Analysis and corresponding solutions for the problems that easily occur in differential pressure orifice flowmeters
Differential pressure orifice flowmeters are widely used in trade settlement due to their design specifications, simplicity, firmness, and good consistency. They do not require real-flow calibration. However, in actual use, this problem often occurs, such as "empty Problems such as “jumping”, over-counting, under-counting, and correction of the impulse tube height, causing unnecessary losses to both the supplier and the user. This article focuses on the “empty jump” phenomenon of differential pressure (orifice) flowmeters and the use of differential pressure. Supervisory methods and preventive measures for some abnormal phenomena when measuring with flow meter. The following is an example analysis of the causes of these problems and proposes solutions.
First, the compressed air flow meter is based on the Bernoulli equation and the flow continuity equation. When the fluid flows through the throttle (or sensor), a differential pressure is generated on both sides of the flow. Directly proportional. Combine the differential pressure transmitter with the flow display instrument to check the zero output. If there is a deviation in the zero position, the possible reasons are as follows.
(1) Static pressure error of differential pressure transmitter.
According to JJG640-1994 compressed air flowmeter verification regulations: one-way static pressure test: add the nominal pressure in the positive pressure chamber, maintain the pressure for 5 minutes and then release the pressure. After 10 minutes, measure the basic error and the return error, and then use the same method for negative pressure Carry out the same test; two-way static pressure test: add 25% of the nominal pressure in the positive and negative pressure chambers at the same time, measure the change of the lower limit value of the output after stabilization, and then increase the pressure to the nominal pressure for the same test. In practice, static pressure is often used. This method is often recommended in products. The specific operation method is: after the verification is completed, first open the balance valve, then open the positive pressure valve, and send the working pressure of the positive and negative pressure chambers to overcome the static pressure effect by adjusting the zero point.
Example: A steam user, the site installation conditions meet the requirements, but the main valve is before the throttling device. After the differential pressure transmitter is calibrated, open the balance valve first, then open the positive pressure valve, and send the positive and negative pressure chambers to a working pressure of 0.65MPa. At this time, the zero point of the differential pressure transmitter is 3.915mA, and the zero point of the differential pressure transmitter is adjusted. Move to the checkout value of 3.960mA, after completion, put the differential pressure transmitter into operation. When the user stops the steam for a period of time, the flow totalizer goes off.
The solutions are as follows:
1. Those who have the conditions shall, as far as possible, use the static pressure value specified in the regulations for static pressure test.
2. It is also possible to increase the working static pressure when inconvenient. Adjust the static pressure screw by adding working pressure to overcome the effect of static pressure. After the static pressure screw is adjusted, use an Allen wrench to loosen the C-shaped reed of the pressure chamber of the differential pressure transmitter. The C-shaped reed and the lead-out shaft are loosened, so that the diaphragm is naturally relaxed, and then tightened again to re-check.
3. There is another reason for this example: the main valve is installed before the throttle device. When the steam is stopped, the pressure in the pipeline is gradually reduced due to the gradual condensation of steam into water, so that the external conditions of working static pressure have changed. If the main valve is installed after the throttling device, although the condensate will also appear in the pipeline after the main valve is closed, the pressure in the pipeline will be the same as the main pipeline from beginning to end, and it is completely feasible to overcome the static pressure by adjusting the zero point.
4. For users who continuously use gas and the pressure is relatively stable, it is also possible to overcome the static pressure by adjusting the working pressure to zero.
(II) Zero point shift caused by the installation position of the differential pressure transmitter deviating from the correct position.
It means that the zero point of the differential pressure transmitter drifts upward after a period of use. When the zero output current of a 0.5-level differential pressure transmitter exceeds 4.08mA, the pressure transmitter itself does not meet the JJG640-1994 compressed air flowmeter verification. Regulations and requirements of JJG882-2004 pressure transmitter verification procedures.
Solution: First select a differential pressure transmitter with good stability, and then shorten the verification period. During the third verification period, check frequently, that is, open the balance valve to observe whether the flow totalizer has “empty jump”. "Please ask the metrology and verification department to perform the verification again.
(3) The liquid column of the positive and negative pressure guiding tubes is unbalanced.
Refers to the "empty jump" phenomenon on the flow totalizer caused by the imbalance of the liquid column in the positive and negative pressure side pressure tube of the compressed air flow meter. Usually refers to the measurement of water vapor, water and other liquids.
There are usually the following situations:
1. If the user stops using steam, the liquid column in the positive and negative sides of the impulse can be unbalanced. If the positive pressure side impulse liquid column is higher than the negative pressure side liquid column, it will not appear on the flow totalizer when using steam. The flow display appears. The solution is that when the user does not use steam for a long time, please provide both parties to open the balance valve until the flow totalizer shows that the flow is zero. When the gas user uses the steam again, both parties should close the balance valve and input use. If the flow totalizer shows that the flow is not zero, the two parties will ask the verification department to perform the verification again after confirmation.
