Standard differential pressure flowmeter

Standard differential pressure flowmeter

1. Measurement principle of flow throttling device

The fluid filled in the pipeline, when it flows through the throttling device (orifice plate, nozzle, etc.) inside the pipeline, will form local contraction at the throttling point of the throttling device, thereby increasing the flow velocity and reducing the static pressure. Therefore, a pressure drop or pressure difference is produced before and after the throttling device. The higher the flow velocity of the medium, the greater the pressure difference generated before and after the throttling device. Therefore, the size of the fluid flow can be measured by measuring the differential pressure.

Set pressure points 1 and 2 at appropriate positions before and after the throttling device. If the throat aperture of the throttling element is d (mm), the inner diameter of the pipeline is D (mm), the area of the flow section is A, the velocity v of the fluid on the flow section, the height h of the fluid position, the mass flow rate of the fluid is Q, and the density of the fluid in the working state is p (kg/m)³）Then, the velocity and pressure parameters of the fluid on the two pressure sections comply with:

In the formula, C - is the outflow coefficient; B - is the diameter ratio of the throttling device; E - is the coefficient of expansion

From the relationship, it can be seen that when the orifice diameter of the throttling element is d and the fluid density p is constant, the flow rate is proportional to the square root of the static pressure difference △ p. The size of the flow rate can be determined based on the magnitude of the static pressure difference measured by the differential pressure instrument. The flow coefficient in the equation is not derived from theoretical equations, but is determined by considering the velocity distribution of the fluid at the cross section and the energy loss of the fluid flowing through the throttling device. The determining factor is the structure of the throttling device. There are manufacturing standards for the structure of throttling devices both internationally and domestically, therefore there are two structural forms of throttling devices: standard and non-standard.

The standard throttling device assumes that an uncalibrated throttling device is geometrically and dynamically similar to a throttling device that has been fully experimentally calibrated, that is, it meets all the requirements of the standard document (GB/T2624 or ISO5167), and its measurement accuracy is within the measurement error specified in the standard. The relationship between mass flow rate and differential pressure can be determined by the above flow equation. The main conditions for achieving geometric similarity include: the structural form of the throttling device, the pressure measuring device, and the manufacturing and installation of the upstream and downstream straight pipe sections of the throttling element in accordance with the various regulations of the standard. The condition for similar dynamics is equal Reynolds numbers.

2. Characteristics and technical indicators of flow throttling devicesStandard differential pressure flowmeter

2.1 Characteristics

1) There are many suitable measurement media and a wide range of applicable working conditions

2) Mature technology with good reliability

3) Manufacturing standardization and good repeatability

4) Long service life and easy maintenance

2.2 Main technical indicators of flow throttling device

The throttling device that meets the structural form and technical requirements specified in GB/T2624 (or ISO5167) standard is a standard throttling device, while others are non-standard throttling devices. The standard throttling device can be calibrated without actual flow, and its accuracy is within the measurement error specified by the standard.

1) The uncertainty of the standard orifice plate outflow coefficient 8c/C is as follows:

When B ≤ 0.6, the uncertainty is ± 0.6%

When 0.6<B ≤ 0.75, the uncertainty is ± B%

(B is the diameter ratio of the throttling element)

2) The uncertainty of the standard nozzle outflow coefficient 8c/C is as follows:

When B ≤ 0.6, the uncertainty is ± 0.8%

When 0.6<B ≤ 0.8, the uncertainty is ± (2B-0.4)%

(B is the diameter ratio of the throttling element)

3) The uncertainty 8c/C of the standard Venturi tube outflow coefficient is as follows:

Venturi tube with coarse casting shrinkage section, uncertainty of ± 0.7%

Venturi tube with mechanical contraction section, uncertainty of ± 0.1%

Venturi tube with contraction section of coarse welded iron plate, uncertainty of ± 1.5%

The throttling device calibrated by real flow can achieve higher measurement accuracy by using the calibrated data in the flow integrator or system.