A valve that passes a room-temperature water test can still leak, seize or require excessive torque after repeated heating and cooling. Before purchase, obtain the valve pressure-temperature chart, seat-leakage limit, hot torque data, cavity-relief arrangement and a list of all nonmetallic parts.
Operating Data
Start with the exact heat-transfer-fluid name. Mineral oil, synthetic aromatic fluid, silicone fluid and food-grade thermal oil can have different viscosity, thermal stability and fire properties.
Record four temperatures:
- Normal operating temperature: the temperature during most operating hours.
- Maximum continuous temperature: the highest temperature expected for long periods.
- Maximum upset temperature: the highest short-term temperature during stopped flow or another fault.
- Minimum startup temperature: the lowest temperature at which the pump and valve may operate.
| Condition | Example value | Main effect on the valve |
|---|---|---|
| Normal temperature | 280°C | Seat wear, packing life and normal torque |
| Maximum continuous temperature | 320°C | Seat creep, gasket relaxation and material strength |
| Upset temperature | 330°C for 30 minutes | Permanent seal damage or thermal seizure |
| Minimum startup temperature | 10°C | High oil viscosity, pressure loss and actuator torque |
| Packing-chamber temperature | Measured or calculated | Stem leakage and packing life |
| Actuator mounting temperature | Measured at the bracket | Actuator seals, switches, solenoids and cables |
In this example, the valve may move from 10°C during startup to a 320°C design condition. That is a 310°C temperature change. The ball, seats, body, stem and packing do not all expand by the same amount, so a valve that turns freely when cold may become tight when hot.
Cold oil can also be much more viscous than hot oil. Higher viscosity increases pipe resistance, pump discharge pressure and the force needed to move oil through the valve cavity. Use the viscosity curve for the actual fluid at 10°C, not only its value at 280°C.
Do not use heater film temperature as the valve temperature. Film temperature is the temperature of the thin oil layer next to a heated surface. A valve normally sees the local bulk temperature, although a valve close to the heater may continue heating after circulation stops.
The packing chamber may be cooler than the oil because the valve neck releases heat to the air. Covering the whole neck with insulation can raise the packing and actuator temperature. For heavily insulated valves, ask for the expected temperature at the gland and actuator bracket.
Pressure at Temperature
A pressure class is not a fixed pressure limit at every temperature. As the body material becomes hotter, its permitted pressure normally falls.
ASME B16.34 covers pressure-temperature ratings, materials, dimensions, examination, testing and marking for many flanged, threaded and welding-end valves.[1]
Do not specify only:
DN100, Class 300, carbon-steel ball valve.
State the full condition:
Design pressure: 18 bar
Design temperature: 320°C
Maximum shutoff differential pressure: 12 bar
Minimum startup temperature: 10°C
The valve body pressure-temperature rating must cover 18 bar at 320°C. Saying that a body is simply “rated for 320°C” is incomplete because pressure and temperature must be checked together.
The seat must also hold 12 bar differential pressure at 320°C. The metal body may remain structurally suitable after a polymer seat has already softened, crept or lost sealing force.
| Valve condition | Upstream pressure | Downstream pressure | Seat differential pressure |
|---|---|---|---|
| Valve open during circulation | 15 bar | 12 bar | 3 bar |
| Valve closed, downstream still pressurized | 15 bar | 5 bar | 10 bar |
| Valve closed, downstream depressurized | 15 bar | 0 bar | 15 bar |
This example shows why normal running differential pressure cannot be used for seat selection. The seat may see only 3 bar while the valve is open but the full 15 bar after the downstream line is drained.
Check all credible pressure conditions:
- Normal pump discharge pressure
- Maximum pump pressure
- Pump shutoff pressure
- Cold-start pressure
- Static head
- Expansion-tank or nitrogen pressure
- Maximum closed-valve differential pressure
- Pressure trapped inside the body cavity
A room-temperature shell or seat test confirms the valve under the stated test conditions. It does not prove long-term hot performance. ISO 5208 defines pressure testing used to check the pressure boundary and closure tightness of metallic valves.[2]
Valve Design
A floating ball valve uses line pressure to move the ball slightly toward the downstream seat. The design is compact and common in smaller sizes, but seat load and operating torque increase with differential pressure.
