Possible diaphragm seal designs (from left): compact design, temperature decoupling, variable capillary length.

© Picture: Computer&AUTOMATION, Source: Endress+Hauser

Diaphragm seals are used in the process industry when conventional pressure gauges reach their limits - be it due to the need for special designs, high temperatures, installation locations that are difficult to access or, last but not least, systems in which strong vibrations occur. They also protect measuring devices from the process media or extreme process conditions - from aggressive, highly viscous, crystallizing or polymerizing media.

However, the so-called temperature effect, which can influence the measurement results in certain cases, must be taken into account when designing the measuring point. This temperature effect occurs due to the design of the diaphragm seal, the interior of which is filled with oil. Therefore, if it is possible to use systems without diaphragm seals when designing pressure gauges, this is recommended. An alternative is, for example, the oil-free and robust ceramic measuring cell, which is suitable for process temperatures up to 150 °C.

Technically challenging

Diaphragm thicknesses or diaphragm thicknesses for different flange sizes

© Endress+Hauser

How does the temperature effect arise in detail? On the one hand, the oil-filled capillary line in the diaphragm seal isolates the measuring electronics from the process to be measured and any critical temperatures (<-25 °C and >+150 °C). On the other hand, the filling oil in the diaphragm seal system also serves as a transmission medium between the process pressure and the sensor element due to its incompressible behavior. When the ambient or process temperature changes, the volume of the oil changes. The sensor element perceives this change in volume as a change in process pressure and evaluates it. A change in temperature in the environment or in the process results in a pressure change in the measurement without the process pressure actually having changed. The oil volume in the diaphragm seal system depends on the length of the design or the decoupling: the longer the decoupling, the greater the oil volume.

To counteract the temperature effect, there are two options: firstly, to keep the oil volume as low as possible and/or secondly, to design the diaphragm seal in such a way that it can absorb the change in volume of the filling oil as well as possible without applying a restoring force to the sensor. The stiffness of the diaphragm is of decisive importance here: with low stiffness, the diaphragm can compensate for a greater change in volume of the filling oil, which improves the measuring accuracy in the event of temperature influences.

Counteracting the temperature effect

For this reason, Endress+Hauser has developed and patented the 'TempC' (temperature compensatory) diaphragm. This membrane has a special embossing and low stiffness in order to react positively to temperature changes. This approach was made possible by design and development technologies such as the finite element method.

The recessed diaphragm provides additional protection for the meter.

© Endress+Hauser

Unlike conventional diaphragms, the 'TempC' diaphragm behaves particularly flexibly when deflected. This means that more volume can be absorbed when the filling oil expands depending on the temperature, without generating a restoring force on the sensor. This reduces the temperature influence by a factor of up to 5. Until now, high accuracies could only be achieved with large diaphragms - and therefore large process connections. With the 'TempC' diaphragm, small process connections can now also be realized. This opens up new areas of application for diaphragm seals - for example, they can be used in pipelines with small nominal diameters. In addition, procurement costs are reduced as other components of a pressure measuring point such as valves, adapters or seals are smaller or no longer required.

From a flange size of DN50, the diaphragm thickness of the 'TempC' increases from 50 µm to 100 µm (for comparison: a human hair is 70 µm thick). This results in greater robustness and corrosion resistance to the process without negatively affecting the rigidity.

Another feature of the diaphragm is its recessed design for flange connections: The fact that the diaphragm is set back from the flange by 2.3 mm means that it is additionally protected against mechanical damage when working with the device: For example, if a fitter places the measuring device on a dirty table with the diaphragm facing downwards, it is protected from smaller particles on the table - conventional diaphragms might already be irreparably damaged.

Excursion into production

The individual processing steps of a stainless steel diaphragm - for example welding and embossing - usually cause stresses in the metal structure. Endress+Hauser has developed welding and embossing processes for the 'TempC' diaphragm that enable optimized processing and machining of the diaphragm. As a result, the diaphragm leaves the production line stress-free and has no inhomogeneities in its structure. In addition, the oil filling of the diaphragm seal offers special possibilities: For example, the diaphragm is placed in its optimum position for each individual diaphragm seal in the thermoforming unit. This allows the optimum amount of oil to be determined individually for each diaphragm seal and the system to be filled accordingly.

Author:
Alexander Hermann is Product Manager Pressure at Endress+Hauser Messtechnik in Weil am Rhein.