Basics of industrial pressure measurement 2

The previous article covered two basic methods of pressure measurement which were manometers/barometers and pressure gauges. We also discussed the different elements used in the pressure gauges which are bourdon tubes, diaphragms, capsules and bellows. Here we will further discuss other aspects of these pressure measurement devices before we discuss other basic instruments used in the industrial pressure measurement.

The construction of these instruments and specifically the wetted parts (parts exposed to process fluids) use different materials for different process fluids and also depend on the measuring range of the instruments. For more sensitivity and for measurement of differential pressure, bellows and capsules are used. Bellows having more effective area are having better sensitivity than capsules. These also come in a variety of different material from bronze, brass to stainless steel, platinum and some alloys like Hastalloy, Inconel, etc. Same is true for Bourdon tube material which also has different construction as a C type, spiral or helical (in order of increasing sensitivity).

Proper selection of wetted material and body parts material is very important to avoid corrosion, fatigue and erosion due to the properties of the fluids as well as in the environment where these are installed. A pressure gauge used in desert high temperature or offshore location may have different specifications as compared to those installed in moderate or controlled environments. As mentioned earlier, different fluids need different material. For toxic or corrosive chemicals special diaphragms of Teflon or other materials may be used.

While the manometers are mostly glass construction, but transparent plastics or other fibers are also sometimes used. We are discussing it because improper material selection sometimes have catastrophic effects and can jeopardize the safety of personnel and plant. The case pressure and proof pressure are also factors to be considered when selection of a pressure gauge is being done.

Proof pressure is the maximum pressure which can be applied to any instrument without compromising its performance. Also maximum pressure is sometimes is indicated on the pressure gauge data sheet. Case pressure on the other hand is the maximum pressure the gauge enclosure can sustain in case of any internal leakage. Normally blow out hole is provided in most cases to release the unwanted pressure.

In our next edition we will be covering some other industrial pressure measuring devices in our newsletter. I would like to thank all who have subscribed and welcome comments and suggestions as well if something is missed. Thanks again for your help and support. Our main aim is to provide the information for those who are new to this field and also for the seasoned ones to jog their memories. Just subscribe to this newsletter INSTRUMENTATION REVISITED and receive updated version every week. Please feel free to share it if you like it. Comments, suggestions, feedback are always welcomed.

(Please note that though the accuracy of the information provided here is based not only on reliable sources, but also based on personal experiences, but it is for providing a general picture and the author is not liable for any loss or damages arising from the use of this information in any instance)