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Pressure-Sensing Line Problems and Solutions

H.M. Hashemian, Analysis and Measurement Services Corp. and Dr. Jin Jiang, The University of Western Ontario
Pages: 1234
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 gasfilled 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

The typical pressure-sensing system design uses a combination of isolation valves. Sensing Line Problems Sensing lines may encounter a number of problems that can affect the accuracy and response time of the pressure-sensing system. The condensate pot helps ensure that this assumption is satisfied by condensing the steam into water at a known point in . In the figure. Sensing line installations are usually designed to allow for the lines’ thermal expansion and vibration without deformation. the transmitter is calibrated with the assumption that the reference leg is filled with a water column of known height. For liquid-sensing lines. The example in Figure 3 shows how a reference leg boil-off can cause sensing line problems. or copper tubing in thicknesses of about 2 mm. a high-point vent must be provided for liquid-sensing lines and a low-point vent must be provided for gas-sensing lines. If the sensing line cannot be sloped. Source: Analysis and Measurement Services Corp. a differential-pressure transmitter is being used to measure the fluid level in a vessel containing water at the bottom and steam at the top. self-venting is accomplished by sloping the sensing line downward so that any gas or air in the line can vent to the process. depending on the application. to ensure drainage by gravity. 2.5 cm to 2 cm) stainless steel.Corp. Reference leg boil-off. attempts are often made to make sensing lines as short as possible. Isolation valves required. reducing the possibility of leaks. and to allow the lines to vent themselves. Sensing lines vary in length. carbon steel. Normally. Because the length of sensing lines affects the overall response time of a pressure-sensing system. and average 10 to 50 meters. The slope of a sensing line might be about 10 cm per meter. They can be as short as a few meters or as long as 200 or 300 meters. Sensing lines are typically made of small-diameter (on the order of 1. Tubing is preferred over piping because it can be installed in one piece. Discussions of the typical problems found in power plants follow.

the ambient temperature may increase. One remedy is to use isolation diaphragms or isolation bellows in the sensing lines (Figure 4). Isolated systems. Source: Analysis and Measurement Services Corp. 3. Steamy scene. Source: Analysis and Measurement Services Corp. This causes the level information to lose accuracy. More specifically. dissolved gases that accumulate over time during normal operation can rapidly . Level measurement instrumentation must include a condensate pot if steam is present. Level measurement problems can occur when noncondensable gases become dissolved in the reference leg of sensing lines. Liquid-level measurement requires isolation diaphragms in the sensing lines. Level measurement problems. 4. Experience has shown that the dissolved gases may reappear during a rapid depressurization of the process below a certain pressure. and pressure may decrease until it causes the water in the reference leg to flash to steam. During certain plant transients or accident conditions.the system.

freezing can occur in fluid sensing lines if the sensing line’s heat tracing. Purging air from voids is difficult. Pressure-sensing lines provide many opportunities for leakage to occur. is aged or damaged. Although sensing lines are usually designed to avoid these problems. leaks. one or more isolation valves. an equalizing valve. Common Sensing Lines Redundant pressure transmitters in some processes sometimes share a sensing line. the dynamic response times of a group of pressure transmitters that share a sensing line may be dominated by the response time of the most compliant transmitter on the common leg. which is used to prevent freezing of the fluid. In cold weather. Any significant leakage or loss of fluid in a sensing line can cause a false pressure indication. Air or gas entrapped in liquid-sensing lines can cause false pressure readings. the pressure information is totally lost. sluggish response. A sensing line may have a root valve. or void in the common leg.come out of solution and displace water from the reference leg. blockages. and freezing. especially under high working pressures. A partial blockage is detrimental only to the dynamic response time of the pressure-sensing system and does not normally affect the static output of the transmitter. For example. Noise from Sensing Lines . an air pocket on the lowpressure side can cause the pressure indication to be higher than normal. This reduces the reference leg level and results in an erroneously high level indication. in differential-pressure measurements. the problem of voids persists. and extraneous noise as a result of acoustic resonances.  Freezing. This could cause all transmitters on the common sensing line to be as slow as the most compliant transmitter. blockage. They also occur due to obstructions caused by isolation and equalizing valves that are improperly aligned or seated or due to sensing lines becoming crimped. blockages. Voids. This problem can go undetected if the freezing causes a normal operating pressure to be locked into the system. they still occur in industrial processes:  Voids. Blockages occur in sensing lines when the chemicals that are used to treat the water and sludge solidify or when other contaminants accumulate. leaks. or freezing in sensing lines can cause errors in pressure measurements and can also affect the dynamic response of the pressure-sensing system. Though one would expect air pockets to dissolve in the liquid under the high pressures common in industrial pressure measurements. Voids. The most compliant transmitter in most cases could be the slowest-responding transmitter. or other connections that can give rise to leaks. The problem with common sensing lines is that they can cause a common mode failure if there is a leak. It can also add a delay in the transmission of the pressure information.  Blockages. In addition to common mode problems.  Leakage. But when the blockage completely blocks the line.