A forged soft-seated floating ball valve can suit clean thermal oil when temperature, pressure and cycling remain inside the verified seat rating.
A trunnion-mounted valve supports the ball at the top and bottom. Spring-loaded seats move toward the ball. This design is often used for larger sizes, higher differential pressure and automated isolation because torque is usually easier to control.
The forged trunnion-mounted ball valve provides an example of a supported ball with spring-loaded seat assemblies.
| Condition | Floating ball | Trunnion ball |
|---|---|---|
| Typical application | Small and medium isolation valves | Medium and large automated valves |
| Ball support | Supported mainly by the seats | Supported by upper and lower trunnions |
| Pressure effect | Pressure pushes the ball onto the downstream seat | Pressure and springs move the seats toward the ball |
| Torque at high differential pressure | Can rise sharply | Usually more predictable |
| Cavity arrangements | Usually simpler | SPE, DPE and hybrid arrangements are common |
One-piece valves have fewer body joints but may be difficult to repair. Two-piece valves provide access to the ball and seats. Three-piece valves can allow the center section to be removed while the pipe ends remain installed.
Every body joint needs a gasket and enough bolt load. Repeated heating and cooling can relax the gasket or bolts, causing a slow external leak. The quotation should identify the gasket material, bolting grade and tightening method.
Bore Size
A full-bore valve has an opening close to the pipe’s inside diameter. It usually creates less pressure loss and is useful where cold oil is thick, flow is high or the line must drain completely.
A reduced-bore valve is lighter and may cost less, but the smaller passage increases local velocity and pressure loss.
The change in flow area can be larger than it first appears. If the internal diameter is reduced by 25%, the remaining flow area is about 56% of the original area. The restriction therefore removes about 44% of the flow area, even though the diameter changed by only 25%.
- Main circulation lines often benefit from full bore.
- Short drain and vent lines may accept reduced bore.
- Piggable lines require a verified full-bore passage.
- Cold-start calculations should use low-temperature oil viscosity.
- The supplier should provide the valve Cv or Kv value.
Full bore is not automatically better. A larger ball and stem may increase valve weight and actuator torque. Compare pressure loss, actuator size and purchase cost before deciding.
Seat Selection
The seat often sets the real temperature limit of a ball valve. Material names alone are not enough. The supplier must provide the complete-valve pressure-temperature chart, hot differential-pressure limit, leakage rate and cycle qualification.
The soft-seated versus metal-seated valve comparison provides an initial screen, but the actual operating data must control the final choice.
| Initial service screen | Seat to check first | Main question |
|---|---|---|
| Clean oil below about 200°C | Verified PTFE-based soft seat | Can it hold the full hot differential pressure? |
| About 200–260°C | Modified PTFE, PEEK or supported composite | Does complete-valve test data cover the condition? |
| About 300°C or higher | Metal seat checked first | What leakage and hot torque are permitted? |
| Oil contains carbon or hard particles | Wear-resistant metal seat | Can particles damage the coating or block the cavity? |
| Frequent operation | Qualified polymer, composite or metal seat | How many hot mechanical cycles were tested? |
These temperature ranges are screening values, not universal material limits. The finished valve rating always takes priority.
PTFE-Based Seats
Reinforced PTFE can provide low friction, tight shutoff and good chemical resistance in clean oil. Fillers can improve wear resistance and reduce deformation.
Performance depends on:
- Filler type and percentage
- Seat thickness and geometry
- Seat support
- Valve size
- Differential pressure
- Ball surface finish
- Thermal and mechanical cycle count
A soft seat does not need to melt before it fails. Under sustained heat and pressure, it can slowly change shape. This creep may reduce contact with the ball or increase friction until the valve leaks or becomes difficult to operate.
PEEK Seats
PEEK has higher mechanical strength than many PTFE-based compounds, but it is still a polymer. Its allowable pressure falls as temperature rises.
Ask whether the stated rating applies to:
- Continuous or short-term exposure
- Full line pressure or reduced differential pressure
- Static isolation or repeated operation
- The actual valve size
- Cold or hot leakage testing
At 320°C continuous service, screen a metal-seated valve first. Only accept a polymer or composite seat when complete-valve qualification covers 320°C and the full shutoff differential pressure.
