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Measure and Control Scheme for Large Diameter and Small Flow

Date: 2019-03-19
浏览次数: 5

In the process pipelines of many enterprises, there are a large number of phenomena of low actual flow rate or low flow rate of fluid. The existence of this phenomenon is mainly due to the following reasons: first, reducing flow rate can reduce pressure loss in fluid flow, reduce driving power and save energy;  The second is to consider the future development and expand the scale, leaving sufficient margin in the design to make the process pipeline larger.  Third, some pipelines have high working pressure. Although the flow rate is not small, the flow rate is low.  ,

The existence of these 'large pipe diameter and small flow' or 'large pipe diameter and low flow rate' brings difficulties to the selection of velocity flowmeter.  In an enterprise, a DN300 water delivery pipeline was originally equipped with a through-hole flowmeter and an industrial water meter. Because the flow rate is too small, there is almost no indication for both meters.  After on-site verification, the sum of the maximum water consumption of the water-using equipment and several water-using equipment is less than 80m3/h, while the minimum flow is only 18m3/h.  At the maximum flow rate, the lower limit flow rate of DN300 flowmeter is less than 0.1m/s.  Therefore, it is suggested to shrink the on-site process pipeline, reduce the diameter of DN300 pipeline to DN100, and select the vortex flowmeter of DN100.  After installation, both the upper limit and the lower limit of water consumption are met.  Another example is a heating furnace that produces high-pressure porcelain bottles. In the heating stage, the amount of natural gas required is 1200Nm3/h(101.325kPa, 20℃). In the heat preservation stage, the amount of natural gas required is 150 Nm3/h. The diameter of the metering main pipe is DN80, the working pressure is 0.3MPa, and the temperature is 25℃.  Orifice flowmeter was originally selected, but vortex flowmeter was selected instead because the flow range could not meet the requirements.  When selecting the meter, the standard state flow rate is converted into the working condition flow rate by the gaseous equation. The converted working condition flow rate ranges from 38.5 m3/h to 308 m3/h. If the vortex flowmeter of N80 is selected according to the original process pipeline in this flow rate range, the upper limit is no problem, but the lower limit cannot meet the requirements.  If DN50 vortex flowmeter is selected, the upper and lower flow rates can meet the requirements.  In practice, DN50 is used instead of shrink tube, and the effect is very good.

Although vortex flowmeter has the characteristic of wide measuring range, its lower limit velocity cannot be too low.  In general, the lower limit flow rate for measuring liquid is 0.3-0.5m/s;  While the lower limit flow rate is 3-5m/s when measuring gas.  In many sites, the lowest flow rate in the process pipeline is much lower than the above value.  In order to meet the measurement requirements, it is necessary to shrink the process piping on site.

There are a large number of similar problems in the field. Shrinking the pipe can effectively solve the actual flow measurement problem.  However, shrinking pipes also adds trouble to customers. Some customers do not have ready-made pipes. It takes a lot of effort to purchase a suitable section of pipes and increases the installation workload and cost.  If the manufacturer directly provides the reduced diameter vortex flowmeter to the customer according to the flow parameters provided by the customer.  For customers, it can not only measure smaller flow, but also simplify installation and save installation costs.  At the same time, for the manufacturer, it provides appropriate instruments to customers, reduces on-site service, satisfies customers and improves the reputation of the enterprise.  Therefore, this is the best plan and a win-win choice.

The practice of providing customers with complete sets of vortex flowmeter by reducing diameter originated in the early 1990s in China.  According to the on-site parameters provided by the customer, a vortex flowmeter assembly with a contraction section and an expansion section is manufactured according to a certain contraction ratio, factory calibration is carried out together with the contraction section and the expansion section, and meter coefficients of the vortex flowmeter assembly are provided to the user.  Now it is an improvement to directly make the components into an integrated instrument.

A DN40 vortex flowmeter has a measuring range ratio of 8: 1 when the accuracy of DN40 experimental pipeline calibration is 1%.  However, after the DN50-DN40 contraction section is added in front of the instrument, it is installed on the DN50 test pipeline for testing. Due to the rectification effect of the contraction section, the range ratio reaches 15:1 when the accuracy reaches 1%.

The DN80- DN50 contraction section and DN80- DN40 contraction section are respectively used for experiments on DN80 test pipelines.  The upstream flow blocking piece of vortex flowmeter is in the form of two 900 elbows in the same plane, and the length of straight pipe section is 3D;  Under the condition that the downstream straight pipe section is 2D, the deviation of instrument coefficient K is only 0.7%.  However, if the vortex flowmeter with the same diameter is not equipped with a contraction section, under the condition of two elbows in the same plane, when the length of the upstream straight pipe section is 8D, the instrument coefficient deviation is 2.0%.

The measuring tube of the reduced diameter vortex flowmeter consists of a contraction section, a measurement section and an expansion section, and adopts a linear contraction section. Although the effect is not as good as that of the Vickers curve contraction section, the processing is simple and the cost is low.  Therefore, there are many direct contraction sections currently used.

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