Pressure Measurement & Control


Diaphragm seals and instrument isolators

May 2000 Pressure Measurement & Control

In Industry, it is often necessary to measure the pressure of a medium that may contain suspended solids. If this is done directly by means of a pressure gauge or other pressure measuring device, the suspended solids will either clog the mechanism of the gauge or the mediummay become contaminated(food and other hygienic materials) when entering the gauge.

It is therefore necessary to seperate the process medium from the pressure gauge itself and, at the same time, be able to measure the pressure. To do this, a diaphragm seal(also called a chemical seal) has to be installed between the process medium and the pressure gauge.

What is a diaphragm seal?

A diaphragm seal is a device designed to separate a pressure measuring instrument from the process media while allowing the instrument to sense the process pressure. The classical diaphragm seal is constructed from an upper housing upon which a flexible membrane (ie diaphragm) is bonded. This upper housing is then clamped to a lower housing.

The upper housing is designed with a variety of connections that are suitable for attachment to various pressure instruments. The subsequent area created between the upper side of the membrane and the pressure sensing element is solid filled (ie without air) with a fluid that thereafter will act as a hydraulic sensing medium. The lower housing is designed to be attached to a variety of process connections and contain the process medium as it pressurises against the underside of the flexible membrane.

This pressure is hydraulically transmitted to the pressure-sensing element when the membrane is displaced by an amount equal to the volume accommodated between the upper side of the membrane and the pressure-sensing element.

The higher the pressure displacement, the more fluid is accommodated by the sensing element, thereby creating more displacement on the diaphragm. Therefore, it is vital that the diaphragm seal be capable of displacing at least as much fluid volume that the pressure sensor requires for normal operation.

It is important to note that the rate of flexibility of some diaphragms when being subjected to an increase or decrease in pressure can create a tolerance of up to 0,5% of applied pressure. This tolerance can be reduced by increasing the diameter of the diaphragm that will help reduce the effects of response time to the pressure instrument and changes in process and ambient temperature.

It is also important to consider the following rule: The lower the measuring range and the higher the process temperature, the larger the diaphragm diameter should be.

Application

Diaphragm seals can be connected to most pressure instruments such as gauges, transmitters (both pressure and differential pressure) and pressure switches regardless of the sensor technology that is used. Some common sensor technologies include:

* Bourdon tube gauge.

* Bellows gauge.

* Piezo-resistive sensors.

* Capacitance sensors.

* Strain gauge sensors.

* Force balance mechanisms.

Diaphragm seals offer protection from a wide variety of process applications where corrosives, slurries or viscous fluids can damage the pressure instrument. This kind of protection includes:

* Prevention of blockage to the pressure sensor when there are suspended solids in the process fluid.

* Prevention of corrosion to the pressure sensor when certain process fluids are not compatible with the sensor material.

* Protection against changes in temperature of the process fluid when freezing or high temperatures can damage the pressure sensor.

* Sanitary isolation from the process fluid to prevent possible contamination from harmful bacteria that may exist in the pressure sensor.

Diaphragm seal ranges

Rhomberg, South African designer and manufacturer of pressure and temperature instruments, offers a range of diaphragm seals that includes the following mountings:

* Threaded: The diaphragm seal connects directly to the process by means of a male or female BSP or NPT thread.

* Flanged: The diaphragm seal is attached to the process by means of a flange as specified according to ANSI, BS or DIN standards.

* In line welded: Various types of welded-in diaphragm seals or isolation rings for flow through applications.

* Internal diaphragm mounting: The diaphragm is welded into a seal that is suitable for flanged, threaded or weld-in process connections.

* Flush mounting: The diaphragm is mounted flush to the process connection by means of either a flanged or hygienic/food industry process connection.

* Capsule seal mounting: A tubular probe with an oval cross-section is immersed into a process fluid with thread connection.

The following diaphragms are available:

* Capsule: Threaded-in capsule design for easy removal or replacement.

* Welded: The diaphragm is welded directly to the upper housing of the seal.

* Bonded: An elastomeric diaphragm (Teflon or Viton) is bonded directly to the upper housing of the seal.

* Clamped: The diaphragm, whether metal or elastomer, is clamped between the upper housing and lower housing of the seal. This is especially suitable for low pressures.

Pressure and temperature limits

Rhomberg's range of diaphragm seals can operate at full vacuum with pressures of the order of hundreds of pascals (millibars), to pressures as high as 100 000 kPa (1000 bar). These diaphragm seals are also capable of operating at temperatures between 500 and 4000ºC. The limits regarding pressure and temperature, however, are determined by the design of the diaphragm seal and the type of fill fluid that is used.

See the temperature and pressure limit cross-reference charts in order to select the correct materials for diaphragm seals.

Diaphragm seal component materials

An extensive range of materials is available to ensure compatibility between the diaphragm seal and the process medium.

Selecting a diaphragm seal

Rhomberg offers a simple 14-point programme that assists the user to select the correct diaphragm seal for an application:

1. Determine what has to be measured (gauge pressure, absolute pressure, differential pressure or vacuum) and how it should be measured (by local indication, signal of a minimum or maximum pressure or electronically by a 0–100% analog or digital signal).

2. Determine what diaphragm material would offer the best resistance to the process fluid.

3. Determine the maximum accuracy required to make the process control function.

4. Determine the process temperatures: minimum, maximum and operating temperatures.

5. Determine the ambient and environmental temperatures.

6. Determine whether a vacuum may occur in the process, for instance fast cooling after sterilisation.

7. Select a fill fluid based on the process and ambient temperature and vacuum. (See points 4, 5 and 6 above.)

8. Highlight any possible variations between process and ambient temperatures. This will enable Rhomberg to assist the user to select a diaphragm with the flexibility to maintain the minimum accuracy required.

9. Highlight the range to be measured and, if applicable, the static pressure. This information is required to determine the minimum diameter of a diaphragm for the application.

10. Determine the required process connection or consider some of the many optional connections available for the application.

11. Determine the gauge connection.

12. Determine the upper housing material.

13. Determine the lower housing/flange material.

14. Determine whether flushing connections are required on lower housings or connection flanges.

Diaphragm seals and their functions are often viewed as complicated. This need not be the case as long as it is kept in mind that the diaphragm seal has been designed to be a simple isolation device. It only becomes complex when allowing for different types of media, the process temperature and other factors that may be unique to a specific application. By following the 14 selection steps described in this article, it is possible to simplify the selection process and to easily determine the correct diaphragm seal for a specific application.



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