Diaphragm Seals, Gauge Guards and Chemical Seals
Diaphragm seals are used to protect pressure gauges, pressure transmitters and temperature gauges from hostile process fluids, and they are also used in sanitary applications i.e. to protect fluids from contamination from the materials used to make the wetted parts of these types of measuring devices. The seal acts as a barrier between the fluid and the instrument.
Sanitary processes are found in the food and pharmaceutical industries where allowing process fluid to accumulate in the pressure port of an instrument would compromise the purity of the fluid. An example of this could be milk getting into the pressure port of a pressure gauge and turning sour.
Diaphragm Seal vs Chemical Seal
Chemical seal, and for that matter gauge guard or even barrier seal, are just other names used for a diaphram seal. To be clear, there is no difference between a chemical seal and a diaphragm seal, nor between a diaphragm seal and a gauge guard.
Protection Against Hostile Process Fluids
A process fluid can be thought of as a hostile fluid for many reasons including being corrosive, abrasive, containing particles or solids, or even just being at a high temperature.
Diaphragm seals are made from materials that are compatible with process fluids, allowing a measuring device to be specified with more standard, less exotic materials of construction. Common materials of construction for diaphragm seals include;
- Stainless Steel
- Tantalum
- Inconel
- Monel
- Hastelloy
- PTFE (Polytetrafluoroethylene)
Diaphragm Seal Fill Fluid
Diaphragm seals use the displacement of an incompressible transfer fluid, known as a fill fluid, to transmit process pressure to the instrument. The fill fluid is displaced by the deflection of the thin diaphragm in the seal. In other words, the fill fluid is the fluid that is in contact with the non-process side of the seal and the wetted parts of the measuring instrument. The choice of fill fluid depends on the specific application, though common fill fluids include;
- Glycerine, or a water and glycrine mixture
- Silicone oil
- Hydraulic oils
Diaphragm Seals and Accuracy
Diaphragm seals, in general, have a negative impact on the accuracy of the measuring device to which it is coupled. This is particuarly true on relatively low pressure measurements. In these instances a larger diaphragm seal will offer more accurate readings than a small diaphrgm seal.
The diaphragm requires additional pressure to displace its fill fluid, which can affect the accuracy of the pressure measured. Further, temperature fluctuations in the process media can cause expansion or contraction of the fill fluid resulting in a loss of accuracy.
Diaphragm Seal Types
Diaphragm seals are availble in four different styles, namely: flange type, threaded type, extended seal, and in line type. The decision on which type to choose is influenced greatly by the requirements of the process, and the type of process connection available.
Diaphragm Seal Installation and Mounting
There are three main mounting techniques for diaphragm seals, each being suitable for specific applications and requirements. These are
- Direct mounting
- Remote mounting
Direct Mounted Diaphragm Seals
In this type of mounting arrangement, the diaphragm seal is directly connected to the process pipe or vessel either via flange or a threaded connection. This is by far the simplest and most common mounting technique you are likely to encounter.
The seal is in close proximity to the pressure source which allows better sensitivity, and helps minimise loss of accuracy.
This type of installation is suitable for applications where the process temperature is not too high.
Remote Mounted Diaphragm Seals
Contrary to what the name suggests, in this type of installation the seals are also directly connected to the process pipe or vessel, however, the associated pressure instrument is remotely located from the seals. A capillary tube, often referred to a filled system, is used to transmit the pressure from the seal to the instrument.
This type of installation is extensively used in Tank or Vessel Level Applications.
The dP transmitter is commonly mounted close to grade, or in close proximity to the high-pressure process connection. For applications under vacuum, the transmitter is mounted below the high-pressure connection to reduce vacuum effects on the transmitter fill fluid.
Capillary legs do introduce measurement inaccuracies. These can be minimised by keeping capillary lengths as short as possible, using capillaries of the same length on both taps when measuring differential pressure, and ensuring that both capillaries experience the same temperature.