Application of differential pressure flowmeter in industrial flow measurement and inspection

At present, with the development of society, the demand for automation instruments in the industry, the measurement level is constantly improving, and the flow measurement instrument has become an indispensable thermal measurement instrument in the production of industrial enterprises. All raw materials and products of liquid and gaseous medium. Accurate metering is required, including water, steam, and air flow in the thermal system.

The advanced digital technology such as intelligent digitalization of flow measurement, the development of science and technology directly promotes the development of the instrument field, and the various needs of the use of instruments in the practice of measurement activities also promote the scientific and technological progress of the instrument industry. The instrument is an important part of the boiler operation control. Everyone knows that it is the eye of the boiler. The good and complete instrument is of great significance to improve the efficiency of the boiler and save fuel to ensure the safe and economic operation of the boiler. The working principle of the pressure transmitter (differential pressure flow transmitter) is introduced, and the frequently occurring faults and treatment methods and the verification of the pressure transmitter are analyzed.

The differential pressure flowmeter is a meter that measures the flow rate based on the differential pressure generated by the flow detecting member installed in the pipe, the fluid condition of the test piece and the geometrical dimensions of the detecting member and the pipe. The differential pressure flowmeter is composed of a throttling device, an upstream and downstream straight pipe section and a differential pressure flow meter.

Measuring principle: When the fluid filled with the pipe passes through the throttling device, the flow will contract at the throttle, the flow rate will be increased, and the static pressure will be lowered, resulting in a pressure difference before and after the throttle. As the flow rate increases, the differential pressure also increases. In contrast, by measuring the differential pressure, the flow rate can be determined, and the meter measured by this principle is called a differential pressure type flow meter.

The composition of the measuring system: the measuring system consists of a throttling device, a pressure guiding conduit, a three-valve group, a differential pressure transmitter and a secondary instrument. The throttling device converts the flow velocity of the fluid in the conduit into a differential pressure signal and sends it to the differential pressure transmitter. The differential pressure transmitter converts the differential pressure signal into a differential pressure standard current signal, and finally the secondary instrument passes the operation on the current signal. Processing shows the flow of fluid.

Differential pressure transmitter and secondary instrument

The differential pressure transmitter is an important part of the differential pressure flowmeter, which converts the differential pressure signal of the throttling device into a current signal to facilitate secondary instrument processing and calculation.

Proper use of flow meters to ensure the accuracy of meter values ​​plays an important role in saving energy and improving economic efficiency. If meter measurement is inaccurate, it will directly affect the interests of the state and consumers, and will also affect people's lives and property. Safety. As far as our heating department is concerned, the boiler is our main heat source. The economical and stable operation of the boiler directly affects the efficiency and safety of the enterprise. We use the advanced DCS system as the core of the boiler control system, and the differential pressure flowmeter is accurate. The data output can ensure that the DCS system is safe and economical for the boiler. The differential pressure flowmeter is installed in a poor working environment. Due to the annual operation, the throttle device or the throttling device and the connected pilot pipe must be changed accordingly.

Here I analyze the experience based on my own years of measurement work:

Frequently occurring faults in differential pressure transmitters:

1: No output signal after installation;

Reasons for inspection: (1) It may not be energized, (2) The power cable and signal cable are connected correctly, and the incorrect operation of the instrument causes the isolation liquid or condensate to be washed away.

2: The output signal is zero;

Reasons for inspection: (1) Whether the positive and negative pressure side root shutoff valve is open, (2) Whether the balance valve is closed, and finally whether the positive pressure side valve, the blowdown valve, and the pressure guiding tube leak.

3: The output signal is full;

Reason for inspection: Check the flow overload and reduce the flow. Whether the negative pressure shutoff valve is open and open the shutoff valve. Whether the negative pressure side valve, the blowdown valve, and the pressure guiding pipe leak repair or replace at the leak point.

4: The output value does not match the flow value;

Reason for inspection: The spacer is not evenly or drained and refilled. The liquid level of the cold suspect is inconsistent or the condensate is lost. If the orifice plate is bent, the orifice plate should be replaced. Whether the surface of the throttle is dirty or rough must be cleaned.

5: The indication value is too high;

Check the reason:

(1) Whether the negative pressure side valve and pipeline are leaking, it is necessary to close the valve in time to handle the leak.

