Differential Pressure Flow meters
Differential pressure flow meters introduce a construction in the pipe that creates a pressure drop across the flow meter. When the flow increases, more pressure drop is created. Impulse piping routes the upstream and downstream pressures of the flow meter to the transmitter that measures the differential pressure to determine the fluid flow.
Differential pressure flow meters use Bernoulli’s equation to measure the flow of fluid. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate.
Types of differential pressure flow meters are:
- Orifice Plates
- Flow Nozzles
- Pitot-static tube Flow meters
- Venturi Tubes
- Spring-Loaded Variable Aperture Flow meters
- Variable Area – Rotameters
Differential pressure flow meters are generally applicable to many flows in most industries, such as mining, mineral processing, pulp and paper, petroleum, chemical, petrochemical, water, and wastewater industries
Brief description of all differential pressure flow meters:
The fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. There is always a permanent pressure loss.
flat metal plate with an opening in the plate, installed perpendicular to the flowing stream in a circular pipe. Orifice differential pressure flow meters can be constructed to measure gas, liquid or steam.
2. Flow Nozzles
A differential pressure transmitter is used to measure pressure between the nozzle and the pipe flow stream. In this flow meter incoming liquid is smoothly but outgoing liquid is to sharp.
The flow nozzle is relative simple and cheap, and available for many applications in many materials.
3. Pitot-static tube Flow meters
This device consisting of a Pitot tube and an annular tube combined with static pressure ports. The pitot tube are one the most used (and cheapest) ways to measure fluid flow.
The differential pressure between the two ports is the velocity head. This indication of velocity combined with the cross-sectional area of the pipe provides an indication of flow rate. Measure either liquids or gases.
4. Venturi Tubes
In the Venturi Tube the fluid flow rate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference.
the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered. A venturi produces less permanent pressure loss than an orifice but is more expensive.
5. Spring-Loaded Variable Aperture Flow meters
Flow rate has a relationship with the differential pressure of the flow meter and the position of the spring-loaded cone. This type of flow meter relates a change in flow rate to the differential pressure across a spring-loaded cone. The cone repositions itself to balance the force. This in turn changes the aperture for the flow.
The float rises until the annular area between the float and tube increases sufficiently to allow a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity factors.
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