the condition occurs.

The following is an example operating profile, not a universal industry requirement:

A seal that survives six four-hour steam-out cycles may not be suitable for continuous operation at 400°C. Thermal cycling can also change gasket compression, seat contact, packing friction, and actuator torque.

Keep Conditions Together

Pressure and temperature must be checked as pairs that occur at the same time.

For example, a line may have:

• 20 bar(g) at 150°C during startup
• 12 bar(g) at 350°C during normal operation
• 6 bar(g) at 420°C during steam cleaning

Check these three pairs separately. Do not combine 20 bar(g) with 420°C unless that condition can actually occur.

The highest pressure may occur at a temperature where the material still has a high allowable pressure. The highest temperature may occur when the system pressure is low. Neither value automatically controls the class.

A useful comparison is:

Shell utilization = Design pressure ÷ Allowable pressure at the same temperature

The result must not exceed 1.00. The condition with the highest utilization is the controlling shell-rating condition. This ratio is a comparison tool, not an ASME formula.

There is no universal rule that every valve must stay below 80% or 90% utilization. Any required extra margin must come from the project specification, owner, or governing design rules.

Use Metal Temperature

ASME pressure-temperature ratings apply to the pressure-containing metal, not automatically to the bulk fluid temperature.

The valve body may be heated or cooled by:

• Steam tracing
• Electric tracing
• Insulation
• Sunlight
• Radiant heat from nearby equipment
• Stagnant fluid
• Thermal stratification
• Heat transfer from a vessel or furnace
• Rapid depressurization

A closed bypass valve near a hot vessel may become hotter than the flowing main line. A traced valve can remain hot after process flow stops.

For example, the process fluid may normally be 280°C while an external tracing system can heat an isolated valve body to 320°C. The pressure-temperature check should use the credible valve-body temperature for that condition.

Find the Design Pressure

Normal operating pressure is only one input. The valve may see higher pressure during pump shutoff, compressor shutdown, relief, static liquid head, or blocked-in heating.

Calculate Static Head

A valve below the liquid surface sees additional pressure from the liquid column.

For water at approximately 1,000 kg/m³:

• 1 m of water adds about 0.098 bar
• 10 m adds about 0.98 bar
• 30 m adds about 2.94 bar

Example:

Selecting the valve from the 6.0 bar vessel-top pressure would miss almost 3 bar at the low-point valve.

For a liquid with a specific gravity of 1.20, a 20 m column adds approximately 2.35 bar:

0.098 bar/m × 20 m × 1.20 = 2.35 bar

Check Pump Shutoff

A centrifugal pump can produce a higher pressure when the discharge is closed than during normal flow.

Example:

The normal pump discharge may be only 7.0 bar(g), but the isolation valve could see about 9.38 bar(g) when the pump runs against a closed outlet. Ignoring shutoff head would understate the pressure by about 2.38 bar.

Use the approved pump curve and actual fluid density for the final calculation.

Check Trapped Liquid

Liquid may be trapped:

• Inside the valve body cavity
• Between two closed valves
• Between a valve and a blind
• Between a valve and a check valve
• Inside a closed, heat-traced branch

When trapped liquid heats up, pressure can rise quickly. The result depends on liquid expansion, compressibility, trapped gas, pipe flexibility, leakage, and vapor formation.

Do not use a fixed rule such as “pressure rises by a certain number of bar per degree.” The result changes greatly with the fluid and trapped system.

Provide a verified self-relieving seat or a separate thermal-relief path when liquid can be trapped and heated.

Hydrostatic test pressure must not be used as the working pressure. A pressure test is applied for a limited time under controlled conditions and does not prove continuous operation at the same pressure and service temperature.

Identify the Material

The correct B16.34 table cannot be selected until the exact pressure-boundary material is known.

Descriptions such as “carbon steel” or “stainless steel” are not enough. Typical specifications include:

• ASTM A216 WCB cast carbon steel
• ASTM A352 LCB low-temperature cast carbon steel
• ASTM A217 WC6 or WC9 cast alloy steel
• ASTM A351 CF8 or CF8M cast stainless steel
• ASTM A105 forged carbon steel
• ASTM A350 LF2 forged low-temperature carbon steel
• ASTM A182 F304 or F316 forged stainless steel

Different materials in the same pressure class can have different allowable pressures at the same temperature.

Cast and forged grades must also be kept separate. ASTM A216 WCB is a casting grade, while ASTM A105 is a forging grade. They should not be treated as the same material because both are carbon steel.

Check the body, closure, cover, end connectors, bolting, and every other component identified by the manufacturer as part of the pressure boundary. If different pressure parts use different materials, ask the manufacturer to confirm the complete pressure-temperature rating.

For more detail on carbon steel, stainless steel, duplex steel, and other body options, see this ball valve material selection guide.

Read the Table Notes

Finding the material name is not enough. Read every note linked to the material group and pressure-temperature table.

