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.
Many 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
Because 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.
When 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.
Connections 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.
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