Flow Nozzle

PRECISE MASS FLOW METERS - Flow Nozzles

The flow nozzles is a flow tube consisting of a smooth convergent section leading to a cylindrical throat area. The throat is the smallest section of the nozzle. Pressure taps are located on the upstream side of the nozzle plate and on the downstream side of the nozzle outlet. They may be in the form of an annular ring, i.e. equally spaced holes connected together which open into the pipeline, or in the form of single holes drilled into the pipeline.

As mentioned above, flow nozzles are primary elements in differential pressure flow meters. These flow meters use the primary elements as an obstruction to generate a pressure drop to calculate the flow rate. This is based on Bernoulli’s principle, according to which any obstruction placed in the path of a flowing fluid will cause the velocity of the fluid to increase and the pressure to decrease in the area of the obstruction.

As the fluid passes through the nozzle, the obstruction causes the velocity of the fluid to increase while its static pressure decreases simultaneously. At the point of maximum convergence, i.e. at the vena contracta, the velocity is at its maximum and the pressure is at its minimum. As the fluid exits the nozzles, its flow expands and the velocity reduces and the pressure rises again. This difference in pressure before and after the primary element is measured using differential pressure transmitters, also called secondary elements. Bernoulli’s equation is used to determine that the pressure differential is proportional to the square of the flow rate. The flow rate is therefore calculated as follows:

Q2 = K * DP

Where:
  Q = flow rate of the fluid 
  K = Constant value for the particular nozzle
  DP = pressure differential

Types of flow nozzles

1. ISA 1932, with corner taps (weld-in type)

Weld-in-type

This type of flow nozzle was developed by the International Organization for Standardization (ISO). It is widely used in high-pressure and high-temperature applications in pipe diameters less than 2 inches. The corner taps are located in the upstream and downstream flanges holding the nozzle. Weld-in type of nozzles is intended for permanent installation in a pipe.

2. A.S.M.E. long radius, low beta ratio (0.20 < β < 0.5), with throat tap (flanged type)

Flanged-type

Beta ratio refers to the ratio of the diameter of the nozzle throat to the diameter of the main pipe (d/D) and represented by the symbol β. This type of nozzle is extensively used in the power industry thanks to the highly accurate measurement capability and easy installation of throat taps which are located in the cylindrical portion of the nozzle. They are fitted into the pipe walls at a distance of one pipe diameter on the upstream side and half pipe diameter on the downstream side. In this configuration, the nozzle is mounted between the pipe flanges.

3. A.S.M.E. long radius, high beta ratio (0.25 < β < 0.8), with radius taps (holding ring type)

Holding-ring-type

This nozzle is installed in pipes without flanges using a holding ring and locating pins made of the same material as the pipe. This prevents the need to weld dissimilar materials. It is recommended for pipes measuring 4 inches and above. 
Some materials used to manufacture flow nozzles include gun metals, stainless steel, bronze, Monel metal, chromium, etc.

No industry can progress without reliable and accurate measurement. The key is measurement, simple as that. Measurement can result in two possible outcomes: If the result confirms your hypothesis then you've made a measurement; If the result is contrary then you've found a problem.

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