Check for:

• Maximum and minimum temperatures
• Required heat treatment
• Product-form limits
• Chemical-composition limits
• Impact-test requirements
• Limits added by the governing piping code

If a material is not listed, do not place it into a similar group based on chemistry or a supplier’s “equivalent material” statement. Use the unlisted-material rules of the governing piping code and obtain a documented engineering review.

Read ASME B16.34

Use the edition required by the project. ASME currently lists B16.34-2025, but an existing plant, contract, or local rule may require an earlier edition.^[4]

Follow this order:

1. Identify the exact body and closure material grades.
2. Confirm whether each pressure part is cast, forged, plate, or fabricated.
3. Find the material in the adopted B16.34 edition.
4. Identify the correct material group.
5. Confirm the rating arrangement allowed for the valve design and end type.
6. Select the proposed pressure class.
7. Find the design metal temperature.
8. Read the allowable pressure.
9. Read all table and material notes.
10. Compare the allowable pressure with the design pressure.
11. Repeat the check for every design condition.

Do not apply a higher Special Class or intermediate rating to a normal Standard Class valve unless the adopted B16.34 edition permits it for that valve design, size, end type, material, examination level, testing, and marking.

Handle Missing Temperatures

A blank cell or dash in a pressure-temperature table provides no usable rating. It does not mean that a low pressure is acceptable.

Do not:

• Continue the last listed pressure
• Copy a value from another material
• Extend the pressure curve beyond its final temperature
• Round the design temperature down

When the design temperature falls between two listed temperatures, use the interpolation method allowed by the adopted standard.

Example calculation:

• Allowable pressure at 300°C: 10.2 bar
• Allowable pressure at 325°C: 9.3 bar
• Design temperature: 315°C

The temperature fraction is:

(315 − 300) ÷ (325 − 300) = 0.60

If linear interpolation is allowed:

10.2 + (9.3 − 10.2) × 0.60 = 9.66 bar

These values only demonstrate the method. Use the licensed table adopted by the project for the final pressure rating.

Compare the Classes

The following values are an illustrative selection example. They are not a replacement for official ASME B16.34 pressure-temperature data.

Class 150 fails startup and steam-out, even though it passes normal operation.

Class 300 passes the illustrative shell check. Startup controls the shell selection because it has the highest utilization of 32.0%.

The valve is still not fully approved. The seat and body seals may not withstand 400°C, and the actuator still needs to be checked at the maximum pressure difference.

Review Small Margins

A result can pass mathematically while leaving little room for pressure or temperature uncertainty.

This comparison may pass, but a 0.2 bar margin can be consumed by a small change in metal temperature, static head, pressure measurement, or operating condition. Check the project’s required margins instead of adding or removing an arbitrary percentage.

Check the Seat

The ASME shell rating does not prove that the valve seat can seal at the same pressure and temperature.

Soft-seat materials may include PTFE, reinforced PTFE, modified PTFE, PCTFE, PEEK, and other engineered polymers. Do not select a seat from the raw material’s maximum temperature alone.

Seat performance also depends on:

• Valve size
• Seat shape and thickness
• Differential pressure
• Contact stress
• Thermal expansion
• Creep and cold flow
• Fluid chemistry
• Operating cycles
• Ball surface condition

Use a manufacturer pressure-temperature curve for the exact valve model, size, class, seat code, and flow direction.

A qualified forged soft-seated floating ball valve can provide tight shutoff and lower torque when the temperature, pressure, and fluid remain within the approved seat range.

Metal seats are normally considered when heat, abrasive particles, erosion, deposits, or frequent cycling can damage a polymer seat. For service near or above 400°C, review the ball coating, seat coating, spring material, thermal expansion, hot leakage, and hot operating torque.

See the high-temperature metal-seated ball valve guide for more detail.

Specify Seat Leakage

Pressure class and seat leakage are separate requirements.

ISO 5208 specifies examinations and tests used to establish the integrity of a metallic valve pressure boundary, closure tightness, and structural adequacy of the closure mechanism.^[5]

State:

• Test standard
• Leakage rate
• Test medium
• Test pressure
• Test direction
• Test temperature
• Holding time

A room-temperature seat test does not prove the same leakage rate at the maximum operating temperature.

Check Packing and Seals

Ask for separate continuous and short-term temperature limits. A seal that survives a four-hour cleaning cycle may not be suitable for continuous service at that temperature.

Graphite packing and gaskets are widely used at elevated temperature, but their performance still depends on oxidation, compression, thermal cycling, and stem surface condition.

ISO 15848-1 provides procedures for evaluating external leakage from valve stem seals and body joints in volatile or hazardous-fluid service.^[6]

Check Differential Pressure

Differential pressure is the pressure difference across the closed valve:

Differential pressure = Upstream pressure − Downstream pressure

The body may contain about 25 bar in both the first and third cases, but the seat load and actuator torque are very different because differential pressure changes from 23 bar to 2 bar.

