Key points which need to consider when select differential pressure flow meter

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Differential pressure flow meter (hereinafter referred to as DPF) is a flow meter that measures the flow rate according to the differential pressure generated by the flow detection parts installed in the pipeline and the known fluid conditions and the geometrical dimensions of the detection parts and the pipeline. DPF consists of a flow detect device ( such as orifice, verabar, V-cone, Venturi, Nozzle, wedge, averaging pitot-tube ) and a secondary device (For example: Multivariable Differential Pressure Flow Transmitter ).

1. Key points of DPF selection:
1.1 Instrument performance
1.1.1 Accuracy, repeatability, linearity, flow range, rangeability

The standard throttling device has strict application range, including pipe diameter, throttling element’s bore diameter, diameter ratio, Reynolds number range, pipe wall roughness, etc. The outflow coefficient and expandability coefficient provided in the standard document can be applied to these ranges of using. It should be noted that the range of using for non-standard throttling element and their calculation formulas are for reference only. In general, it is better to do real flow calibration for use them reliably. The uncertainty of the standard nozzle’s outflow coefficient is much greater than that of the standard orifice plate, because it is more difficult to replicate the profile throttle parts accurately, if the standard nozzle is calibrated with real flow, high accuracy can be obtained. The accuracy of DPF is largely depends on the conditions of on-site. If the throttling element’s manufacture quality meet the requirement, there are two main factors affecting the accuracy of DPF: the determination of the fluid’s physical parameters and whether the fluid’s flow characteristics meet the requirements of the standard.

The accuracy of the whole flow meter also depends on the accuracy of differential pressure transmitter and flow display instrument. At present, there is a tendency to use high-precision differential pressure transmitter as far as possible. It can be seen in the calculation formula of flow measurement that when the accuracy of other parameters is not high, using the high-precision differential pressure transmitter can not play much role. The function of flow display instrument is mainly to monitor the stability of operation parameters, and its data conversion accuracy is generally no problem. Therefore, if want to improve the accuracy of measurement, a comprehensive estimate should be made so that the choice of the most technically economical option can be made.

DPF is a kind of instrument with strict requirements from design, manufacture to installation & using. The mistake in any link will cause a big error. If strictly follow the standard, the accurate measurement can be guaranteed. The repeatability of DPF is lower than that of other flow meters (electromagnetic, positive displacement, turbine, vortex , etc.). The reason is that the output signal is analog value, easy to be interfered, especially at the link of pressure tapping tube, the signal is easily to be interfered to produce fluctuation, which result in low repeatability, so the improvement of accuracy is affected.

The output signal of DPF is in square relation with the flow rate, it is a nonlinear instrument, which is the reason that the range ratio is narrow. In fact, the outflow coefficient of throttling element is stable in a wide Range of Reynolds number, therefore, at present using of two (or more) range of differential pressure transmitter can broaden its range ratio (greater than 10:1). The throttling element of elastic loading variable-area & variable-head which has been put into use in recent years applies other working principles to increase its range ratio (up to 100:1).

1.1.2 Pressure loss

The large pressure loss is one of DPF’s weaknesses, but there are some options available after careful analysis. Orifice plate and nozzle are the throttling element of larger pressure loss in DPF throttling elements, but there are also differences. At the same flow rate and β value, the pressure loss of the nozzle is only 30%-50% of the pressure loss of the orifice plate, that is to say, the nozzle is lower pressure loss. Various flow tubes (venturi, Doyle, Rolos, universal Venturi, etc.) are low-pressure loss throttling element, their pressure loss is only 20% of the pressure loss of the orifice plate, and even as low as 5%-10%. The development and application of these throttling elements is a direction of future efforts. Of course, another type of dynamic pressure head DPF ( velocity-averaging pitot tube flow meter) is known for low pressure loss.

 

2. Fluid characteristics

2.1 Determination of fluid physical parameters

The physical parameters of fluid include density, viscosity, isentropic index and humidity, etc. Some of these parameters directly enter the flow rate equation, and some affect the outflow coefficient and expansibility coefficient.

