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How to perform large-scale range measurement on the throttling device

How the throttling device performs a large range range measurement summary:
In actual use, the throttling device is transmitted by the differential pressure transmitter signal, and there is a large measurement error when measuring a fluid with a large range. The use of a replacement orifice plate, the establishment of two sets of measuring devices and the use of two differential pressure transmitters can expand the measurement range of the orifice measuring fluid.
I. Introduction
The throttling device with the orifice plate and the nozzle as the throttle member is widely used in the current industrial production, and has the advantages of simple structure, convenient installation and maintenance, and long service life. The measuring range of the measuring fluid (the ratio of the maximum flow to the minimum flow) is generally 4..1, and in actual use, the measuring range of the throttling device required by some users is much larger than this ratio, and may reach 10..1 or more. Large, when measuring small flow, the numerical error of the orifice plate measurement is extremely large, and sometimes the minimum flow rate is basically not measured. This phenomenon is particularly evident in steam measurement. When the thermal power plant supplies steam to users, the amount of steam used in summer and winter is different. The amount of steam used in winter is much larger than that in summer. In this way, the orifice plate designed and manufactured according to the maximum flow rate in winter will be inaccurate when measuring the flow rate in summer. This article will discuss this issue in accordance with the standards ISO5167-1 and GB/T2624-93.
Second, the basic principle of flow measurement
When the pipe-filled fluid flows through the throttling device in the pipe, the stream will contract at the throttling, increasing the flow rate, lowering the static pressure, and creating a static pressure difference before and after the orifice. The greater the flow rate of the fluid, the greater the pressure differential. Depending on the relationship between the differential pressure and the flow rate, the orifice can be used to measure the flow of the fluid.
According to the standards ISO5167-1 and GB/T2624-93, the relationship between fluid flow and differential pressure is determined by:
Qm= ε d2 (1)
Qv= ε d2 (2)
Where: Qv - working state ** product flow (unit: m3 / s)
Qm — mass flow in working condition (unit: Kg/s)
C
- outflow coefficient
β — diameter ratio β=
D — upstream pipe inner diameter (mm) under working condition
ε - expansion coefficient
d — diameter of the throttle opening in the working condition (unit: m)
ΔP — differential pressure (unit: Pa)
ρ — density of fluid (unit: Kg/m3)
The formula for calculating the outflow coefficient C is given by the STOLZ equation:
C=0.5959+0.0312β2.1-0.184β8+0.0029β2.5(106/ReD)0.75
ReD is the pipeline Reynolds number
It can be seen from the above formula that under the premise of determining the opening value of the orifice plate, the parameters affecting the flow rate are the outflow coefficient, the expansion coefficient, the density, and the difference. Under the premise that the fluid state is determined to be constant, the outflow coefficient, expansion coefficient and density are fixed. The relationship with the flow value is the differential pressure value. The relationship is square. When the flow changes, the differential pressure will be The relationship of squares changes accordingly.
From the formula, we can see that when the measured minimum flow rate is 1/10 of the flow rate of the division, the differential pressure value is only 1/100 of the differential pressure value, and the differential pressure value measured by the differential pressure transmitter at this time. It is already very small, and it has been difficult to measure accurately.
Second, the method of expanding the flow measurement range of the throttling device
According to the above principle, when the throttling device is installed on a large-scale pipeline, the following methods can be used to achieve accurate measurement in different ranges.
1.
Segmentation measurement by changing the orifice plate
When performing a large range of measurement, the range of the throttle device can be segmented, that is, two throttling devices are designed for different flow measurement ranges according to the actual measurement needs, one for large flow measurement, one for Used for small flow measurements. For measuring the measurement of steam with a range ratio of 100000kg/h to 5000kg/h, the turndown ratio is 20..1, and the range ratio can be divided into 100000kg/h~20000kg/h and 20000kg/h~5000kg. /h to design the throttle, the measurement range ratio of the two throttling devices is 5..1 and 4..1 respectively, and the measurement accuracy of the throttling device can fully meet the measurement requirements. In this method, only two throttling devices need to be manufactured, and the throttling device can adopt the same differential pressure value by designing different opening ratios, so that the throttling device only needs to be equipped with a differential pressure after installation. The transmitter does not need to adjust the differential pressure value of the differential pressure transmitter.
When measuring by this method, it is necessary to replace the throttle device of the corresponding flow range according to the actual situation. After selecting the corresponding throttle, the pipeline needs to stop running, then replace the replacement throttle with the original throttle, and restart the system after installation.