mechanical snubbers are sometimes used in pressure-sensing lines. steam line resonances. acoustic resonances. One advantage of electronic filters is that they remove not only any mechanical or acoustic noise in the system but also any electrical noise. they increase the system’s response time. they do not protect the sensing element of the pressure transmitter from mechanical fatigue caused by the excessive high-frequency vibration that process pressure fluctuations impose. A water-filled sensing line of about 30 meters has about 21 milliseconds of sonic delay. The disadvantage of electronic filters is that. however. An alternative to snubbers is electronic low-pass filters with adjustable response times. Sensing Lines’ Effect on Pressure Transmitters’ Response Time The response time of a liquid-filled sensing line has two major components: a sonic delay and a hydraulic delay.Noise arises in sensing lines because of process fluctuations. The hydraulic delay in a sensing line depends mainly on the volume of fluid that must move in the sensing line in order to bring a pressure change from the process to the transmitter. Figure 5 shows a sensing line leading to a pressure transmitter that has a sensing element that must move a portion or all of the distance x in order to indicate the applied pressure. control system malfunctions. Another advantage is that they can be designed to have a precise roll-off frequency. vibration in the sensing line. and resonances caused by undissolved air pockets in liquid-filled sensing lines. The distance x traveled by the sensing element depends on the pressure transmitter’s design. like snubbers. Therefore. unlike snubbers. Snubbers reduce the effect of noise by increasing the dynamic response time of the pressure-sensing system. The sonic delay is also referred to as acoustic delay. they must be used cautiously in those cases where response time is important. The sonic delay corresponds to the time that it takes for the pressure signal to travel at the speed of sound through a completely filled (solid) sensing line from the process to the transmitter. These filters can provide any level of noise reduction. . To reduce the effect of noise.

however. The distance x was used to illustrate the relationship between sensing line delays and a pressure transmitter’s design characteristics. Transmitter compliance is a characteristic parameter of a transmitter that should be specified by the manufacturer. In some pressure transmitters. . a larger volume of fluid must move through the sensing line in order to indicate a given pressure change. Therefore. the response time of the overall pressure-sensing system from the process to the transmitter output is a strong function of the sensing line’s length and diameter. not the distance x. Table 1 shows the compliances of three different pressure transmitters. Transmitter compliance is determined by the physical installation of the component and its volume.5. The parameter that combines these two factors is the transmitter compliance. the longer the sensing line. In reality. the controlling factor in a sensing line’s hydraulic delay is the volume change inside the transmitter. Another controlling factor is the pressure that is required to induce the volume change. In other transmitters. the more time is required for the fluid to move the required distance and also overcome the additional resistance to flow. The movement of the sensing element requires a corresponding movement of the fluid in the sensing line. For the transmitter with a larger compliance. the sensing element is a bellows that must move an appreciable amount to indicate the applied pressure. such as some manufactured by Barton. such as some manufactured by Rosemount. the sensing element is a diaphragm that moves very little to indicate the applied pressure. Source: Analysis and Measurement Services Corp. For such transmitters. Easy calculation. which is defined as the ratio of the transmitter volume change to the pressure change that is required to attain the volume change.

