Steam flow meter measurement error and solution

Steam flow measurement is difficult, mainly because steam is a special medium. With the change of working conditions (such as temperature and pressure), superheated steam often turns into saturated steam to form a steam-liquid two-phase flow medium. For steam with frequently changing phase flow, using current flow meters to measure gas flow will definitely lead to inaccurate measurement. The solution to this problem is to maintain the superheat of steam and minimize the water content of steam, such as strengthening the insulation measures of steam pipes and reducing the pressure loss of steam to improve the accuracy of measurement. However, these methods cannot completely solve the problem of inaccurate steam flow measurement. However, the fundamental solution to the problem is to develop a flow meter that can measure two-phase flow media. 

Five main factors should be considered when selecting a flow meter: the characteristics of the measured fluid, the production process, the installation conditions, the maintenance requirements, and the characteristics of the flow meter. For steam metering, the above five factors should also be considered. Here, we focus on the characteristics of the flow meter, the installation conditions, the maintenance requirements, and several issues that should be paid attention to when selecting a flow meter. 

At present, the instruments for measuring steam flow mainly include vortex flowmeter, differential pressure flowmeter, split rotor flowmeter, Annubar flowmeter, float flowmeter, etc. Here we mainly discuss vortex flowmeter, orifice flowmeter and elbow flowmeter. 

Vortex flowmeter 

is a new type of flowmeter successfully developed based on the Karman vortex principle. It has been rapidly developed since the 1970s. It is one of the alternatives to orifice flowmeter. It has the following characteristics: simple and firm structure, no moving parts; very convenient maintenance, low installation cost; wide measurement range, range ratio can reach 1:10; small pressure loss, low operating cost; wide application range, gas and liquid flow can be measured. 

However, the vortex flowmeter has certain limitations: the vortex flowmeter is a velocity flowmeter, and the stability of the vortex separation is affected by the flow velocity, so it has certain requirements for the straight pipe section, generally 10D in the front and 5D in the back; when measuring liquids, the upper limit flow rate is limited by pressure loss and cavitation, generally 0.5~8m/s; when measuring gases, the upper limit flow rate is limited by the change in the compressibility of the medium, and the lower limit flow rate is limited by the Reynolds number and the sensitivity of the sensor, and the steam is 8~25m/s; the stress-type vortex flowmeter is more sensitive to vibration, so when installing the flowmeter on a pipeline with large vibration, the pipeline must have certain shock-absorbing measures; the stress-type vortex flowmeter uses piezoelectric crystals as detection sensors, so it is limited by temperature, generally -40~+300℃. 

Orifice flowmeter 

Orifice flowmeter has a long history of application, has international standards, high theoretical accuracy, and is widely used. However, after decades of application, it has been found that the orifice flowmeter has the following shortcomings: many factors in the application (design parameters do not match the operating parameters, insufficient upstream straight pipe section, non-concentricity between the orifice plate and the pipeline, contamination of the A surface of the orifice plate, wear of sharp angles, etc.) have a very large impact on its measurement accuracy, increasing its measurement error; installation is more troublesome, and the workload of maintenance and disassembly and cleaning is large; the flow range ratio is 1:3, which has great limitations; if the installation is not correct, steam leakage is likely to occur; the pressure loss is large, and the operating cost is high. 

Elbow flowmeter 

Elbow flowmeter features: 

1. Simple structure and low price. The elbow sensor is actually a 90-degree standard elbow. There is no flow sensor with a simpler structure than it. With the development of the mechanical processing industry and the continuous improvement of industry standardization and standardized management, the standard mechanism elbow used as an elbow sensor is getting better and better in quality and lower in price. 

2. Without any additional throttling parts or inserts, the power consumption of fluid transportation in the pipeline can be greatly reduced, saving energy, especially for those large systems, large pipe diameters, and low pressure head measurement objects. 

Here is a simple example to illustrate. In order to maintain the normal operation of an orifice flowmeter installed in a heating pipeline with a flow rate of thousands of tons per hour, it takes about tens of thousands of degrees of electricity in a heating season, which is equivalent to tens of thousands of yuan. Here, only the pressure loss of the orifice flowmeter is considered to be several thousand Pa, which is far more than this value during actual operation. Even this pressure loss of several thousand Pa, the additional operating costs it causes cannot be ignored. The data given in Table 1 are the data of the operating costs of a single orifice flowmeter in a heating season, including the additional power consumption of the circulating pump caused by the orifice flowmeter, the power consumption cost, the equivalent standard coal volume, and the coal purchase cost when the flow range of the main pipeline of the current thermal network is different. The operating days are calculated as 120 days, the electricity price is 0.35 yuan/degree, and the standard coal price is 200 yuan/t. 


For a medium-sized heating network with a flow rate of 4000m3/h, when the orifice pressure loss is 30 kPa, only one orifice flowmeter consumes 96,000 kWh of electricity and the operating cost is 31,200 yuan. For a large heating network with a flow rate of 10,000m3/h, the additional power consumption reaches 240,000 kWh and the operating cost is 78,000 yuan. However, the elbow flowmeter has no additional resistance loss. If the elbow flowmeter is used instead of the orifice flowmeter for measurement, the operating cost can be greatly reduced and considerable economic benefits can be obtained; 

3. The elbow flowmeter sensor is wear-resistant and insensitive to trace wear. 

4. It can be installed by direct welding, which completely solves the trouble of leakage on site. 

5. It has strong adaptability, wide measuring range, and no strict requirements for straight pipe sections. As long as the flow rate of the fluid in the pipeline can be measured by an orifice plate, vortex street, or average velocity tube flowmeter, it can be measured by an elbow flowmeter. In terms of high temperature resistance, high pressure resistance, impact resistance, vibration resistance, moisture resistance, and dust resistance, the elbow flowmeter is far superior to other flowmeters. For example, 

high temperature, high pressure, impact, and vibration are very limited in the usability of the vortex flowmeter. This is related to its measurement principle. The actual measurement results produce a large additional error that affects the measurement accuracy and causes inaccurate measurements. All these problems do not exist for the elbow flowmeter. 

The range ratio of the elbow flowmeter can reach 1:10. For steam, its applicable range is 0-70m/s, which can better meet the requirements of steam flow measurement. Due to its special measurement principle, the elbow flowmeter does not have strict requirements on the straight pipe section in practical application. Generally, it only requires 5D in the front and 2D in the back, which is far lower than the requirements of other flow measurement devices; 

6. The elbow flowmeter has high accuracy and good reproducibility. The measurement accuracy can reach 1%, and the reproducibility accuracy can reach 0.02%. After one installation, it no longer needs to be disassembled and assembled repeatedly. Therefore, its installation accuracy can also be best guaranteed. 

In summary, the selection of steam flow meters is very important. Accurately measuring steam flow is a problem that all production departments need and generally care about. With the development of the economy, the call for improving the measurement level is getting higher and higher. Therefore, we should do some meticulous technical work based on the actual production situation and explore a successful way to measure steam flow.