Metal Seats
Metal seats are normally checked when:
- The temperature is outside the verified polymer-seat range.
- The oil contains carbon, sludge or hard particles.
- The valve operates frequently.
- High differential pressure causes soft-seat deformation.
- The project requires severe-service construction.
A forged metal-seated ball valve uses hard-faced and matched metal sealing surfaces for high-temperature, abrasive and frequent-cycle service. The product range lists service temperatures up to 350°C, subject to confirmation of the exact size, class, material and pressure-temperature rating.
For a metal seat, check:
- Ball and seat base materials
- Coating or hardfacing process
- Coating thickness and hardness
- Maximum pressure-temperature range
- Ball roundness and surface finish
- Lapping method
- Permitted leakage rate
- Hot operating torque
A harder coating is not automatically better. A brittle coating may crack. A large difference in expansion between the coating and base material may also cause separation during repeated heating and cooling.
Metal seats normally have higher torque and more leakage than soft seats. Do not request “zero leakage” without stating the test medium, pressure, direction, holding time and permitted rate.
Do not confuse a normal metal seat with the secondary metal contact in a fire-safe soft-seated valve. A metal-seated valve uses metal contact during normal operation. A fire-safe soft-seated valve normally seals on the polymer seat and uses secondary metal contact only after fire damages that seat.
Thermal Growth
The ball, seats and body expand by different amounts. Too much seat preload can make the hot valve seize. Too little preload can allow hot leakage.
A valve may turn normally at 20°C but become tight at 280°C. The supplier should explain how seat springs, flexible seat lips, clearances and material combinations control the change.
A useful qualification test records cold torque before heating, hot torque at the operating temperature and cold torque again after cooling. A large permanent increase after cooling may show seat damage, coating damage or packing overload.
Stem Sealing
The stem is a common external leak path because hot oil has low viscosity and can pass through small clearances.
A suitable stem assembly should include:
- High-temperature packing
- A correctly machined packing chamber
- A smooth, undamaged stem surface
- A rigid gland follower
- Evenly adjustable gland bolts
- A blowout-resistant stem
- Correct stem and actuator alignment
Flexible graphite is commonly used for high-temperature packing, but graphite quality alone does not guarantee a tight stem. Ring arrangement, packing density, stem finish and gland load also matter.
The packing must be selected by the expected packing-chamber temperature. The oil may be at 300°C while the gland is cooler because the valve neck releases heat. Heavy insulation can raise the gland temperature and shorten packing life.
Live Loading
Belleville springs can maintain packing load as the packing settles during thermal cycling. This is called live loading.
Too little spring load allows leakage. Too much load increases stem friction and actuator torque. The supplier should state:
- Spring arrangement
- Installed spring height
- Target gland load
- Permitted adjustment range
- Replacement procedure
Overtightening may stop a small leak for a short time, but it can score the stem and prevent full actuator travel. The ball valve leakage troubleshooting guide explains how to separate stem, body-joint and seat leakage.
Inspection Example
The inspection frequency should match the plant risk and manufacturer instructions. A practical startup plan may include:
- Record the initial gland position and cold torque before startup.
- Inspect for leakage during the first 3–5 thermal cycles.
- Check the hot valve again after 24–72 operating hours.
- Inspect visible stem and body joints monthly during early service.
- Compare torque and gland position during a planned 6- or 12-month shutdown.
These intervals are examples, not fixed industry requirements. A valve close to the heater or operating every day may need more frequent checks than a normally open valve on a return line.
Where low external emissions are required, ISO 15848-1 covers type testing of stem seals and body joints.[3] ISO 15848-2 covers production acceptance testing.[4]
A type-test certificate does not mean every supplied valve completed the full thermal-cycle test. State separately whether production acceptance testing is required. CARILO’s ISO 15848 testing guide explains the difference between type qualification and production records.
Cavity Pressure
When a ball valve closes, liquid can become trapped between the two seats. Heating the trapped oil can create a rapid pressure rise because liquid cannot easily compress.
Consider a valve that closes when the trapped oil is at 30°C. If the system later heats the valve to 280°C, the trapped oil experiences a 250°C temperature rise inside a nearly fixed cavity. The actual pressure depends on the oil, cavity volume, seat movement and relief design, so one universal pressure value cannot be used.
