Diaphragm Seal Systems for Safety Applications

Diaphragm seals enable pressure, differential pressure and absolute pressure measurement devices to be used even in the most demanding applications. WIKA diaphragm seals can be produced in various designs and from durable materials compatible with a large proportion of pressure gauges. Although a SIL classification is not possible for diaphragm seals, since diaphragm seals contain only mechanical components, security-related technical values exist in WIKA diaphragm seals.

Functional Safety

High-quality components must be used to avoid risks to people, the environment and equipment. Reliable components in control and instrumentation technology (C&I) protect critical processes in the chemical industry. In this context, circuit breakers, safety circuits or safety functions are typically used. The required safety characteristics for the components used are specified by standards such as IEC 61508 (functional safety in general) and IEC 61511 (functional safety in the process industry). In this context, the term Safety Integrity Level (SIL) is also used. The failure rate of a component is determined by the manufacturer and provided to the user. An important tool in this regard is FMEDA (Failure Modes, Effects and Diagnostic Analysis). For this purpose, the statistical values and functional relationships of the components are evaluated together. The results are quantified data about the probability of failure or the reliability of the components. To define data relating to the installation of diaphragm seals, the functional principle must be explained.

Operating Principle of the Diaphragm Seal System

Pressure measurement instruments can be adapted to the most difficult conditions in the process industry with the aid of diaphragm seals. In this case, a diaphragm made from a suitable material separates the measuring environment from the measuring element. A fill fluid adapted to the measurement task transfers the hydraulic pressure to the measuring element. The process side of the seal is isolated by a flexible membrane (See Figure 1).

Figure 1: Installation of a diaphragm seal to a pressure gauge with capillary tube.

The space between the diaphragm and the pressure gauge is completely filled with a fill fluid. Process pressure is transmitted through the elastic membrane to the fluid and hydraulically to the measuring element of the pressure measurement device. For example; manometer, transmitter or pressure switch. In this way, pressure can be measured reliably. The diaphragm seal can be connected directly to the measuring device either directly or via a flexible capillary line. At high temperatures, a cooling element can be inserted between the seal and the device.

Materials

Standard diaphragm material is corrosion-resistant 316L stainless steel (1.4404 / 1.4435). If this material does not show sufficient resistance to corrosive environments, the parts of the diaphragm seal in contact with the medium can be coated with plastics such as PTFE, ECTFE, etc. Another alternative would be to use wikaramic® ceramic coating as continuous protection against wear. For particularly critical applications, diaphragms can be produced from materials such as tantalum or Hastelloy C. Such a measurement arrangement can be used without problems at extremely high temperatures up to +400°C. The only limitation stems from the maximum working temperature of the system fill fluid or special material. There are more than 20 different special materials available for users to choose from. The most common special materials for chemical seals in chemical and petrochemical applications are Hastelloy C4, Hastelloy C276, tantalum and Monel 400.

Safety Values for Diaphragm Seal Systems

IEC 61508 applies to all applications of electronic systems that have a significant impact on the safety of persons, the environment and equipment. Safety-related requirements depend on the likelihood of a hazardous event occurring and its potential consequences. The higher the expected severity of harm and the higher the probability of occurrence, the higher the SIL classification from SIL 1 to SIL 4. This classification is carried out by the plant operator using a "risk graph". According to IEC 61508, the entire safety circuit must be considered, i.e. all components used in the circuit (sensors, logic processors, actuators). To perform such a calculation and risk assessment, the structure of each component of the diaphragm seal must be known. For example, for a standard design consisting of a diaphragm seal body and membrane, the probability of failure is relatively low since there is only one connection point between the diaphragm seal and the body. If significant designs such as an extended diaphragm seal (see Figure 3) need to be implemented, the sum of failure rates increases with each additional component or each connection point. It should not be forgotten that a failure can occur with a certain probability for each component and connection. Figure 2: Model 990.27 diaphragm seal IPT-10 process pressure transmitter. Diaphragm material is tantalum.

Figure 3: Extended diaphragm model 990.29

  For the evaluation of a complete safety-related system, a greatly simplified categorization of the diaphragm seal with relevant safety parameters is listed in the table below (see table). More detailed characteristic values are available for diaphragm seal types and can be provided if required for an evaluation.

Conclusion

In order to reduce risk, the plant operator must perform a calculation for the entire system by taking into account all components in the existing system. This calculation also serves as proof of the risk reduction of the safety system. Table 1: * DU = Failure rate dangerous undetected/FIT = Failure in Time   Jennifer Breunig Diaphragm Seal Systems Product Manager WIKA         Heiko Kern Diaphragm Seal Systems Design Manager WIKA         Translator Gül Selvi Sales Support Specialist WIKA Instruments Industrial Measurement Devices Trade Ltd. Co.