Installation conditions

To apply the standard document (GB / T 2624 or ISO 5167) in the outflow coefficient and expandability coefficient, must be put into use with the standard throttle device to achieve geometrical similarity and dynamics similar to the installation conditions on site is to achieve the two similar important factors, therefore, the installation of the throttle device should be given sufficient attention.

　　In the installation conditions before and after the necessary length of the straight pipe before and after the throttle is often difficult for the selector, in the reasoning flowmeter in the throttle DPF requires a long straight pipe section. In addition, the type of field flow stopper is much more than the standard documents include, especially the so-called combined flow stopper (two kinds of flow stopper interval is very short) is more difficult to solve, in accordance with GB/T 2624-93 (or ISO 5167-1) encountered in such cases can be used to add a flow adjuster to solve the problem, but the addition of the flow adjuster required by a very long straight section of the pipe (about 42D). In this case, the following options are available: the use of a throttle device that requires a shorter straight pipe length, such as a classical venturi or other flow tube; the use of real flow calibration to determine the outflow coefficient under field conditions, which can be either on-line or off-line.

　　We have talked about the pilot line is the weak link of the throttle DPF, in recent years the emergence of one-piece throttle DPF better solve this problem, such as China's northern winter steam flow measurement is often for the pilot line insulation against freezing brainstorming, one-piece DPF to solve it. Most of the flowmeter has a one-piece and separate two types, their use has its own characteristics, according to the actual situation to be selected, throttle DPF is no exception. One-piece DPF differential pressure transmitter must be adapted to the site of the harsh environmental conditions, in some cases, such as pipeline vibration or strong electromagnetic interference or separate type is better.

Environmental conditions

The two parts of the DPF, the differential pressure transmitter and the flow indicator transmitter, have microprocessors and electronic components, and their requirements for environmental conditions are the same as those for general electronic instrumentation, which have been discussed in other chapters of this book and will not be repeated here.

Economic factors

The economic factors include acquisition cost, installation cost, operation cost, calibration cost, maintenance cost and spare parts.

　　(1) Purchase cost

　　The acquisition cost of the detection parts of DPF is relatively cheap, but it is not necessarily cheap to consider the remaining two parts: the differential pressure transmitter and the flow display instrument as a whole. In addition, it may also need to purchase some auxiliary equipment, such as condensers, gas collectors, settlers and isolators should also be estimated.

　　(2) installation costs

　　Separation type DPF installation is more troublesome, mainly differential pressure signal piping and auxiliary equipment installation, for corrosive dirty media such as isolation system, the cost is higher.

　　(3) Operating costs

　　For large-diameter pipeline measurement, energy consumption generated by the operating costs may be a large number, of course, such as the choice of low-pressure loss throttle (classic venturi, etc.) is also a way to reduce costs, but the acquisition of throttle devices and high purchase costs, should be carefully accounted for.

　　(4) Calibration fee

　　One of the advantages of DPF is to save the test pieces of the calibration fee, not only the manufacturer, the user can also be free from the actual flow calibration trouble, this point is far-reaching significance. Of course, the remaining two parts of the DPF calibration fee should also be considered, they are relatively convenient and cheap.

　　(5) Maintenance

　　DPF test pieces of maintenance costs less, the remaining two parts have a certain maintenance costs.

　　(6) spare parts

　　DPF differential pressure and display instrumentation versatility, for large and medium-sized enterprises to use a larger number of flow meters can focus on the selection of certain model specifications to save the number of spare parts. In recent years, the promotion of fixed-value throttle parts so that the throttle device to get rid of the situation of the right number, the purchase of test parts is very convenient, can reduce the number of spare parts.

　　The comprehensive calculation of the above costs can be more accurately determine its economy.

Principles of selection of standard throttling devices

The test piece of the throttled DPF is of course the standard throttling device, in order to select the most suitable standard throttling device, the selection should be considered from the following aspects:

　　1) Limitations of pipe diameter, diameter ratio and Reynolds number range;

　　2)Measurement accuracy;

　　3)Allowable pressure loss;

　　4) the required minimum length of straight pipe section;

　　5) sensitivity to erosion, abrasion and fouling of the measured medium;

　　6)complexity and price of the structure;

　　7) the ease of installation;

　　8) long-term stability of use.

According to the above aspects, standard throttle device selection principles can be summarized as follows.

　　Standard throttling device of various types of throttling devices applied to the pipe diameter, diameter ratio and Reynolds number range have certain limitations in the national standard GB/T 2624-93 (or international standard ISO 5167-1) has detailed provisions, such as orifice plate flow measurement can be applied to a larger range of pipe diameters than the nozzle and Venturi nozzle, the classic Venturi tube between the various types of pipe diameter range of large differences and so on.

　　The accuracy of the various types of standard throttling devices is determined by the uncertainty of the outflow coefficient with respect to the expandability coefficient for the same differential pressure and density measurement accuracy. Various throttle parts of the outflow coefficient of uncertainty varies greatly, compared to the outflow coefficient of the orifice plate of the uncertainty of the smallest, the contour of the throttle parts (nozzles, venturi) larger. The reason for the larger profile throttles is the poor quality of the fitted database on which the outflow coefficient equations given in the standard are based. However, an individual calibration of the contour throttles also results in a high degree of accuracy.

　　At the same differential pressure, the pressure loss of a classical venturi and venturi nozzle is about 1/4-1/6 that of an orifice plate with a nozzle, whereas the pressure loss of a nozzle at the same flow rate and the same β-value is only 30-50% of that of an orifice plate.

　　For the same type of flow stopper and diameter ratio, the necessary straight section length of a classical venturi is much smaller than that of an orifice plate and a nozzle.

　　Measurement of easy to make the throttle fouling, wear and deformation of the measured medium, the contour of the throttle than the orifice plate flow measurement is much more superior.

　　In processing and manufacturing and installation, the most simple orifice plate, followed by nozzle, venturi nozzle and the most complex classic venturi, its cost also increases. The larger the pipe diameter, the more significant this difference.

　　Orifice plate is easy to take out to check the quality of the throttle parts (using replaceable orifice plate throttle device), nozzle and venturi need to cut off the fluid, remove the pipe to check, more trouble.

　　Small diameter (DN50-DN100) throttling device, take the pressure port size and take the pressure of the location of the impact of significant, then the use of ring chamber pressure has certain advantages.