## Flow Measurement With Orifice,Venturi And Rotameter

Orifice Plate :

With an orifice plate, the fluid flow is measured through the difference in pressure from the upstream side to the downstream side of a partially obstructed pipe. The plate obstructing the flow offers a precisely measured obstruction that narrows the pipe and forces the flowing fluid to constrict.

The orifice plates are simple, cheap and can be delivered for almost any application in any material.

Turndown Ratio :

Turndown ratio can be expressed as:

TR = qmax / qmin (1)

where

TR = Turndown Ratio

qmax = maximum flow

qmin = minimum flow

Maximum and minimum flow is stated within a specified accuracy and repeatability for the device.
Example – Turndown Ratio for an Orifice Meter

The turndown ratio – TR – for an orifice meter with maximum flow of 12 kg/s and a minimum flow of 3 kg/s can be calculated as:

TR = (12 kg/s) / (3 kg/s)

= 4
normally expressed as turndown ratio of 4:1

This is a typical turndown ratio for a orifice plate. In general a orifice plates has turndown ratio between 3:1 and 5:1.

Venturi Tube :

Due to simplicity and dependability, the Venturi tube flowmeter is often used in applications where it’s necessary with higher TurnDown Rates, or lower pressure drops, than the orifice plate can provide.

In the Venturi Tube the fluid flowrate is measured by reducing the cross sectional flow area in the flow path, generating a pressure difference. After the constricted area, the fluid is passes through a pressure recovery exit section, where up to 80% of the differential pressure generated at the constricted area, is recovered.

With proper instrumentation and flow calibrating, the Venturi Tube flowrate can be reduced to about 10% of its full scale range with proper accuracy.

Rotatometer :

The rotameter consists of a vertically oriented glass (or plastic) tube with a larger end at the top, and a metering float which is free to move within the tube. Fluid flow causes the float to rise in the tube as the upward pressure differential and buoyancy of the fluid overcome the effect of gravity.

The float rises until the annular area between the float and tube increases sufficiently to allow a state of dynamic equilibrium between the upward differential pressure and buoyancy factors, and downward gravity factors.

The height of the float is an indication of the flow rate. The tube can be calibrated and graduated in appropriate flow units.

The rotameter meter typically have a TurnDown Ratio up to 12:1. The accuracy may be as good as 1% of full scale rating.

Magnetic floats can be used for alarm and signal transmission functions.