Diaphragm seals in chemical/ petrochemical / offshore plant /facilities

Diaphragm seals, also known as chemical or remote seals, are used for pressure measurements when the process medium must not come in contact with pressurized parts of the measuring instrument.

Example of diaphragm seals used in measurement of dp across a filter.
Image source: http://www.aplisens.de/produkty/pdf/APRE-2200.pdf
Diaphragms are used in:

1. The medium is corrosive. Diaphragm seals protect the pressure measuring element (e.g., the interior of the Bourdon tube) against corrosive medium.
2. The medium is highly viscous and fibrous. Diaphragm seals prevent measuring problems due to dead spaces and constrictions in the bores of the measuring instrument (pressure channels, Bourdon tubes)
3. The medium has a tendency towards crystallization or polymerization. Diaphragm seals will stop the crystallization/ polymerization from reaching the measuring instrument.
4. The medium has a very high temperature. Diaphragm seals and capillaries minimize the effect of high temperatures, reducing high temperature errors in the display of the measuring instrument and averting damage due to heat exceeding the upper limits for the thermal loading of the instrument components.
5. The pressure measuring point is in an awkward position, inhibiting the installation of the measuring instrument or prohibiting the accurate reading of the display. By using a diaphragm seal and a capillary, the measuring instrument can be conveniently installed in a location where it can be easily viewed.
6. Hygienic standards require special requirements. Diaphragm seals remove dead space in the measuring instrument and fittings.
7. The medium is toxic or harmful to the environment. The design of the diaphragm seal helps prevent leakage into the environment.

Advantages of diaphragm seal:

1. Longer service life of the measuring assembly;
2. Lower mounting costs
3. Elimination of maintenance.

Common error during design and installation of diaphragm seal
1)        Failure to use Low Volume Nipples:
Always use a low volume nipple with high quality threads when a nipple is required to connect the diaphragm seal to the instrument. This will help eliminate temperature induced errors and reduce the possibility of fill fluid leakage.
2)        Fill Fluid Vaporization:
The fill fluid can vaporize and destroy the diaphragm seal system if the process or ambient temperatures exceed the capabilities of the fill fluid. The potential for problems increases with high operating temperatures at low pressure ranges. Always ensure the fill fluid will work within the pressure and temperature range of the application.
3)        Improper Filling:
Overall performance of a diaphragm seal system can be dramatically affected by improper filling of the system. The diaphragm may bulge outward or the static pressure exerted by the fill fluid on the measuring instrument may induce gross measurement errors if the system is overfilled. The system may experience a lack of response or non-linear reading if the system is under filled.
4)        Improperly Sized Diaphragm:
The diaphragm seal may not be capable of driving the measuring instrument if the diaphragm is too small. There may be problems with instrument resolution while measuring small pressure changes and the system may be susceptible to temperature errors caused by contraction and expansion of the fill fluid.
5)        Slow Response Time:
Longer capillary lines were used than were necessary for the application, consideration was not given to ambient temperature effects, incorrect fill fluid was specified, or incorrect capillary internal diameter was used. Always consult vendor for assistance in determining the response time of a diaphragm seal system in an application.
6)        Unequal Capillary Lines:
Unequal capillary lines are not recommended for differential pressure instruments since the system may be susceptible to zero shifts resulting from fill fluid expansion and contraction.
 
Reference:

1. http://www.aplisens.de/produkty/pdf/APRE-2200.pdf