Case analysis of using electromagnetic flowmeter to measure air conditioning chilled water-EEWORLD

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Electromagnetic flowmeters are now widely used in industrial production, especially in the field of water flow measurement. Even many water companies use electromagnetic flowmeters to measure large-caliber water. This article is a case analysis of air conditioning chilled water measurement. The specific situation is as follows: An airport hotel obtains chilled water from the airport energy center for air conditioning. In winter, the chilled water is not used, and the shut-off valves on the chilled water pipes in and out of the hotel are closed, but the electromagnetic flowmeter indicates full scale.

(1) Cooling is stopped but the chilled water pipe is still full of water

In cooling systems, energy companies generally use water as a refrigerant to provide cooling to users. In addition to fresh water and salt water, these refrigerants sometimes also use a mixture of fresh water and alcohol and ethylene glycol to meet the needs of different temperature levels. In cooling systems for air conditioning purposes, fresh water is generally used, and the water supply temperature is usually 5-6°C. These fresh waters are treated with chemicals to prevent the oxygen in the water from corroding the inner wall of the steel pipe.

In winter, although the cooling supply has been stopped and the fresh water in the pipes has stopped circulating, the pipes are still filled with fresh water to prevent air from invading the pipes and causing rust on the inner walls of the pipes.

(2) The adaptability of electromagnetic flowmeter to empty pipe

Most of the electromagnetic flowmeters currently used on site have poor adaptability to empty pipes, that is, they do not allow the measuring tube to be empty. Once the measuring tube is empty, the electrode is exposed to the air and the large internal resistance of the signal source will induce interference from the excitation coil, causing the flow indication to rise to the full scale.

In this case, the full scale of the electromagnetic flowmeter should also be caused by the empty measuring tube.

(3) Why does empty traffic flow occur?

The air conditioning chilled water system should normally be filled with water, but because the hotel was overhauling its equipment, the water was drained, and after the maintenance was completed, the valve was forgotten to be opened to let in water, resulting in an empty pipe.

(1) Empty pipe detection and empty pipe zeroing

Electromagnetic flowmeters are designed to measure the volume flow of conductive liquids. The measurement principle is based on Faraday's law of electromagnetic induction. According to this law, when a conductive fluid moves through a magnetic field to cut magnetic lines of force, an induced electromotive force is generated in a direction perpendicular to the magnetic field and the flow velocity. This electromotive force is given by equation 4.24 in section 4.14. The principle is shown in Figure 4.31.

For many years, people have been looking for methods to detect and zero empty traffic pipes.

Figure 4.32 shows a method that has been successfully used by a foreign company.

This method is based on conductivity measurement. In Figure 4.32, the resistance between the two electrodes can be expressed by equation (4.26):

Electromagnetic Flowmeter

Where: Re—inter-electrode resistance;

K—constant;

Ef—electrode coefficient (related to the electrode area and the inner diameter of the measuring tube);

q - electrical conductivity.

From equation (4.26), we can get equation (4.27):

The symbols in the formula have the same meaning as in formula (4.26).

For tap water, acid and alkali solutions, Re is about 1OΩ-1kΩ when the tube is full, and when the tube is empty, the liquid level is lower than the electrode, and in principle Re is infinite. Since the inner wall of the tube between the two electrodes may be wet, in fact Re is only a limited resistance value.

But the resistance value is much larger than that in normal operation. Generally, the Re value to judge whether the pipe is empty is 3 times the resistance value in normal operation.

Once the instrument determines that the conduit is empty, the single chip microcomputer in the instrument will take two measures. One is to set the output to zero, and the other is to send out an empty pipe alarm signal, thereby preventing incorrect measurement or incorrect control when the conduit is empty.

(2) Air traffic control fullness and safety

Empty pipe indicating fullness is a major defect unique to electromagnetic flowmeters, which can easily give people an illusion. Especially when the flowmeter output signal is used for automatic adjustment, this illusion can easily cause the regulating valve to run to the limit position, so safety should be paid attention to.

(3) Partial pipe detection

The electromagnetic flowmeter is a type of velocity flowmeter. The signal it gives is actually the average flow velocity of the liquid flowing through the measuring tube. When the measuring tube is full of liquid, equation (4.28) holds true:

Where, qv—volume flow rate;

A—Measuring pipe flow cross-sectional area;

v—average flow velocity.

However, if the pipe is not full, part of the cross-sectional area of the pipe will be occupied by the gas, so that the actual flow cross-sectional area of the liquid is smaller than A, which will eventually lead to a higher flow indication.

Electromagnetic flowmeters are widely used in water supply and drainage. When water flows in a horizontal pipe, it often releases gas due to temperature changes and gathers at the top of the horizontal pipe. If there is gas at the top of the electromagnetic flowmeter measuring pipe, the high-precision characteristics of the instrument will be greatly reduced. Therefore, partial pipe detection has long been a research topic.

The non-full pipe detection is also based on the conductivity detection method. In the electromagnetic flow sensor shown in Figure 4.33, one (or a pair) non-full pipe detection electrode is added. When the liquid in the conduit is full, the resistance between the top electrode and the other electrodes is very small; when the pipe is not full, there is a gas barrier between the top electrode and the conductive liquid, so the electrode resistance increases significantly, thereby making a non-full pipe judgment.

When the electromagnetic flowmeter is not full, the solution is generally to install an exhaust valve at the top of the upstream pipe of the flow measurement tube.

Reference address：Case analysis of using electromagnetic flowmeter to measure air conditioning chilled water

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