An Orifice Meter is basically a type of flow meter which is used to measure the rate of flow fluids (mainly Liquids or Gases), using the Differential Pressure Measurement principle.
There is basically an Orifice Plate installed in the orificemeter which provides obstruction to the fluid flow. Here, the streamline contracts because of the area contraction due to orifice which is placed between the pipe by flange.
There is a vena -contacta considered as minimum area -cross section.
A differential pressure is developed across the Orifice Plate which is directly proportion to the flow-rate of the liquid or gas.
When a liquid / gas, whose flow-rate is to be determined, is passed through an Orifice Meter, there is a drop in the pressure between the Inlet section and Outlet Section of Orifice Meter. This drop in pressure can be measured using a differential pressure measuring instrument.
Since this differential pressure is in direct proportion to the flow-rate as per the Bernoulli’s Equation hence the differential pressure instrument can be configured to display flow-rate instead of showing differential pressure.
Working:
- The working principle of Orifice Meter is the same, as that of Venturi meter
- The orifice plate, is being fixed at a section of the pipe, creates an obstruction to the flow by providing an opening in the form of an orifice to the flow passage.
Orifice-meter-flow
Fig 1 Flow through an Orificemeter
- The area A0 of the orifice is much smaller than the cross-sectional area of the pipe. The flow from an upstream section, where it is uniform, adjusts itself in such a way that it contracts until a section downstream the orifice plate is reached, where the vena contracta is formed, and then expands to fill the passage of the pipe.
- One of the pressure tapings is usually provided at a distance of one diameter upstream the orifice plate where the flow is almost uniform (Sec. 1-1) and the other at a distance of half a diameter downstream the orifice plate.
- Vena-contracta is made at a distance of D/2 from orifice plate in downstream direction.
- Considering some assumptions as the fluid is ideal and the downstream pressure taping to be at the vena contracta (Sec. c-c), Now applying Bernoulli’s theorem in between Sec. 1-1 and Sec. c-c,
where and are the piezometric pressures at Sec.1-1 and c-c respectively.
- From the equation of continuity,
where Ac is the area of the vena contracta.
- With the help of Eq. (1), Eq. (2) One can write ;
Correction in Velocity
- As we know measured value of the piezometric pressure drop for a real fluid is always more due to friction than that assumed in case of an inviscid flow, a coefficient of velocity Cv (always less than 1) has to be introduced to determine the actual velocity Vc when the pressure drop in above velocity Eq. , is substituted by its measured value in terms of the manometer deflection ‘∆h’
Hence, where ‘∆h’ is the difference in liquid levels in the manometer and ρm is the density of the manometric liquid.
Volumetric flow rate
- If a coefficient of contraction Cc is defined as, Cc = Ac /A0, where A0 is the area of the orifice, then Eq.(5) can be written, with the help of Eq. (4),
where, Coefficient of discharge of orificemeter (Cd) is 0.61.
Orifice Plate
An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a pressure drop and thus measure the flow.
A linearly extending section similar to the Inlet section. Here also the diameter is the same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure the Pressure of the media at this discharge.
As shown in the adjacent diagram, a gasket is used to seal the space between the Orifice Plate and the Flange surface, prevent leakage.
April 25, 2025
Orificemeter – Flow Measuring Devices (Flowmeters)
maximios Engineering
An Orifice Meter is basically a type of flow meter which is used to measure the rate of flow fluids (mainly Liquids or Gases), using the Differential Pressure Measurement principle.
There is basically an Orifice Plate installed in the orificemeter which provides obstruction to the fluid flow. Here, the streamline contracts because of the area contraction due to orifice which is placed between the pipe by flange.
There is a vena -contacta considered as minimum area -cross section.
A differential pressure is developed across the Orifice Plate which is directly proportion to the flow-rate of the liquid or gas.
When a liquid / gas, whose flow-rate is to be determined, is passed through an Orifice Meter, there is a drop in the pressure between the Inlet section and Outlet Section of Orifice Meter. This drop in pressure can be measured using a differential pressure measuring instrument.
Since this differential pressure is in direct proportion to the flow-rate as per the Bernoulli’s Equation hence the differential pressure instrument can be configured to display flow-rate instead of showing differential pressure.
Working:
Orifice-meter-flow
Fig 1 Flow through an Orificemeter
Correction in Velocity
Hence, where ‘∆h’ is the difference in liquid levels in the manometer and ρm is the density of the manometric liquid.
Volumetric flow rate
where, Coefficient of discharge of orificemeter (Cd) is 0.61.
Orifice Plate
An Orifice Plate is inserted in between the Inlet and Outlet Sections to create a pressure drop and thus measure the flow.
A linearly extending section similar to the Inlet section. Here also the diameter is the same as that of the outlet pipe for an end connection for an outgoing flow. Here we measure the Pressure of the media at this discharge.
As shown in the adjacent diagram, a gasket is used to seal the space between the Orifice Plate and the Flange surface, prevent leakage.