High media temperatures are among the most common reasons why a pressure sensor, process transmitter or pressure gauge cannot be mounted directly on a process line. In steam, thermal oil, bitumen, chemical and plastics plants, temperatures at the process connection may be significantly higher than the permissible temperature of the pressure measuring instrument itself.
A diaphragm seal separates the measuring instrument from the process medium and transmits the pressure via a flexible diaphragm and a system fill fluid. It protects the measuring instrument from direct contact with the medium, but it does not automatically solve the temperature problem. Heat can still be transferred to the measuring instrument through the diaphragm seal body, the fill fluid and the mechanical connection.
There are three basic mounting methods for thermal isolation: direct mounting of the measuring instrument on the diaphragm seal, a cooling element between the diaphragm seal and the measuring instrument, or a capillary line for remote mounting. The technically appropriate solution does not depend solely on the maximum media temperature. The ambient temperature, mounting position, measuring range, fill fluid, response time, diaphragm size, process medium and actual heat input at the installation point are equally important.
Contents
- How does a diaphragm seal protect the measuring instrument?
- Distinguishing between process, ambient and instrument temperature
- How does heat reach the pressure sensor?
- Direct mounting: fast and compact
- Cooling element: compact thermal isolation
- Capillary line: mounting the measuring instrument remotely
- Comparison of direct mounting, cooling element and capillary
- Selecting a fill fluid for high temperatures
- Diaphragm material and process medium
- Temperature and response time of the measuring system
- Insulation, installation point and heat dissipation
- Special considerations for steam
- Correctly specifying a diaphragm seal system
- Practical example: Hot thermal oil
- Which products are suitable?
- Conclusion
- Frequently asked questions
How does a diaphragm seal protect the measuring instrument?
A thin metallic diaphragm is located on the process side of the diaphragm seal. The space behind this diaphragm, the connection to the measuring instrument and the measuring element form a closed hydraulic system that is completely filled with a system fill fluid. When process pressure acts on the diaphragm, it is transmitted through the fluid to the connected pressure measuring instrument.
The process medium therefore normally comes into contact only with the diaphragm, the process connection and, where applicable, other wetted parts of the diaphragm seal. The actual measuring cell of a pressure sensor or process transmitter remains separated from the medium. This is particularly important for hot, aggressive, highly viscous, crystallising or contaminated media.
The diaphragm seal system is evacuated, filled, sealed and matched to the connected measuring instrument as a complete unit. The diaphragm seal, connection and measuring instrument must therefore not be separated from one another like ordinary threaded components. If the closed system is opened, fill fluid may escape or air may enter. Professional refilling and recalibration are then required.
Distinguishing between process, ambient and instrument temperature
When a diaphragm seal for high temperatures is requested, only the maximum media temperature is often specified. This value alone is not sufficient for reliable system design. At least three temperature ranges must be considered separately.
The process temperature is the temperature of the medium directly at the diaphragm seal diaphragm. Among other factors, it determines the requirements for the diaphragm material, process connection, seal and fill fluid. The normal operating value is not the only decisive factor. Heating cycles, cleaning, sterilisation, steam purging and temporary temperature peaks must also be considered.
The ambient temperature affects the measuring instrument, cooling element and capillary line. A cooling element can only dissipate heat effectively to the surroundings if the surrounding area is significantly cooler and sufficient air circulation is available. If the measuring point is located in an enclosed, poorly ventilated or already very hot area of the plant, the cooling effect will be reduced.
The permissible instrument temperature applies to the connected pressure gauge, pressure sensor or process transmitter. Electronic components, displays, connectors and seals in particular often have lower temperature limits than metallic process connections. Even if the diaphragm of the diaphragm seal is suitable for a high temperature, the electronics must not be thermally overloaded.
How does heat reach the pressure sensor?
Heat input occurs mainly through thermal conduction via the metallic components and the system fill fluid. In addition, thermal radiation from hot pipelines, vessels or furnaces, as well as heated ambient air, can place a thermal load on the measuring instrument.
