A diaphragm seal is a device consisting of a diaphragm welded (or firmly clamped) between two suitable flanges and properly gasketed to prevent leakage of liquid or gas. The diaphragm is a dividing membrane or thin partition that 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.
Diaphragm seals are available in various styles: one-piece construction, two-piece construction, flanged and saddle types are most common. Excluding the one-piece construction unit, material choices can be made for the upper housing, diaphragm, and lower housing. Particular attention should be made to the diaphragm seal with respect to the maximum/minimum process pressure and the pressure instrument to which the diaphragm is being fitted to. Further options involving cooling towers and fill fluid can be used to accommodate high temperature applications.
The most important factor with regard to diaphragm seal performance is the integrity of the filled system. Regardless of the appropriate sizing of the diaphragm seal and proper fill fluid selection, if the filled system contains entrapped gases or pockets of air, the performance will tend to be inaccurate and unpredictable.
Consideration for selecting a diaphragm seal is preceded by understanding the requirements of the pressure sensing element of the instrument. A primary concern is establishing the sensing element’s displacement requirement. We can summarize the following:
Bourdon tube – diaphragm seal can usually be appropriately sized to the dimensions of the Bourdon tube. Typically, the higher the pressure, the lower the volumetric displacement requirement is to drive the sensor its full span. Inversely, low pressure Bourdon tubes may pose limitations on the smaller diameter sizes of the diaphragms that can be used.
Helical tube – lower displacement requirement than the equivalent pressure range Bourdon tube. Extremely low displacement requirement even in low pressure ranges.
Capsule type – contact Winters for exact displacement requirements. The nature of this type of sensor is such that only the highest displacing diaphragm seals will work.
Bellows type – contact Winters for exact displacement requirements. If there is a diaphragm seal that has adequate displacement capability to drive the bellows its full range, the large volume of fill fluid’s head pressure usually shifts the bellows out of its operating range. Bellows sensors are rarely compatible with diaphragm seals.
The pressure sensing instrument is isolated from the process fluid by means of a flexible diaphragm. The void between this diaphragm and the instrument’s pressure element is filled with an incompressible fluid. This fluid transmits pressure applied to the flexible diaphragm to the sensing element of the pressure instrument.
Volumetric displacement is the amount of fluid that a flexible diaphragm will displace when pressure is applied. For proper function, the diaphragm seal must have greater displacement capability than what the instrument’s sensing element requires for its full range actuation.
Diaphragm displacement curve is a term relative to the non-linearity of the relationship between the pressure applied to a diaphragm and the amount of fluid it displaces. At ambient temperatures an instrument can be calibrated to perform in a linear fashion through its full displacement requirement. As fill fluid increases in volume because of a rise in temperature, the operating range of the diaphragm will shift to a less linear portion of the diaphragm’s movement. The instrument may be very accurate at ambient temperatures but non-linear under elevated temperature conditions.
Fill fluids used by diaphragm seal manufacturers are offered because of their low coefficient of expansion with changes in temperature. Care should be taken not to exceed the maximum operating temperature range of the fill fluid. If the fill fluid reaches its vapourization temperature, this filled system may not indicate accurately and will typically exhibit irreparable damage to the sensing diaphragm.
Note: When selecting a diaphragm seal, always refer to ASME B40.100 (2013)