2. During the verification process, more water is wasted, which causes the liquid column on the positive pressure side to be higher than the liquid column on the negative pressure side. The solution is to open the balance valve and allow the two liquid columns to balance, and observe that the difference becomes zero until it meets the requirements. , And then quickly close the balancing valve. Otherwise, it may take several hours for the two liquid columns to equilibrate. Similarly, if the negative pressure side is higher than the positive pressure side, the output value will be reduced. After the verification is completed, it is necessary to open the balance valve to balance the positive and negative liquid columns.
3. Leakage on the negative pressure side, leakage of the drain valve. The solution is to replace the pressure guiding pipe and the sewage valve.
4. There is an error on the empty pack. The solution is to redesign the installation.
(D) the impact of weather on the compressed air flow meter.
Due to the weather, the differential pressure transmitter or pressure transmitter may freeze, resulting in damage to the differential pressure transmitter or pressure transmitter. The solution is to accompany a heat pipe in the pipeline that introduces the pressure guiding tube, and try to install the differential pressure transmitter and pressure transmitter indoors. If the weather is cold, compare with the past. Contact the verification department.
2. In the steamed soup trade settlement, the indicator value of the flowmeter is directly related to the trade value. Some thermal users try to reduce the indicator value of the flowmeter in order to achieve the purpose of reducing the payment of steam. The problem is getting more serious, which directly causes a series of abnormal phenomena. This article monitors some abnormal phenomena when using differential pressure flow meters and measures to prevent them:
1. The leakage of the high-pressure side pressure-conducting pipe is artificially caused to reduce the differential pressure, thereby reducing the flowmeter display value. Specific approach: Welding at the high-pressure pipe, the connection of the three valve groups, the sewage valve and other places caused leakage. This method is difficult to detect on the monitoring system, unless the amount of "stealing steam" is too large to be found. It is easy to find in the winter through surprise field inspections, but difficult to find in summer.
2. The balance valve in the three-valve group is artificially opened, which causes the differential pressure to decrease, thereby reducing the indication of the flow meter. This method is difficult to detect on the monitoring system, unless the "stealing steam" is too large to find out, because the steam does not leak outwards, it is also difficult to find it by surprise on-site inspection. Locking the three valve block in the box is a common precaution.
3. Artificially open the high-pressure side drain / exhaust valve of the differential pressure transmitter, causing the differential pressure to decrease. The phenomenon and characteristics are the same as 1. Locking the transmitter in the box is a common method.
4. Modify the zero point or range of the differential pressure transmitter, and increase the zero cutoff, so that the small flow meter is cut off. Increasing the range directly results in a smaller flow display value. These adjustments can be adjusted directly on the transmitter. A small screwdriver can be used for this; and for intelligent differential pressure transmitters, it can be more convenient to connect the handheld programmer anywhere on the power supply line of the differential pressure transmitter and adjust it arbitrarily. Transmitter is the most common device in industrial instrumentation. Most people who have a slight understanding of the instrument can adjust it. Unless you forget to recall it when checking the meter, it is difficult to find. The adjustment amount is not too much and it is difficult to monitor the system. It was found that on-site inspections are also difficult to find. It is a common precautionary measure to lock the differential pressure transmitter together with its connection with the secondary meter in the box.
5. Make the pressure channel of the pressure transmitter leak, causing the pressure indication to decrease, thereby reducing the density indication, causing the flow meter to be low, which can be found on site inspection.
6. A small resistor is connected in series on the temperature measuring thermal resistance line, which causes the temperature indication value to increase, thereby reducing the density indication value, and causing the flow to be low. This method is difficult to find unless the resistance in the series is too large and too much. . "Stealing Steam"
7. Modify the parameters in the pressure transmitter in the same way as the differential pressure transmitter, so that the pressure indication value is reduced, thereby reducing the density indication value, resulting in a low flow rate. The phenomenon characteristics are the same as the differential pressure transmitter.
8. Intentional use of gas in the case of over-range and small flow, using the small range of the differential pressure flow meter to make the large flow small due to the inherent output signal limitation of the differential pressure transmitter when the range is exceeded. The more the range is, the smaller the indication is. This method of using gas intentionally avoiding the normal measurement range of the compressed air flow meter is a * "safe" method of "stealing steam", but through careful analysis of the monitoring data Can be noticed.
9. Disconnect the power supply of the differential pressure transmitter to make the flow output to zero, causing the illusion of no gas. This method can be found if there is a 4-20mA abnormal alarm function on the monitoring system.
10. Reduce the horizontal height of the high-pressure pipe, and use the difference between the liquid level of the condensate in the high / low pressure-conducting pipe to reduce the differential pressure, so that the flow indicator is small. This method has a small amount of "stealing steam".
11. Add a pressure regulator (relay) to the power supply of the flow totalizer to the differential pressure transmitter to make it not work or work at intervals. This method is difficult to detect on the meter. This method Can be detected on the monitoring system, observe its pressure and temperature changes, you can see whether steam is being used.
Compressed air flow meters are of immediate interest to both the supplier and the user, and sometimes cause intense conflicts. Strict supervision and prevention of abnormal situations should require the personnel of the heat management office to strictly supervise, strengthen inspections, regular inspections, reasonable settings, daily analysis, and eliminate loopholes. The key is to establish and implement the post responsibility system of instrumentation personnel and hot network management personnel, adhere to strict supervision and resolutely prevent the occurrence of abnormal phenomena.