Orifice plate
Construction
An orifice plate consists of a circular, thin, flat plate with a hole (or orifice) machined through its centre to fine limits of accuracy. The orifice has a diameter less than the pipeline into which the plate is installed and a typical section of an installation is shown in Figure 23.2(a). Orifice plates are manufactured in stainless steel, monel metal, polyester glass fibre, and for large pipes, such as sewers or hot gas mains, in brick and concrete.
Principles of operation
When a fluid moves through a restriction in a pipe, the fluid accelerates and a reduction in pressure occurs, the magnitude of which is related to the flow rate of the fluid. The variation of pressure near an orifice plate is shown in Figure 23.2(b). The position of mini- mum pressure is located downstream from the orifice plate where the flow stream is narrowest. This point of minimum cross-sectional area of the jet is called the ‘vena contracta’. Beyond this point the pressure rises but does not return to the original upstream value and there is a permanent pressure loss. This loss depends on the size and type of orifice plate, the positions of the upstream and downstream pressure tappings and the change in fluid velocity between the pressure tappings that depends on the flow rate and the dimensions of the orifice plate.
In Figure 23.2(a) corner pressure tappings are shown at A and B. Alternatively, with an orifice plate inserted into a pipeline of diameter d, pressure tappings are often located at distances of d and d/2 from the plate respectively upstream and downstream. At distance d upstream the flow pattern is not influenced by the presence of the orifice plate, and distance d/2 coincides with the vena contracta.
Advantages of orifice plates
(i) They are relatively inexpensive.
(ii) They are usually thin enough to fit between an existing pair of pipe flanges.
Disadvantages of orifice plates
(i) The sharpness of the edge of the orifice can be- come worn with use, causing calibration errors.
(ii) The possible build-up of matter against the plate.
(iii) A considerable loss in the pumping efficiency
due to the pressure loss downstream of the plate.
Applications
Orifice plates are usually used in medium and large pipes and are best suited to the indication and control of essentially constant flow rates. Several applications are found in the general process industries.
Venturi tube
Construction
The Venturi tube or venturimeter is an instrument for measuring with accuracy the flow rate of fluids in pipes. A typical arrangement of a section through such a device is shown in Figure 23.3, and consists of a short converging conical tube called the inlet or upstream cone, leading to a cylindrical portion called the throat. A diverging section called the outlet or recovery cone follows this. The entrance and exit diameter is the same as that of the pipeline into which it is installed. Angle β is usually a maximum of 21°, giving a taper of β/2 of 10.5°. The length of the throat is made equal to the diameter of the throat. Angle a is about 5° to 7° to ensure a minimum loss of energy but where this is unimportant a can be as large as 14° to 15°.
Pressure tappings are made at the entry (at A) and at the throat (at B) and the pressure difference h which is measured using a manometer, a differential pressure cell or similar gauge, is dependent on the flow rate through the meter. Usually pressure chambers are fitted around the entrance pipe and the throat circumference with a series of tapping holes made in the chamber to which the manometer is connected. This ensures that an average pressure is recorded. The loss of energy due to turbulence that occurs just downstream with an orifice plate is largely avoided in the venturimeter due to the gradual divergence beyond the throat.
Venturimeters are usually made a permanent installation in a pipeline and are manufactured usually from stainless steel, cast iron, monel metal or polyester glass fibre.
Advantages of venturimeters
(i) High accuracy results are possible.
(ii) There is a low pressure loss in the tube (typically only 2% to 3% in a well proportioned tube).
(iii) Venturimeters are unlikely to trap any matter
from the fluid being metered.
Disadvantages of venturimeters
(i) High manufacturing costs.
(ii) The installation tends to be rather long (typically 120 mm for a pipe of internal diameter 50 mm).