Comparison between Cone Flow Meter and Orifice Flow Meter

As a flow measurement instrument, differential pressure flowmeter has the characteristics of simple structure, long service life and wide adaptability, so it is widely used in power, chemical, coal, metallurgy, steel and other industries. It is estimated that differential pressure flowmeter accounts for about 60%~70% of flow meters. As an emerging force in the 1980s, cone flowmeter has been gradually accepted by the market with its unique advantages. This article will introduce the principle and characteristics of cone flowmeter in detail, as well as the comparison with orifice plate.

1 Principle Introduction

1.1 Overview

The cone flow meter (see Figure 1) is a new type of high-precision flow meter that can measure various Reynolds numbers and meet the application conditions of various media. Its operating principle is the same as other types of differential pressure principles, all based on the principle of energy conservation in a sealed pipe. The cone flow meter has a unique structural design, so its performance is better.

The cone flowmeter hangs a cone throttling piece at the center of the pipeline. The cone hinders the flow of the medium and reshapes the flow velocity curve. A negative pressure area can be formed immediately downstream of the cone. There is a pressure difference between the positive pressure upstream of the pipeline and the negative pressure downstream after throttling by the throttling piece. The positive and negative pressures are taken out with the pressure tapping port. The positive pressure port is located upstream of the pipeline, and the negative pressure port is located at the end of the cone. By measuring the differential pressure between the two, the flow rate in the pipeline can be calculated according to the Bernoulli equation. The cone is located in the center of the pipeline, which can optimize the flow velocity curve of the measured medium, so the measurement accuracy is high and the straight pipe section requirements upstream and downstream of the instrument are low.

1.2 Principle of operation

The cone flowmeter is a differential pressure flowmeter. So far, the flowmeter designed based on the differential pressure principle has been used for more than 100 years. The differential pressure principle is based on the energy conversion principle in the sealed pipeline, that is, the stable fluid and flow rate are proportional to the square root of the medium flow rate in the pipeline. When the pressure decreases, the speed will increase. When the medium approaches the cone, its pressure is p1. When the medium passes through the throttling area of ​​the cone, the speed will increase and the pressure will decrease to p2. Both p1 and p2 are led to the differential pressure transmitter connected to the back through the pressure tapping port of the cone flowmeter. When the flow rate changes, the differential pressure value between the two pressure tapping ports of the cone flowmeter will increase or decrease. When the flow rate is the same, if the throttling area is large, the differential pressure value generated is also large. The β value is equal to the throttling area of ​​the cone divided by the cross-sectional area of ​​the inner diameter of the pipe (which can be converted into the diameter ratio between the two).

1.3 Reshaping the flow velocity curve

The calculation formula used by the cone flowmeter for flow calculation is the same as that of other differential pressure flow meters, but the structure of the throttling device is completely different from other meters. It is realized by hanging a cone in the center of the pipeline. The cone can force the medium in the center of the pipeline to flow around the cone, which has many advantages compared with other traditional differential pressure meters. In addition, if the medium passes through a very long pipeline without any obstruction or interference in the pipeline, its flow velocity distribution is very uniform.

The flow velocity of the medium passing through the pipe diameter is different at each point. The flow velocity near the pipe wall is almost zero, and the flow velocity in the center of the pipe is the largest. This is caused by the friction of the pipe wall on the medium. Since the cone is suspended in the center of the pipeline, it is directly in contact with the high-speed area of ​​the fluid, forcing the fluid in the high-speed area to mix with the fluid in the low-speed area, thereby making the flow velocity uniform and reducing the fluid velocity in the high-speed area. This is also the main reason why the cone flowmeter can measure low-flow fluids. Since the throttling elements of other types of differential pressure flowmeters do not contact the high-speed medium in the center of the pipe, there may be no differential pressure signal when the medium flow rate is very low.

Under normal working conditions, it is difficult to evenly distribute the flow rate. Any changes in the pipeline may affect the fluid, such as elbows, valves, reductions, expansions, pumps, tees, etc. For other instruments, this is a difficult problem to solve, but the cone of the cone flowmeter reshapes the upstream flow velocity distribution curve, so that the flow rate basically reaches the ideal state.

The cone flow meter can make the fluid evenly distributed under extremely harsh conditions (such as single elbow or double elbow pipe just upstream of the meter) to ensure higher measurement accuracy.

2 Advantages of cone flowmeter and comparison with traditional orifice flowmeter

2.1 Accuracy

Cone flowmeter has high accuracy: the accuracy can reach ±0.5%.

The orifice flowmeter has low accuracy: the accuracy is basically below ±3%.

2.2 Repeatability

Cone flowmeter has good repeatability: better than ±0.1%. Because the cone shapes the flow velocity, the flow velocity reaches an ideal state, with fewer interference sources, so the repeatability is good.

The orifice flowmeter relies on the straight pipe section to shape the fluid. The straight pipe section shaping can only achieve a state that is relatively close to the completely ideal state of the fluid, and the throttling of the orifice plate itself destroys the ideal state of the fluid. Therefore, there are many interference sources and poor repeatability.

2.3 Installation Requirements

The installation requirements of cone flowmeter are low: 0d~3d in front and 0d~1d in the back. Whether it is a pump, compressor, valve or elbow (a single elbow or two double elbows not on the same plane), it has basically no effect on the measurement accuracy.

The installation requirements of orifice flowmeter are high: generally 10 days before and 5 days after. In complex working conditions, it should be increased to 20 days before and 5 days after.

