Improper pressure-sensing line design or installation is often found to be the cause of poor sensing system accuracy and response time. Here’s how to identify and solve those pesky pressure sensor problems in short order.
Sensing lines (also referred to as impulse lines) are used to enable the location of pressure transmitters away from the process being measured so as to reduce the temperature effects on the transmitter’s performance and operating life. High ambient temperatures can affect a transmitter’s mechanical components and also shorten the life of its solid-state electronics. Locating a transmitter away from the process can also reduce the adverse effects of vibration and facilitate access to the transmitter for replacement or maintenance.
Figures 1 and 2 illustrate two different views of sensing lines. As these figures show, sensing lines connect a pressure transmitter to the process. Depending on the application, there may be one or two sensing lines for each transmitter. Both liquid-filled and gas-filled sensing lines are used in industrial processes. Liquid-sensing lines typically contain the process liquid or oil, depending on the sensing line’s design and application. Gas-sensing lines typically contain steam, air, nitrogen, or other gases, and there is sometimes a transition in sensing lines to another medium, such as oil or water. A diaphragm, bellows, or condensate pot is used in the sensing line for the transition from one medium to another.
1. Get it right the first time. Example of a proper pressure transmitter installation. Source: Analysis and Measurement Services Corp.
2. Isolation valves required. The typical pressure-sensing system design uses a combination of isolation valves. Source: Analysis and Measurement Services Corp.
Sensing lines are typically made of small-diameter (on the order of 1.5 cm to 2 cm) stainless steel, carbon steel, or copper tubing in thicknesses of about 2 mm. Tubing is preferred over piping because it can be installed in one piece, reducing the possibility of leaks.
Sensing lines vary in length, depending on the application. They can be as short as a few meters or as long as 200 or 300 meters, and average 10 to 50 meters. Because the length of sensing lines affects the overall response time of a pressure-sensing system, attempts are often made to make sensing lines as short as possible.
Sensing line installations are usually designed to allow for the lines’ thermal expansion and vibration without deformation, to ensure drainage by gravity, and to allow the lines to vent themselves. For liquid-sensing lines, self-venting is accomplished by sloping the sensing line downward so that any gas or air in the line can vent to the process. The slope of a sensing line might be about 10 cm per meter. If the sensing line cannot be sloped, a high-point vent must be provided for liquid-sensing lines and a low-point vent must be provided for gas-sensing lines.
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