Comparison between Regular Volume Tubes and Small Volume Tubes

1. Overview

The conventional volume meter refers to a unidirectional or bidirectional ball-type volume meter, domestic model LJG.

The small volume meter refers to a piston-type volume meter.

The small volume meters from the United States were introduced to our country around the late 1980s, with manufacturers such as Smith, Brooks, Waugh, and Calibron. The Kaifeng Instrument Factory also produced small volume meters in the early 1990s.

As a manufacturer of flow calibration devices, the Kaifeng Instrument Factory finds it inconvenient to comment on the performance of these two types of volume meters to avoid misleading users. Below is a comparison of the performance of the two types of volume meters introduced by BOPP, a flow calibration device manufacturer in Germany.

2. Performance Comparison of Ball-Type Volume Meters and Small Volume Meters

2.1 Ball-Type Volume Meter

a. Advantages

· High reliability.

· Easy to operate.

· Can be used to calibrate flow meters with low pulse resolution, requiring the flow meter to output 10,000 pulses between the detection switches.

· Fewer calibration times at a flow point.

· Insensitive to solid particles in the tested liquid.

· Low maintenance costs.

· Easy replacement of the elastic ball in the displacer.

· Good repeatability, better than 0.02%.

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

b. Disadvantages

· Large footprint.

· Unidirectional type has a hydraulic system.

2.2 Small Volume Meter

a. Advantages

· Small footprint.

· Short calibration time, with the shortest calibration time of 0.5 seconds in one run.

· The measuring cylinder uses non-electroplated nickel, which has strong corrosion resistance.

b. Disadvantages

· Standard volume is small (the largest standard volume in existing products is 300 liters), not suitable for calibrating flow meters with low pulse resolution.

· More calibration runs, which offsets the advantage of short calibration time; according to American product samples, typically 15 calibrations are required for one flow point.

· The sealing element at the sliding seal has a relative speed of 1.5 m/s with the moving part, making the sealing element prone to wear and difficult to replace.

· Sensitive to solid particles in the tested liquid, which can easily damage the piston sealing element in the measuring cylinder.

· High maintenance costs.

· Using the pulse insertion method can lead to reduced repeatability.

· When using the pulse insertion method, it is necessary to know the standard deviation of the pulse period output by the flow meter, denoted as бt. According to the national standard GB/T17286.3, the number of pulses collected in one calibration, denoted as N, is calculated as N=500(бt)².

Based on N and the flow meter coefficient K, the standard volume is determined, and бt should be provided by the flow meter manufacturer. However, flow meter manufacturers usually do not provide this, which complicates the determination of small volume specifications.

Note: The British IP standard stipulates the following regarding this issue:

If a flow point fails 15 calibrations, an additional 30 can be added; if it still fails after 30, the conclusion is that the flow meter is not suitable for calibration with a small volume meter (without stating that the flow meter is unqualified).

2.3 Standard Deviation of Pulse Interval бt

When selecting a piston-type small volume meter, it is essential to know the standard deviation of the time interval of the pulses output by the flow meter, initially specified in ISO7278/3 (BS6886) as follows:

Standard deviation of pulse interval: the minimum number of 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 our country.

N=500(бt)²

According to the British IP standard, high-quality turbine бt is ±1%, while using gear-driven pulse generators can result in бt as high as ±10%.

Appendix A, this is a test by the American small volume standard drafting group, where the pulse irregularity in the figure differs from бt; irregularity = (maximum period - minimum period) / 2 average period.

A1—Turbine flow meter, transmitter without regulator.

A2, A3, A4—Transmitters with gear regulators.

3. Comparison of Calibrating Flow Meters with Small Volume Meters and Conventional Volume Meters

Around 1993, to revise the national volume meter calibration procedure JJG209, the main drafters including Xie Jiji conducted an experiment at the Yanshan Petrochemical Plant in Beijing.

Tested flow meter: DN100 oval gear flow meter, product from Japan's Tokimec.

Volume meter:

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

b. Ball-type volume meter DN150, domestic, standard volume V=728L.

c. Spherical volume tube DN250, domestic, standard volume V=2750L

Measured medium: diesel

Measured medium viscosity: 2.5CP

Verification flow rate: 46.4m3/h

Volume tube SVP150 DN150 DN250

Instrument constants 10.0155 9.98053 10.0063

Pulse count 570.62 7280 27521

Standard deviation Sk 0.0151 0.00234 0.000058

Repeatability δk 0.39% 0.051% 0.0013%

Number of verifications 30 12 12

It can be seen that the small volume tube verification is unqualified, and the flowmeter is not necessarily unqualified; it only indicates that using a small volume tube with a capacity of only 57L for this flowmeter is inappropriate.

Test records are in Appendix B, and the verification records using the DN250 volume tube were lost due to improper data management.

4. Regarding the verification of the Coriolis mass flowmeter

Theoretically, the mass of a liquid equals the product of the liquid density and the liquid volume. Since the volume tube's volume accuracy can reach 0.05%, new technology has led to the successful development of high-precision online densitometers. Therefore, using a volume tube and online density to verify mass flowmeters is theoretically feasible. Thus, our country's mass flow verification procedure JJG897—95 also provides this verification method.

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

Where: M—liquid mass

ρt—density of the liquid at temperature t

Vt—volume of the liquid at temperature t

ρ20—density of the liquid under reference conditions, given by density

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

Reference conditions—20℃, 101.325Kpa

If we consider the liquid to be incompressible

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

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

Where: a---temperature correction coefficient of density, according to national professional standards

ν—temperature compensation coefficient of density

Currently, imported densitometers have reference conditions at a temperature of 60℉(15.55℃), and density is converted from ρ15.5 to ρ20. This work can only be resolved through experiments. Liquids can be compressed, which also involves pressure correction. These technical details are not provided in JJG897-95 and cannot be provided. Therefore, using a volume tube and online densitometer for online verification of mass flowmeters is not a theoretical issue, but rather a practical one, accumulating experience to formulate operational procedures.

Internationally, oil product trade is settled based on volume, and the calculation standards for oil product volume are already very mature. The use of volumetric flowmeters and volume tubes to form flow verification systems has been widely adopted by various countries. However, the method of using mass flowmeters is still in research and development abroad, with no mature and recognized verification standards. In the JJG-897 procedure, the verification method for the Coriolis mass flowmeter is only general guiding principles and lacks operability. There has been no precedent in the country for using mass flowmeters as trade handover. Therefore, considering reliability and comparability, oil product handover still follows the GB9109.1~5 standards to avoid trade disputes.

The offline verification of mass flowmeters usually uses the weighing method, which has been widely adopted in France. In China, Qilu Petrochemical has 380 Coriolis flowmeters, and to verify these flowmeters, they have built a weighing verification device.

For reference only!