This problem can occur when:
- A cold closed valve is heated with the system.
- The line is heat traced.
- One side contains hotter oil.
- The valve is close to the heater.
- Flow stops while external heating continues.
- Both seats seal pressure inside the cavity.
SPE Seats
A self-relieving single-piston-effect seat is designed so that sufficient cavity overpressure can move the seat away from the ball and release pressure toward a specified side of the pipeline.
The relief pressure depends on seat area, spring force, friction and pressure direction. A drawing should show which side receives the relieved pressure.
DPE Seats
A double-piston-effect seat can seal when pressure comes from the line or the cavity. This improves isolation but may trap cavity pressure, so an independent relief device is normally required.
A hybrid arrangement may use one SPE seat and one DPE seat. This provides a cavity-relief path while keeping a second pressure-assisted barrier.
Other relief methods include:
- A pressure-balancing hole in the ball
- An external cavity-relief valve
- A vent connected to a closed return system
A drilled ball makes the valve directional. Installing it backward may remove the intended relief path. Field drilling can also change the shutoff function and invalidate existing test qualification.
Relief holes and small tubing can become blocked by carbon or sludge. Inspection must confirm that the path remains open. Relief discharge should go to a suitable safe location rather than beside the hot valve.[5]
The body cavity may remain pressurized after both pipe sections have been depressurized. Drain and vent the cavity through an approved connection before loosening any pressure-containing part.
Fire Safety
Thermal oil is combustible. Operating above the oil’s flash point does not automatically cause a fire while the fluid remains inside a closed system. Risk rises when oil leaks, forms a spray or mist, or reaches an ignition source.
Fire-Safe Construction
A fire-safe valve may include:
- Secondary metal seat contact
- Flexible-graphite stem packing
- Flexible-graphite body seals
- A blowout-resistant stem
- Metal control of external leakage paths
An anti-static device and cavity-pressure relief are important safety features, but they do not prove that the valve has passed a fire test.
Fire-Tested Valve
ISO 10497 specifies fire type-testing requirements for soft- and metal-seated isolation valves. It checks seat leakage, external leakage, cavity-pressure relief for relevant designs and operation under defined fire-test conditions. Powered actuators are generally outside its scope.[6]
API 607 is the API fire-test standard for quarter-turn valves and valves fitted with nonmetallic seats.[7]
Check the complete fire-test report for:
- Valve model and body design
- Tested size and pressure class
- Seat construction
- Stem packing and body gasket
- Material group
- Test standard and edition
- Seat and external leakage results
- Post-fire operation
- Permitted qualification range
A test on one valve does not automatically cover every size, class or body design. A two-piece body should not automatically qualify a three-piece body with different joints. A soft-seat test should not automatically qualify a normal metal-seated design.
Fire testing permits limited leakage under severe conditions. It does not promise zero leakage or prove that a valve exposed to a real fire can return to service without inspection.
Shutdown Assembly
A fire-tested valve does not prove that the actuator can move during a fire. The complete shutdown package may also require:
- A defined fail-open or fail-closed position
- A spring-return actuator or another stored-energy source
- Protected air tubing
- Protected solenoids
- Fire-resistant cables
- A reliable instrument-air supply
- Emergency shutdown logic
- A verified closing time
Strategically located remotely operated isolation valves can reduce the amount of fluid released after loss of containment.[8]
Closing speed must also be checked. A faster valve is not always safer if rapid closure creates excessive pressure surge in the thermal-oil loop.
Actuator Sizing
The actuator must produce enough torque in the worst operating condition, not only during normal hot circulation.
Check:
- Break-to-open torque
- Running torque
- End-to-close or reseating torque
- Maximum shutoff differential pressure
- Pressure direction
- Cold-start viscosity
- Maximum operating temperature
- Packing load
- Seat material
- Contamination and long idle periods
| Condition | Possible torque increase |
|---|---|
| 10°C cold startup | Thick oil and higher hydraulic resistance |
| 12 bar shutoff differential pressure | Greater seat load |
| 320°C design temperature | Ball and seat thermal expansion |
| Several months without movement | Static friction and deposits |
| Live-loaded packing | Higher stem friction |
| Metal seat | Higher preload and sliding friction |
ISO 5115 defines design and information responsibilities for part-turn valve, actuator and mounting-kit assemblies.[9]
For a pneumatic actuator, use the minimum air pressure available at the valve, not the normal compressor-header pressure. Check both the air stroke and the spring stroke.
