Critical decisions for selecting pressure transmitters

There are many considerations when it comes to selecting a pressure transmitter, and few are examined in school. Here’s advice to avoid tough lessons as you learn on the job.
By Wally Baker November 13, 2015

Figure 1: An in-line pressure transmitter has a single process connection. Courtesy: Emerson Process ManagementMany of today’s smart process transmitters are multitalented devices. They can provide a great deal more information and perform more functions beyond simply providing a pressure reading. This extra data can include temperature, device history, calibration information, diagnostics, and more. A smart transmitter can detect internal problems, be recalibrated and re-ranged remotely, and in some cases even spot process anomalies such as plugged impulse lines.

In addition to measuring pressure values directly, pressure measurements can be used to determine or infer flow rates, fluid levels, product density, and other parameters. As a result, pressure transmitters are deployed more widely than any other type of process instrument.

Wireless technology adds another layer of capability to pressure transmitters, making it possible to measure and gather information in locations that previously were not economically feasible to reach due to the costs associated with conduit, cable I/O cabinets, and other infrastructure needed for wired transmitters.

With this additional ability to "measure anything, anywhere," an engineer has to decide what needs to be measured and why. What justification is there for a given measurement, and which device is best suited to meet his or her goals?

This article suggests a series of questions and other considerations to help guide a designer charged with making critical decisions regarding pressure applications and transmitter selections.

The pressure is on

The first questions to ask should include:

  • What is the normal operating pressure of the application? Select a transmitter that is optimized for that range.
  • What are the highest and lowest pressures expected during normal operation? Accuracy can generally degrade somewhat at the extremes, but the device must remain repeatable under these conditions and must not suffer any damage nor require recalibration.
  • What maximum pressure will the device experience under the worst conditions? For safety reasons, the transmitter must be able to withstand a pressure as high as the pressure rating of the vessel or pipe to which it is attached without bursting. All attached piping, flanges, manifolds, and other accessories that will be exposed to the same maximum pressure must be rated to this threshold. Some devices can continue to operate after an overpressure incident and retain accuracy, while others may need recalibration or even replacement, but the primary consideration here is safety.

Connecting to the process

Figure 2: A coplanar mounted transmitter has two process connections. Courtesy: Emerson Process ManagementBecause pressure transmitters are also used in flow and level applications, they are often integrated with other components, such as orifice plates for measuring flow or another pressure sensor for monitoring level in a tank. Three types of mountings are available to connect transmitters to components or the process: (1) in-line, where the transmitter is mounted directly to a single process penetration; and (2) coplanar and (3) biplanar, where the transmitter connects to the process via two connections.

An in-line mounted transmitter (see Figure 1) has a single connection, usually at the bottom of the unit, for measuring gage or absolute pressure. In-line mounted transmitters are usually lightweight and do not often require a mounting bracket.

A coplanar mounted transmitter (see Figure 2) has two process connections for differential pressure (DP) on the bottom of the unit. This transmitter is lightweight and is usually installed on a single process flange. The coplanar connection enables measurement of differential, absolute, or gage pressure-type applications.

A biplanar configuration is a more traditional way of connecting to the process (see Figure 3) and has two ports on the side of the lower part of the unit. This is the traditional process connection used for DP measurement, and it also supports gage and absolute pressure. It is heavier and more challenging to connect than in-line or coplanar designs. A transmitter used in a DP flow application can also be mounted directly to a flange containing an orifice plate.

Figure 3: A biplanar connection, with two ports on the side, supports gage, differential, and absolute pressure measurements. Courtesy: Emerson Process ManagementWhen considering a connection type, consider if there is an existing connection point or if it will it be necessary to create a new tap into the process. Either of these may require a process shutdown, which can be costly and potentially dangerous. It is also possible to hot tap a process and add a transmitter while running, but this requires highly trained personnel. Other things to consider regarding the connection are:

  • Can a flange be added to make the connection? If so, what type of flange is appropriate for the application?
  • What threading is present?
  • Is a manifold already installed?

All of the mounting considerations for a monitoring point also apply to a control point. But for control, a connection that is more maintainable is ideal. For example, if there is sediment buildup, it is important to be able to clean out and purge the connection point. By adding a three- or five-way manifold, or a bleedable flange, you can easily purge any buildup.

Remote seals

Figure 4: An electronic connection between two pressure transmitters eliminates the need for impulse lines that can clog with sediment or freeze in the winter. Courtesy: Emerson Process ManagementConnections to a pressure transmitter often involve impulse lines that run from the process tap to the transmitter. These can plug up with sediment or freeze in the winter. One solution to plugged impulse lines is a remote seal and capillary system, where process pressure is transmitted to the pressure sensor via an oil-filled capillary. The system acts as an extension of the pressure transmitter and protects its diaphragm from hot, cold, or corrosive processes, as well as from viscous materials or those containing suspended solids that might plug impulse piping. In hygienic applications, remote seals also make it easier to clean process connections and prevent contamination between batches, as well as avoid the maintenance often needed with wet-leg and dry-leg installations.

There are also electronic remote sensor arrangements for measuring tank level. Instead of having a single DP transmitter installed with impulse line connections to the bottom and top of the tank, two transmitters are used. One is located at the bottom of the tank, with the other one located at the top. The two sensors are connected electronically, instead of via a wet leg/dry leg or capillary (see Figure 4). This is useful for tall tanks because it eliminates the need for long impulse lines. A caveat to this method is that accuracy is impacted in tanks with high static pressures relative to the DP measurement for level. When considering such an arrangement, it is best to consult a factory expert to help select the best technology.

