Maintenance of differential pressure flowmeter

The working principle and structure of Differential Pressure Flowmeter

Differential pressure flowmeter measurement system as shown in the figure above, fluid flow in the pipeline, through the throttling device, due to the sudden reduction of flow area, the flow rate will inevitably produce local contraction, speed up the flow rate, according to the principle of conservation of energy, dynamic pressure and static pressure energy can be converted to each other under certain conditions, speed up the result of the static pressure energy will inevitably lead to decline. Therefore, a static pressure difference is generated between the upper and lower reaches of the throttling device, and the size of this static pressure difference is related to the flow rate of the fluid flowing through the pipe. By measuring the pressure difference before and after the throttling device through the Differential Pressure Transmitter, the measured flow rate can be known.

Differential pressure flowmeter fault inspection, judgment and treatment

The differential Pressure Transmitter is used in conjunction with the flow integrator DCS card, and the signal loop is shown in the figure below

In the figure, A is a conventional instrument circuit, powered by a power box or isolation grid, and B is a DCS circuit, powered by DCS card. The fault phenomena and inspection methods of the two circuits are the same. The fault inspection and treatment are introduced in combination with the differential pressure steam flowmeter measurement system diagram and differential pressure flowmeter measurement circuit diagram.

(1) Minimum or no traffic display

The flow display is minimal, in fact, there is no flow display value, that is, the differential Pressure transmitter has no current signal output.

The reasons are that the sampling valve is not opened, the pressure guide pipe is blocked, the condensate of the steam flow is not completely condensed, the blowdown valve is not closed, the power supply and connection cable of the differential pressure transmitter are faulty, and the components of the differential pressure transmitter are damaged.

Process reasons: There is really no fluid flow in the process pipe.

Flow display minimum or no display Check and handle as follows:

① Observe whether the display instrument and differential pressure transmitter head display value, the dial head indicates zero or the LCD dial head does not have any display, indicating that the differential pressure transmitter power supply is interrupted, check whether the power supply switch is closed, whether the fuse is blown, and whether the connecting cable is broken.

② Use a multimeter to measure the voltage of the 24VDC power supply box or the power supply voltage of the DCS card. If the power supply is normal, then measure the voltage between XS terminals 1 and 2, or the voltage between the wiring terminals of the differential pressure transmitter. If the voltage is 18-23V, it indicates that the connection between the line and the differential pressure transmitter is basically normal.

The measured voltage is "0V", which may be the measurement circuit is open, and there will be no current in the circuit; Or if there is a short circuit in the circuit, the current in the circuit will be large. You can disconnect any of the wires and string them into a multimeter to measure the current.

(3) If there is no current, check whether the signal wiring is loose or disconnected, whether the terminal is oxidized or corroded, and tighten the screw if necessary. Check the input and output current of the safety gate to determine whether the fault is in front of or after the safety gate, there is no current, it should be a differential pressure transmitter fault.

④ For the newly installed or replaced transmitter, check whether the range is correct and whether the parameters of the intelligent differential pressure transmitter are set correctly. If the range setting is large, the display of the meter may be small.

⑤ The process pipeline has flow, but the differential pressure transmitter only has a current signal of about 4mA, the flow output current is not up, there is no flow display on the display instrument, and the pressure pipeline can be checked.

The sampling valve is not opened, the blowdown valve or the balance valve is not closed, the balance valve is leaking or not tight, the pressure guide is blocked, and the high pressure chamber of the differential pressure transmitter is leaking. The smart transmitter can be checked with a hand operator to check whether the transmitter is set to a fixed output 4mA mode.

The differential pressure transmitter on the site can be checked by artificially increasing the differential pressure, quickly opening the blowdown valve of the negative pressure tube, and then quickly closing it, using sudden blowdown to reduce the pressure of the negative pressure tube, indirectly increasing the pressure difference of the positive and negative pressure guide tube for the above operation, the differential pressure transmitter head has increased changes, indicating that the transmitter can work.

(2) The flow rate is displayed below zero

Differential pressure flowmeter flow is displayed below zero, indicating that the current input to the flow integrator or clamp is less than 4mA, check whether the differential pressure transmitter can be zeroed, the zero position has no effect may have a problem with the differential pressure transmitter. The differential pressure transmitter can wither, and the differential pressure can be artificially increased to check whether the output current of the differential pressure transmitter can be increased.

Determine that the differential pressure transmitter is normal, but the display is still below zero, the current signal can be used to check the safety gate and isolator, determine whether it is normal, and find out the fault symptomatic treatment. There is no problem in the above check, and then check whether the current signal polarity is reversed; Whether the positive and negative pressure guides are connected incorrectly; Whether the positive pressure pipe is seriously leaking. Most of the problems can be found by careful observation and can be dealt with symptomatically.

(3) The traffic display is high

High flow is that the displayed flow is significantly more than the normal flow, that is, the output current of the differential pressure transmitter is high.

Process parameters deviate from the design conditions will also make the flow display high, such as saturated steam flow, when the working pressure is lower than the design pressure, the change in steam density will make the steam flow display high. The operating temperature of water flow is lower than the design temperature, and the change of water density will also make the flow display high.