The actual temperature at the measuring instrument therefore depends on more than just the media temperature. The length and geometry of the connection, material, mounting position, air movement, pipe insulation, operating duration and distance from other heat sources also influence the result. A short temperature peak may need to be assessed differently from a process that operates continuously at a high temperature for several days.
Arrangements in which the measuring instrument is positioned directly above a hot connection and the heated air rises towards the instrument housing are particularly critical. Completely enclosing or insulating the entire assembly can also prevent heat from being dissipated to the surroundings.
Direct mounting: fast and compact
With direct mounting, the pressure measuring instrument is connected directly to the diaphragm seal. This keeps the fill volume and the hydraulic transmission path small. This design normally offers the fastest response to pressure changes and causes the lowest additional temperature influence compared with a long capillary.
Direct mounting should therefore generally be preferred when the process and ambient temperatures remain within the permissible limits of the complete system. It is also suitable for plants in which rapid pressure changes, short cycle times or pressure peaks need to be detected with minimal delay.
Direct mounting can nevertheless be problematic with hot media. Heat is transferred quickly to the measuring instrument through the short metallic connection. If the permissible instrument temperature is exceeded, the zero point, sensitivity, indication and service life may be affected. A diaphragm seal with a temperature-resistant diaphragm alone does not provide sufficient protection for the electronics.
Strong vibrations or a difficult-to-access installation point also count against direct mounting. In such cases, spatial separation of the measuring instrument may be necessary even if the temperature is theoretically still within the instrument specification.
Cooling element: compact thermal isolation
A cooling element is installed between the diaphragm seal and the measuring instrument. Its elongated design or enlarged surface area reduces heat conduction and dissipates some of the absorbed heat to the surroundings through convection and thermal radiation. The measuring instrument can therefore remain significantly cooler than with direct mounting.
This solution is particularly useful when only limited additional installation space is available and the measuring instrument is still to remain directly at the measuring point. Compared with a capillary line, the arrangement remains compact, mechanically straightforward and generally more responsive.
However, the cooling effect must not be treated as a fixed, universally applicable temperature reduction. A process temperature of, for example, 250 °C cannot automatically be converted into a specific temperature at the measuring instrument. The actual cooling depends on the design, ambient temperature, air circulation, mounting position and operating duration.
The ZKE 153 cooling element is a specific example of a compact cooling section for pressure measuring instruments. The pressure, connection and temperature data stated on the product page apply to this component. Its suitability for a particular measuring point must nevertheless be assessed on the basis of the complete diaphragm seal system and the connections used.
A cooling element should not be completely enclosed within the pipe insulation. Its surface must be able to dissipate heat to the surroundings. If it is tightly insulated together with the hot pipeline, the intended cooling effect may be largely lost.
Capillary line: mounting the measuring instrument remotely
A capillary line enables spatial separation between the diaphragm seal and the measuring instrument. The diaphragm seal remains directly on the hot process, while the pressure sensor, process transmitter or pressure gauge is mounted in a cooler, less vibration-prone and more accessible location.
This mounting method offers the greatest degree of design flexibility. It is useful when the temperature reduction provided by a cooling element is insufficient, the measuring instrument cannot be read directly at the process, or strong vibrations and mechanical loads are present. With electronic transmitters, the display, connection terminals and cable glands can also be moved out of the hot area of the plant.
However, the capillary line increases the fill volume of the system. Temperature changes along the line cause the volume of the system fill fluid to change. This volume change must be absorbed by the diaphragm seal diaphragm and may cause an additional zero-point shift. The lower the measuring range and the smaller the diaphragm, the more significant this effect can become in relation to the measuring span.
The length, internal diameter, fill fluid and temperature also determine the response time. A long capillary should therefore not be selected merely as a precaution. It should only be as long as is actually required for safe and accessible installation.
