What is a Flow nozzle? Principle, Construction, Advantages, Disadvantages, and Applications.

A flow nozzle is actually a short cylinder with the approach being elliptical shape to a narrow throat. It is typically installed in-line and used at the inlet to and the outlet of a pipe or outlet from the tank.

It can be installed in any position, although the horizontal orientation is preferred. Vertical down flow is preferred for wet steam gases or liquid-containing solids.

For proper performance, the location of pressure taps on either side of the flow nozzle is at one pipe diameter upstream, and 0.5 pipe diameter downstream from the inlet face. The most common flow nozzle is that the flange type.


On flowing the fluid through the meter, there will be an increase in the velocity (i.e. kinetic energy) and a decrease in the pressure. The rate of flow is then measured at the expense of this pressure drop.

Construction- Types

Flow nozzle is a modified, shorter version of the Venturi meter that is suitable for the circular cross-section with converging-diverging profiles without any strain portion in between.

As shown in the below figure there are three types of flow nozzles available in the market.

  • Flange type
  • Weld in type
  • Throat tap

Flow nozzles are made of 304 or 316 Stain less steel, Hastelloy, Carbon steel.

Characteristics of flow nozzles

The rangeability is 4-1.

Delta P lies between orifice plates and venturi tubes.

Typical accuracy is 1-2 % of full range.

Flow nozzles maintain their accuracy for long periods, even in difficult service.

They are dimensionally more stable than orifice plates, particularly in high temperature and high-velocity services.

Required upstream straight pipe length is 10-30 diameters, almost like those of orifice meters.


When the fluid enters the nozzle its velocity gradually increased in the most frictionless fashion. The fluid leaves the nozzle as a jet. The vena contract is formed at the outlet edge of the nozzle. The new nozzle is not fitted with a diverging cone, thus an appreciable head is lost. The head lost is very much similar to the orifice. 

Flow nozzles have a higher flow capacity than orifice plates. There is a major difference between the flow nozzle and the venturi tube is that although they have similar inlet nozzles, the flow nozzle has no exit section.

Flow nozzles can handle flows having larger solid contents and be used for higher velocities, greater turbulence, and high-temperature applications. They are often being used with fluid or steam with applications containing some suspended solids, and an application where the product is being discharged from service.

Flow nozzles are more economical, but they provide less accuracy than venturi tubes and have a higher unrecoverable pressure loss.


  • High-velocity application.
  • Operate in higher turbulence.
  • Used with fluids containing.
  • More cost-effective than Venturi tubes.
  • It is physically smaller than the venture tube.


  • More expensive than orifice plates.
  • It is difficult to remove compared to orifice plates.
  • Higher unrecoverable pressure loss.


  • Used to measure very high flow rates of superheated steam.
  • Flow nozzles are highly accurate in measuring gas flows and are often used in laboratories as stands for calibrating other gas flow meters.