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Instrumentation Technologies and Best Practices: Pressure

PressureI recently had the chance to moderate a panel discussion with Emerson experts Kevin Branger, Todd Wallace, and Rick Fox that addressed important advances and best practices in specifying, installing, operating, and maintaining Rosemount instrumentation. We’re bringing you a series on this informative panel discussion, and this installment focuses on pressure measurement.

We started off the discussion by providing a brief overview of pressure technology. It is a simple and fundamental process measurement consisting of three variants: absolute pressure is measured relative to a perfect vacuum; gage pressure is measured relative to atmospheric pressure; and differential pressure is the difference in pressure between any two points. Pressure instrumentation can also be used to measure level in a tank and flow in a pipe.

The panel discussed some of the recent technology innovations in pressure instrumentation. Pressure gauges are one of the simplest and oldest types of instruments. They provide local indication of the pressure using a needle and dial display. Traditional pressure gauges are purely mechanical and are notorious for poor reliability and frequent failures. New advances in pressure gauge technology are now available that replace the mechanical components with proven solid-state sensor technology. This provides a step change in reliability, overpressure, and vibration capabilities, resulting in a device that can last 10 or more years. These devices also have status indication for easy verification that the gauge is functional. WirelessHART™ capabilities can even make it possible to “know before you go” by having remote insight to the gauge readings and health without having to be physically at the device.

Another fundamental instrument is the pressure transmitter, which provides continuous process insight by being directly connected to a PLC or DCS through a wired or wireless connection. They are used across all industries in a variety of applications ranging from indoor controlled environments to outdoor installations in remote and extreme environments. The panel talked about the unique challenges that arise from making accurate pressure measurements in hot process applications. New thermal range expander technology can operate in temperatures up to 770oF. Engineered with two different fill fluids, the technology eliminates the need for heat tracing and can be used in various process applications. In cold climates, the capabilities of pressure transmitters have been extended for operation down to -76oF without the need for heated enclosures.

The panel also talked about technology enhancements in the realm of multi-sensing / multivariable transmitters. Multivariable transmitters are advantageous in that they combine multiple sensors and measurements into a single device. This results in simpler installations with fewer pipe penetrations and fewer devices to install, wire, and commission. Multivariable transmitters are commonly used in flow applications where static pressure and temperature measurements are made in addition to the DP measurement to maximize the accuracy of the flow calculation. 

In level applications, conventional installations commonly use a single transmitter with lengths of impulse piping or remote seals and capillaries. For taller vessels that require longer lengths of impulse piping or capillary, best practice is now to use electronic remote sensor technology that calculates differential pressure electronically using two pressure sensors linked together with an electrical cable. This type of system eliminates the temperature-induced measurement errors and time-response challenges that can be commonplace with traditional installations. This solution also provides multivariable capabilities by providing the individual pressure measurements of the sensors in addition to the calculated DP. This eliminates the need for a separate pressure transmitter that is commonly installed to measure the vessel pressure.

Diagnostic capabilities have also been enhanced in recent years. Most of today’s “smart” transmitters have internal diagnostics that can send a notification if the device is malfunctioning or needs service. However, there are now transmitters available that extend the diagnostics coverage beyond the device to include the loop wiring and the process itself for enhanced safety and reduced unplanned downtime.

The session concluded with the panelists sharing some of the more common questions they receive on pressure instruments. Surprisingly, the most common question is about troubleshooting HART communication with a transmitter. The panel mentioned that this is often due to not having a large enough resistor in the loop, and they advised making sure the loop resistance is well above the 250-ohm minimum threshold. AC noise interference with the HART signal can also be a cause. The panel also explained the different calibration functions that exist in a pressure transmitter, how to configure a device for flow or level applications using the “scaled variable” feature, and installation best practices for draft range applications.

Overall, pressure is a simple and straightforward measurement to make. However, due to the breadth of industries and applications where pressure measurements are made, there are a variety of challenges and issues that can arise. More information on pressure measurement can be found here. Do you have any questions on pressure measurement technologies and best practices that weren’t addressed here? Ask them in a comment below and one of our experts will answer it for you!