Flow Measurement Basics

    Flow measurement can be defined as the quantification of movement of a fluid. The flow measurement is assumed as the oldest recorded work in the instrumentation field. In industrial field, flow measurement is of great importance as from physiological processes to rocket science, the characteristic of flow is required.

    Generally, flow is measured in two ways, volumetric basis and on the basis of weight. A flow of solids is usually measured in terms of mass per unit time or weight per unit time. Liquid flow is measured volumetrically or in the basis of weight. Gaseous flow is normally measured volumetrically.

There are various methods used to measure the flow rate of steam, water, lubricants, air, etc.

Categories

• Measuring the flow precisely where the accuracy of the analyzer depends on it.
• Measuring the flow where it is necessary to know the flow rate but it is not critical (e.g., fast loop flow)
• Checking that there is flow present but measurement is not required (e.g., cooling water for heat exchangers.

Types of Methods:

 

Orifice Plate	Flange Taps	Corner Taps
Vena Contracta taps	Pipe Taps	Venturi tubes
Flow Nozzle	Elbow taps	Pitot tubes
Annubar

 

Orifice Plate

Advantages of orifice plates include:  

• High differential pressure generated
• Exhaustive data available
• Low purchase price and installation cost
• Easy replacement

Disadvantages include:

• High permanent pressure loss implies higher pumping cost.
• Cannot be used on dirty fluids, slurries or wet steam as erosion will alter the differential pressure generated by the orifice plate. 

Venturi Tube

 Advantages of venturi tube meter:

• Less chance of getting stuck with sediment.
• The discharge coefficient is high.
• Its behaviour can be predicted perfectly.
• It can be installed vertically, horizontally, inclined.
• They are more precise and can be used for a wide range of flows.
• About 90% of the pressure drop can be recovered.

Disadvantages of venturi tube meter:

• They are large in size and, therefore, where space is limited, they can-not be used.
• Initial costs, installation and expensive maintenance.
• Requires a long placement length. That is, the venturi meter must be driven by a straight tube that has no connections or misalignments to avoid turbulence in the flow, for satisfactory operation.
• It cannot be used in pipes of less than 7.5 cm in diameter.
• Maintenance is not easy.
• It cannot be modified to measure the pressure beyond a maximum speed 

Pitot Tube

Advantages of Pitot tube:

• Small and do not contain any moving parts
• Low cost
• Low permanent pressure loss.
• Ease of installation into an existing system

Disadvantages of Pitot tube:

• Foreign material in a fluid can easily clog pitot tube and disrupt normal reading as a result.
• Low accuracy
• Low rangeability

 

Sources of Flow measurement errors:

• Erosion
• Over-ranging Damage to DP cell
• Vapour Formation in the Throat
• Clogging of Throat
• Plugged or Leaking in sensing lines

Orifice Plate

The orifice plate is the most common form of restriction that is used in flow measurement. An orifice plate is basically a thin metal plate with a hole bored in the center. It has a tab on one side where the specification of the plate is stamped. The upstream side of the orifice plate usually has a sharp, edge.

When an orifice plate is installed in a flow line (usually clamped between a pair of flanges), increase of fluid flow velocity through the reduced area at the orifice develops a differential pressure across the orifice. This pressure is a function of flow rate.

Flange Taps

Flange taps are the most widely used pressure tapping location for orifices. They are holes bored through the flanges, located one inch upstream and one inch downstream from the respective faces of the orifice plate.

The upstream and downstream sides of the orifice plate are connected to the high pressure and low-pressure sides of a DP transmitter. A pressure transmitter, when installed to measure flow, can be called a flow transmitter.

Corner Taps

Corner taps are located right at upstream and downstream faces of the orifice plates.

Vena Contracta Taps

 Vena contracta taps are located one pipe inner diameter upstream and at the point of minimum pressure, usually one-half pipe inner diameter downstream.

Pipe Taps

Pipe taps are located two and a half pipe inner diameters upstream and eight pipe inner diameters downstream. When an orifice plate is used with one of the standardized pressure tap locations, an on-location calibration of the flow transmitter is not necessary. Once the ratio and the kind of pressure tap to be used are decided, there are empirically derived charts and tables available to facilitate calibration.

Venturi Tubes

For applications where high permanent pressure loss is not tolerable, a venturi tube can be used. Because of its gradually curved inlet and outlet cones, almost no permanent pressure drop occurs. This design also minimizes wear and plugging by allowing the flow to sweep suspended solids through without obstruction.

        

Flow Nozzle

A flow nozzle is also called a half venturi.

The flow nozzle has properties between an orifice plate and a venturi. Because of its streamlined contour, the flow nozzle has a lower permanent pressure loss than an orifice plate (but higher than a venturi). The differential it generates is also lower than an orifice plate (but again higher than the venturi tube). They are also less expensive than the venturi tubes. 

Elbow Taps

Centrifugal force generated by a fluid flowing through an elbow can be used to measure fluid flow. As fluid goes around an elbow, a high-pressure area appears on the outer face of the elbow. If a flow transmitter is used to sense this high pressure and the lower pressure at the inner face of the elbow, flow rate can be measured. Figure 8 shows an example of an elbow tap installation.

Pitot Tubes

Pitot tubes also utilize the principles captured in Bernoullis equation, to measure flow. Most pitot tubes actually consist of two tubes. One, the low-pressure tube measures the static pressure in the pipe. The second, the high-pressure tube is inserted in the pipe in such a way that the flowing fluid is stopped in the tube. The pressure in the high-pressure tube will be the static pressure in the system plus a pressure dependant on the force required stopping the flow. Pitot tubes are more common measuring gas flows & liquid flows

Annubar

An annubar is very similar to a pitot tube. The difference is that there is more than one hole into the pressure measuring chambers. The pressure in the high-pressure chamber represents an average of the velocity across the pipe. Annubars are more accurate than pitots as they are not as position sensitive or as sensitive to the velocity profile of the fluid.