Wednesday, March 28, 2012

Restriction Orifice Sizing

These flow meters can successfully monitor a wide variety of fluids under diverse conditions if you select the right orifice shape and the type of pressure taps.

orifice meter measuring pressure at a particular flow rate

To control an industrial process, it is essential to know the amount of material entering and leaving key process steps. To monitor this, the engineer can choose from a wide variety of flow meters. Despite this wealth of choice, simple orifice meters are often selected because of their low cost, compactness, and suitability for a wide range of flow rates.

Here, we will explore how such meters work, how to choose the right type of orifice, and the appropriate pressure taps.

How Flow Meters Work

The underlying principle behind an orifice meter is Bernoulli's energy equation for streamline flow which states that when a flow is contracted or expanded, the total energy of the fluid remains constant. During contraction, kinetic energy increases and potential energy decreases, whereas potential energy increases at the cost of kinetic energy in expansion. A change in potential energy in reflected in a change of static pressure.

Reduction of the cross-section of the flowing stream in passing through an orifice increases its velocity head at the expense of its pressure head. Bernoulli's equation thus provide a basis for correlating the increase in velocity head with the decrease in the pressure head. Due to the sharpness of the downstream side of the orifice plate it forms a free-flowing jet in the downstream fluid, as shown in figure 6.1. The point of minimum area of the flow stream, the vena contracta, is located at a position along the pipe that depends on the flow rate. Minimum pressure is observed at the vena contracts position. Pressure taps (the proper positioning of which will be discussed later) at the upstream and downstream sides allow the measurement of the differential pressure across the orifice.

Orifice Choices

The type of orifices can be classified by their geometry. Each has advantages, disadvantages, and typical applications.

 orifice shapes concentric, square, conical, quadrent, eccentric, segmential, differential pressure

Concentric Orifices

This is the type most commonly used in the chemical process industries (CPI). The concentric orifice plate is a flat metal sheet with a circular hole. It is installed in the pipeline with the hole or orifice concentric to the pipe

A square or sharp edged orifice is a clean-cut hole with straight walls perpendicular to the plate's face. Erosion or other damage to the inlet shape can make such orifices unsuitable for some flow measurements. To avoid damage to orifice edges, they can be made conical or rounded on the upstream side of the plate. These variants are called conical-edged and quadrant-edged orifices, respectively. They provide a constant discharge coefficient even at a low Reynolds number.

Concentric orifices are useful for clean liquids, gases, and low-velocity vapours. They should not be used for liquids containing solids, wet steam, and the like.

Eccentric and segmental: These are frequently used for gas metering when there is a possibility that entrained liquids or solids would otherwise accumulate in front of a concentric circular orifice. Such a build-up can be avoided if the opening is placed on the lower or upper side of the pipe. For liquid flow with entrained gas, the opening is placed on the upper side. In such situations, the pressure taps should be located on the opposite sides of the pipe from the opening.

If the upper or lower segment of the pipe is blocked by a plate with a straight square edge, then the wall of the pipe acts as a boundary of the orifice resulting in an increase of the discharge or orifice coefficient. In some of the commercially available segmental orifices, the plate can be raised or lowered like a gate, thus eliminating the need to change the orifice plate.

Nomenclature of terms in orifice

The eccentric orifice has the advantages of the segmental orifice, but it does not allow free drainage over as much of the pipe circumference. In all other respects, the eccentric orifice is superior to the segmental. Both designs can be used in horizontal pipes for liquids containing granular solids, wet steam, and oil-containing water. They, however, should be used for liquids containing solids that are sticky or that have a density close to the liquids.

Annular  Orifices

This orifice consists of a dish supported concentrically in a pipe section by supporting spiders. Upstream and downstream pressures are transmitted through the central shaft to a differential pressure transmitter through the central shaft to a differential pressure transmitter. The flow coefficient is constant above a pipe Reynolds number of 10,000. An annular orifice provides free drainage for heavy materials at the bottom of the pipe while allowing gas or vapours to pass along the top of the pipe.

Such orifices are used for gas metering when there is a possibility of entrained liquids or solids and for liquid metering with entrained gases present in small concentrations. They avoid the problem of dirt build-up in front of an orifice in liquid streams and of liquid build-up in a most gas stream. Their disadvantages are lack of available standard equations, and dependence on pipe dimensions to define flow area, which is a more serious drawback at a high ratio of orifice to pipeline diameter ( b> 0.9).

Integral Orifices

For very small pipe sizes, an orifice can be installed integrally with a differential pressure transmitter. This provides a compact installation with an accuracy of 2­5%. Such a combination is used particularly for flow studies.


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