Do not size the actuator for the full class rating unless that full pressure difference can occur or the project specification requires it.

In a floating ball valve, line pressure pushes the ball toward the downstream seat. As differential pressure rises, seat load and operating torque normally rise.

In a trunnion-mounted valve, the ball is supported by bearings and the seats move toward the ball. This arrangement is often used for larger sizes and higher pressures, but the supplier must still confirm reverse pressure, seat action, leakage, cavity relief, and torque.

See this trunnion-mounted ball valve selection guide.

Check the Body Cavity

A closed ball valve can trap fluid between its two seats. Heating can raise the cavity pressure above the pressure on either pipeline side.

Common trunnion-seat arrangements include:

• Single-piston-effect seat: The seat normally seals against line pressure but can move to release excessive cavity pressure toward the pipeline.
• Double-piston-effect seat: Pressure from either side can push the seat toward the ball, improving isolation but often preventing automatic cavity relief through that seat.

The exact action depends on the valve design. The manufacturer must state:

• Upstream and downstream seat type
• Cavity-relief direction
• Pressure needed to open the relief path
• Performance during reverse flow
• Need for an external relief device

For toxic, flammable, reactive, or solidifying fluids, define where relieved fluid may safely discharge.

Check Flanges and Gaskets

The connected assembly cannot exceed the rating of its lowest-rated part.

ASME B16.5 covers pressure-temperature ratings, materials, dimensions, tolerances, marking, and testing for covered flanges and flanged fittings from NPS ½ through NPS 24.^[7]

ASME B16.47 covers large steel flanges from NPS 26 through NPS 60 within its stated classes and material limits.^[8]

Confirm:

• Valve and mating-flange class
• Flange standard and series
• Flange material
• Facing type
• Bolting material
• Gasket type
• Temperature limit

A Class 300 valve connected to a Class 150 mating flange does not create a Class 300 joint.

ASME B16.20 covers materials, dimensions, tolerances, and marking for covered ring-joint, spiral-wound, and metal-jacketed gaskets.^[9]

For butt-weld valves, check end material, wall thickness, bore transition, welding heat, and any required postweld heat treatment. Welding or heat treatment can damage seats, packing, O-rings, coatings, and bearings.

Check the Actuator

Use torque data for the exact valve model, size, class, seat, packing, temperature, fluid, and maximum differential pressure.

Check:

• Break-to-open torque
• Running torque
• End-to-close torque
• Seating torque
• Reverse-pressure torque
• Actuator output at each important stroke position

The following actuator figures are an example only:

The 1.40 ratio is not a universal requirement. Use the margin required by the project, valve manufacturer, and actuator manufacturer.

For pneumatic actuators, use the minimum guaranteed air pressure. For example, an actuator may produce adequate torque at 6 bar air supply but fail the required torque check when the guaranteed minimum supply is only 4.5 bar.

The actuator, solenoid, switch box, and positioner may have much lower temperature limits than the valve body. An extended stem, mounting bracket, thermal spacer, or heat shield may be needed.

This valve torque curve guide explains how break torque, running torque, and actuator output should be compared.

Check Product Standards

API 608 applies to covered metal ball valves with flanged, threaded, and welding ends. API lists the seventh edition as published in April 2025.^[10]

API 6D defines requirements for valves used in pipeline and piping systems. API has published the twenty-fifth edition.^[11]

Use API 6D only when its scope and the project requirements apply. An API 608 process valve and an API 6D pipeline valve are not automatically interchangeable.

ISO 10497 specifies fire type-testing requirements for soft- and metal-seated isolation valves. Passing a fire test does not increase the valve’s normal pressure-temperature rating.^[12]

Prepare the Datasheet

Common Selection Errors

• Using the class number as a psi value
• Checking only normal operating pressure
• Ignoring pump shutoff pressure or static liquid head
• Combining pressure and temperature values that cannot occur together
• Using fluid temperature without checking metal temperature
• Using a general carbon-steel or stainless-steel description
• Ignoring table notes and material limits
• Using the shell rating as the complete-valve rating
• Selecting a seat from a raw polymer temperature limit
• Ignoring reverse pressure and maximum differential pressure
• Assuming every ball valve automatically relieves cavity pressure
• Using hydrostatic test pressure as working pressure
• Using an old pressure table without checking the project edition
• Extending a pressure-temperature curve beyond its published range

Conclusion

Select the ball valve class from the worst real pressure-temperature pair, not from normal pressure or the class number. A 30 m water column alone adds about 2.94 bar, and a pump with 3 bar suction pressure plus 65 m shutoff head can reach about 9.38 bar. In the sample class check, Class 150 failed at 14 bar and 200°C, while Class 300 passed the shell calculation. Final approval still requires the exact material group, model-specific seat curve, maximum differential pressure, cavity-relief direction, flange and gasket limits, and actuator torque at minimum supply pressure.