With the above method, since it is only necessary to replace a single throttle, the cost is low. However, when the throttle is replaced, the system must be stopped. If the replacement is frequent, it will affect the user's production. It can only be used in places where the flow measurement changes stably. For example, in winter and summer, there are different flow values. The unit can be used for maintenance once every six months. The opportunity to replace it. It is not applicable to occasions where traffic changes frequently.
2.
Establish parallel pipeline installation two sets of throttling devices
Two sets of throttling measuring devices are installed side by side at the installation of the throttling device, and the throttling members of the two measuring ranges are respectively used for measurement. The basic principle is similar to the first method, but this method does not require the replacement of the throttle.
Although this method can achieve a large-scale measurement and does not have the impact on production caused by frequent replacement of the throttle, the cost caused by this method is higher, because it is necessary to rearrange one when using this method. Pipeline (according to the requirements of ISO5167-1 and GB/T2624-93 standards, this section of pipe has considerable length requirements), adding a set of throttling devices and corresponding secondary instruments such as valves and differential pressure transmitters. This method is not recommended for large-scale measurement if not necessary.
3.
Transmitter with different differential pressure scales using the same set of throttling devices
In addition to the above two methods, it is also possible to adjust the measurement range of the differential pressure transmitter by setting the opening value of the segment to achieve the purpose of measuring the fluid in different ranges.
It can be known from the above formula that when the working state does not change, the opening and differential pressure values of the regulating throttle can keep the flow rate constant. According to this principle, the range of the throttle device can be performed when performing the large-scale ratio measurement. For the measurement of steam with a range ratio of 100000kg/h to 5000kg/h, the range ratio can be divided into 100000kg/h to 20000kg/h and 20000kg/h to 5000kg/h for throttling. Design, but keep the opening of the throttle constant. If the differential pressure is set to 100KPa when measuring 100000kg/h, the turndown ratio is 20..1 when the measurement is not segmented, and the maximum differential pressure of the differential pressure transmitter is 100KPa, and the ratio of the minimum measured differential pressure is 0.25KPa. At this time, when the measurement is performed according to the maximum measurement differential pressure, the differential pressure transmitter is basically not detected at the minimum flow rate. In the segmentation measurement, the measuring range ratio of the throttle device of 100000kg/h to 20000kg/h range is 5..1, the maximum differential pressure is 100KPa, and the ratio of the minimum differential pressure is 4KPa, and the range is from 20000kg/h to 5000kg/h. The measuring range ratio of the section throttling device is 4..1, the maximum differential pressure is 4KPa, and the ratio of the minimum measured differential pressure is 0.25KPa. At this time, the differential pressure transmitter can meet the measurement accuracy requirements.
When using this method for measurement, two different differential pressure range transmitters should be used for measurement. In the above example, a transmitter with a differential pressure range of 100KPa and a differential pressure range of 4KPa is required. At the same time, take two pairs of pressure-receiving holes on the throttle to install the transmitter (the specific installation method can be operated according to the requirements of ISO5167-1 and GB/T2624-93), and use different differential pressure transmissions at different flow rates. The device performs measurement. When the flow rate is large, the transmitter with a large measuring range is used. When the flow rate is small, the transmitter with a small measuring range is used, and the replacement of the two transmitters is adjusted by opening and closing the valve.
In this way, two transmitters are required, which will result in higher costs, but it is not necessary to stop for the replacement of the throttle, so it has considerable applicability.
Fourth, the conclusion
Through the three methods described in this paper, the throttling device can be adapted to the purpose of measuring a large range of ratios, but the three methods have their advantages and disadvantages.
When the first method is used, the cost is small, but the replacement of the throttle is troublesome and affects the user, and cannot be used in a situation where the flow changes frequently.
The use of the second method results in higher costs because of the need to construct a length of pipe and the corresponding throttling devices, valves and secondary instruments, but this approach does not cause a pause in production operations. Moreover, the internal leakage of the valve will not be caused due to the frequent opening and closing of the valve, resulting in misalignment of the meter.
In the third way, by using two transmitters to achieve the purpose, the cost of one transmitter will be increased, but the adjustment of the flow measurement can be achieved only by opening and closing the valve to achieve the purpose, flexible and convenient operation. , has a certain applicability.
The three methods described in this article have their own advantages and disadvantages. Different customers can judge which method to use according to the actual needs of production when making choices.

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