Selection of Steam Flow Meter and Energy Saving

Selection and energy saving of steam flowmeter:

Without accurate measurement, accurate management is impossible. It is the basis and basis for effective management. It is usually easier to install meters for water and electricity. However, since the temperature, pressure and dryness of steam will change with the transportation process, it is often difficult to measure steam very accurately. Technical reasons have caused us to ignore steam measurement. In fact, whether in hospitals or other places where steam is widely used, such as factories, the waste of steam is more serious than that of water and electricity. With the development of measurement science and the enhancement of managers' energy-saving awareness, steam measurement has received more and more attention.

The benefits of steam measurement are:

(1) When steam consumption is abnormal, the cause can be quickly found, losses can be reduced, and accurate basis can be provided for technical improvement;
(2) The efficiency of all steam-using equipment can be monitored, providing a basis for preventive maintenance, thereby ensuring the normal operation of production;
(3) The flowmeter itself cannot save energy, but improved management based on accurate measurement data can effectively save energy by 5% to 20%.

1 Example of how to choose a steam flowmeter

The West China Hospital of Sichuan University currently has 4 10t/h gas and oil boilers, with an annual steam output of 52,400 tons, natural gas consumption of 5.12 million m3, and natural gas costs of 8.8 million yuan. The steam metering management and energy saving and consumption reduction work have received great attention from the hospital leaders.

Steam is mainly used for disinfection, preparation, central air conditioning, central kitchen, pulp washing, etc. in hospitals, and the steam load changes are very large. For example, the main pipeline of the steam unit for sanitary hot water in the hospital is DN100, the steam pressure is 10bar, and the maximum flow rate can reach 3825kg/h based on the flow rate of 25m/s. But in fact, sometimes only one device is turned on in the entire steam unit, and the minimum steam consumption is about 100kg/h. Therefore, the actual required flowmeter range ratio is 3825 ÷ 100 = 38:1.

The range ratio of the standard orifice steam flowmeter is generally 4:1, which obviously cannot meet the requirements.

The range ratio of the vortex flowmeter can reach up to 40:1, which can meet the requirements at first glance. However, the vortex flowmeter measures the velocity of the fluid by measuring the number of vortices generated when the fluid passes through the vortex generator. When the flow velocity of the fluid is lower than the lower limit (generally 2m/s), no vortex is generated. Therefore, when the maximum steam flow velocity is 25m/s, its actual range ratio is only 25/2=12.5:1, which cannot meet the requirements.

Through analysis and comparison, we chose the spring-loaded Gilflo variable area flowmeter with a range ratio of up to 100:1. The differential pressure signal output by this type of flowmeter has a linear relationship with the measured flow rate, so it is also called a linear orifice differential pressure flowmeter.

2 Introduction to Gilflo variable area flowmeter

2.1 Working principle

Referring to Figure 1, when the fluid flows through the Gilflo flowmeter, the centrally positioned cone pushes the load spring to move axially, causing the annular area to change. The change in the annular area makes the output signal (differential pressure) linearly related to the flow rate, greatly expanding its range ratio. The differential pressure transmitter measures the differential pressure and converts it into a 4-20mA signal and outputs it to the flow computer. Through the processing and calculation of the flow computer, the instantaneous flow rate, total flow rate, temperature, pressure and other information of the fluid are displayed.

2.2 Features

Giflo flowmeter has the following features:

(1) Large range ratio. The typical Giflo differential pressure flowmeter can measure in the range of 1%FS to 100%FS. For measurement objects with large flow changes, one flowmeter can solve the problem.

(2) Linear differential pressure output with high accuracy. Since each unit is calibrated with water and multiple compensations are performed, and the differential pressure signal is linearly related to the flow rate, when the measured flow rate is relatively small relative to the full-scale flow rate, the amplitude of the differential pressure signal is also large, which is conducive to improving the measurement accuracy. The actual accuracy can reach ± ​​1%.

(3) Low straight pipe section requirements. Due to the variable area design of the orifice plate, it becomes a measuring mechanism that works under high Reynolds number conditions. It can be measured in the position close to the downstream of the elbow and tee (the typical installation requirements are that the upstream straight pipe section is ≥6 times the pipe diameter, and the downstream straight pipe section is ≥3 times the pipe diameter).

(4) Good vibration resistance. In harsh places with large vibration, vortex flowmeters are prone to "making something out of nothing" and high readings. Gilfo linear orifice plate has better vibration resistance than vortex flowmeter.

2.3 Issues to note

Since the temperature, pressure and dryness of steam will change continuously during the transportation process, in addition to selecting the appropriate type of flowmeter, we also need to pay attention to the following key issues in the measurement of steam systems:

(1) Density compensation. For saturated steam, its density corresponds to pressure one by one. Therefore, if there is no actual density compensation function, when the steam pressure fluctuates, the displayed value will inevitably have errors.

(2) Install a steam-water separator. The dryness of steam will change continuously with heat dissipation during transportation, producing condensed water. If correction is not considered in the measurement, it will have a great impact on the accuracy of the measurement. At present, there is no instrument that can detect steam dryness online, so it can only be set through empirical parameters. In addition, condensed water will also cause corrosion, water hammer and other problems, which will affect the measurement results and even damage the flowmeter. Therefore, to improve the measurement accuracy of steam flow, a steam-water separator must be installed upstream of the flowmeter.

(3) Install a filter. During the installation process of the steam pipeline, it is inevitable that there will be some impurities, such as gravel, welding slag, etc. If the filter is not installed, these impurities may cause various problems during operation, affect the normal operation of the equipment, and reduce the service life of the equipment. Therefore, we installed a 100-mesh filter upstream of each steam flow meter.

3 Effect after installation

Since 2005, we have successively installed 9 Gilflo spring-loaded variable area flow meters at each boiler outlet, central kitchen, pulp washing room, etc. After 3 years of operation, the measurement results are very stable and accurate, providing accurate and valuable data for the management and energy-saving transformation of our hospital. Through these data, we have made continuous improvements, the efficiency of the steam system has gradually improved, and the energy consumption has decreased year by year, which has produced huge economic benefits. While saving fuel costs for the hospital, it has also reduced smoke emissions and saved a lot of resources for the society.

According to the above transformation and data, our hospital used 810,000 m3 less natural gas in 2006 than in 2005, saving 1.4 million yuan in funds, and the system is safer and more reliable than before the transformation, and the effect is very obvious. It can be said that accurate measurement is the premise and guarantee of energy saving and consumption reduction.