(2) There is gas on the negative pressure side, or the positive pressure pipeline point is higher than the negative pressure pipeline, causing additional error, rearranging the pipeline, and discharging the gas in the negative pressure pipe.

(3) The pipeline on the negative pressure side is blocked, and the valve is closed.

(4) The negative pressure side stop valve is not fully opened, open in time

6: The indication value is low;

Check the reason:

(1) The balance valve is not closed, and it is closed in time.

(2) There are leak points in the balance valve and positive pressure side pipeline, and it is necessary to repair or replace it in time.

(3) There is gas on the positive pressure side, or the pipeline on the positive pressure side is lower than the negative pressure tube, causing additional error

(4) The positive pressure side stop valve is not fully opened, and the shut-off valve is opened in time.

(5) The right angle edge of the inlet of the orifice plate becomes dull, the damage is serious, and the new orifice plate is processed or replaced in time.

Transmitter verification:

1. The appearance of the transmitter;

(1) The nameplate of the transmitter should be complete and clear, and the main technical indicators such as product name, model, specification and measuring range should be indicated. The high and low pressure chambers should be clearly marked. It should also indicate the name or trademark of the manufacturer, the factory number, and the date of manufacture.

(2) The components of the transmitter should be intact, the fasteners should not be loose or damaged, and some should be flexible and reliable.

(3) The surface of the outer casing and parts of the newly manufactured transmitter shall be smooth, intact, free of rust and mildew. There must be no debris such as chips or residues inside. Transmitters that are used and repaired are not allowed to have defects that affect usage and metering performance.

2. The measuring part of the pressure transmitter's sealed transmitter is subject to the upper limit pressure (the differential pressure transmitter is the rated working pressure), and there must be no leakage or damage. 3. The basic error of the pressure transmitter has The zero drift and the sensor error are large. Therefore, after the pressure transmitter is installed on site, the calibration should be performed. The basic error of the transmitter should not exceed the allowable error.

3. The measurement error of the pressure transmitter is calculated as follows. â–³A=Ad-As

In the formula, the measurement error mAV or kpa of each verification point of â–³A-pressure transmitter

The actual output value of each test point of the upper or lower stroke of the Ad pressure transmitter, mAV or kpa

Theoretical output value of each test point of AS-pressure transmitter, mAV or kpa

4, choose verification account

The selection of verification points should be basically evenly distributed according to the range. Generally, it includes not less than 5 points including the upper limit and the lower limit. Pressure transmitters superior to 0.1 and 0.05 should be no less than 9 points.

5. Identification method:

From the lower limit value of the smooth input pressure signal to each verification point, read and record the output value up to the upper limit; then, the pressure signal is smoothly changed to the respective verification points in reverse, and the output value is read and recorded until the lower limit, which is one cycle. This is done in two cycles.

6. The calibration of the conventional differential pressure transmitter and the intelligent differential pressure transmitter is different, because the smart transmitter has a micro-processing chip between the input pressure source and the generated 4-20 mA current signal, in addition to the mechanical and electrical circuits. The operation of input data. Therefore, the adjustment is different from the conventional method. Here I will briefly introduce how to adjust the instruments produced by the company's manufacturers:

(1) When the zero point is out of tolerance, you need to adjust the zero point. When the range zero point is 0Mpa, press and hold the DOWN button for about 5 seconds to release the display “GOOD”. When the range zero point is not 0MPa, press ENTER5 seconds to enter the menu, enter SUB6, input Password 040820, then press Back, and finally press DOWN and MOVE for 5 seconds at the same time, display "00.

(2) When the pressure is full, it needs to be full. Press ENTER for 5 seconds to enter the menu, press to enter SUB6, enter the password 040821, then press to return, and finally press UP5 seconds to display GOOD. Zero adjustment has almost no effect on fullness, but it has an effect on zero when adjusting the fullness. It can be seen that the fullness of the adjustment should be back to zero in time to see if the zero is within the allowable error.

In the process of measuring differential pressure transmitters, there are many items that need to be measured, and various flow meters, etc. The accuracy of these measurement data is an important technical basis for ensuring safety and economic production, and is the management and reduction of enterprises. The basic guarantee of cost. Therefore, it is very necessary to do a good job in measurement management. It is still very important to do a good job in the selection, maintenance in use and verification of subsequent supporting transmitters.