Comparative Study of V-cone Flowmeter and Orifice Flowmeter

0 Preface

For a long time, orifice flowmeters have been widely used due to their mature technology, high degree of standardization, simple structure, and easy maintenance. Orifice flowmeters are also often used in offshore oil projects. Taking the gas outlet measurement of the test separator of the oil production platform as an example, since the medium is gas, the pressure is generally high, the medium composition is complex, and the flow range varies greatly with the years, it is determined that the orifice flowmeter is the most suitable for this occasion. In particular, the replaceable orifice plate that has appeared in recent years has solved the disadvantage of the low range ratio of the original orifice flowmeter and has been widely used. However, the replaceable orifice plate is expensive, generally at least 5 times the price of the orifice flowmeter with the same diameter. So is there a better product to replace it? The author believes that the v-cone flowmeter is the preferred product to replace it.

1 v cone flow meter advantages

The v-cone flowmeter is a flowmeter that has only appeared in China since around 2000. Its working principle is the same as that of the orifice flowmeter. When the medium passes through the v-cone, due to the existence of the flow-blocking component v-cone, the flow area of ​​the fluid changes and the flow velocity changes. According to the Bernoulli equation, the change in flow velocity causes a change in pressure, and there is a certain relationship between the change in pressure and the flow velocity. The purpose of measuring flow is achieved by measuring the pressure difference. Although the principle is the same as that of the orifice plate, the most essential difference is that the orifice plate is a center contraction type throttling device, while the v-cone is a side wall contraction type throttling device.

As we all know, the narrow measuring range is the main reason that restricts the use of orifice plates. The main reason for the narrow measuring range is that the flow coefficient of the orifice plate changes greatly with the change of the Reynolds number, and the sudden throttling of the orifice plate causes a relatively large vortex behind the orifice plate. The existence of the vortex directly leads to a relatively large signal noise, thereby limiting its measuring range. The V-cone flowmeter overcomes the above shortcomings.

As shown in Figure 1, the v-cone flow sensor is composed of an internal throttling part v-cone coaxial with the pipeline and a pressure-taking device. When the fluid passes through, the flow state of the fluid does not change suddenly like the orifice plate, but the medium shrinks toward the side wall along the direction of the cone. The cone plays a role in adjusting the flow state of the medium, reshaping the flow curve, and making the flow of the fluid more stable. In addition, due to the existence of inertial force, the medium does not directly flush the edge of the cone when passing through the edge of the v-cone. Therefore, the edge of the v-cone is not as easy to wear as the orifice plate, and the service life of the v-cone is long. Due to the unique structure of the v-cone, the v-cone flowmeter has many advantages over the orifice flowmeter.

1.1 Wide range ratio

Mccrometer, the earliest manufacturer of V-cones, conducted a series of tests and research and concluded that the signal-to-noise ratio of V-cones is much smaller than that of orifice plates. Due to the small signal-to-noise ratio, V-cones can accurately measure small flow rates even when the measured differential pressure is under small working conditions. Therefore, it is not necessary to increase the damping of the transmitter to improve the influence of signal noise like the orifice plate. Because of this, the range ratio of V-cones is much larger than that of orifice plates. Professor Sn Singh of the Indian Institute of Technology conducted experiments using oil and water at different Reynolds numbers and concluded that within a certain measurement range, the outflow coefficient of V-cones is almost independent of the Reynolds number. The author has also participated in the calibration of hundreds of V-cone flowmeters. It can be seen from the data that generally within the accuracy range of 1.0, the range ratio of V-cones can reach 10:1 using a single differential pressure transmitter.

1.2 Low straight pipe section requirements

The self-rectification function of the V-cone makes its straight pipe section requirement lower than that of the orifice plate. Professor Sn Singh of the Indian Institute of Technology used oil and water respectively, and placed valves of different openings at different upstream locations for testing. The final conclusion was: if the upstream interference occurs 10 days upstream of the V-cone flowmeter, or farther away, then this asymmetric velocity distribution will not affect the value of the discharge coefficient, and the discharge coefficient will remain at the same value obtained under ideal conditions; if the location of the upstream interference source is less than 10 days (such as 5 days) from the V-cone flowmeter, the maximum change in the discharge coefficient is 6%. It can be seen that as long as there is a 10-day straight pipe section in front of the V-cone, no matter what kind of throttling device is in front, it will not affect its measurement. ISO-5167 stipulates that the straight pipe section requirements of the orifice plate vary greatly depending on the different throttling devices in front, and generally require at least 20 days. For example, the gas outlet of the test separator mentioned above generally uses a 4-inch pipeline. For the V-cone, only a 1-meter straight pipe section is required in front and a 0.5-meter straight pipe section is required in the back.

