Selection of Flow Instruments

General principles

1 Scale selection

The scale of the instrument should meet the requirements of the scale modulus of the instrument. When the scale reading is not an integer, it can also be selected as an integer for the convenience of reading conversion.

(1) Root scale range

• The maximum flow shall not exceed 95% of full scale;
• Normal flow is 70%~85% of full scale;
• The minimum flow shall not be less than 30% of the full scale.

(2) Linear scale range

• The maximum flow shall not exceed 90% of full scale;
• Normal flow is 50%~70% of full scale;
• The minimum flow shall not be less than 10% of the full scale.

2 Instrument accuracy

The flowmeter used for energy measurement shall comply with the provisions of the General Rules for the Allocation and Management of Energy Measuring Instruments in Enterprises (Trial).
(1) Measurement for settlement of fuel in and out of the plant, ± 0.1%;
(2) Measurement for technical and economic analysis of workshop team and process, ± 0.5%~2%;
(3) For industrial and civil water metering, ± 2.5%;
(4) For steam metering including superheated steam and saturated steam, ± 2.5%;
(5) For the measurement of natural gas, gas, and domestic gas, ± 2.0%;
(6) Oil metering for key energy-consuming equipment and process control, ± 1.5%;
(7) Measurement of other energetic working mediums (such as compressed air, oxygen, nitrogen, hydrogen, water, etc.) used for process control, ± 2%.

3 Flow unit

• Volume flow: m3/h, l/h;
• Kg/h, t/h for mass flow;
• The volume flow of gas under the standard state is Nm3/h (0 ℃, 0.1013MPa).

Selection of general fluid, liquid, and steam flow measuring instruments

1 Differential pressure flowmeter

(1) Throttling device

① Standard throttling device
For flow measurement of general fluid, a standard throttling device (standard orifice plate and standard nozzle) shall be selected. The selection of a standard throttling device must comply with the provisions of GB2624-8l or international standard ISO5167-1980. If there are new national standards, the new regulations shall be implemented.

② Non-standard throttling device

• A Venturi tube can be selected if the following conditions are met:
• An accurate measurement under low-pressure loss is required;
• The measured medium is clean gas and liquid;
• The inner diameter of the pipe is 100~800mm;
• Fluid pressure is within 1.0MPa.

• A double orifice plate can be selected if the following conditions are met:
• The measured medium is clean gas and liquid;
• Reynolds number is greater than (equal to) 3000 and less than (equal to) 300000.

• 1/4 round nozzle can be selected if the following conditions are met:
• The measured medium is clean gas and liquid;
• Reynolds's number is greater than 200 and less than 100000.

• Round hole plate can be selected if the following conditions are met:
• Dirty media (such as blast furnace gas, mud, etc.) where the measured medium may produce sediment before and after the orifice plate;
• Must have horizontal or sloped piping.

③ Selection of pressure tapping mode

It shall be considered that the whole project shall adopt a unified pressure tapping method as far as possible.
Generally, corner connection or flange pressure tapping is adopted.
According to the service conditions and measurement requirements, other pressure measurement methods such as diameter distance measurement can be used.

(2) Selection of differential pressure range of differential pressure transmitter

The selection of differential pressure range shall be determined according to the calculation. Generally, it is better to select according to the different working pressure of the fluid:
Low differential pressure: 6kPa, 10kPa;
Medium differential pressure: 16kPa, 25kPa;
High differential pressure: 40kPa, 60kPa.

(3) Measures to improve measurement accuracy

For fluid with large temperature and pressure fluctuation, temperature and pressure compensation measures shall be considered;
When the length of the straight pipe section of the pipeline is insufficient or rotary flow occurs in the pipeline, fluid correction measures shall be considered and rectifiers with corresponding pipe diameters shall be added.

(4) Special differential pressure flowmeter

① Steam flow meter
For the flow of saturated steam, when the required accuracy is not higher than class 2.5 and it is calculated locally or remotely, the steam flowmeter can be used.

② Embedded orifice plate flowmeter

For the microflow measurement of clean liquid, steam, and gas without suspended solids, when the range ratio is not greater than 3 ∶ l, the measurement accuracy is not required, and the pipe diameter DN is less than 50 mm, the embedded orifice flowmeter can be used. When measuring steam, the steam temperature shall not be greater than 120 ℃.

2 Rotator flowmeter

When the required accuracy is not higher than 1.5 and the range ratio is not greater than 10 ∶ 1, the rotameter can be selected.

(1) Glass rotameter

The glass rotameter can be used for local indication of medium and small flow, small flow, clean, transparent, non-toxic, non-combustion and explosion hazard, non-corrosive and non-adhesive fluid flow with pressure less than 1MPa and temperature less than 100 ℃.

(2) Metal tube rotameter

① Ordinary metal tube rotameter
For medium and small flow measurements of fluid that is easy to vaporize, condense, toxic, flammable, explosive and does not contain magnetic substances, fibers, and wear substances, and is non-corrosive to stainless steel (1Crl8Ni9Ti), when local indication or remote transmission signal is required, ordinary metal tube rotameter can be used.

② Special metal tube rotameter

Metal tube rotameter with jacket
When the measured medium is easy to crystallize or vaporize or has a high viscosity, the jacketed metal tube rotameter can be selected. A heating or cooling medium is used in the jacket.
Corrosion-proof metal tube rotameter
For corrosive medium flow measurement, an anti-corrosion metal tube rotameter can be used.

(3) Rotor flowmeter

It is required to be installed vertically with a gradient of no more than 5 °. The fluid shall be installed from bottom to top with low vibration and easy observation and maintenance. Upstream and downstream block valves and bypass valves shall be set. For dirty media, a filter must be installed at the inlet of the flowmeter.

3 Velocity flowmeter

(1) Target flowmeter

For liquid flow measurement with high viscosity and a small number of solid particles, when the accuracy is required to be no higher than 1.5 and the range ratio is not greater than 3:1, the target flowmeter can be used.
A target flowmeter is generally installed on a horizontal pipeline. The length of the front straight pipe section is 15-40D, and the length of the rear straight pipe section is 5D.

(2) Turbine flowmeter

Clean gas and kinematic viscosity shall not be greater than 5 × For the flow measurement of 10-6m2/s clean liquid, when it is required to measure accurately and the range ratio is not greater than 10 ∶ 1, turbine flowmeter can be used.
The turbine flowmeter shall be installed on the horizontal pipeline to fill the whole pipeline with liquid and shall be equipped with upstream and downstream stop valves and bypass valves, as well as a filter at the upstream and a drain valve at the downstream.
Length of straight pipe section: not less than 20D upstream and 5D downstream.

(3) Vortex flowmeter (Karman vortex flowmeter or vortex flowmeter)

A vortex flowmeter can be selected for large and medium flow measurements of clean gas, steam, and liquid. Low-speed fluid and viscosity greater than 20 × 10-3pa ˙ S Vortex flowmeter shall not be used for liquid measurement. When selecting, the pipeline velocity shall be checked.
The flowmeter has the characteristics of small pressure loss and convenient installation.
Requirements for straight pipe section: 15~40D for upstream (depending on piping); When a rectifier is added upstream, the upstream shall not be less than 10D; At least 5D downstream.

(4) Water meter

When the equivalent range ratio of the local accumulated water flow is required to be less than 30 ∶ 1, a water meter can be used.

The water meter is installed on the horizontal pipe, and the length of the straight pipe section is required to be no less than 8D upstream and 5D downstream.

There is another part of the knowledge about the selection of flow measurement instruments.