A diaphragm separator is a thin-walled, flexible fitting between the pressure measuring instrument and the process medium. The diaphragm separator or diaphragm seal isolates the process fluid from the pressure instrument and comes in different connection forms, such as flange, thread or tri clamp.
What comes in contact with the fluid is always the membrane, which is very sensitive, of low thickness and in different diameters; it is this membrane that comes in contact with the process fluid. The complete operation of the diaphragm separator system is based on Pascal’s Principle, which states that the pressure exerted on a fluid (filling fluid) is transmitted uniformly in all directions through the separator diaphragm. The fluid never enters the instrument mechanism.
The volume contained between the instrument sensor and the diaphragm separator is filled with different liquids such as silicone, vegetable oil or special inert liquids for high temperature.
In some chemical applications or in sanitary processes, to avoid corrosion or contamination due to the accumulation of fluids in the system, it is necessary to isolate the pressure measuring instrument, to protect it and to extend its useful life.
Some reasons to use a diaphragm separator:
- In applications with aggressive products that generate corrosion, the use of the diaphragm separator is the most indicated solution, allowing the use of a conventional instrument decreasing costs. The diaphragm separator can be manufactured in a variety of materials and/or plastic coatings, which eliminate or drastically reduce the problems caused by corrosive media.
- In sanitary applications, where the product is not allowed to deposit and produce bacteria, this is the best solution; it allows to make the optimal cleaning required by the regulations.
- Protect pressure measuring instruments and prevent the release of harmful emissions into the environment.
When the fluid is highly viscous and it is impossible for it to enter the sensing element of the pressure gauge, switch or transmitter, or it enters slowly; the diaphragm separator is the only solution.
- When the fluid tends to crystallize or polymerize, especially at process stops, it can block the fluid from entering the instrument and prevent correct measurement.
- When the fluid is a mass or slurry.
- When the fluid is very hot, it can alter or deform the sensor element of the pressure gauge.
- When the measuring point is located in a place that is difficult to access or is not visible. In this case a diaphragm separator with capillary is required for remote measurements.
- To protect the sensor element of the instrument from high temperatures, a diaphragm separator with capillary or cooling tower is recommended to dissipate the heat.
What to consider for a good selection
1.- Fluid characteristics
As the diaphragm and the lower body of the diaphragm separator are exposed to the fluid, it is important to select the materials of these components to be compatible with the fluid. Tables are available to assist in the selection of these materials, however, it is up to the customer to specify the appropriate materials. If the fluid is very dense or contains suspended solids, this is necessary for the correct selection of the diaphragm material.
For the upper body of the separator, the material used is SS 316L, this material is one of the most versatile stainless steels.
Almost every measuring system that includes a diaphragm separator, pressure gauge and capillary, if applicable, will be filled with a volume of liquid at a temperature of approximately 70°F (20°C). This temperature is referred to as the filling liquid temperature. This liquid will expand or contract according to changes in ambient temperature, causing alteration in the sensing element of the instrument, adding an error. This is why it is so important that the filling and assembly of the diaphragm seals be done by qualified personnel.
To avoid this error, the ambient temperature and the temperature of the fluid to be measured must be specified; only then, a good selection of the diaphragm seal and the necessary accessories can be made.
Special calibration techniques can be used to ensure the best possible accuracy. A cooling tower or capillary of a certain diameter and length is suggested. A pressure gauge, switch or transmitter cannot work with a fluid temperature > 212°F (100°C), to overcome this, some accessories should be used to protect it.
When selecting a diaphragm separator, it is very important to take into account the pressure to be measured. Separator diaphragms are manufactured in different diameters, from ½” to 6″. The larger the diameter of the diaphragm, the more sensitive it is and can sense lower pressures < 15 PSI. Membrane diameters < 1 ½” are ideal for sensing pressures > 15 PSI up to high pressures.
The above indicates that the correct selection of diaphragm diameter is very important, getting it right will ensure that the diaphragm seal performs well. If we review the pressure equation, P = F/A, we can deduce that pressure is inversely proportional to area.
4.- Type of pressure gauge
From the experience of several manufacturers, taking into account the most common dial diameters that exist, 1 ½”, 2 ½”, 4″, 4 ½”, 6″, it can be assumed as a good criterion, that for low pressures < 15 PSI, the use of 2 ½” dial pressure gauges is recommended and for pressures > 15 PSI and < 1000 PSI, any of these sizes will do; for pressures > 1000 PSI, the use of 4″, 4 ½” and 6″ dial pressure gauges is recommended.
5.- Process connection
The process connection of a diaphragm seal must be specified by the customer. Most process connections are threaded, flanged or tri-clamp.
The most commonly used threads are ¼” or ½” NPT, in some applications metric threads are used. For applications with aggressive fluids, the use of plastic diaphragm seals or PTFE coated SS 316L membranes is recommended.
6.- Filling fluid
WIKA offers a wide range of liquids that allow temperatures from – 130°F (-90°C) to 750°F (400°C). It is very important, in order to select the filling liquid, to know the type of process. For example, in food applications, the filling liquid must be a vegetable oil, in case it breaks the membrane and the filling liquid leaks, it will not contaminate the food fluid in a dangerous way.
There are also special liquids for oxidizing media such as oxygen and chlorine.
7.- Mounting position
The mounting position is important for diaphragm separator systems that include a capillary. The level difference between the diaphragm separator and the pressure gauge causes a hydrostatic pressure to act on the sensing element.
a) For gauges installed above the level of the diaphragm separator the needle on the gauge dial will be lower than the actual measurement.
b) For gauges installed below the level of the diaphragm separator, the pointer on the gauge dial will be larger than the actual measurement.
The diaphragm separator system can be calibrated to compensate for the error caused by hydrostatic pressure if the level difference is known in advance, by using smaller diameter capillaries and reducing the viscosity of the fillable liquid in the system.