Differential Pressure Transmitter Fault Diagnosis and Analysis in Application
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2.1 Differential Pressure Transmitter Working Principle The differential pressure from the double sided pressure tube directly acts on the diaphragms of the transmitter's sensors on both sides of the diaphragm and is conducted to the measuring element through the sealing liquid in the diaphragm. The measuring element will measure the The differential pressure signal is converted to its corresponding electrical signal and passed to the converter. After amplification and other processing, it becomes the standard electrical signal output. Differential pressure transmitter several application measurement methods:
(1) In combination with a throttling element, the liquid flow rate is measured using differential pressure values ​​generated before and after the throttling element.
(2) Measure the height of the liquid using the pressure difference created by the liquid's own gravity.
(3) Directly measure the pressure difference of liquids in different pipes and tanks.
The installation of differential pressure transmitters includes the laying of pressure guiding tubes, the laying of electrical signal cables, and the installation of differential pressure transmitters.
2.2 Differential Pressure Transmitter Fault Diagnosis In the process of measurement, the transmitter often has some faults. The timely judgment and analysis of the faults are very important for the ongoing production. Based on our experience in routine maintenance, we summarized some judgment analysis methods and analysis procedures.
(1) Investigation method: Review the fire, smoke, odor, power supply changes, lightning, wetness, misoperation, and mis-maintenance prior to failure.
(2) Intuitive method: to observe the external damage of the circuit, the leakage of the pressure guiding tube, the overheating of the circuit, and the status of the power switch.
(3) Test method:
Disconnection detection: Separate the suspected faulty part from other parts to see if the fault disappears. If it disappears, determine the fault. If not, proceed to the next step. For example: The smart differential pressure transmitter can not communicate with Hart remotely. Disconnect the power from the meter body and use the on-site power supply method to power the transmitter for communication to see if the cable is overlaid with an electromagnetic signal of about 2 kHz to interfere with the communication.
Short-circuit detection: In the case of ensuring safety, the relevant part of the circuit is directly shorted, such as: the output value of the differential transmitter is too small, the pressure tube can be disconnected, and the differential pressure signal is directly induced from the pressure-receiving valve at a time. To the two sides of the differential pressure transmitter, observe the output of the transmitter to determine the plugging and leakage connectivity of the pressure-conducting pipeline.
Replacement detection: Replace the suspected defective part and determine the fault location. For example, if you suspect that the transmitter circuit board has failed, replace it temporarily to determine the cause.
Branch detection: The measurement loop is divided into several parts, such as: power supply, signal output, signal transmission, signal detection, according to sub-section inspection, from simple to complex, from the table and in, narrow the scope, to find the fault location.
3. Typical Fault Cases 3.1 The blockage of the pressure guiding pipe The blockage of the positive pressure guiding pipe is taken as an example to analyze the failure phenomenon of the pressure pipe blocking. In the maintenance of the instrument, due to the poor discharge of the pressure tube of the differential pressure transmitter, or the media is dirty or sticky, the positive and negative pressure tube clogging is easy to occur. The performance characteristics are: the output of the transmitter drops, rises or does not occur. change. When the flow increases, the effect on the output of the transmitter (the output signal of the transmitter itself):
Set the original flow rate to F1, P1=P1+-P1-, F'1=K, and F'1 is the transmitter output value before the change.
Let the increased flow rate be F2, (ie: F2F1), P2=P2+-P2-, F'2=K, and F'2 is the transmitter output value after the flow rate is increased. Because the positive pressure tube is blocked, when the actual flow rate is F1, F2 respectively, P1+=P2+; when the flow rate increases, P2- appears the following change: because the actual flow rate increases to F2, compared with the original flow rate F1, in the pipeline The static pressure also increases accordingly. The increase value is set to P0. At the same time, P2- decreases due to the increase of the fluid velocity in the pipeline. The decrease value is P0. The relationship between P2- and P1- is:
P2-=P1-+P0-P0?
Then: P2=P2+-P2-=P1+-(P1-+P0-P0?)=P1+(P0?-P0)
Then: F' now = K = K
P When P0=P0?: F'2=K=KF'2=F'1 The transmitter output does not change.
When P0P0?: F'2=K=KF'2F'1 The transmitter output becomes larger.
When the P0F'1 transmitter output becomes smaller.
The effect on the output of the transmitter (the output signal of the transmitter itself) when the flow rate is reduced.
Set the original flow to F1, P1=P1+-P1-, F'1=K, F'1 is the transmitter output value before change.
Let the reduced flow rate be F2, (ie, F2F1), P2=P2+-P2-, F'2=K, F'2 is the transmitter output value after the flow rate is reduced.
As the positive pressure pipe is blocked, when the actual flow rate is F1 and F1, respectively, P1+=P2+; when the actual flow rate decreases from F1 to F2, the static pressure in the pipeline is also reduced accordingly, and the reduction value is set to P0. When the actual flow rate drops to F2, the P2-value also increases due to the decrease in the flow rate of the fluid in the tube, and the increase value is set to P0'.
At this point, the relationship between P2- and P1- is:-
P2-=P1--P0+P0'
P2=P2+-P2-=P1+-(P1--P0+P0')=P1+(P0-P0')
F'2=K=K
∴ When P0=P0? Then: F'2=K=KF'2=F'2 The transmitter output does not change;
When P0P0? Then: F'2=K=KF'2F'1 The transmitter output becomes larger;
When P0
Under normal circumstances, the main reason for the blocking of the pressure guiding tube is mainly due to the measurement of irregular discharge of the pressure guiding tube or the measurement of viscous medium and particulate matter.
3.2 Leakage of pressure guiding pipe Leakage of positive pressure guiding pipe is used to analyze the failure phenomenon of leakage of pressure guiding pipe. As shown in Fig. 1, the flow measurement method of the purification air total pipeline used for the control instrument valve of a heating furnace of the Laiwu Iron and Steel Group Company is: throttle orifice + differential pressure transmitter. The flow rate of the wind when the device is in normal production is basically stable, but in the later production process, the flow of the wind is found to be much lower than the normal value.
After inspection, the configuration of the secondary instrument (DCS) and the circuit of the electrical signal are working properly. The transmitter sends a calibration room with a normal calibration. So the suspected problem arises in the pilot pressure. After inspection, the positive pressure guiding tube is not properly welded and causes leakage. To, after plugging after plugging, flow measurement returned to normal.