Rethink Local Pressure Measurement

Wireless gauges offer a number of important advantages but also pose challenges.

By Wally Baker, Emerson Process Management

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Pressure gauges provide crucial information for many process operations. Traditionally, plants have relied on Bourdon tube gauges for local pressure measurements. Typically, staff responsible for a particular unit view and record readings. The data then go to the control room so operators can verify the unit is running correctly, optimize performance and ensure safety. In addition, the readings, often manually entered into a data management system, provide useful information for scheduling and performing maintenance.

Wireless pressure gauges are a relatively recent innovation. They use digital electronics to transmit continuous and automatic readings, and offer other benefits. Some, for instance, feature new pressure-sensing technology that reduces the number of mechanical components. However, the wireless instruments cost far more initially than Bourdon tube gauges and require a learning curve. So, here, we’ll examine when and why you should consider the wireless option.

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Bourdon Tube Challenges

Bourdon tube gauges are a low-cost method to measure pressure. This means the gauges frequently don’t get adequate attention (Figure 1). Moreover, a variety of factors can compromise their performance.

Process operations with high pressures or pressure spikes, such as those that often occur during startups or upsets, can cause these gauges to leak or burst, and possibly create safety issues. Meanwhile, in some cases, the process fluid entering the Bourdon tube is becoming hotter or more corrosive, resulting in gauge failure.

In addition, a gauge’s Bourdon tube and other mechanical components can fatigue and fail due to significant pulsations, such as those coming from a positive-displacement pump directly upstream. To combat these process pulsations, plants sometimes use liquid-filled gauges to dampen needle movements to decrease wear and tear. These can reduce overuse of the needle gearing and subsequent premature failure of the needle assemblies but cost more. Filling the gauge with liquid also can create problems if the liquid solidifies at low ambient temperatures, which can cause the gauge to break. Many installations address this issue with a remote seal system — but this adds significant cost.

A Bourdon tube gauge can show an incorrect reading with no clear indication of malfunction. To verify operation, a technician typically taps the gauge, for example with a wrench, to see if the needle bounces and springs back to an appropriate reading. This shows the gauge needle isn’t stuck but doesn’t confirm the gauge is working properly.

Getting the correct pressure information to where it can be utilized by facility personnel also is a challenge with traditional gauges. Many plants require operators to record readings during their rounds — with key data then entered manually into an asset management or other data storage system. This approach introduces a significant delay (hours or maybe even days) between when the data are collected and entered and thus available for use, as well as the possibility of errors such as not writing down the correct reading or making data-entry mistakes into the system.

Coping With The Challenges

A wireless pressure instrument (Figure 2) addresses many issues that undermine the performance and life of a Bourdon tube gauge. Table 1 summarizes five key challenges and how wireless gauges handle them.

Let’s look at the last item in the table, expanded use of data, in a bit more detail. The wireless instrument transmits the local pressure reading to control, asset management and other host systems via a wireless mesh network such as WirelessHART. This advantage applies to any type of process measurement where a wireless instrument replaces a gauge.

Data are transmitted from each wireless instrument node, such as a pressure gauge, via the mesh network to a wireless gateway (Figure 3). WirelessHART uses the mesh network to provide redundant paths of communication as each node can receive and retransmit wireless information.

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