2.4 Long-term stability

Cone flowmeter has good long-term stability: the fluid flows through the cone without sudden fluctuations, but forms a boundary layer along the cone and guides the fluid away from the back angle of the cone. Therefore, the cone angle is not worn by unclean fluid, the β value can remain unchanged for a long time, and long-term accurate measurement is guaranteed.

The orifice flowmeter has poor long-term stability: As the fluid flows through the sharp edge of the orifice, the interception produces high-speed friction, causing the orifice to wear, and dirt changes the size of the orifice, causing the β value to change, and it cannot maintain long-term accurate measurement.

2.5 Signal stability

The signal of the cone flowmeter is stable: the "signal fluctuation" is 1/10 of the orifice plate. The fluid flows through the flowmeter to form very short vortices, which make the flowmeter produce high-frequency and low-amplitude signals, and the signals flow to the center at the tail of the cone and cancel each other out, so the interference is small.

The orifice flowmeter signal is unstable: the vortices generated by the fluid flowing through the plane are long. These long vortices cause the orifice plate to produce low-frequency and large-amplitude signals, which have large signal interference and will seriously interfere with the accuracy of the differential pressure reading.

2.6 Pressure loss and range

The cone flowmeter has low pressure loss and wide range: the range ratio is usually 15:1~50:1. This is because the streamlined design of the cone greatly reduces the pressure loss, which can be as small as 0.06 kpa. Among all differential pressure flowmeters, only the cone flowmeter has a pressure loss close to that of the Venturi. Since there is no sharp edge, the permanent pressure loss caused by the cone flowmeter is constant and smaller than that of the orifice. At the same time, the extremely stable signal makes the lower limit of the differential pressure range much lower than that of ordinary differential pressure flowmeters, so the range can be extended to the lower limit, and the signal linearity can be maintained at a Reynolds number as low as 8,000. If curve correction is used, it can still be measured and good repeatability can be guaranteed under lower Reynolds number conditions.

Orifice flowmeter has large pressure loss and small range: usually the range ratio is 3:1~5:1. This is due to the obstruction of the plane and the sharp edge, so the pressure loss is large, resulting in large fluctuations and interference of small signals, and small ranges cannot be measured.

2.7 β value range and differential pressure

Cone flowmeter has a wide range of β values: The unique geometry of the cone flowmeter allows a wide range of β values. The standard β values ​​are 0.450, 0.550, 0.650, 0.750 and 0.850, and specific β values ​​can be set to ensure a specific differential pressure output and a larger differential pressure signal. The full-scale differential pressure signal ranges from 0.1 kPa to tens of kPa, ensuring the accuracy of the measurement.

2.8 Pipeline scope and form

Cone flowmeter has wide pipeline range: There are two basic types of cone flowmeters, namely pipeline type and insertion type. The pipeline cone flowmeter ranges from 1/2" to 60", while the insertion cone flowmeter ranges from 6" to 72".

2.9 Effects of dirt and maintenance

The conical flowmeter has no stagnant area and is maintenance-free for a long time: the conical streamlined thorough purge design avoids the retention of residues, condensates or particles in the fluid, and can keep the cone clean for a long time. Because the cone shapes the fluid, the flow rate of the fluid on the pipe wall is accelerated, and the dirt retention at the positive pressure port is reduced. The chamfered design accelerates the fluid to leave after flowing through the cone, and the negative pressure port will not be contaminated. Therefore, no maintenance and cleaning is required for a long time (2 to 3 years), and accurate measurement can be guaranteed.

Practice has proved that cone flowmeters can be successfully used in the measurement of particularly dirty media such as coke oven gas and residual oil, and have been widely used in the measurement of coke oven gas flow in steel plants.

The orifice flowmeter is a flat surface where dirt easily accumulates. It generally needs to be cleaned once every three months to ensure its accuracy.

2.10 Can measure high temperature and high pressure media

Cone flowmeters use different materials for different temperatures and pressures. The highest operating temperature can reach 700°C, and the maximum pressure can reach 30 MPa.

2.11 Gas-liquid two-phase medium (wet gas)

Measuring wet gas (gas containing water) fluids has always been a difficult point in flow measurement. The cone flowmeter uses a unique cone design, which allows most of the water contained in the gas to move quickly along the cone from the higher temperature in the center of the pipe to the lower temperature around it, thereby generating condensation, which drips along the cone to the bottom of the pipe. Since the cone shapes the fluid, the flow rate of the fluid on the pipe wall is accelerated, and the water also passes quickly, and no large amount of condensation will be generated at the pressure tapping port. Therefore, the gas contains a small amount of water, which generates less interference signals to the pressure tapping port of the flowmeter, improving the measurement accuracy. However, it is better that the water content in the gas does not exceed 5%, and should not exceed 10% at most.

The orifice flowmeter has a fluid blocking design, which produces dripping in the center of the orifice. At the same time, the fluid on the tube wall is blocked, the flow rate is slowed down, and the condensation near the positive pressure port will affect the pressure signal; the eddy current generated by the fluid after flowing through the orifice plate will diverge toward the tube wall in a wave-like manner, affecting the pressure signal of the negative pressure port. Therefore, moisture will have a greater negative impact on the measurement accuracy of the orifice plate, thereby affecting the measurement. Therefore, it is often difficult to achieve the expected effect when measuring moisture using orifice plates and other methods.

In summary, in the foreseeable future, cone flowmeters will play an increasingly important role.