A brief discussion on instrument technology in DCS industrial control system

In order to ensure the operation quality of the DCS system, the instrument signals input into the system must be accurate. Therefore, the installation must be carried out in strict accordance with the GBJ93-96 "Automatic Control Installation Atlas" and the instrument manual. The following requirements are put forward for the installation and use of the instrument:

1. Pressure transmitter

1. Pressure and negative pressure devices are installed on the straight pipeline of the flow beam, and should not be installed in the bends, corners, dead ends of the pipeline and the places where the flow beam is vortex, which are enough to distort the static pressure head. 2. When installing pressure or negative pressure devices, since they are perpendicular to the flow beam, the pressure pipe should not extend into the interior of the fluid pipeline or equipment, the pressure port should have a smooth outer edge and should not have sharp edges. The continuous conduit and accessories should be cut neatly and burrs should be removed.

3. The location of the pressure pipe on the horizontal and inclined pipeline should be at the upper part of the pipeline when the fluid is gas. When the fluid is liquid, it should be at the lower half of the pipeline at an angle of 0-450 to the horizontal center line or on the center line of the pipeline. When the fluid is steam, it should be at the upper half of the pipeline at an angle of 0-450 to the horizontal center line or on the center line of the pipeline.

4. All pressure taking devices must be equipped with a primary door, which should be close to the pressure taking device. 5. The horizontal section connecting the pressure pulse pipeline should maintain a certain slope, and the direction of inclination should be able to ensure the discharge of air or condensate. The pipeline slope requirement for the pressure pulse pipeline should not be less than 1:100. The pressure pulse pipeline should be equipped with a drain valve at the pressure gauge to flush the pipeline and exhaust air. 6. The pressure pulse pipeline should be purged before installation to ensure that the pipeline is clean and unobstructed. The valves on the pipeline should be tested for tightness before installation. After the pipeline is laid, the tightness test should be carried out again. The pressure pulse pipeline should be filled with water before driving (note that bubbles are not allowed to be poured in when filling water to affect the measurement). 7. For the placement of the intelligent transmitter, please refer to the installation and use instructions of the random transmitter.

2. Temperature transmitter

1. The temperature sensing element should be placed in a place that can represent the temperature of the medium being measured. The temperature sensing element should not be placed near valves, elbows and dead corners of equipment. The temperature sensing element should be placed at an angle, and its tilt direction should make the temperature sensing end face the fluid. 2. When installing a heat sensing element on a pipe with a diameter of less than 76mm, the method of expanding the tube should be used. 3. For easy disassembly and to prevent dirt from falling into the thermometer, when installing a platinum resistance thermometer on a horizontal pipe, it is generally installed horizontally or slightly tilted downward. 4. The temperature sensing part of the platinum resistance thermometer should be installed in the center of the pipe, and the distance between the temperature end and the center of the pipe should be 50-70mm.

3. Electromagnetic flowmeter

1. The environmental conditions (temperature, humidity, electromagnetic field, etc.) of the installation location must meet the requirements of the manual. 2. Length of straight pipe section. According to the situation of the pipeline where the flow meter is located, the necessary straight pipe length must be guaranteed. Generally, the upstream pipeline maintains a straight pipe section of more than 10D, and the downstream maintains 3D-4D. D is the diameter of the pipe. 3. The addition of chemical additives will affect the conductivity of the liquid and should not be injected upstream of the meter. If it must be injected upstream, a reactor or straight pipe section of sufficient volume must be maintained so that the conductivity of the liquid flowing through the flow meter is in a stable state. 4. Installation method: Regardless of horizontal, vertical, or inclined installation, it must be ensured that the measuring tube of the meter is always full of liquid and no bubbles exist (see the instrument manual). For liquids containing solid particles or slurries, it is recommended to install vertically, with the flow direction from bottom to top. The plane where the electrode is located must be parallel to the ground.

5. Grounding, A. The working grounding of the flow sensor is to ensure that the reference potential is the same as the measured object, and the liquid level potential should be the same as the ground potential. The electromagnetic flowmeter is equipped with a grounding ring, which is used to establish liquid grounding by contacting the liquid. According to the different materials of the pipeline where the flowmeter is located, the grounding connection method should be correctly adopted. The working grounding should adhere to the "single-point grounding" principle. (See "Chemical Automation and Instrumentation" 1998, Issue 4, Pages 48-49, "Grounding Measures for Electromagnetic Flowmeters") The grounding resistance should be ≤100 ohms. B. Shield grounding. The signal cable output from the flow converter to the secondary instrument DCS system has its shielding layer grounded on the control room side and grounded through the grounding bus.

6. Reasonable process regulations and correct operation. Prevent bubbles generated during the production process from flowing through the electromagnetic flowmeter.