Working principles and usage requirements of commonly used steam flow meters

Commonly used flow meters for steam flow measurement include differential pressure flow meters, vortex flow meters, rotor steam flow meters, V-cone flow meters, elbow flow meters and flute-shaped velocity averaging tube flow meters. The working principles and uses of these steam flow meters The requirements are different for each. The editor of Study.com will share with you the knowledge related to these instruments.

1. Differential pressure flow meter

In the measurement of steam flow, the differential pressure flowmeter still ranks first. It is based on authoritative standards and has test parts that are easy to copy; it has a simple and solid structure, reliable performance, and a long service life; it can be used in a wide temperature and pressure range. Wide; low price, no need for real flow calibration and other advantages. The complete set of differential pressure flow meters is composed of standard throttling devices, differential pressure transmitters and flow totalizers from different manufacturers, so it is flexible and convenient to use. Differential pressure flow meters are particularly suitable for measuring high-temperature, high-pressure saturated steam and superheated steam. However, there are also shortcomings, such as: the installation is complicated, easy to leak, and maintenance and disassembly are labor-intensive; the range ratio is only 3:1, and the pressure loss is large; due to changes in measurement conditions during use, the process parameters deviate from the design values. will produce large measurement errors.

2. Vortex flowmeter

The vortex flowmeter has the advantages of simple structure and no need for pressure guiding tubes; large measurement range, range ratio up to 10:1; small pressure loss, etc., and its proportion in saturated steam flow measurement is increasing. The ease of installation is similar to that of orifice plates, and it has certain requirements for straight pipe sections. But there are also shortcomings, such as: regular verification cannot be done by ordinary users and needs to be disassembled and sent for inspection; its stability is affected by the flow rate. The stress-type vortex flowmeter is more sensitive to vibration and is easily affected by the vibration of the pipeline or equipment. The measurement error is also limited by temperature, which usually cannot exceed 300°C (the vortex flowmeter is suitable for saturated steam flow measurement, but cannot be used for superheated steam flow measurement). It is not suitable for measuring multi-phase flow, such as saturated steam with high humidity.

3. Rotor steam flow meter

Rotor-type steam flowmeter is widely used in saturated steam measurement in small and medium-sized enterprises. It is a purely mechanical measuring instrument. It has the advantages of simple, solid structure and low maintenance; only the inner orifice plate needs to be replaced to adjust the range, and it has a pressure compensation function for manual adjustment, no need for manual adjustment. However, there are also shortcomings, such as: the straight pipe section has certain requirements: the adaptability range of the pipe diameter is limited, and it can only be installed in a horizontal position; the accuracy is not high, and meter reading can only be done manually on site, which is not convenient for measurement management.

4. Linear orifice plate differential pressure flow meter

Linear orifice differential pressure flow meters have been used in enterprises. It is also called an elastically loaded variable area variable pressure head orifice plate, and its flow rate is linearly related to the differential pressure. Its range ratio can reach 100:1, which is very suitable for measurement occasions with large flow changes; it has high measurement accuracy, low requirements for straight pipe sections and good anti-vibration performance. However, there are also shortcomings, such as: each instrument needs to be calibrated with water. Changes in fluid temperature will lead to changes in fluid density, and will also lead to changes in the inner diameter of the pipe, the diameter of the orifice plate and the geometric size of the piston, resulting in measurement errors. Because there are movable parts, maintenance is complicated, and the gap between the orifice plate and the piston is very small. A filter must be installed upstream of the instrument to prevent impurities in the pipeline from blocking the piston.

5. V-cone flowmeter

The measurement principle of the v-cone flowmeter is the same as that of the differential pressure flowmeter, and it is also a throttling differential pressure flowmeter. It adopts the gradual contraction and throttling method of the side wall, that is, "the contraction of the fluid in the center of the pipe becomes the contraction of the side wall." It has extremely low requirements for installing straight pipe sections and almost no straight pipe sections are required; the range ratio can reach 10:1; the pressure loss is only 1/3 of the orifice plate. It is not afraid of vibration, resistant to high temperature and high pressure, and can measure saturated steam and superheated steam. The geometric size of the throttling part can remain unchanged for a long time, and it can work stably for a long time without calibration. When measuring steam flow, a dedicated three-valve group is connected between the transmitter and the flowmeter, eliminating the need for a pressure guiding pipe. However, there are still some shortcomings. For example, there is still the problem of pressure loss. When measuring high-temperature superheated steam, it is still necessary to install a condenser or extend the pipe to protect the instrument. When measuring steam flow, it is still necessary to compensate for temperature and pressure.

6. Elbow flowmeter

The elbow flowmeter is an elbow with fixed geometric dimensions and fixed shape. Its structure is simple without any additional parts and throttling parts, so there is basically no pressure loss. The range ratio can reach 10:1, high precision, maintenance-free measuring device, long life, high temperature resistance, high pressure resistance, vibration resistance, etc. However, there are also shortcomings. For example, because the differential pressure generated by measurement is low, generally around 3000pa, the selection of a differential pressure transmitter is highly demanding. When measuring steam flow, temperature and pressure compensation are still required.

7. Flute-shaped velocity averaging tube flow meter

The flute-shaped velocity averaging tube flowmeter is based on the pitot tube velocity measurement principle. It outputs a differential pressure signal and is used in conjunction with a micro-differential pressure transmitter that measures differential pressure. It can measure the flow of steam, liquid, and gas. Its structure is simple and the pressure is high. The loss is small, the length of the upstream and downstream straight pipe sections is required to be short, the installation is easy, the range ratio can reach 10:1, it is resistant to high temperature and high pressure, is not affected by wear, and has no leakage, etc. However, there are also shortcomings. For example, the requirements for on-site installation conditions are high, and the differential pressure generated is small, some are only 20-30pa. A high-precision micro-differential pressure transmitter must be used. Due to structural reasons, the differential pressure of each pressure port is The flow rate is different, and there is a certain pressure difference between the pressure holes. There is a flow of medium between the pressure holes. If the medium is present, it may be blocked and cause measurement errors. The measured fluid flows in the round tube, and due to fluid separation, Different points lead to different pressure distributions caused by the round pipe when facing the fluid, resulting in unstable flow coefficient and unstable flow rate.