A diaphragm seal is a device consisting of a diaphragm welded (or else bolted) between two flanges and suitably gasketed to prevent leakage of liquid or gas.

The diaphragm is a dividing membrane or thin partition, which acts as a barrier to a liquid or gas that may clog or corrode the sensing element of a pressure instrument. It can be used with pressure gauges, pressure transmitters or pressure switches.

The physical barrier (seal) between the instrument and the medium eliminates clogging and crystallization, which minimizes corrosion. A diaphragm seal also protects the instrument’s sensing element from high temperatures by dissipating and/or conducting heat.

How does filling work?

The pressure sensing instrument (pressure gauge) is isolated from the process fluid by the flexible diaphragm seal. The empty space between this diaphragm and the pressure element of the instrument must be filled with an incompressible fluid, usually a type of oil. This fluid transmits the pressure applied to the flexible diaphragm to the sensing element of the pressure instrument (Pascal’s Principle).

Pascal’s Principle: The pressure exerted on an incompressible fluid at equilibrium inside a vessel with non-deformable walls is transmitted with equal intensity in all directions and at all points in the fluid.

Diaphragm seal applications

• In processes where fluids are corrosive and would attack the pressure sensing element.
• In processes where the fluids are solids laden or viscous and would block the pressure sensing element.
• Where the process fluid would solidify due to a change in process temperature.
• Where the process fluid must be contained in sanitary piping systems.
• In processes where the fluids are at too high a temperature to enter the pressure instrument.

Types of diaphragm seals

Since the possibilities of using diaphragm separators are numerous, they cannot be covered by only one model. Over time, different designs proved to be particularly beneficial for certain cases. In view of the above, there are currently three basic types:

• Membrane separator.
• Tubular separator.
• Bulb separator.