Among these parameters density is the most important. The requirement to accuracy of density is the same as to that of differential pressure. But it is difficult to accurately determine density, which is an important factor affecting the improvement of DPF accuracy. The reason for the poor accuracy of density is that density is usually determined by the functional relationship which includes composition, pressure, and temperature, but the accuracy of the functional relationship is often uncertain, especially for mixtures, it generally lacks precise functions.

In this case, it is better to use densimeter to measure, but unfortunately, the current specification of densimeter can not meet the actual needs, and the price is expensive, the reliability is not high also hinder its widespread use.

Viscosity can be less accurate, because it is used to calculate the Reynolds number, the Reynolds number is not sensitive to the effect of outflow coefficient. Another function of viscosity is to determine whether the measured medium is the Newtonian fluid, presently the measured medium be requested to be the Newtonian fluid by the flow measurement standards. Due to the development of petrochemical industry, more and more non-Newtonian fluids need to be measured, so the fluid’s viscosity will cause more attention in the future.

In addition to mixed fluids, the determination of fluid physical parameters has encountered difficulties under the condition of high pressure, high temperature, low temperature, etc. Many physical parameters lack the data under the high pressure, high temperature, low temperature, etc.

2.2 Fluid corrosion, abrasion, deposit, dirt, etc

These characteristics pose a great threat to the reliability of the flow meter using. DPF is used to determine the relationship between the flow rate and the output signal by geometric dimension, and keeping the geometric dimension constant becomes the key factor to guarantee the measurement accuracy in long-term use.

During using, geometric changes is not easy to detect, that is to say, the flow characteristics have changed and is not known, this is very dangerous. How to deal with this difficult problem is still being explored, for example, using the replaceable orifice plate is a measure which is taken for facilitate inspection, in addition, using of parallel measuring pipeline can do regularly checking and cleaning and so on. The above fluid characteristics need to be understood in order to take preventive measures, advise to do some tests if the situation is ambiguous.

 

3. Installation

In installation conditions, the necessary straight pipe length before and after throttling parts often embarrass the users, and the straight pipe required by the reasoning DPF is relatively long. In addition, the flow resistance devices on-site are far more than that the standard documents includes, especially the so-called combined flow resistance parts (the distance between two kinds of flow resistance parts is very short) is more difficult to solve, according to GB/T 2624-93 or ISO 5167-1, encountered this situation, can install the flow regulator to solve, however, installing the flow regulator require that the straight pipe length shall be very long (about 42D). In this case, the following solutions are selectable: use the throttling device which require shorter straight pipe length, such as classic venturi pipe or other flow pipe; the outflow coefficient under field conditions is determined by the real-stream verification method, which can be on-line or off-line.

Pressure tapping pipe is the weak link of throttling DPF, in recent years, the emergence of integrated throttling DPF has better solved this problem. For example, the heat preservation and anti-freezing for the pressure tapping pipe is very troublesome for the steam flow measurement in winter in north China, the integrated DPF has completely solved this difficulty. Most flow meters have integrated type and separated type, they have their own characteristics, type selection shall be according to the actual situation of the site, throttling DPF is no exception. The differential pressure transmitter of the integrated DPF must be adapted to the harsh environmental conditions in the field. In some cases, such as strong pipeline vibration or strong electromagnetic interference, it is better to use the separated type.

 

4. Environmental conditions

The differential pressure transmitter and flow indicator of DPF have microprocessor and electronic components, their requirements for environmental condition are the same as those of general electronic instruments.

 

5. Economic aspect

Economic factors include purchase cost, installation cost, operation cost, calibration cost, maintenance cost and spare parts cost.

5.1 Acquisition expenses

The test part of DPF are relatively cheap to purchase, but integrated with the differential pressure transmitter and the flow display instrument , the whole DPF is not necessarily cheap. In addition, it may need to purchase some auxiliary equipments, such as condensing pot, collector, settler and isolator should be considered according to the process condition of on-site.

