Differential Pressure Technology
Differential pressure flowmeters have a cross section change in the flow pipe, which causes the velocity of the flowing fluid to change. A change in velocity occurs whenever there is a change in flow cross-section; i.e., with a decrease in velocity, an increase in pressure.
Governing Equation
In these two equations
Q and
Vavg are unknowns. Upon finding
Vavg, the volume flow rate
Q can be obtained.
Types of Differential Pressure Flowmeters
Orifice: flat metal plate with an opening in the plate, installed perpendicular to the flowing stream in a circular pipe. As the flowing fluid passes through the orifice, the restriction causes an increase in velocity and decrease in pressure. There is always a permanent pressure loss. No dirty liquid allowed.
Venturi: a restriction with a relatively long passage with smooth entry and exit. Produces less permanent pressure loss than orifice but is more expensive. Often used in dirty streams because there is no build-up of the foreign material.
Nozzle: smooth entry and sharp exit. Permanent pressure loss is on the same level as an orifice, but with the ability to handle dirty and abrasive fluids. Primarily used in steam service.
Pitot-Static Tube: a device consisting of a Pitot tube and an annular tube combined with static pressure ports. 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.
Elbow meter: a differential pressure is caused by centrifugal force between the inside diameter and the outside walls of the pipe elbow. It does not introduce any additional pressure loss other than that caused by the elbow.
Wedge meter: a wedge-shaped element that is perpendicular to the flow at the top of the conduit which means that the bottom part is unrestricted. Therefore, it is useful in slurry measurement.
V-Cone flowmeter: consists of a V-shaped cone element placed at the center of the pipe, leaving an annular space for the passage of fluid. Lower permanent pressure loss than orifice flowmeter.
Spring-Loaded Variable Aperture: with the change in flow rate, the differential pressure across the spring-loaded cone varies. The cone repositions itself to balance the force. And this in turns changes the aperture for the flow. Flow rate has a relationship with the differential pressure of the flowmeter and the position of the spring-loaded cone.
Laminar flow element: Flow rate is linearly proportional to the differential pressure and inversely proportional to the viscosity of the flowing fluid. A flow can be made into laminar by passing through a bundle of small diameter tubes.