A diaphragm separator is a thin-walled, flexible fitting between the pressure measuring instrument and the process medium. The separator or diaphragm seal isolates the process fluid from the pressure instrument, it comes in different connection forms, such as flange, thread or tri clamp.
What comes into contact with the fluid is always the membrane, very sensitive, low thickness and in different diameters; It is this membrane that comes into contact with the process fluid. The entire operation of the diaphragm separator system is based on Pascal’s Principle, which states that the pressure exerted on a fluid (filling liquid) is transmitted uniformly in all directions through the separator membrane. The fluid never enters the mechanism of the instrument.
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 temperatures.
In some chemical applications or in sanitary processes, to prevent corrosion or contamination due to the accumulation of fluids in the system, it is necessary to isolate the pressure measuring instrument, protect it and 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 while reducing 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 healthcare applications, where the product is not allowed to settle and produce bacteria, this is the best solution; allows you to do the optimal cleaning required by the
- Protect pressure measuring instruments and prevent the escape of harmful emissions into the environment.
- When the fluid is highly viscous and it is impossible to enter the sensing element of the manometer, switch or transmitter, or it enters slowly; the diaphragm separator is the only solution.
- When the fluid tends to crystallize or polymerize, especially during process stops. It can block the fluid from entering the instrument and prevent a correct measurement.
- When the fluid is a mass or a sludge.
- When the fluid is very hot, it can alter or deform the pressure gauge sensing element.
- When the measurement point is 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 sensing element of the instrument from high temperatures, it is recommended to use a diaphragm separator with a capillary or cooling tower to dissipate the heat.
What to take into account for a good selection
1.- Fluid characteristics
Since the diaphragm and 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. There are tables to help the selection of these materials, however, the client is the one who must specify the appropriate materials. It must be taken into account 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.
2.- Temperature
Almost every measurement system that includes a diaphragm separator, manometer and capillary, if applicable, will be filled with a volume of liquid at a temperature of approximately 70°F (20°C). This temperature is called 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 suitable and qualified personnel.
To avoid this error, the ambient temperature and the temperature of the fluid to be measured must be specified; Only in this way can a good selection of the diaphragm seal and the necessary accessories be made.
Special calibration techniques may 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 at a temperature > 212°F (100°C), to solve this, some accessories must be used to protect it.
3.-Pressure
When selecting a diaphragm separator, it is very important to take into account the pressure to be measured. The separator membranes are manufactured in different diameters, from ½” to 6”. The larger the diameter of the membrane, 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 the membrane diameter is very important; when done correctly, the proper functioning of the diaphragm seal is guaranteed. If we review the pressure equation, P = F/A, we can deduce that pressure is inversely proportional to area.
4.- Manometer type
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 manometers is recommended and for pressures > 15 PSI and < 1000 PSI, any of these sizes will work; for pressures > 1000 PSI, the use of 4”, 4 ½” and 6” dial pressure gauges is recommended.
5.-Connection to the process
The connection to the process 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-lined SS 316L membranes is recommended.
6.- Filling liquid
WIKA offers a wide range of liquids that allow temperatures from – 130°F (-90°C) to 750°F (400°C). It is very important, to select the filling liquid, to know the type of process. Por ejemplo, en aplicaciones alimenticias, el líquido de llenado debe ser un aceite vegetal, en caso que rompa la membrana y el líquido de llenado fugue, no contaminará , de manera peligrosa el fluido alimenticio.
There are also special liquids for oxidizing media such as oxygen and chlorine.
7.- Mounting position
Mounting position is important for diaphragm separator systems that include a capillary. The difference in level between the diaphragm separator and the manometer causes a hydrostatic pressure to act on the sensing element.
a) For pressure gauges installed above the level of the diaphragm separator, the needle on the pressure gauge face will be lower than the actual measurement.
b) For gauges installed below the level of the diaphragm separator, the needle on the gauge face 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, using smaller diameter capillaries and reducing the viscosity of the system’s fillable liquid.