During installation, the minimum bending radius specified by the manufacturer must be observed and the line must be secured without mechanical stress. It must not be kinked, crushed, used as a carrying handle or attached to components subject to severe vibration. It must also be protected against later entrapment beneath insulation cladding or maintenance panels.
In addition, the difference in height between the diaphragm seal and the measuring instrument creates a hydrostatic pressure component due to the fluid column in the capillary. This influence can be considerable with low measuring ranges. The intended mounting height must therefore be known at the ordering and calibration stage.
Comparison of direct mounting, cooling element and capillary
| Mounting method | Typical advantage | Typical limitation | Suitable application |
|---|---|---|---|
| Direct mounting | Small fill volume, fast response, compact design | High heat input into the measuring instrument, transmission of vibrations | Moderate temperatures, rapid pressure changes, short machine cycles |
| Cooling element | Compact temperature reduction without remote instrument mounting | Cooling effect depends on the surroundings, air circulation and mounting position | Hot pipelines and vessels with limited installation space |
| Capillary line | Significant spatial separation from heat, vibration and difficult-to-access measuring points | Larger fill volume, longer response time, temperature influence and elevation effect | Very hot processes, strong vibration, remote indication or transmitter mounting |
The technically best solution is not automatically the one with the greatest distance from the process. An unnecessarily long capillary may impair measuring performance more severely than a suitably sized cooling element. Conversely, a cooling element is unsuitable if there is hardly any air circulation at the installation point or if the entire area of the plant remains permanently very hot.
Selecting a fill fluid for high temperatures
The system fill fluid transmits the pressure from the diaphragm to the measuring instrument. It must cover the full temperature range of the diaphragm seal system. The maximum process temperature is not the only factor to be considered. The lowest ambient temperature during shutdown, storage or start-up may also be decisive.
At low temperatures, the viscosity of many fill fluids increases. As a result, the system responds more slowly to pressure changes. At high temperatures, thermal expansion, ageing, decomposition and the vapour pressure of the fluid may become critical. Particularly with vacuum or absolute-pressure measurements, it must be ensured that the fill fluid does not outgas or boil under the prevailing pressure and temperature conditions.
The fill fluid must also be suitable for the application. In food, pharmaceutical or cosmetic applications, a fluid approved for the relevant contact may be required. Separate requirements apply to oxygen applications, strongly oxidising media or special safety requirements. Selection must therefore not be based solely on a temperature table.
A high-temperature fill fluid does not automatically make a diaphragm seal system suitable for every high temperature. The diaphragm, seals, process connection, capillary and measuring instrument must also be approved. The permissible temperature is limited by the weakest component of the complete system.
Diaphragm material and process medium
The diaphragm is directly exposed to the high process temperature and the medium. Its material must therefore be suitable both thermally and chemically. Stainless steel can be used with many industrial media, but it is not sufficient for every acid, alkali, saline solution or chloride-containing application.
Depending on the medium, materials such as Hastelloy, Monel, tantalum or coated or lined versions may be required. Selection depends on concentration, temperature, pH value, flow, cleaning chemicals and possible shutdown conditions. A material approval at room temperature cannot automatically be transferred to a process operating at 200 °C.
With bitumen, polymer melts and hot oils, adhesion or solidification of the medium must be considered in addition to chemical resistance. A flush diaphragm avoids narrow pressure channels in which the product could accumulate. During cooling, however, solidifying medium must not mechanically deform the diaphragm or overload it through changes in volume.
Temperature and response time of the measuring system
The response time of a diaphragm seal system is influenced by the diaphragm, system fill fluid, fill volume, connection and connected measuring instrument. Direct mounting normally provides the fastest transmission response. A cooling element increases the transmission path, while a capillary line extends it further.
At low ambient temperatures, the viscosity of the fill fluid increases. As a result, a capillary that responds sufficiently quickly during warm operation may become significantly slower during a cold plant start. This effect is particularly relevant with long or very narrow capillaries.
