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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

Isolation valves required. self-venting is accomplished by sloping the sensing line downward so that any gas or air in the line can vent to the process. Sensing lines vary in length. reducing the possibility of leaks. The condensate pot helps ensure that this assumption is satisfied by condensing the steam into water at a known point in . Sensing lines are typically made of small-diameter (on the order of 1. or copper tubing in thicknesses of about 2 mm.5 cm to 2 cm) stainless steel. 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. Reference leg boil-off. Discussions of the typical problems found in power plants follow. and to allow the lines to vent themselves. In the figure. attempts are often made to make sensing lines as short as possible. Sensing Line Problems Sensing lines may encounter a number of problems that can affect the accuracy and response time of the pressure-sensing system. Sensing line installations are usually designed to allow for the lines’ thermal expansion and vibration without deformation. depending on the application. The example in Figure 3 shows how a reference leg boil-off can cause sensing line problems. to ensure drainage by gravity. They can be as short as a few meters or as long as 200 or 300 meters.Corp. 2. The slope of a sensing line might be about 10 cm per meter. Source: Analysis and Measurement Services Corp. Tubing is preferred over piping because it can be installed in one piece. If the sensing line cannot be sloped. and average 10 to 50 meters. a high-point vent must be provided for liquid-sensing lines and a low-point vent must be provided for gas-sensing lines. Because the length of sensing lines affects the overall response time of a pressure-sensing system. Normally. the transmitter is calibrated with the assumption that the reference leg is filled with a water column of known height. carbon steel. For liquid-sensing lines. The typical pressure-sensing system design uses a combination of isolation valves.

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

This could cause all transmitters on the common sensing line to be as slow as the most compliant transmitter. they still occur in industrial processes:  Voids. especially under high working pressures. or void in the common leg. blockages. blockages. Purging air from voids is difficult. A sensing line may have a root valve. Common Sensing Lines Redundant pressure transmitters in some processes sometimes share a sensing line. In addition to common mode problems. is aged or damaged. In cold weather. For example. But when the blockage completely blocks the line. The most compliant transmitter in most cases could be the slowest-responding transmitter. This problem can go undetected if the freezing causes a normal operating pressure to be locked into the system. leaks. Voids. Blockages occur in sensing lines when the chemicals that are used to treat the water and sludge solidify or when other contaminants accumulate. Voids. Although sensing lines are usually designed to avoid these problems. It can also add a delay in the transmission of the pressure information. Pressure-sensing lines provide many opportunities for leakage to occur. or freezing in sensing lines can cause errors in pressure measurements and can also affect the dynamic response of the pressure-sensing system. an air pocket on the lowpressure side can cause the pressure indication to be higher than normal. or other connections that can give rise to leaks. 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. This reduces the reference leg level and results in an erroneously high level indication. and extraneous noise as a result of acoustic resonances. Noise from Sensing Lines . leaks.  Freezing. The problem with common sensing lines is that they can cause a common mode failure if there is a leak. in differential-pressure measurements. an equalizing valve.  Leakage.come out of solution and displace water from the reference leg. one or more isolation valves.  Blockages. Though one would expect air pockets to dissolve in the liquid under the high pressures common in industrial pressure measurements. sluggish response. Air or gas entrapped in liquid-sensing lines can cause false pressure readings. blockage. the pressure information is totally lost. and freezing. 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. which is used to prevent freezing of the fluid. freezing can occur in fluid sensing lines if the sensing line’s heat tracing. 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. the problem of voids persists. Any significant leakage or loss of fluid in a sensing line can cause a false pressure indication.

To reduce the effect of noise. The sonic delay is also referred to as acoustic delay. Snubbers reduce the effect of noise by increasing the dynamic response time of the pressure-sensing system. An alternative to snubbers is electronic low-pass filters with adjustable response times. mechanical snubbers are sometimes used in pressure-sensing lines. One advantage of electronic filters is that they remove not only any mechanical or acoustic noise in the system but also any electrical noise. The distance x traveled by the sensing element depends on the pressure transmitter’s design. unlike snubbers. . and resonances caused by undissolved air pockets in liquid-filled sensing lines. control system malfunctions. Therefore. 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. acoustic resonances. they increase the system’s response time. however.Noise arises in sensing lines because of process fluctuations. A water-filled sensing line of about 30 meters has about 21 milliseconds of sonic delay. 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. The disadvantage of electronic filters is that. steam line resonances. 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. like snubbers. 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. vibration in the sensing line. Another advantage is that they can be designed to have a precise roll-off frequency.

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

Table 1. the data in Figure 6 only serves as an illustration of the effect of sensing line blockage on transmitter response time. As such. Examples of compliance values for representative pressure transmitters. Source: Analysis and Measurement Services Corp. This figure is based on laboratory test data. . Source: Analysis and Measurement Services Corp. It is understood that the snubber may not correctly simulate the effect of a real blockage in a pressure-sensing line. Figure 6 shows how the response times of representative pressure transmitters are increased as a function of sensing line blockages. Different responses. This data was obtained in laboratory experiments in which a snubber (Figure 7) was used to simulate sensing line blockages for the tests. 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. 6.

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. For example. The effect of the void in the sensing line is manifested by a resonance on the PSD and a lower break frequency. It is obvious from the data in Figure 6 that different transmitters are affected differently by blockages. depending on their compliance value. . 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. Give a blockage the cold shoulder. Source: Analysis and Measurement Services Corp. Source: Analysis and Measurement Services Corp. while the response time of the Rosemount transmitter increases by only about 10% for the same amount of blockage. 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. Sensing an air leak.7. The power spectral density (PSD) for a pressure-sensing system can identify air leakage in a sensing line. Snubbers are used to simulate sensing line blockages. 8.

Representative results of validation of noise analysis technique for response time testing of pressure transmitters and associated sensing lines. then with 30 meters of sensing line tubing. . NUREG/CR5851 [March 1993]). "Long-Term Performance and Aging Characteristics of Nuclear Plant Pressure Transmitters. 9. 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. Hashemian. 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.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. In each case.M. the blockage in this case increased the transmitter’s response time by at least an order of magnitude. Nuclear Regulatory Commission. Clearly. and finally with a snubber in the sensing line. Source: Analysis and Measurement Services Corp. Table 2.S. The noise was generated for this experiment in a laboratory test loop that was designed to simulate process fluctuations for research purposes. As shown by these results. The power spectral density (PSD) from online testing of a Barton transmitter can identify a sensing line blockage. Line blockages also sensed. the response time of the transmitter and the attached sensing line was measured by the conventional ramp method and by the noise analysis technique. Source: Analysis and Measurement Services Corp." U. (See H. The transmitter was tested alone. Table 2 shows representative results of such tests that involved a Barton pressure transmitter.

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