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Comparison of conventional and small volume tubes

(Summary description)  Conventional volume tube refers to unidirectional or bidirectional ball volume tube, domestic model LJG.

Comparison of conventional and small volume tubes

(Summary description)  Conventional volume tube refers to unidirectional or bidirectional ball volume tube, domestic model LJG.


  1 Overview

  Conventional volume tube refers to unidirectional or bidirectional ball volume tube, domestic model LJG.

  Small volume tube refers to piston volume tube.

  The small-volume tube in the United States was introduced to China in the late 1980s. The production plants are Smith; Brooks; Waugh; calibron, etc. Kaifeng Instrument Factory also produced small-volume tubes in the early 1990s.

  Kaifeng Instrument Factory, as a flow inspection device manufacturer, is not convenient to comment on the performance of these two types of volume tubes, so as not to mislead users. A comparison of the performance of the two volume tubes by a German manufacturer of flow verification equipment BOPP is now presented.

  2. Performance comparison between spherical volume tube and small volume tube

  2.1 Ball volume tube

  A. Advantages

  · High reliability.

  · Easy operation.

  · It can be used to verify the flowmeter with low pulse resolution. It only requires the flowmeter to output 10,000 pulses between the detection switches.

  · The number of verifications at a flow point is low.

  · Not sensitive to the reaction of solid particles in the test liquid.

  · Low maintenance costs.

  · Displacer—Easy replacement of elastic ball.

  Good repeatability, better than 0.02%.

  · Using four detection switches can reduce the number of verifications.

  B. Disadvantages

  · Large area.

  · One-way type with hydraulic system.

  2.2 Small volume tube

  A. Advantages

  · Small footprint.

  · The verification time is short. The shortest verification time in one run is 0.5 seconds.

  · The measuring cylinder adopts electroless nickel plating, which has strong corrosion resistance.

  B. Disadvantages

  · The standard volume is small, (the largest standard volume among existing products is 300 liters). It is not suitable for the verification of flow meters with low pulse resolution.

  · The number of verification runs is large, which offsets the advantage of a short verification time. According to the introduction of the US product sample, a flow point is usually verified 15 times.

  · The relative speed between the seal and the moving part at the sliding seal is 1.5m / s, so the seal is easy to damage and difficult to replace.

  · Sensitive to solid particles in the liquid being tested, which may easily cause damage to the piston seal in the measuring cylinder.

  · High maintenance costs.

  · The pulse insertion method will reduce the repeatability.

  · When using the pulse interpolation method, it is necessary to know the standard deviation бt of the pulse period of the flowmeter output. According to the national standard GB / T17286.3, the number of pulses N collected in one test is calculated as follows: )2

  确定 Determine the standard volume according to N and the flowmeter coefficient K. бt should be produced by the flowmeter manufacturer. But the flow meter manufacturers usually can't provide it, which makes it difficult to determine the small volume specifications.

  Note: The British IP standard stipulates this issue as follows:

  If a flow point test fails 15 times, it can be increased by 30 times. If 30 times still fail. The conclusion is that this flow meter is not suitable for verification with a small volume tube. (Without saying that the flowmeter is unqualified)

  2.3 Standard deviation of pulse interval бt

  When using a piston-type small-volume tube, we must first know the standard deviation бt of the time interval of the output pulse of the flow meter.

  偏差 Standard deviation of pulse interval Minimum pulses collected in a single stroke

  Бt (%) N

  0.5 -------------- 100

  1.0 -------------- 500

  2.5 -------------- 2500

  5.0 -------------- 10000

  10.0 ------------- 40000

  15.0 ------------- 90000

  Now ISO7278-3—1988 has been adopted in China

  N = 500 (бt) 2

  介绍 According to the British IP standard, high-quality turbine бt is ± 1%, and gear transmission pulse generator бt is as high as ± 10%.

  Appendix A, this is the test of the American small volume standard preparation group. The pulse irregularity in the figure is different from бt.

