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Principle and fault diagnosis of differential pressure transmitter

Jan 17, 2019


Principle and fault diagnosis of dp transmitter



1. Working principle of differential pressure transmitter:

The differential pressure from the double-sided pressure guiding tube acts directly on the double-sided isolating diaphragm of the transmitter sensor, and is transmitted to the measuring component through the sealing liquid in the diaphragm, and the measuring component converts the measured differential pressure signal into corresponding The electrical signal is transmitted to the converter, and is converted into a standard electrical signal output by amplification or the like.

dp transmitter

2. Several application measurement methods of differential pressure transmitter:

(1). In combination with the throttling element, the flow rate of the liquid is measured using the differential pressure value generated before and after the throttling element.

(2). Measure the height of the liquid using the pressure difference created by the gravity of the liquid itself.

(3). Directly measure the pressure difference between different pipelines and tank liquids.


3. Fault diagnosis and analysis in application

Transmitter During the measurement process, some faults often occur, and the timely determination and processing of faults is critical to the ongoing production. Based on the experience in routine maintenance, we summarize some judgment analysis methods and analysis processes.

1. Investigation method: Review the fire, smoke, odor, power supply change, lightning strike, moisture, misoperation, and mis-maintenance before the fault occurs.

2. Intuitive method: observe the external damage of the circuit, the leakage of the pressure guiding tube, the overheating of the circuit, the state of the power supply switch, etc.

3. Detection method:

1) Open circuit detection: separate the suspected fault part from other parts to see if the fault disappears. If it disappears, determine the fault. Otherwise, you can go to the next step. For example, the intelligent differential pressure transmitter cannot be normal Hart remote communication. The power supply can be disconnected from the watch body, and the transmitter can be powered by the on-site power supply to check whether the cable is superimposed with the electromagnetic signal of about 2 kHz to interfere with the communication.

2) Short-circuit detection: In the case of ensuring safety, the relevant part of the circuit is directly short-circuited. For example, if the output value of the differential transmitter is too small, the pressure-conducting tube can be disconnected, and the differential pressure signal is directly from the outside of the pressure-receiving valve. Directly to the differential pressure transmitter on both sides, observe the transmitter output to determine the plugging and leakage connectivity of the pilot line.

3) Replacement test: Replace the suspected faulty part and judge the fault location. For example, if the transmitter board is suspected to be faulty, you can temporarily replace it to determine the cause.

4) Division detection: divide the measurement loop into several parts, such as: power supply, signal output, signal transmission, signal detection, check by sub-section, from simple to complex, from the table to the inside, narrow the scope, find the fault position.

dp transmitter

4, Failure analysis of several typical measurement loops

Below I only use the pressure guiding tube fault as an example to analyze the differential voltage transmitter measuring circuit fault.

1. Congestion of the pressure guiding tube: In the maintenance of the instrument, due to the untimely discharge of the pressure regulating tube of the differential pressure transmitter, or the dirty or sticky medium, the positive and negative pressure guiding tube blockage is a common occurrence.

When the actual flow rate is reduced from F to F, the static pressure in the pipeline is correspondingly reduced, and the reduction value is P0. Meanwhile, when the actual flow rate drops to F, the P-value is also due to the fluid flow rate in the tube. Decrease and increase, set the rise value to P0'.

That is: △ P = (P + - P0) - (P - + P0 ') At this time the transmitter output value should be reduced.

2. Positive pressure tube leakage:

In fact, when the leakage is very small, it is difficult for the process operation or the instrument maintenance personnel to find out for various reasons. Only when the leakage amount is large, the measured flow rate has a large error compared with the actual flow rate. Even if the actual flow rate rises, it is always △ P after leakage << △ P before leakage, after F leakage <3. Balance valve leakage:

Let the pressure before the leak be P1, the pressure after the leak is P2, P1= P1+- P1- , F1 is the output value of the transmitter before the balance valve leaks, and F2 is the output value of the transmitter after the balance valve leaks.

We assume that the fluid flow in the pipeline is analyzed without change, and the pressure of the leak is PS.

Then: the static pressure of the positive and negative pressure guiding tubes after leakage is:

P2+= P1+-PS, P2-= P1-+ PS

P2= P2+- P2- = P1-2 PS,

According to the relationship between differential pressure and flow rate, F24. Transmitter measurement error under the condition of gas flow pressure guiding tube effusion:

Set the pressure of the positive pressure guiding pipe to P0+, the pressure of the negative pressure guiding pipe to P0-, and the pressure of the positive pressure of the differential pressure transmitter to P1+, transmitter> differential pressure transmitter The negative pressure is P1-.

P0= P0+- P0-

P1= P1+- P1-

Under normal measurement:

P0= P1

Set the flow rate under normal measurement state to F, then F=K

Here K is a constant coefficient.

If the density of the liquid water is ρ, then in the case where the height of the positive pressure tube effusion is h+ and the height of the negative pressure tube is h-:

P1+= P0++ρgh+

P1-= P0-+ρgh-

P1= P1+- P1-= (P0+) +(ρ h+) -(P-+ρ h-) = P+ρ (h+-h-)

Then the transmitter output is:

F=K

When h+>h-, the actual differential pressure measured by the transmitter increases and the output flow signal increases.

When h+ is here, due to the pressure-receiving method of the positive pressure conduit, as the time increases, h+ is gradually greater than h-, and the measured flow rate also increases.