How to improve the measurement accuracy of steam flow meters

Steam flow meters have a wide range of applications in the petroleum and chemical industries, but the measurement accuracy of steam has always been a difficult point in measurement. Currently, there are many misunderstandings among many users. Believe that the brand's voice and high-quality flow meter can ensure the accuracy of the measurement. In fact, this is a one-sided idea, it will ignore the dependence of the actual situation and increase accounting costs. In fact, the measurement of a steam flow meter is affected by many factors. Scope issues and pipe diameters are the first two points that should be paid attention to during the instrument modeling stage:

1. Scope issues. Through a large amount of experimental data, it is proved that the main reason for the abnormal steam flow meter measurement is the error range of the steam flow meter. Everyone knows that in the steam season, the steam consumption is quite large, and in the off-season, the steam consumption will be very small, and the large gap between the steam consumption makes it difficult to manufacture the general steam flowmeter. Nener is enough to adapt. Therefore, when you choose a steam flow meter, you must first determine the flow measurement range of the steam flow meter, and then select a steam meter that meets the relevant operating parameters, otherwise it will be difficult for the steam flow meter to perform its due role.

2. The pipe diameter problem. When designing the throttling device, the steam flow meter basically uses the nominal nominal pipe diameter value provided by the process, but in fact, there is an error between the nominal nominal pipe diameter value and the actual pipe diameter value, especially for coiled pipes. It is said that sometimes there is a large gap between the nominal nominal pipe diameter value and the actual pipe diameter value, which makes the measurement error increase, and finally the measurement of the steam flow meter is difficult to achieve the expected effect. The current national standard has long stipulated that the pipe diameter D value used to calculate the throttle valve diameter ratio should be the average value of the 0.5D length and inner diameter of the upstream pressure port. The average value of the inner diameter should be the average of the inner diameters measured in at least two cross-sections of the vertical axis, and the value of the inner diameter should be ± Ω; 0.3 Therefore, it is recommended to measure the pipe diameter before designing the steam flow meter. In order to reduce the calculation error of the steam flow meter.

Steam flow meter technical parameters

1. Diameter: DN25~DN1000 (mm)

2. Accuracy: gallons; 1S

3. Measuring ratio: standard 1:13, extended 1:30

4. Working pressure: le; 42.0MPA

5. Medium temperature: -40oC~450oC

6. Medium viscosity: ≤30CP (equivalent to heavy oil)

7. β value: 0.2 to 0.8

8. Connection method: flange or clip-on

9. Flange standard:

DNle; 600mm, PN2.0~PN26, HG20616; PN32, HG20618;

DNgt; 600mm, PN2.0~PN15, HG20623.

It can also be manufactured according to the flange standard provided by the user.

10. Material: main body, orifice plate, pressure pipe, three valve group: stainless steel; straight pipe section and connecting flange: carbon steel or stainless steel

11. Installation method: horizontal or vertical.

12. Intelligent differential pressure transmitter: output: 4~20mA.DC or digital signal; power supply: 24V.DC;

Features of steam flow meter:

1. Integrated structure, flow measurement

2. High measurement accuracy and wide ratio range

3. The minimum pressure difference can reach 30mmH2O

4. High temperature and high pressure resistance, suitable for various working conditions

5. Intelligent transmission, automatic compensation for temperature and pressure conditions

6. Open digital platform, multi-function serial interface, field bus structure, HART communication protocol

Summarize a series of measures that can effectively improve the accuracy of steam measurement, and share these experiences with you:

1. Manufacturing and installation of standard throttling device H

Flow meters suitable for steam applications include: steam flow meters, orifice flow meters, turbine flow meters, including bypass flow meters, variable flow meters, tampered load and flow meters, direct online (DIVA) flow meters, dual-tube flow meters Meter and vortex flow meter. The orifice flowmeter has simple structure, high precision, low wood forming rate and wide application.