  A1—Turbine flowmeter, transmitter without regulator

  A2, A3, A4—transmitters with gear adjusters

  3. Comparison of small volume tube and conventional volume tube verification flowmeter

  About 1993, in order to revise the main drafter of China's volume tube inspection regulations JJG209, Xie Jiji, etc., conducted a test at Beijing Yanshan Petrochemical Plant

  Checked flowmeter: DN100 waist flowmeter, Japan Tokiko products

  Volume tube:

  A. SVP150, American Smith product, standard volume V = 57L

  B. Ball volume tube DN150, domestic, standard volume V = 728L

  C. Ball volume tube DN250, made in China, standard volume V = 2750L

  Measured medium: diesel

  粘度 Viscosity of measured medium: 2.5CP

  Verification flow: 46.4m3 / h

  Volume tube SVP150 DN150 DN250

  Instrument constant 10.0155 9.98053 10.0063

  Chirp pulses 570.62 7280 27521

  Standard deviation Sk 0.0151 0.00234 0.000058

  Repeatability δk 0.39% 0.051% 0.0013%

  次数 Number of checks 30 12 12

  From this, it can be seen that the small volume tube is unqualified, and the flow meter is not necessarily unqualified, but it is indicated that it is not appropriate to use a small volume tube with a volume of only 57L.

  The test records are shown in Appendix B. Due to improper data storage, the DN250 volume tube verification records were lost.

  4. Verification of Collioli mass flowmeter

  In theory, the mass of a liquid is equal to the product of the density of the liquid and the volume of the liquid. Because the volumetric accuracy of the volume tube can reach 0.05%, the new technology promotes the success of high-precision online density meters. Therefore, it is theoretically feasible to use a volume tube and an on-line density verification mass flow meter. Therefore, China's mass flow verification regulations JJG897-95 also gives this verification method.

  M = ρtVt = ρ20V20 ------------------------------- (1)

  In the formula: M—Liquid mass

  Ρt——the density of liquid at temperature t

  Vt—volume of liquid at temperature t

  Ρ20—Density of liquid under reference conditions, given by density

  V20—Volume of liquid under reference conditions, given by volume tube

  Reference condition—20 ℃ , 101.325Kpa

  If we consider liquids to be incompressible

  Ρt = ρ20—a (t-20) = ρ20 [1-a / ρ20 · (t-20) = ρ20ν ------------ (2)

  ν = 1-a / ρ20 · (t-20) ------------------------------------- ------------- (3)

  In the formula: a --- temperature correction coefficient of density, check according to national professional standards

  Ν—temperature compensation factor for density

  The current reference conditions for imported densitometers are that the temperature is 60 ° F (15.55 ° C), and the density is converted from ρ15.5 to ρ20. This work can only be solved by experiments. The liquid can be compressed. This involves the correction of pressure. These technical details are not given by JJG897-95, and it is impossible to give them. Therefore, it is not a theoretical problem to perform on-line verification of mass flowmeters using volume tubes and on-line densitometers. Instead, it is through practice and accumulated experience to formulate good operational procedures. .

  In the international arena, oil trade is settled by volume, and the volume calculation specifications of oil products are very mature. A volumetric flow meter and volume tube are used to form a flow verification system, which has been widely used in various countries. The mass flowmeter method is currently under research and development in foreign countries, and there is no mature and recognized verification standard. In the JJG-897 procedure, the verification method for Coriox mass flowmeters is only a general guiding principle, but lacks Operability, there is no precedent in China for using a mass flow meter as a custody transfer. Therefore, from the aspects of reliability and contrast, the transfer of oil products still follows the GB9109.1 ~ 5 standard, and no trade disputes will occur.

  Offline calibration of mass flow meters usually adopts the weighing method, which is widely used in France abroad. Qilu Petrochemical has 380 Kelioli flowmeters in China. In order to verify these flowmeters, it has built a weighing check device.

  for reference only!

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