. This figure is based on laboratory test data. Source: Analysis and Measurement Services Corp. Source: Analysis and Measurement Services Corp. This data was obtained in laboratory experiments in which a snubber (Figure 7) was used to simulate sensing line blockages for the tests. 6. It is understood that the snubber may not correctly simulate the effect of a real blockage in a pressure-sensing line. Examples of compliance values for representative pressure transmitters. Different responses. Figure 6 shows how the response times of representative pressure transmitters are increased as a function of sensing line blockages. As such. the response time of transmitters with larger compliances is more significantly affected by any void or obstruction in the sensing line. The response time differences between different transmitter designs with a sensing line blockage can be profound. Furthermore. the data in Figure 6 only serves as an illustration of the effect of sensing line blockage on transmitter response time.Table 1.

For example. depending on their compliance value. Figure 8 shows power spectral densities (PSDs) for a pressure-sensing system that was tested in a laboratory experimental setup with and without air in the sensing line. 8. Sensing an air leak. At this lower break frequency the PSD roll-off begins measuring that the transmitter’s response time is larger with the void in the system. Snubbers are used to simulate sensing line blockages. The power spectral density (PSD) for a pressure-sensing system can identify air leakage in a sensing line. Source: Analysis and Measurement Services Corp. Give a blockage the cold shoulder. It is obvious from the data in Figure 6 that different transmitters are affected differently by blockages. Source: Analysis and Measurement Services Corp. the response time of the Barton transmitter shown in Figure 6 increases by almost 200% when the blockage advances to near 65% of the diameter.7. The effect of the void in the sensing line is manifested by a resonance on the PSD and a lower break frequency. while the response time of the Rosemount transmitter increases by only about 10% for the same amount of blockage. .

the blockage in this case increased the transmitter’s response time by at least an order of magnitude. (See H. the response time of the transmitter and the attached sensing line was measured by the conventional ramp method and by the noise analysis technique. Table 2. NUREG/CR5851 [March 1993]). "Long-Term Performance and Aging Characteristics of Nuclear Plant Pressure Transmitters.Figure 9 compares two PSDs for a pressure transmitter tested in a power plant before and after the sensing line was cleared of a blockage. then with 30 meters of sensing line tubing. The power spectral density (PSD) from online testing of a Barton transmitter can identify a sensing line blockage. 9.S." U. Source: Analysis and Measurement Services Corp. the noise analysis technique identifies the response time of the transmitter and its sensing line with good accuracy and accounts for the effect of sensing line length and the blockage (simulated by the snubbers) on the response time. Validation of Noise Analysis Technique for Online Detection of Sensing Line Problems The validity of the noise analysis technique for online detection of sensing line blockages has been established by numerous laboratory and in-plant demonstration tests involving a variety of pressure transmitters. Representative results of validation of noise analysis technique for response time testing of pressure transmitters and associated sensing lines. Clearly. Line blockages also sensed. The noise was generated for this experiment in a laboratory test loop that was designed to simulate process fluctuations for research purposes. In each case. Nuclear Regulatory Commission.M. and finally with a snubber in the sensing line. As shown by these results. Hashemian. . Source: Analysis and Measurement Services Corp. Table 2 shows representative results of such tests that involved a Barton pressure transmitter. The transmitter was tested alone.

one of the main advantages of response time testing with the noise analysis technique is that its results will include the effects of sensing lines.M. or drain the sensing lines. Jin Jiang (jjiang@uwo. That is. In fact.ca) is a professor of electrical and computer engineering at the University of Western Ontario. This can be done by using the noise analysis technique for in-situ testing of pressure transmitters’ response times described earlier. . any response time result for pressure transmitters that is obtained by the noise analysis technique will inherently account for the length and diameter of sensing lines as well as for any blockages. back fill. Another solution is to test or monitor for the presence of voids or blockages in the sensing lines online. leaks. Hashemian (hash@ams-corp . Dr. —H.com) is president and CEO of Analysis and Measurement Services Corp. or freezing that may be present in the sensing lines.Solving Sensing-Line Problems Remedies that remove voids and/or blockages in sensing lines are to periodically blow down. voids.