Learn more about environmental considerations for pressure transmitters as well as communication protocol choices.

Environmental considerations

Figure 5: A wireless instrumentation network makes it easy and economical to add data points across an organization. Courtesy: Emerson Process ManagementThe environment in which the pressure transmitter will be operating is important. Questions to ask include:

  • What is the operating temperature? If using a remote seal system, be sure to choose a fill fluid compatible with both the process temperature and the ambient temperature. At low temperatures, impulse lines can freeze, or the fill fluid can gel. At high temperatures, the fill oil in a remote seal system can boil or degrade. There are fill fluids specially formulated to meet a wide range of temperatures.
  • Will there be significant mechanical vibration? Transmitters should always be installed to minimize vibration, shock, and temperature fluctuations.
  • Does the process involve significant pressure pulsation? A transmitter installed at the discharge end of a positive-displacement pump can be damaged by rapid pulsations, degrading measurement accuracy and shortening sensor life. A pulsation damper or snubber may be required. It can be as simple as a porous metal filter or an adjustable needle valve inserted in the impulse piping.
  • Will the device be used in a hazardous area? If so, what approvals will be required? Relevant approvals can include ATEX, IECEx, CSA, and FM. Many devices are available with combination approvals that make them suitable for a variety of hazardous areas.
  • What about washdowns? In food, beverage, and pharmaceutical applications, the transmitter may be subjected to washdowns with hot and aggressive chemicals and must be rated for such service.

It’s also important to know how often the transmitter will need recalibration, and what will be involved in performing this task. It may require that the device be physically removed and sent to an instrument laboratory, which may require a process shutdown. If the transmitter can be isolated with shutoff valves, the disruption will be considerably reduced. In addition, some transmitters now offer longer stability specifications, thus reducing maintenance costs since they require fewer calibrations.

Choices of communication protocols

The simplest and most common way for a transmitter to deliver its output is analog, via a 4-20 mA current loop: 4 mA indicates the device’s lower range value, and 20 mA indicates its upper range value, which might, for example, indicate an empty and full tank, respectively. A current loop requires that each transmitter have wiring leading back to the control room (a distance that can reach a mile or more, in some facilities), plus its own input point in the control system, which can increase significantly the cost of adding measurement points to a process.

In the 1980s, the HART protocol became available. HART superimposes a low-level, digital signal on the 4-20 mA output allowing two-way communication between the control system and the transmitter at a speed of 1,200 bits/s. The digital signal contains information from the device including device status, diagnostics, measured or calculated values, and others. HART has the advantage of using the existing 4-20 mA field wiring, which makes it simple and inexpensive to set up.

An alternative wiring architecture uses a digital data bus interface such as EtherNet/IP, Foundation fieldbus, or Profibus PA. These bus protocols allow much more flexible installation, as it’s no longer required to wire each transmitter back to the controller since multiple transmitters can exist on one drop of wires. They also permit large amounts of information to be transmitted at relatively high rates of speed.

The wireless option

One way to reduce wiring costs substantially for transmitters is to skip all wired connections, analog or digital, and use a wireless system. Alternatively, wireless can be used to add capabilities to an existing wired system. While HART is an industry standard, of the 30 million wired installed HART instruments, fewer than 10% have remote access to secondary data. Technicians use HART to check and re-range transmitters via handheld units.

There are now wireless adapters that can be plugged into a HART-equipped 4-20 mA transmitter to make it part of a wireless network—thus providing configuration data, status information, calibration dates, and process data, including the main pressure value and other data, such as temperature.

While the first wireless networks in plants were used almost exclusively for asset monitoring purposes, the advent of IEC 62591 (WirelessHART) and other wireless instrumentation protocols made wireless delivery of process-variable information increasingly common for monitoring and control applications. This system takes the form of a self-organizing mesh network, with a gateway as its connection point to the plant’s main control system via a digital bus.

Wireless instrumentation network protocols use a variety of approaches to move data from the individual devices to a gateway. With WirelessHART, each transmitter (see Figure 5) constitutes a network node that acts as both a source of data and as a router. A message from one node is passed from node to node until it reaches the gateway, and a message from the gateway to a particular node can be similarly passed via multiple paths in the mesh until it reaches its intended recipient. This redundancy provides for reliable communication because data movement does not depend on a single path.

A wireless network makes it possible to expand a plant’s instrumentation without running any wiring at all. One simply connects the gateway to the control system. Wireless transmitters can be battery-powered or powered locally with hard-wiring, becoming nodes on the network.

Wally Baker is the global pressure content marketing manager for Emerson Process Management. Courtesy: Emerson Process ManagementWireless pressure transmitters make it possible to measure pressure, flow, and level in locations once deemed economically unfeasible. With this capability, engineers can consider using wireless transmitters for control, monitoring, or safety reasons just about anywhere in a process plant.

Given the complexity of device selection and deployment, don’t be afraid to ask for help from industry vendors. Many of the application engineers working for instrumentation manufacturers have years of industry experience and have dealt with a range of challenging measurement situations. They often can provide what amounts to free consulting services in terms of technology selection.

– Wally Baker is the global pressure content marketing manager for Emerson Process Management. Edited by Peter Welander, contributing content specialist, Control Engineering, pwelander@cfemedia.com.

Key concepts:

  • Pressure sensors are the most widely deployed industrial measuring devices.
  • There are countless configurations designed for specific applications.
  • Making an appropriate selection depends on understanding the needs of the process.

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