Application of electromagnetic flowmeter in circulating water system

Introduction

In petrochemical chemical plants, the circulating water system is a public engineering system of the plant. It is not only an important link in environmental protection, but also a key component of petrochemical chemical plants. Therefore, the selection and use of good circulating water flow meters is also one of the key tasks of chemical plants in the start-up stage. This article introduces the use of DWM2000L electromagnetic flowmeters used in the circulating water system of the solution styrene butadiene rubber plant in Shanghai Caojing Chemical Industry Zone.

1 Circulating water system of solution styrene butadiene rubber plant

The solution styrene butadiene rubber plant in Shanghai Caojing Chemical Industry Zone was built and started in July 2006 with the process package technology provided by Asahi Kasei Corporation of Japan. It can produce 15 brands of solution styrene butadiene rubber and low-cis rubber. The normal amount of circulating cooling water in the plant is 4000t/h, which is used to cool the materials in the main process device. Part of the circulating cooling water enters the cooler to reduce the temperature of the materials, and part of the circulating cooling water enters the condenser to cool the gas phase materials into the liquid phase. Two cooling towers of the same capacity are set up in the device, with a diameter of 7000mm, a power of 75kw, and a capacity of 2000m3/h; three high-power circulating water pumps are set up to supply circulating cooling water to the main process device. A sewage pipeline is set up in the circulating water system for regular discharge of polluted wastewater; a make-up water pipeline is set up for regular replenishment after discharge; and a dosing pipeline is set up to inhibit the growth of bacteria in the circulating water and prevent adhesion and corrosion of heat exchange equipment.

Electromagnetic flowmeters from Cologne are used on the above pipelines, among which OPTIFLUX 4300c ex/IFC300c ex are selected for the sewage and make-up water pipelines, and DWM2000L plug-in electromagnetic flowmeter is selected for the circulating water pipeline. The process flow diagram of the circulating water system is shown in Figure 1.

Figure 1 Schematic diagram of the process flow of the circulating water system

2 DWM2000L Electromagnetic Flowmeter

2.1 Measurement Principle

According to Faraday's law of electromagnetic induction, the conductor moving in a magnetic field can generate an induced voltage U0. The measured liquid water is a conductor, and its movement direction is perpendicular to the magnetic induction intensity B. The voltage induced between the two electrodes (platinum electrode and ground electrode) is:

U = K × B × V × D

, where K is a constant; B is the magnetic induction intensity; V is the medium flow rate; D is the electrode spacing.

The voltage U is converted into a 4~20mA electrical signal output proportional to the flow rate and independent of the load through the transmitter. The measurement principle diagram is shown in Figure 2.

Figure 2 Measurement principle diagram

2.2 Technical Parameters

The DWM2000L insertion electromagnetic flowmeter is suitable for flow measurement of conductive liquids and slurry suspensions. It has a common type with a protection level of IP66, equivalent to NEMA 4; a submersible type with a protection level of IP68, which can be used in open pipes and flooded places. The DWM2000L is an extended sensor with a length of more than 3m. The technical parameters are shown in Table 1. [page]

Table 1 Technical Parameters

2.3 Installation

When selecting the installation location, the length of the upstream and downstream straight pipe sections of the instrument must be met first. Secondly, avoid large motors and equipment that easily generates magnetic fields to prevent interference. Also, choose a location that is easy to install and maintain. The installation diagram is shown in Figure 3. (a) in Figure 3 is the correct installation method, and (b) is incorrect. In addition, when installing the extended type, be sure to mark the insertion depth first. The general insertion depth is 1/8 of the pipe diameter. After the insertion depth is met, turn the sensor as a whole so that the flow direction mark is consistent with the fluid direction. Finally, lock the locking piece and tighten the safety chain. For the submersible type, users should pay special attention to not opening the IP68 cable interface. The manufacturer has made special sealing treatment on the cable connection before leaving the factory.

Figure 3 Installation diagram

2.4 Setting

① Determine the position of the electronic components. Open the converter housing, make the direction of the arrow on the electronic components consistent with the flow direction of the medium, and then tighten the locking piece. If the direction of the arrow is inconsistent with the flow direction of the medium, an erroneous measurement result will be generated.

② Set the reference speed. Set the reference speed through 2 DIP switches (1m/s and 0.1m/s). The speed can be calculated according to the formula V= 354×(Q/D×D), where V=m/s, Q=m3/h, D=pipe inner diameter mm.

③ Set the time constant. Select the time constant of 5, 8 or 10s through the DIP switch.

④ Set the alarm status. The normally closed contact NO means that when the speed is greater than the reference speed, the circuit is disconnected; the normally open contact NC means that when the speed is greater than the reference speed, the circuit is closed; when the circuit is disconnected, the LED light is on.

After setting, pay special attention to the installation of the rubber seal when connecting the IP68 housing. The seal must not be damaged, and the inner surface of the seal must not be damaged in the slightest to prevent water from entering the IP68 housing and damaging the instrument.

3 Practical application of DWM2000L electromagnetic flowmeter

In April 2006, the on-site installation engineer installed the meter under the guidance of the manual, and conducted linkage debugging with the device in June. With the start-up of the device, the meter was put into normal operation. So far, there has been no maintenance workload and deviation. The circulating water volume reflects the production and operation status of the device in a timely manner.

Differential pressure flow measurement instruments are also used in the circulating water system, including the Verabar flowmeter. The Verabar flowmeter uses the differential pressure principle to measure flow, adopts a non-contraction throttling design to reduce pressure loss, and can measure various media such as gas, liquid and steam. The range ratio is greater than 10:1. The installation is simple. It only needs to open a small hole in the pipe and weld a base without cutting the pipe. The measurement system has very high requirements for the accuracy of the parameters of the measured medium. Compared with the electromagnetic flowmeter used in the circulating water system, the measurement principle does not target the conductive characteristics of water, the range ratio is not large enough, and the installation cannot meet the IP68 submersible requirements. The pressure pipe is easy to be blocked, and the changes in pressure difference, maximum flow and working temperature are easy to cause measurement errors.

4 Conclusion

The product features of DWM2000L electromagnetic flowmeter are large measurable range, wide application in sewage, cold water, tap water treatment, electronic unit can be replaced without interrupting work, low power consumption, wetted parts are stainless steel or ceramic structure, no moving parts, maintenance-free. Practice has proved that it is appropriate to use electromagnetic flowmeter in circulating water system.

References
1 Liang Guowei, Cai Wuchang. Flow Measurement Technology and Instruments [M]. Beijing: Machinery Industry Press, 2002.