Diaphragm Seals - General Information0 pages

Data sheet
Date
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INFO SEALS
08/2013
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GENERAL INFORMATION ABOUT DIAPHRAGM SEALS

INTRODUCTION
Modern chemical processing and process manufacturing procedures demand ever greater accuracy and reliability from their
pressure measuring instruments. In order to minimise costs, both reject levels and production downtime must be kept to an
absolute minimum. In some applications where there are chemical or hygiene considerations, it is necessary to isolate the pressure
measuring instrument usually a gauge or a transmitter from the process medium.
This isolation is achieved by using a pressure-sensitive diaphragm made from a material resistant to the process medium enclosed
in a system fitted to the base of the instrument. The space between the diaphragm and the measuring element in the instrument
has to be evacuated under a vacuum and filled with a suitable fill fluid and sealed. The process pressure exerts a force on the
outside face of the diaphragm. As the diaphragm flexes under this force it pushes inwards and attempts to compress the fill fluid
within the instrument.
This fill fluid is designed to withstand compression so that the movement is channelled directly into the measuring element
producing a resultant reading on the gauge.
The entire operation of the diaphragm seal system is based on Pascal's Principle which states that a pressure exerted on a fluid is
transmitted undiminished through that fluid and in every direction.
For this process to work the displacement force of the diaphragm must exceed the force required to move the measuring element in
the instrument itself. In practice this means that the smaller the force required to move the measuring element the easier it is to
construct an accurate seal system.
APPLICATION
Knowing in which situations to install a diaphragm seal rather than a standard instrument is critical to maintaining the integrity of the
process. A diaphragm is usually required under the following circumstances.
•

The process medium is corrosive, and would chemically attack the working parts of a standard pressure gauge or
transmitter.

•

The process medium is viscous or contains solid particles, either of which could result in the gauge's pressure inlet
becoming blocked and preventing any pressure from reaching the measuring element.

•

The process medium is prone to solidifying over time (e.g. it may freeze as the temperature drops, or it may set as it
dries, or it may be subject to polymerization), in which case a standard gauge's internal moving parts could be rendered
immobile.

•

The process medium is so corrosive that even pressure elements made from exotic materials such as Monel or Hastelloy
do not provide adequate protection. The only solution is to apply a special coating to the base material to prevent
corrosion. Application of these special coatings to the internal movement of a standard pressure gauge or transmitter
would be impossible, but a diaphragm seal fits the bill perfectly.

•

It is important to eliminate the formation of bacteria on or in the process connection in applications where hygiene is of
paramount importance such as in the food and beverage industries.

_______________________________________________________________________________________________
PCI (Process System Solutions) Ltd, Unit 7 Slaidburn Crescent, Southport, Merseyside, PR9 9YF, UK,
Tel: +44 (0) 1704 509090, Fax: +44 (0) 1704 509091,
email: sales@pci-instruments.com, Website: www.pci-instruments.com