1.3 Small pressure loss

Pressure loss is the irreversible pressure, which is one of the important indicators to be considered when selecting a flow meter. In today's society where energy is scarce and energy conservation and emission reduction are advocated, this parameter is particularly important. Since the shape of the edge of the V-cone determines the flow state of the fluid, the pressure loss of the fluid is much smaller than the pressure loss when using the orifice plate. And because of the self-rectification function of the V-cone, there is no need to add a rectifier in front of it, while the orifice plate requires a longer straight pipe section to adjust the flow state of the fluid. The space on the offshore oil platform is small and the land is expensive. It is difficult to meet the requirements of the straight pipe section. It is often necessary to add a rectifier to ensure the flow state of the medium. However, the disadvantage of the high pressure loss of the rectifier is that it consumes more energy. Even if the rectifier is not considered, at the same flow rate, the pressure loss of the V-cone is generally only 1/2~1/3 of the orifice plate.

1.4 High precision

Since the V-cone structure has no moving parts, the V-cone has good repeatability, generally less than 0.1%. Due to its wide range ratio, that is, its linearity is good, its accuracy is high. According to GB17167-2006 "General Rules for the Equipment and Management of Energy Metering Instruments in Energy-consuming Units", the accuracy level requirements for flow meters used for energy measurement are: coal gas and natural gas are 2.0 levels. Therefore, in the gas measurement at the outlet of the test separator, within the range of 10:1, the overall error of the V-cone plus the differential pressure transmitter and the secondary meter will not be less than 1.5 levels, which can fully meet the measurement requirements.

1.5 Good long-term performance

Because the contour shape of the cone restricts the flow of the medium, the fluid forms a boundary layer along the cone, which means that the edge of the cone is not easily worn by unclean fluid, so that the opening ratio β remains unchanged, allowing the flow meter to maintain accuracy for a long time without calibration.

Disadvantages of 2 V Cone Flow Meter

Although the V-cone flowmeter has many advantages, it still has some shortcomings.

2.1 Calibration required

The orifice plate is the only flowmeter that can guarantee its measurement accuracy by ensuring the geometric dimensions. The V-cone is a new type of flowmeter. Although many manufacturers and researchers have done a lot of experiments, the national standards that the V-cone flowmeter can follow are limited so far. In addition, due to confidentiality and patent restrictions, there is a lack of available literature on the structural design of the V-cone flowmeter. Each manufacturer produces according to the enterprise standard, and it can be said that the products are mixed, and not every manufacturer's products are reliable. Due to insufficient test data, each V-cone flowmeter must be strictly calibrated before leaving the factory. This also increases its cost.

2.2 High price

The price of a v-cone flowmeter of the same specification is higher than that of an orifice flowmeter, but lower than that of a replaceable orifice flowmeter. The price of a v-cone is relatively high due to the large amount of materials used, complex processing technology, and high calibration costs. However, when considering a flowmeter, the overall evaluation of comprehensive factors such as the initial purchase price and installation costs, the reliability of the instrument, long-term maintenance costs, and permanent pressure loss is the basis for selecting a flowmeter. Only by comprehensively considering and weighing these important factors can a reasonable selection be made. As mentioned above, except for the initial purchase cost of a v-cone flowmeter being higher than that of an orifice flowmeter, the other costs should be the same as or lower than those of an orifice flowmeter. Especially in terms of the energy consumption required to compensate for pressure loss, a v-cone flowmeter is far superior to an orifice flowmeter. Therefore, in the long run, the comprehensive cost of using a v-cone flowmeter is lower than that of an orifice flowmeter.

The purpose of using a replaceable orifice plate is to increase the range ratio of the orifice plate. However, it is expensive. If the V-cone flowmeter is equipped with two transmitters with different ranges, the range ratio expansion is much greater than that of a replaceable orifice plate flowmeter, and its price is much lower than that of a replaceable orifice plate.

2.3 Low differential pressure

Since the outflow coefficient of the V-cone flowmeter is greater than that of the orifice flowmeter, the differential pressure value of the orifice plate is higher than that of the V-cone at the same flow rate. The transmitter of the orifice flowmeter generally uses a medium differential pressure gauge, while the V-cone flowmeter generally uses a low differential pressure gauge.

3 Conclusion

In summary, it can be seen that in many cases, using a V-cone flowmeter instead of an orifice flowmeter should have more advantages than disadvantages. For the gas measurement at the outlet of the offshore platform test separator mentioned at the beginning of the article, the author believes that choosing an orifice flowmeter or a replaceable orifice flowmeter is not the best choice. A V-cone flowmeter should be more suitable for this working condition. Of course, for other working conditions, specific problems need to be analyzed specifically and cannot be generalized.