Be aware of what types of transients could occur in the line where the transmitter is mounted. This is particularly important if a pressure transmitter is installed near a pump discharge. Pulsation dampeners are generally used to damp out process fluid pulsations, more often for signal fluctuations but possibly for protection against mechanical vibrations that might damage the transmitter.
One dampening technique is installing a pigtail in the impulse piping between the instrument and the process root valve connection, and some transmitters are available with built-in dampening. Signal pulsations are generally filtered by a dampener rather than time-delayed, so the output is an average of the pulsations. This raises the question of whether the transmitter will provide a representative signal.
Chemical seals serve several purposes in a transmitter installation:
* Preventing contact of potential noxious or corrosive process fluid with the transmitter.
* In certain cases, serving as a winterizing function in lieu of the electrical or steam tracing devices.
* Allowing remote mounting of a transmitter for better maintenance access (using a diaphragm and capillary-type seal). This can also alleviate some of the burdens associated with area classification if the diaphragm and capillary seal allows the instrument to be mounted outside the hazardous area. It is considered good practice to allow no more than 25 ft. of capillary tubing from the diaphragm to the transmitter. A downside to a capillary can be additional loop deadtime.
* Preventing solids or slurries from plugging the measurement element.
* Providing a larger surface area or process connection, which can improve sensitivity and help minimize plugging.
It is critical that the liquid in the seal be capable of withstanding the temperature at the process connection and the ambient temperature without freezing or gelling. The fluid should also be compatible with the process fluid so there is no major contamination problem should the seal break. Work with the transmitter or seal manufacturer to select the correct fluid.
Pay attention to the construction of the seal, and in general, look for fully-welded seals. In sanitary services, consider an approved self-cleaning seal. If vacuum is present, use a seal designed and constructed for that kind of service.
Purging is used to keep the instrument clear of process fluid that can cause plugging of the impulse lines or the transmitter. Purging is commonly used in applications involving solids, process fluids that are subject to solidification or plugging, and acid or basic fluids. The purge fluid must be compatible with the process and delivered at a higher pressure than the process, with check valves for backflow prevention. Reference 7 includes information on determining purge flow rates.
Before selecting the sensor type, if the specifier understands the measured process fluid and its characteristics in various temperatures and pressures, knows the conditions the transmitter will experience, and provides adequate room to install and maintain the transmitter, there's an excellent chance the proper pressure measurement technology will be applied.