If it is not the reason of the process, the flow integrator and differential pressure transmitter should be checked, and the zero position should be checked for deviation. For newly installed or replaced throttling devices or differential pressure transmitters, check whether the range Settings of the transmitters and DCS are correct, and whether the transmitters and throttling devices are compatible. It should also check whether the negative pressure pipe leaks, and the negative pressure pipe of the measurement liquid has gas, and should be discharged.

There is no flow in the process pipeline, and the flow integrator still has flow display or cumulative value, which is also a high flow display. First of all, it is necessary to determine whether the differential pressure transmitter or flow integrator has a problem, check whether the zero position of the instrument is correct, when the zero position is correct, the output of the intelligent transmitter can be set by hand as any current value, observe the flow display, there is an output and can be correctly displayed, then the transmitter and display instrument are normal, you can focus on checking the measuring pipeline of the instrument to determine the cause of the differential pressure aberration. Find out the cause and deal with it.

(4) The traffic display is low

The low flow rate is that the displayed flow rate is significantly less than the normal flow rate, that is, the output current of the differential pressure transmitter is low.

The reason of the instrument is the differential pressure measurement signal transmission distortion, the blockage and leakage of the pressure guide pipe, valve, isolator and condenser, and the liquid level of the positive and negative pipes of the isolator and condensate water are not equal, which will cause the differential pressure signal distortion and make the flow measurement error.

Process parameters deviate from the design conditions will also make the flow display low, such as superheated steam flow, when the operating temperature is lower than the design temperature, the change in steam density will make the flow display low. The operating temperature of water flow is higher than the design temperature, and the change of water density will make the flow display low.

Excluding the reasons of the process, the display instrument and the transmitter can be checked to check whether the zero position of the flow integrator or the differential pressure transmitter is deviated; For newly installed and replaced throttling devices or transmitters, check whether the range Settings of the transmitters and display instruments are correct, and whether the transmitters and throttling devices are compatible. You can focus on checking the measuring pipe, such as whether the positive pressure pipe leaks, and if the positive pressure pipe for measuring the liquid flow has gas, it should be discharged. The balance valve is not tightly closed or internal leakage will also make the flow display low, the differential pressure transmitter is working normally, while closing the high and low pressure valves of the three valve groups, so that the differential pressure transmitter maintains a fixed pressure difference, record the display value, after a period of time, observe the change of the display value, no change, the balance valve has no internal leakage, the display value drops, indicating that the balance valve internal leakage. There must be no leakage during the inspection, otherwise misjudgment will be made.

(5) Traffic display fluctuation

The flow parameter fluctuates frequently, which is normal fluctuation, different from the displayed value instability. During the check, switch the control system to manual and observe the fluctuation state. Manual flow fluctuations are still frequent, mostly due to the process. The fluctuation is reduced, which may be the cause of the instrument or improper PID parameter setting. Adjusting the damping time of the differential pressure transmitter can also reduce the flow fluctuation.

The flow fluctuates obviously, showing sometimes no, high and low, first check whether the screw of the terminal of the measurement loop is loose, oxidation, corrosion caused by poor contact. If you suspect that there is a problem with the differential pressure transmitter or flow integrator, you can disconnect the connection between the differential pressure transmitter and the flow integrator, close the three valve groups in the state of flow, so that the transmitter maintains a fixed differential pressure, observe whether the output current of the transmitter is stable, to judge whether the transmitter is faulty or input a fixed current value to the flow integrator, and observe whether it is stable. To judge whether the flow integrator is normal.

The failure rate of measuring pipelines or accessories is much higher than that of differential pressure transmitters. The reason for measuring pipes and accessories cannot be ignored. The liquid or steam flow should be checked whether there is gas in the pressure guide tube and transmitter measuring room, and the gas flow should be checked whether there is liquid in the pressure guide tube and transmitter measuring room, and the gas and liquid can be discharged through the blowdown valve, or discharged with the drain cock in the transmitter measuring room.

The heat tracing temperature of the pressure tube is too high or too low, so that the measured medium appears vaporization or condensation, which is also one of the reasons for the fluctuation of the flow display. There are impurities in the pressure guide pipe, and the dirt appears to be blocked and not blocked, which will also cause the display fluctuation. The solution is to blow down or flush the pressure guide pipe.

(6) Slow response of flow display

Flow display slow response, first drain and flush the pressure tube, and check whether the pressure tube and valve are blocked. If there is no problem, try to reduce the damping time of the differential pressure transmitter. If there is no change, the transmitter should be checked and treated.

(7) Maximum traffic display

If the newly installed or replaced instrument has the largest flow display, it should be checked whether the flow integrator is set correctly and whether the range of the differential pressure transmitter is consistent with the design. If there is no problem, go to the site to check whether the negative pressure tube has serious leakage.

The output current of the differential pressure transmitter exceeds 20mA, and the meter can be stopped to check the zero position. Most of the zero positions of the transmitter will be abnormal if it is faulty. The circuit board of the transmitter can be replaced to determine the fault.

The actual flow of the process exceeds the upper limit of the measuring range of the instrument, which is a common problem in production. Only redesign the orifice plate for replacement.