Back to Basics - 2 (Pressure Series)
Muhammad Junaid Atta
Controls and Functional Safety Engineer | Expert in Designing Reliable Systems and Ensuring Safety Compliance - CPEng (AUS), RPEV, RPEQ, CPEng (NZ) PE (PK) FSEng (TüV Rheinland) CMRP (SMRPCO) AMP (Exida)
Accessories (Siphons, Pulsation Dampers and Chemical Seals)
Protection of pressure sensing instruments from high temperature, high pressure and corrosion.
Siphons
In most of the steam service, it is required to prevent steam in the pressure sensing element. This prevention is achieved by installing a siphon (pigtail) between the gauge and the process connection.
Pigtail siphons are meant for vertical applications when installed in a piping configuration. If it is in a horizontal application, it is referred to as a coil siphon and has a slight dip in its loop design to provide for the trapped water in a horizontal position.
Pulsation Damper
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It is required to use a pulsation damper where sudden change in pressure is expected. Pulsation damper is always installed in between pressure sensing element and the process connection.
It is not recommended to install pulsation damper where it is required to measure high frequency pulses.
Chemical Seals
Chemical seals are used?if the process conditions no longer allow direct interaction with a pressure sensor. Following are the three reasons why chemicals seals area used.
1) Due to the sudden change in process changes it is possible that process fluid can freeze or settle down in the sensing line, chemical seals avoids it.
2) Chemical seals are used to measure pressure of extremely corrosive process fluids to avoid direct intact of corrosive process fluid with pressure sensor. The chemical seals are made to withstand the corrosive effect of the process fluid.
3) It also prevents slurries or viscous polymer from entering, plugging the sensing element.
Filling fluids are selected to provide low thermal expansion and low viscosity and to avoid danger of process contamination in case of diaphragm rupture.
The FDA approvals shall also be considered while selecting the filling fluids, like in food or in pharma applications are subject to FDA review & fill fluids must be selected based on FDA approvals. Refer to the external link to know the commonly used fill fluids in chemical seals.