A certain delay may be uncritical for slowly changing process pressures, level measurements or static monitoring tasks. With rapid pump cycles, extrusion processes, dosing applications or safety-relevant pressure increases, however, it must be verified that the complete diaphragm seal system provides the required dynamic response.
A slow indication is not necessarily an instrument fault. It may be the expected result of the capillary length, fill fluid, temperature and measuring instrument volume. The required response time should therefore be specified during system design.
Insulation, installation point and heat dissipation
Pipe insulation should not be extended over the entire pressure measurement assembly without careful consideration. Depending on the application, the process connection may be insulated, but a cooling element requires an exposed surface for heat dissipation. The measuring instrument should also not be enclosed beneath a shared insulation cover together with the hot pipeline.
With capillary systems, it must be decided which part of the line is to be kept warm and which part is to be deliberately cooled. A partially insulated capillary can create large temperature gradients. For differential-pressure measurements with two diaphragm seals, both capillary lines should have comparable lengths, routing and ambient temperatures wherever possible. Unequal heating of the two sides may otherwise cause an additional differential-pressure error.
External heat sources must also be considered. A transmitter may overheat despite being positioned at a sufficient distance from the process if it is installed directly next to a furnace, an uninsulated steam line or beneath a hot factory roof. The temperature at the intended installation point should therefore ideally be determined under actual operating conditions.
Special considerations for steam
For clean water vapour, a diaphragm seal is not automatically the simplest solution. The temperature is often kept away from the measuring instrument by means of a suitable impulse line, a siphon or a condensate chamber. The resulting condensate forms a thermal barrier between the hot steam and the pressure measuring instrument.
A diaphragm seal may nevertheless be useful if the medium is contaminated, deposits occur, hygienic or completely closed separation is required, or the design of the measuring point does not permit a conventional condensate line. A suitable diaphragm material may also be required for chemically contaminated steam or aggressive condensate.
Start-up conditions, condensate hammer, drainage, mounting position and the risk of freezing must also be considered in steam applications. Selection should therefore not be based solely on the normal operating temperature.
Correctly specifying a diaphragm seal system
For a reliable design, the medium, chemical composition, normal and maximum pressure, possible vacuum conditions, operating and peak temperature, and the lowest ambient temperature should be known as a minimum. The process connection, required measuring range, measuring instrument, required accuracy and required response time must also be specified.
For thermal design, the intended installation point, pipe insulation, air circulation, distance from other heat sources and duration of temperature exposure are also required. With a capillary line, the length, routing, elevation difference and method of securing it must be considered.
A practical procedure consists of five steps:
- First, the medium, process pressure and all temperature conditions are recorded in full.
- The process connection, diaphragm material and fill fluid are then selected.
- Next, it is checked whether direct mounting is thermally and mechanically permissible.
- If direct mounting is unsuitable, the cooling element and capillary are compared with regard to temperature, response time and installation space.
- Finally, the complete system is calculated and calibrated for the measuring range, temperature error, mounting position and required dynamic response.
The maximum temperature of an individual component must not be confused with the permissible temperature of the complete system. Final approval should always relate to the specific ordered combination.
Practical example: Pressure measurement on hot thermal oil
The pressure in a thermal-oil line is to be measured in a process plant. The medium reaches 240 °C during continuous operation. The pressure changes relatively slowly and the measuring instrument is to cover a range from 0 to 16 bar. Additional hot pipelines run directly next to the measuring point, and the pipe is fully insulated.
A directly mounted pressure transmitter is initially ruled out because heat would be transferred directly to the electronics through the short process connection. A compact cooling element appears possible in principle. However, an inspection of the installation point shows that it would be positioned partly beneath the existing pipe insulation and that there is very little air circulation. The actual cooling effect would therefore be difficult to predict.
The selected solution is a flush diaphragm seal suitable for thermal oil, fitted with a short capillary line. The transmitter is mounted laterally on a cooler support structure. The capillary is kept as short as possible, protected against vibration and not insulated together with the hot pipeline.
