Diaphragm seals are used commonly to isolate and protect pressure measuring instruments from the process medium in chemical processing and extreme process applications. The protection will result in greater reliability and accuracy in an operation, minimise production downtime and prolong the lifespan of the pressure measuring instrument.
- Protects the sensing elements from harsh medium (aggressive, corrosive, and viscous medium, extreme temperature)
- Prevents direct contact and contamination for sanitary requirements
- Eliminates clogging and crystallization; minimise corrosion
- Accessibility or obstacles; Hard-to-reach location that is not suitable for direct mounting
A diaphragm seal is a thin, flexible wall that is typically connected to a pressure gauge, pressure transmitter or a pressure switch. The space between the diaphragm and the measuring instrument is filled with a system fluid (transmitting fluid), which hydraulically transmits pressure from the flexible diaphragm.
Process Connection:
Threaded: The seal utilizes a male or female NPT to make a direct connection with a process.
Flanged: Flanges are available in various types such as flat face, raised face, or ring joint flange.
Sanitary: Connections are ideal for applications that demand high hygiene. These connections must comply with Clean in Place (CIP) methodologies as well as FDA requirements.
Accessories:
Capillary: A capillary is connected between the diaphragm seal and pressure measuring instrument to eliminate or to minimise temperature effects from the hot fluid to the measuring instrument. In some cases, remote mounting with the capillary is used for hazardous or hard-to-reach location.
Cooling towers: Cooling towers are intended to protect the instrument from excessively high process medium temperatures. Recommended for process temperatures +100˚C.
Gauge Pressure: The pressure which is measured above atmospheric pressure. Examples of industrial applications: open tank level, line pressure, blanket pressure in a tank
Absolute Pressure: The pressure which is measured above vacuum. Examples of industrial applications: open tank level, line pressure, blanket pressure in a wet/dry tank, plant atmospheric reference
Differential Pressure: Difference between two applied pressures points. Example of industrial applications: closed tank level, density, filter health, DP flow
Single Seal: A transmitter with one remote seal can be used for open tank level measurement. The seal is on the high side of the transmitter, and the seal element must be located near the bottom of the tank. The transmitter can be located either above or below the seal element. The low side of the transmitter must be vented to atmosphere.
Double Seals, Pressure Service: The transmitter can be located between the seals, above both seals or below both seals on a pressure service installation.
The preferred location is mid way between the seal elements. This location minimizes the required capillary length, and usually provides the most uniform distribution of ambient temperatures across the capillary length.
Single Seal, Dry Leg Installation: When a transmitter with one seal is used to measure level in a pressurized tank, a compensating leg must be connected between the vapor space at the top of the tank and the low side of the transmitter. If the process vapor is not readily condensable, or if the compensating leg is at a higher temperature than the tank interior, a dry leg can be used. A trap installed at the bottom of the leg minimizes the possibility of condensate collecting in the transmitter body.
Single Seal, Wet Leg Installation: When the process vapor is condensable, a wet leg is recommended. The wet leg is filled with a suitable liquid to maintain a constant pressure on the low side of the transmitter.
This approach avoids the problem of process vapor condensing and collecting in the compensating leg resulting in serious measurement error. The leg can be filled with process liquid or any liquid with a low vapor pressure relative to the process. A filling tee is required at the top of the leg. The wet leg can be avoided if the transmitter can be located near the top of the tank, so that condensate drains back into the tank.
A gauge pressure transmitter with a remote seal can be used for measurement of pressure in either a process pipe or tank. Locate the remote seal in the side or top of the pipe to avoid collection of sediment on the seal diaphragm.
Pressure Service: When the operating pressure range has a minimum value above atmospheric pressure (0 psig), the transmitter can be placed in any convenient location, either at the same elevation as the remote seal, above or below the seal.
Vacuum Service: When the operating process pressure range includes pressures below atmospheric, it is essential that the transmitter be located below the elevation of the seal element to ensure that the pressure in the transmitter body is always greater than 0 psia.
