Nozzle flow meter

1. Overview of nozzle flowmeter

Nozzle flowmeter is also called standard nozzle flowmeter, standard nozzle, ISA1932 nozzle, and standard orifice plate, venturi tube flowmeter, and venturi nozzle belong to the standard throttling device stipulated by national standards. It conforms to the national standard GB/2624-1993, so there is no need for real-flow calibration like the standard orifice plate, as long as the design and processing according to the national standard can achieve the design accuracy, but the premise also requires the user to be able to provide more accurately: common flow, medium, work The five parameters of temperature, working pressure, inner and outer diameter of the process tube.

2. Measuring principle of nozzle flowmeter

The measuring principle of the nozzle is based on the throttling principle of fluid mechanics. When the fluid in the pipeline is filled with the fluid flowing through the nozzle in the pipeline, the flow rate will be locally contracted in the nozzle, which will increase the flow rate and reduce the static pressure, so it will be generated before and after the nozzle. The pressure drop or pressure difference, the greater the flow of the medium, the greater the pressure difference generated before and after the nozzle, so the size of the fluid flow can be measured by measuring the pressure difference.

Because the nozzle adopts the arc-shaped contour structure, its pressure loss is small, the straight pipe section required is short, and the accuracy is high.

Flow calculation formula:

In the formula: ,——respectively the mass flow rate (㎏/s) and volume flow rate (m3/s);

⇒Outflow coefficient;

⇒Expansibility coefficient;

⇒The diameter of the throttle opening, m;

⇒Diameter ratio,

⇒pipe inner diameter, m;

⇒The density of the measured fluid, ㎏/m;

⇒Differential pressure, Pa;

3. Features of nozzle flowmeter

1. Simple structure and easy installation.

2. The pressure loss of the nozzle is smaller than that of the orifice plate, and the length of the straight pipe is also required to be shorter.

3. No need for real-flow verification, stable performance.

4. Can withstand high temperature and pressure, and impact resistance.

5. The corrosion resistance is better than the orifice plate, and the service life is longer.

6. ​​High precision, good repeatability, stable outflow coefficient.

7. The arc-shaped structure design can measure various liquids, gases, steam and various dirty media.

8. Integral forging processing technology, high cost.

4. Classification of nozzle flow meters

(1) Standard nozzle (ISA1932 nozzle)

•Applicable medium; especially suitable for high temperature and high pressure steam and water, and can also be used for various gases and liquids

•Nominal diameter: DN50—500mm (DN>500 can also be designed and produced)

•Working pressure: ≦42MPa

•Working temperature: -50℃ — 650℃

•Take pressure method: corner connection (separate ring chamber, flange ring chamber integrated or direct drilling) to take pressure

•Nozzle installation method: flange (direct drilling and pressing parts), fastener clamping type

Flange, ring chamber, fastener clamp type

Clamping part (and pressing part) welding type

•Executive standard: IS05167; GB/T2624

•Accuracy: Meet the standard: ±1.0%

•Material: Flange or clamping parts: Carbon steel, alloy steel, stainless steel, etc. can be selected according to the temperature of the medium

nozzle: stainless steel 304; 316 etc.

(2) Long diameter nozzle

•Applicable medium; suitable for high temperature and high pressure steam and water, and can also be used for various gases and liquids

•Nominal diameter: DN50—600mm

•Working pressure: ≦42MPa

•Working temperature: -50℃-650℃

•Pressing method: Diameter (D-D/2) pressure

•Nozzle installation method: equipped with and welded upstream and downstream pipe sections and then set pressure holes according to the diameter (D-D/2)•

•Executive standard: IS05167; GB/T2624

•Accuracy: Meet the standard: ±2.0%

•Material: Carbon steel and alloy steel can be selected for upstream and downstream pipe sections according to the temperature of the medium; stainless steel nozzles; stainless steel 304; 316, etc.

5. Main technical parameters

1. Nominal diameter: 19.99mm≤DN≤630mm

2. Nominal pressure: PN≤32Mpa

3. Working temperature: -50℃≤t≤550℃

4. Reynolds number range: 0.30≤β≤0.441, 70000≤ReD≤107

When 0.44≤β≤0.8, 20000≤ReD≤107

5. Throat diameter ratio: 0.30≤β≤0.80

6. Accuracy: 0.5 grade, 1.0 grade, 1.5 grade, 2.0 grade

7. Reference standard: GB/T2624-2006, JJG640-94

8. Connection method: flange connection, welding

6. The structure of the nozzle flowmeter

• The standard nozzle is composed of the entrance plane part A perpendicular to the axis, the entrance constriction formed by the arc-shaped curved surfaces B and C, the cylindrical throat E and the protective groove F required to prevent edge damage. The pressure is taken The upstream pressure-taking port adopts a corner joint to take pressure, and the downstream pressure-taking port can be set according to the corner joint to take pressure, or it can be set far downstream. Total length of nozzle

•When 0.30≤β≤2/3, the nozzle length=0.6041d

•When 2/3≤β≤0.80, the nozzle length={0.4041+(0.75/β-0.25/β2-0.5225)0.5}d

•The upstream surface of the ISA 1932 nozzle is a nozzle composed of a plane perpendicular to the axis, a constricted section defined by two arcs whose profile is a circle, a cylindrical throat and a groove. ISA 1932 nozzle has only one method of tapping pressure.

•The upstream surface of the long-diameter nozzle is composed of a plane perpendicular to the axis, a constricted section with a 1/4 elliptical profile, a cylindrical throat, and possible grooves or bevels. There is only one pressure method for long diameter nozzles: D-D/2.