Because the process pressure changes only slowly, the longer response time compared with direct mounting is acceptable. For a rapid control loop, however, it would be necessary to reassess whether a suitable cooling element or a specialised high-temperature pressure sensor would be the better solution.
Which products are suitable?
The diaphragm seal category includes various diaphragm, tubular, threaded, flanged and hygienic versions. Depending on the application, the systems can be directly mounted, equipped with a cooling element or connected to the measuring instrument via a capillary line.
The ZKE 153 cooling element can be used with suitable connections to thermally isolate a pressure measuring instrument. Its specific suitability must be assessed on the basis of the pressure, media temperature, connection, mounting position and complete measuring system.
The pressure sensors and differential-pressure sensors category includes various electronic measuring instruments for industrial pressure measurement. For high process temperatures, the sensor should not be considered in isolation, but should be specified together with the diaphragm seal, fill fluid, cooling element or capillary.
| Product category | Function in the high-temperature system | Important selection criteria |
|---|---|---|
| Diaphragm seals | Separation of the process medium from the measuring instrument | Diaphragm, material, process connection, fill fluid and temperature |
| ZKE 153 cooling element | Reduction of heat input into the measuring instrument | Connections, pressure, media temperature, air circulation and installation space |
| Pressure sensors and differential-pressure sensors | Electronic measurement of the transmitted process pressure | Measuring range, accuracy, instrument temperature, dynamic response and installation point |
Conclusion: Plan temperature management for the complete diaphragm seal system
A diaphragm seal does not automatically provide complete protection for a pressure sensor against high temperatures. Process heat can still be transferred to the measuring instrument through the diaphragm, fill fluid and metallic connection. The process temperature, ambient temperature and permissible instrument temperature must therefore be considered together.
Direct mounting offers the fastest response and the lowest additional system influence, but it is only possible if sufficient resistance to temperature and vibration is available. A cooling element provides compact thermal isolation, but requires unrestricted air circulation. A capillary line provides the greatest distance from the heat source, but increases the fill volume, temperature influence, elevation effect and response time.
The correct solution can only be determined from the complete application. The media temperature, surroundings, measuring range, diaphragm material, fill fluid, installation point, capillary length and required dynamic response must be assessed together. The earlier this information is available, the more reliably sensor overload, zero-point shifts and unnecessarily slow pressure measurement can be avoided.
Frequently asked questions about diaphragm seals at high temperatures
What temperature can a diaphragm seal withstand?
There is no universally applicable temperature limit for all diaphragm seals. The permissible temperature depends on the diaphragm material, process connection, seals, fill fluid, mounting method and connected measuring instrument. Approval of the complete system is always decisive.
When is a cooling element better than a capillary line?
A cooling element is suitable when a compact design is required, sufficient air circulation is available and the achieved temperature reduction is adequate for the measuring instrument. It generally provides a faster response than a longer capillary line.
When should a capillary line be used?
A capillary is suitable when the measuring instrument must be positioned well away from heat, vibration or a difficult-to-access measuring point. The length, elevation difference, temperature profile and required response time must be known during system design.
May a cooling element be insulated?
The heat-dissipating surface of a cooling element should normally remain exposed. Fully enclosing it within the hot pipe insulation reduces air circulation and therefore its cooling effect. The specific mounting instructions provided by the manufacturer must be observed.
Why does a diaphragm seal with a capillary respond slowly?
The response time is influenced by the capillary length, internal diameter, fill volume and viscosity of the system fill fluid. At low ambient temperatures, the fluid often becomes more viscous, which can slow the response further.
Is a diaphragm seal always required for steam?
No. With clean water vapour, a suitable condensate line or siphon may provide a simpler form of thermal isolation. A diaphragm seal becomes relevant, for example, where hygienic requirements, deposits, aggressive condensate or completely closed media separation are involved.
Can a diaphragm seal subsequently be separated from the sensor?
No. The diaphragm seal, fill fluid, connection and measuring instrument form a closed system. Once separated, professional evacuation, refilling and recalibration are required.