When using the standard throttling device to measure the steam flow, the design, processing, inspection, installation and use of the standard orifice throttling device must be carried out in strict accordance with the technical specifications stipulated in the SY/T6143-2004 standard. The right-angle edge of the orifice plate will be worn by the medium for a long time, especially the medium containing impurities will wear more seriously, which will affect the accuracy of the orifice plate flowmeter, and the time should be adjusted to the edge orifice and the inner wall of the measuring tube. Check the roughness.

Ensure the length of the straight pipe before and after the 1.1 orifice plate

The correct choice is a prerequisite to ensure the accuracy of the flowmeter. This needs to consider the medium type, medium state, medium temperature, pipe diameter, installation method and output signal. Once the type is selected, special attention should be paid to providing sufficient straight pipe sections upstream and downstream of the orifice flowmeter during the installation process, so that the flow pattern of the flowmeter will not be disturbed. The requirements for determining the length of the straight pipe section on the upstream side of the on-site throttle shall be considered in the three directions. First, consider that the first pipe seat on the upstream side of the throttle and the straight pipe section of the throttle meet the requirements; second, consider the throttling. Whether the distance between the straight pipe section between the first choke member and the second choke member on the upstream side meets the requirements; finally, consider that the straight pipe section between the choke member and the orifice plate above the second choke member is Apricot, meet the requirements. For the requirement to determine the length of the straight pipe section on the downstream side of the throttling element, it is only necessary to consider whether the first pipe block on the downstream side of the throttling piece and the straight pipe section of the throttling piece meet the requirements. When the length of the straight pipe section is less than the zero additional uncertainty value, and is equal to or less than the 0.5f additional uncertainty value, the measurement uncertainty of the throttling device should arithmetically add 0.5r~1.2 straight pipe section and roughness

The roughness of the orifice plate, standard nozzle, long-diameter nozzle and straight pipe section of Venturi nozzle refers to the roughness specification in the range of 10D upstream of the throttle. The inner surface of the upstream pipe of the orifice plate should meet the roughness requirements.

The length of the upstream venturi is at least equal to 2D, and the upstream buckle has a roughness Ra / Dlt; = 3.2 * 10-4.

1.3 The roundness of the pipe

The pipe must be round and must be rounded over the length of the shortest straight pipe section required. The straight pipe section can be a seam steel pipe, but the internal weld is parallel to the pipeline axis, which meets the special requirements of all throttle valves. The height of the weld is not greater than the allowable step length. Welds should not be located within 30 degrees; an industry centered on any single pressure faucet.

2. Use interchangeable holes and fixed value throttling

The fixed value throttling device changes the existing throttling loading method according to the calculation result to deal with its aperture. The orifice diameter measuring tube is equipped with a limited number of throttling components, and the aperture series is selected according to the priority number, and each path is selected. There are 35 different aperture ratios (3 orifice plates).

3. Adopt a reasonable flow accumulation plan

According to steam measurement conditions, a comprehensive solution of pressure, temperature and flow should be used for compensation or full compensation, and intelligent differential pressure transmitters, pressure transmitters, temperature transmitters and flow computers should be used for online detection. The system can compensate for temperature and pressure changes, thereby improving measurement accuracy and reducing additional errors in flow measurement caused by flow pulsation (or fluctuation). Orifice flowmeters usually have a 1:3 turndown ratio, and the actual measured steam flow changes sometimes exceed this range. In this case, the measurement accuracy is significantly reduced. If a fixed-value negative flow device is used, the wide-flow intelligent differential pressure transmitter can be used together with the flow computer to easily expand the flow range or migration range to achieve traditional standards. The intelligence of orifice flowmeter.

4. in conclusion 

If the user's measurement of steam flow is an orifice flowmeter, he must pay attention to the design, manufacture, installation and use must be carried out in strict accordance with the specifications. Each node error will cause a large error in the measurement result, but the development of orifice flowmeter is earlier, the technology is mature, and the degree of standardization is the highest. As long as we strictly follow the SY/T6143-2004 standard, the accuracy of the measurement results can be guaranteed.