Fluid Flow Measurement

FLUID FLOW MEASUREMENT

CHARACTERISTICS OF FLUID FLOW

A mathematical formula can be used to illustrate how an increase in the velocity of fluid flow can cause a decrease in the pressure exerted on the walls of a pipe. In this formula, only velocity and pressure are considered. Other factors, such as friction, viscosity, and height are ignored. Without considering these factors, the formula is simply

2 2 PA+K (VA) = PB + K(VB)

P represents the pressure exerted by the fluid on the walls of the pipe. PA is the pressure on the walls of the pipe at point A and PB is the pressure at point B. V represents the velocity of the fluid through the pipe. VA is the velocity of the fluid flow at point A and VB is the velocity at point B. K is a mathematical constant.

Types of Fluid Flow The velocity of a fluid also affects the kind of flow that occurs through a pipe. The two basic terms used to describe fluid flow are "laminar" and "turbulent." The figure below illustrates the difference between laminar flow and turbulent flow. At relatively low overall velocities, fluid flow tends to be laminar, or streamlined. When flow is laminar, the fluid tends to flow smoothly in layers. The velocity of the layers at the center of the flow is higher than the velocity of the outer layers. The decreased velocity of the outer layers is due to friction between the outer layers of the fluid and the walls of the pipe. At higher velocities, fluid flow tends to become turbulent. When flow is turbulent, the layers of fluid are erratic instead of streamlined. Turbulence may have an adverse effect on the accuracy of fluid flow measurements by causing variations in pressure on the walls of a pipe at any given point.

155 FLUID FLOW MEASUREMENT

Types of Fluid Flow

Orifice Plates Orifice plates are simple devices that are installed in pipes to restrict the flow of fluid through the pipes. The figure below shows an example of a typical orifice plate. It is basically a flat, round plate with an orifice, or hole, in it. Orifice plates are usually categorized according to the shape of their opening and/or its location. For example, the orifice plate shown in below is called a concentric orifice plate because it has a round opening located in the center of the plate.

Concentric Orifice Plate

The figure below shows two other types of orifice plates: an eccentric orifice plate (Figure A) and a segmental orifice plate (Figure B). The orifice in an eccentric orifice plate is always positioned off center. In a segmental orifice plate, the orifice is a part, or segment, of a circle.

Typical Orifice Plates

Figure below is a cutaway view of a concentric orifice plate mounted between two flanges. As shown in the figure, a flange is a rim around a pipe that provides a means of bolting two sections of pipe together.

Orifice Plate Installed Between Two Flanges

The change in pressure across an orifice is indicated by the pressure curve shown in blue in Figure below. The pressure exerted against the walls of the pipe in the figure is greatest at point A and, because of the orifice plate, lowest at point B, where the velocity of the fluid is the greatest. This point is called the vena contracta. From point B to point C, the pressure exerted against the walls of the pipe again increases, because the velocity of the fluid decreases. At point C, the pressure exerted against the walls of the pipe is almost equal to the pressure exerted at point A. The pressure at point C is still somewhat lower, however, because some of the energy in the fluid flow has been used to accelerate the fluid past the orifice plate. This causes a loss of pressure that is not completely regained. The difference between the pressure on the walls of the pipe at point A and the pressure on the walls of the pipe on the opposite side of the orifice plate is the differential pressure, or delta P. The high pressure on the upstream side of the orifice plate is compared to the low pressure on the downstream side. The difference between these pressures changes when the flow rate changes. By measuring this differential pressure, fluid flow through a pipe can be determined. The specific points of measurement are determined by the system designer.

Effect of Orifice Plate on Pressure

The differential pressure created by an orifice plate is measured by placing taps on either side of the plate. A tap is a hole in a pipe with a tube installed in it. When a measuring device is connected to the taps as shown in Figure below, the delta P can be measured.

Tap Placement for Orifice Plate Installation