5.2 Installation cost

The installation of separated DPF is more troublesome, mainly the installation of differential pressure signal pipeline and auxiliary equipment. For the corrosive and dirty media, need to adopt the isolation system, the cost is higher.

5.3 Running cost

For large diameter pipeline measurement, the operating cost of energy consumption may be a large number, of course, select the low-pressure loss throttling device (such as classic venturi pipe, etc.) is a way to reduce the cost, but the purchase cost of throttling device is high, so it should be carefully calculated cost.

5.4 Calibration cost

One of the advantages of DPF is that it can save the calibration cost of test part, and not only the manufacturer but also the user can avoid the trouble of real-stream calibration, which is of great significance. Of course, the calibration cost of the other two parts of DPF should also be considered. They are relatively convenient and cheap.

5.5 Maintenance cost

The test part of DPF has less maintenance cost, and the other two parts have certain maintenance cost.

5.6 Spare part

Differential pressure transmitter and display instrument have strong commonality. For large and meddle-size enterprises which need use a large number of flow meters, some models and specifications can be centrally selected to save the number of spare parts. In recent years, the promotion of fixed-value throttling parts makes the throttling device can get rid of the situation of matching tag number, and the purchase of detection parts is very convenient, which can reduce the number of spare parts.

The comprehensive calculation of the above costs can accurately determine the economic cost.

 

6. Selection principle of standard throttling device

6.1 The first choice for DPF’s test part is of course the Standard throttling device, in order to select the most suitable standard throttling device, the following aspects should be considered:

1) Limit conditions of pipe diameter, diameter ratio and Reynolds number range; 2) Measurement accuracy,
3) Allowable pressure loss,
4) The minimum straight pipe length which is required for installation,
5) Sensitivity to erosion, wear and dirt of the measured medium,
6) Complexity of structure and price,
7) Convenience of installation,
8) Long-term stability of using

6.2 According to the above aspects, the selection principle of standard throttling device can be summarized as follows.

1) The pipe diameter, diameter ratio and Reynolds number range of various types of throttling parts used in standard throttling devices have certain limit, which are specified in national standard GB/T 2624-93 or international standard ISO 5167-1. For example, orifice plates can be used in a larger pipe diameter range compared with nozzles and venturi nozzles, the application diameter range of various types of classic venturi varies greatly among.

2) The accuracy of various types of throttle parts in standard throttle device has relation with the uncertainty of outflow coefficient and expansibility coefficient under the same measurement accuracy of differential pressure and density. The uncertainty of outflow coefficient of various throttle parts varies greatly. In comparison, the uncertainty of orifice plate’s outflow coefficient is the smallest. And the outflow coefficient’s uncertainty of the arch throttle parts (nozzle, venturi pipe) is larger, the reason is that the quality of the fitting database on which the outflow coefficient formula given in the standard is based is poor. However, high accuracy can be obtained by individual calibration of the arch throttle device.

3) Under the same differential pressure, the pressure loss of the classic venturi tube and venturi nozzle is about 1/4 to 1/6 of that of the orifice plate and nozzle. At the same flow rate and the same beta value, the pressure loss of the nozzle is 30%-50% of the orifice plate.

4) Under the same choke type device and diameter ratio, the necessary straight pipe length of the classic venturi tube is much smaller than that of the orifice plate and nozzle.

5) The arch throttles are much better than orifice plates for measuring the medium which is prone to contaminate, wear the throttles and make throttles deform.

6) In processing, manufacturing and installation, the orifice plate is the simplest, followed by the nozzle, venturi nozzle and classic venturi tube is the most complex, and the cost is increasing in turn base on complexity. The larger the pipeline diameter is, the more significant the difference is.

7) The orifice plate is easy to take out and check the quality (adopt the throttle device of replaceable orifice plate). The nozzle and venturi pipe need to cut off the fluid and disassemble from the pipeline to check, which is quite troublesome.

8) For the throttling devices of small and middle size diameter (DN50-DN100), the size and position of pressure tapping have significant influence, so the ring chamber pressure tapping has certain advantages.

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