For an electric actuator, check starting torque, duty cycle, motor temperature, torque switches and stall protection.
Ask whether the valve torque already includes a 25%, 30% or another service allowance before adding a new safety factor. Adding several margins without coordination can produce an oversized actuator that damages the stem or seats when the valve is blocked.
CARILO’s valve torque curve guide explains break torque, running torque, reseating torque and actuator margin.
Materials and Hidden Parts
Carbon steel is common for thermal-oil valve bodies. Stainless steel may be selected for external corrosion, cleanliness or fluid compatibility, but it is not automatically better in every position.
| Part | What to check |
|---|---|
| Body | Pressure-temperature rating, material quality and external corrosion |
| Ball | Roundness, surface finish, hardness and coating adhesion |
| Stem | Hot strength, surface condition and maximum allowable torque |
| Bolting | Hot strength, stress relaxation and tightening method |
| Gaskets | Temperature limit, compression retention and oxidation resistance |
| Bearings | Temperature limit, friction and fluid compatibility |
A metal-seated valve may still contain temperature-sensitive parts. Request a list covering:
- Stem and body O-rings
- Thrust washers
- Bearings
- Seat back seals
- Sealants and lubricants
- Actuator seals
- Position-indicator plastics
- Solenoid and cable insulation
The list should show each material, its location and its verified temperature limit. A metal seat does not make a valve suitable for 320°C if an internal bearing or O-ring is rated for a lower temperature.
Installation
Pipe strain can distort the valve body, move the seats out of alignment and bend the stem.
- Remove welding slag, rust and construction debris.
- Check the required flow and cavity-relief direction.
- Align the pipe before tightening the valve.
- Do not use flange bolts to pull misaligned pipe together.
- Support heavy actuators where required.
- Cycle the valve before pressurization.
- Confirm open and closed indication.
For welded-end valves, follow the manufacturer’s welding procedure. It should state the required valve position, maximum body temperature, cooling method and post-weld testing.
Insulation should not permanently cover:
- Packing gland
- Gland bolts
- Body vent and drain
- Cavity-relief connection
- Actuator bracket
- Nameplate
- Body-joint inspection area
Oil-soaked insulation can hide leakage and keep combustible oil against a hot surface. Find the leak, make the line safe and replace the contaminated insulation.
Testing
Different tests prove different parts of valve performance.
| Test | What it proves |
|---|---|
| Shell test | Pressure-boundary integrity at the test condition |
| Seat test | Closure leakage at the stated pressure and medium |
| Functional test | Full travel, indication and actuator operation |
| Fire test | Limited leakage and operation during defined fire exposure |
| Fugitive-emission test | External stem and body-joint leakage |
| Thermal-cycle test | Seat durability and torque after temperature changes |
ISO 23632 provides design-validation testing for metallic valves, including procedures used to examine seat performance and operating torque through mechanical and thermal cycling.[10]
Thermal-Cycle Test Example
A project-specific qualification plan for a 320°C valve might include:
- Record initial cold leakage and torque at ambient temperature.
- Heat the valve to 320°C under the specified internal pressure.
- Hold the maximum temperature for 1–2 hours per cycle.
- Complete 20–50 thermal cycles for an initial project qualification.
- Operate the valve at least once during each hot cycle.
- Record torque every 5 or 10 cycles.
- Check hot leakage at the design temperature.
- Check cold leakage again after the final cooling cycle.
- Do not adjust the packing during testing unless the procedure allows it.
- Inspect the seats, ball, stem and packing after the test.
The numbers above are an example, not a universal standard. A safety-critical valve or a valve expected to complete thousands of cycles may need a longer qualification programme.
API 608 covers metal ball valves with flanged, threaded and welding ends. API published the seventh edition in 2025.[11]
API 608, API 598 or ISO 5208 compliance does not alone prove long-term suitability for a specific thermal-oil temperature. The ball valve testing guide explains the difference between shell, seat, torque and functional testing.
Inspection and Fault Finding
Inspect the valve more often when it operates at high temperature, cycles frequently or is located near the heater.
