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How does a DP flow transmitter work?
Dec 11, 2018

How does a DP flow transmitter work?


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.


How does a DP flow transmitter work?

ⅰ.Measurement principle

When the pipe-filled fluid flows through the throttling device, the stream will contract at the throttle, causing the flow rate to increase and the static pressure to decrease, resulting in a pressure differential across the throttle. As the flow rate increases, the differential pressure also increases. Therefore, by measuring the differential pressure, the flow rate can be determined, and the meter measured by this principle is called a differential pressure type flowmeter.

ⅱ. the composition of the measurement 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 the secondary instrument passes the operation processing on the current signal. , showing the flow of fluid.

ⅲ.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 throttle 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 consumers and will also affect people's lives and property. . 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 ensures that the DCS system is safe and economical for the boiler. The differential pressure flowmeter is installed in the harsh working site. Due to the annual operation, the throttle device or the throttling device and the connected pressure guiding pipeline must undergo corresponding changes. Here, based on the experience of the metering work for many years, :

ⅳ.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/negative pressure side root shutoff valve is open, (2) Whether the balance valve is closed, and whether the positive pressure side valve, the blowdown valve, and the pressure guiding tube are leaking.

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 condensate level is inconsistent or the condensate is lost. If the orifice plate is bent, replace the orifice plate. Whether the surface of the throttle is dirty or rough must be cleaned.

5: The indication value is too high

Reasons for inspection: (1) Whether the valve or pipeline at the negative pressure side leaks, the valve should be closed 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

Reason for inspection: (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 pipeline, 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.

How does a DP flow transmitter work?

ⅴ.Transmitter verification

  1. Appearance of the transmitter (1) The nameplate of the transmitter should be complete and clear. 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 the movable parts 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.  Sealing of pressure transmitter The measuring part of the transmitter shall not leak or damage when it is subjected to the upper limit of measurement (differential pressure transmitter is rated working pressure).

  3. the basic error of the pressure transmitter

    The instrument has zero drift and large sensor error. Therefore, after the pressure transmitter is installed on site, it should be adjusted. The basic error of the transmitter should not exceed the allowable error.

The measurement error of the pressure transmitter is calculated according to the following formula. △A=Ad-As

△A——measurement error mAV or kpa of each test point of pressure transmitter

Ad——The actual output value of each checkpoint of the pressure transmitter on the stroke or down stroke, mAV or kpa

As——the theoretical output value of each test point of the pressure transmitter, mAV or kpa

4, choose the verification point

The selection of verification points shall be basically evenly distributed according to the range, generally including not less than 5 points including the upper limit value and the lower limit value. 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 smoothly change the pressure signal to each verification point in reverse, and read and record the output value to 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 briefly introduce how to adjust the instrument produced by Wei'an factory. (1) When the zero point is out of tolerance, you need to adjust the zero point. When the zero point of the range is 0Mpa, hold down the DOWN button for about 5 seconds to release the display “GOOD”. When the span zero is not 0Mpa, press ENTER for 5 seconds to enter the menu, enter SUB6, enter the password 040820, then press to return, press DOWN and MOVE for 5 seconds, and 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, 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 should be back to zero in time to see if the zero is within the allowable error.

ⅵ.the conclusion

In the daily production and operation of heating enterprises, there are many projects that need to be measured. There are weighing instruments, heat meters, water meters, electricity meters, and various flow meters for trade settlement. The accuracy of these measurement data is to ensure safety. The important technical basis for economic production is the basic guarantee for strengthening the operation and management of heating enterprises and reducing costs. Therefore, it is very necessary to do a good job in the measurement management of heating companies. Orifice flowmeters are still widely used in the heating industry, especially in heat source plants, due to their unique stability. Therefore, the selection of the orifice flowmeter, the maintenance in use, and the verification of the subsequent supporting transmitters are still in place.