Theory of Head Meters
Head-type flow measurement derives from Bernoulli’s theorem, which states that, in a flowing stream, the sum of the pressure head, the velocity head, and the elevation head at one point is equal to their sum at another point in the direction off low plus the loss due to friction between the two points. Velocity head is defined as the vertical distance through which a liquid would fall to attain a given velocity. Pressure head is the vertical distance that a column of the flowing liquid would rise in an open-ended tube as a result of the static pressure. This principle is applied to flow measurement by altering the velocity of the flowing stream in a predetermined manner, usually by a change in the cross-sectional area of the stream. Typically, the velocity at the throat of an orifice is increased relative to the velocity in the pipe. There is a corresponding increase in velocity head. Neglecting friction and change of elevation head, there is an equal decrease in pressure head. This difference between the pressure in the pipe just upstream of the restriction and the pressure at the throat is measured. Velocity is determined from the ratio of the cross-sectional areas of pipe and flow nozzle, and the difference of velocity heads given by differential pressure measurements. Flow rate derives from velocity and area.
The point where the fluid flow profile constricts to a minimum cross-sectional area after flowing through the orifice is called the vena contracta, and it is the area of minimum fluid pressure. The vena contracta corresponds to the narrow throat of a venturi tube. The precise location of the vena contracta for an orifice plate installation will vary with flow rate, and also with the beta ratio (β) of the orifice plate, defined as the ratio of bore diameter (d) to inside pipe diameter (D):
beta ratio (β) = Bore diameter (d) / Inside pipe diameter (D)
Types of Orifice Plates
There are 4 types of Orifice Plates that we will discuss:
Concentric Orifice Plate (Hole is in the center)
Eccentric Orifice Plate (Hole is off center)
Segmental Orifice Plate
Others – such as Conical and Quadrant, and Edged Entrance types.
Concentric Orifice Plate :-
This is generally the most commonly used type of orifice plate with the orifice bore positioned along the pipe centreline. When designing and configuring orifice plates, there are several factors which must be followed to ensure accurate and reliable measurement. The orifice upstream edge should be sharp and square. The minimum plate thickness is standardized based on orifice bore, inside pipe diameter etc. The beta ratio (d/D) should be within recommended limits to ensure conformance with recommended practices. The plate surface flatness should also be within the acceptable tolerance limits.
Eccentric Orifice Plate.
The eccentric orifice plate is similar to the concentric device in that it also exhibits a circular opening/bore. The bore position however is offset from the centreline of the pipe. When the bore is offset and oriented downwards of the pipe centreline, this enables flow of impurities in the measured fluid through the orifice without forming deposits and sediments in front of the aperture.
Eccentric orifice plates are often used in pipelines transporting heterogeneous mixtures, fluids carrying small amounts of non-abrasive solids or gases with small amounts of liquid or where there is the potential for formation of sediment, tar deposits etc.
Segmental Orifice Plate.
This device is essentially a thin plate with an opening in the shape of a circle segment which is comparable to a partially opened gate valve. This device is generally used for measuring liquids and gases carrying non-abrasive impurities including light slurries or exceptionally dirty gases.
The predictable accuracy of both segmental and eccentric orifice plates is not as high or reliable as the concentric orifice plate device.
Design Pressure :- For plates, limited by readout device only; integral orifice transmitter to 1500 PSIG(10.3 MPa).
Design Temperature :- This is a function of associated readout system, only when the differential-pressure unit must operate at the elevated temperature. For integral orifice transmitter, the standard range is − 20 to 250 ° F ( − 29 to 121 ° C).
Sizes :- Maximum size is pipe size
Rangeability:- If one defines rangeability as the flow range within which the combined flow measurement error does not exceed ± 1% of actual flow, then the rangeability of conventional orifice installations is about 3:1 maximum. When using intelligent transmitters with automatic switching capability between the “high” and the “low” span, the rangeability can approach 10:1.
Price :- plate only is $100 to $300, depending on size and materials. For steel orifice flanges from 2 to 12 in. (50 to 300 mm), the cost ranges from $250 to $1200. For flanged meter runs in the same size range, the cost ranges from $500 to $3500. The cost of electronic or pneumatic integral orifice transmitters is between $1500 and $2500. The cost of d/p transmitters ranges from $1000 to $2500, depending on type and “intelligence.”
Orifice plate advantages and disadvantages
Advantage of Orifice Plate :-
- The standard throttle pieces have been recognized by international regulatory organizations, without real flow calibration, you can put into use immediately.
- Simple structure, strong, stable and reliable performance, lower price.
- Wide application range, including all single-phase fluid (liquid, gas, steam), local mixed phase flow, can be use even the general process diameter, working conditions (temperature, pressure).
- The detection pieces and differential pressure display instrument can be bought separated from different manufacturer, so that it can realize the professional consumption management.
Limitations/ Disadvantage of orifice Plate :-
- The repeatability and accuracy in the flow sensor is not very clear due to the impact of many factors;
- The range is narrow, because the flow coefficient in associated to the Reynolds number, the general range are only 3: 1 ~ 4: 1;
- It has a requirement of longer straight pipe, especially for larger diameter types
- Large pressure loss;
- The sensor is sensitive to the corrosion, wear, scaling, dirt, the precision is difficult to guarantee with long-term use, it needs to remove for a strong inspection once a year;
- Adopt the flange connection, it will easily occur some problems, greatly increasing the maintenance workload.
Orifice Plate Technical Specification
Orientation of Orifice Plate as below
2. For gas service
3. For vertical Lines
4. For liquid lines on pipe rack
5. For steam & gas lines on pipe