Pressure and differential pressure (DP) measurements are important for regulating inventory, flow, and product quality and are critical for ensuring safe operation. Installation errors and the drift of today’s smart pressure and DP transmitters have decreased by an order of magnitude by compensation of temperature, static pressure, and non-ideal sensor effects. Most of the remaining concerns are associated with impulse lines.
Pressure control of boilers, prime movers (e.g. pumps, fans, and compressors), and headers is important for supply to consistently meet demand. Pressure control in reactors, columns, evaporators, and extruders is the key to reducing variability in product quality. Pressure control is critical for increasing safety and decreasing emissions by preventing excursions that would activate safety instrumentation systems (SIS) and relief devices. Pressure measurements for SIS actions must be fast, reliable, and accurate.
The driving force for flow across system resistances and control valves is differential pressure and pressure for critical flow. Pressure and DP measurements can monitor the pressure profile in piping systems, the performance of filters, and the coating of heat exchange surfaces. Portable wireless transmitters can generate process diagnostics and identify installed valve flow characteristics. DP measurements in clean rooms can maintain a differential of 0.25 inches water column for positive air flow out of the room to prevent contamination in pharmaceutical and electronics manufacturing.
DP is by far the prevalent instrument used for level and flow measurement despite newer technologies, such as radar and Coriolis primarily due to the low cost of a DP. Material balance control, residence time control, flow ratio control, and vessel-sump- tank inventory control most often depend on DP measurements. High levels can cause spills and carry-over of the process into vent systems. Low levels can damage pumps.
Given the significant role of pressure and DP measurements in manufacturing, what can be done to get the best implementation? The following checklist is not intended to cover all the installation and specification requirements but some of the major application details to be addressed for automation component. The following list assumes the materials of construction have been properly specified, the sensor will work safely and reliably with acceptable accuracy for the maximum possible temperature, and the electrical connections and enclosure will meet electrical area classifications and codes. For a detailed understanding see Chapters 3-5 in the ISA book Essentials of Modern Measurements and Final Elements in the Process Industries. Reliability and precision (noise, repeatability, resolution, and threshold sensitivity) are most important.
- Is the transmitter fast enough for application (e.g. damping < 0.2 sec for compressor control)?
- For gas, is transmitter mounted above process connection to prevent accumulation of liquids?
- For liquid, is the transmitter below process connection to prevent trapping gases?
- Do impulse lines have a continuous slope with no bends or smooth long radius bends?
- Do impulse lines have vent and drain valves?
- Does a DP have an equalization valve?
- Does process pressure connection design prevent appreciable velocity head?
- Do transmitter and impulse lines need freeze protection?
- If heat tracing is used, are high temperatures prevented that could alter fluid composition in impulse lines or transmitter (e.g. vaporization, reactions, or formation of tars and polymers)?
- For plugging services, can impulse lines be purged or eliminated?
- For purged impulse lines is purge flow and purge pressure high enough?
- For purged impulse lines is purge flow and purge pressure adjustable and indicated?
- For purged impulse lines is liquid purge needed to eliminate transients from compressibility of gas purge during fast static pressure disturbances and to prevent solids build-up from drying at bubbler tip?
- Can impulse lines be eliminated by direct mount of transmitter or use of capillary system?
- Should an extended diaphragm be used to minimize fouling of a direct mounted level DP?
- Does diaphragm area need to be increased to improve threshold sensitivity?
- Does diaphragm area need to be decreased to increase speed of response?
- Is capillary length minimized to increase speed of response?
- Are the capillary systems at the same temperature (e.g. sun versus shade)?
- For DP measurement with low static pressure, can DP be computed from dual direct mounted transmitters to eliminate the need for impulse lines?
- Can a smart transmitter be used to detect plugged impulse lines?
- Can wireless transmitters be used to provide portability for process troubleshooting?