- Look for oil around the stem and body joints.
- Check whether the gland follower remains level.
- Record opening time and operating torque.
- Inspect removable insulation for oil staining.
- Check vents, drains and cavity-relief lines for blockage.
- Check actuator bolts, tubing, switches and cables.
A rising torque trend often gives earlier warning than complete failure.
| Torque pattern | Likely cause |
|---|---|
| High only at the start | Static friction, deposits or long idle period |
| High through the full stroke | Overtight packing, bearing damage or body distortion |
| High near closing | Seat preload, thermal growth or incorrect travel stop |
| High only when hot | Ball and seat thermal expansion |
| High only when cold | High oil viscosity |
| Higher in one direction | Directional seat load or pressure difference |
Before blaming the seat for internal leakage, confirm that:
- The actuator reaches full closure.
- The travel stop is set correctly.
- The bypass valve is closed.
- The valve is installed in the correct direction.
- Downstream trapped oil is not expanding.
- Cavity pressure is not releasing through a self-relieving seat.
- The pressure gauge is reading correctly.
Do not loosen body bolts, drains, vents or packing parts while the valve is hot and pressurized. HSE guidance describes planned isolation methods for maintenance on equipment containing hazardous fluids.[12]
Worked Example
Consider a DN100 automated isolation valve with these conditions:
- Normal temperature: 280°C
- Design temperature: 320°C
- Minimum startup temperature: 10°C
- Design pressure: 16 bar
- Maximum shutoff differential pressure: 12 bar
- Two complete open-close-open cycles per day
- Normal position: open
- Emergency position: fail closed
Two full cycles per day equal about 730 cycles per year. Because each full cycle contains two 90° strokes, the valve completes about 1,460 quarter-turn strokes per year. Over 10 years, this equals about 7,300 full cycles or 14,600 individual strokes.
| Check | Required decision |
|---|---|
| Body | Confirm that its pressure-temperature rating covers 16 bar at 320°C. |
| Seat | Screen a metal seat first. Accept a polymer only with complete-valve proof at 320°C and 12 bar differential pressure. |
| Leakage | State the test medium, pressure, direction, duration and permitted rate. |
| Cycling | Confirm that the seat and actuator can support about 7,300 full cycles over 10 years. |
| Packing | Use packing rated for the actual gland temperature and control the live-loading setting. |
| Cavity | Confirm the SPE, DPE, hybrid, drilled-ball or external-relief arrangement. |
| Fire safety | Check that the fire-test report covers the supplied body, seat and packing design. |
| Actuator | Check cold-start, hot, maximum-differential and long-idle torque. |
| Startup inspection | Inspect the gland and body joints during the first 3–5 hot cycles and after 24–72 hours. |
RFQ Checklist
| Item | Information required |
|---|---|
| Fluid | Exact thermal-oil product and current condition |
| Temperature | Normal, continuous maximum, upset and minimum startup |
| Pressure | Operating, design, pump shutoff and maximum differential |
| Valve | Size, pressure class, bore, ends and body construction |
| Seat | Material, direction, hot rating and leakage limit |
| Cavity relief | SPE, DPE, hybrid, drilled ball or external relief |
| Stem sealing | Packing material, gland temperature and live loading |
| Operation | Cycles per day, design life, normal position and fail position |
| Actuator | Minimum air pressure or power supply, torque and closing time |
| Testing | Shell, seat, fire, emissions, torque and thermal-cycle tests |
| Documents | Drawings, material list, torque data, test reports and manuals |
The supplier should return a completed data sheet, pressure-temperature chart, sectional drawing, seat direction, cavity-relief drawing, nonmetallic-parts list, torque curve, maximum allowable stem torque and applicable test reports.
Conclusion
A thermal-oil ball valve must be checked at the real pressure and temperature, not selected from pressure class or seat name alone. At 320°C continuous service, evaluate a metal seat first and confirm that the body still covers the design pressure. A valve operating twice per day completes about 7,300 full cycles in 10 years, so hot leakage and torque should be checked after thermal cycling. Also verify the packing-chamber temperature, cavity-relief direction, hidden nonmetallic parts and actuator torque at cold startup. Before ordering, require measurable leakage limits, a complete pressure-temperature chart and test reports covering the supplied design.
