A Comparison Between Orifice Plate And Venturimeter

Flow is one of the crucial process variables that is needed to be measured, transmitted, manipulated and controlled in order to ensure safety and success of a given process in process control industries.
Orifice plates and venturimeters are the two most widely used flow meters in industries for flow measurement.

In this article, the orifice meters and venturimeters are compared and its similarities and differences are listed down.

Similarities

1. Orifice plates and venturimeters come under the category of Variable head flow meters or Differential flow meters or Obstruction type meters. They are the Primary elements of differential flow meter. The primary elements are placed in the path of the fluid and introduce an obstruction to the fluid passage in order to produce a differential pressure.

2. Orifice plates and venturimeters are used in conjunction with secondary elements of differential flow meters. Secondary elements are differential pressure measuring devices namely, manometer, bellow meter, force balance meter and ring balance meter.

3. Orifice plates and venturimeters are called as volumetric flow detectors as they measure the volume of flow rate. Mass flow rates can be calculated by measuring temperature and pressure.

4. Orifice plates and venturimeters work on Bernoulli's principle which states that the difference in pressure created across the obstruction gives the measure of the fluid flow rate. The flow rate is proportional to the square root of the differential pressure.

5. It is difficult to employ these meters to measure pulsating flow.

Differences

Orifice plate	Venturimeter
Orifice plates are the simplest element of flow measurement. Orifice has a precisely measured hole in the middle and is inserted in the pipeline to reduce pipe diameter and thereby to offer obstruction. Orifice Plate	Venturimeter consists of a straight inlet section, a converging conical inlet section, a cylindrical throat and a diverging conical outlet section. The throat section is of a small cross section and it provides constriction to the fluid. Venturimeter
The high pressure tap is located before the orifice plate and the low pressure tap is located after it.	The high pressure tap is taken at the inlet section and the low pressure tap is located at the middle of the throat.
The different types of orifice plates are,  1. Concentric 2. Eccentric 3. Segmental 4. Quadrant edge.	The various types of Venturimeters are,  1. Long-form or classic 2. Short-form 3. Eccentric 4. Rectangular
It is made of stainless steel, nickel, monel, phosphor bronze.	It is usually made of cast iron or steel.
Orifice cause high permanent pressure loss of about 60 – 70%.	The diverging conical outlet section is where the pressure head is recovered. The permanent pressure loss or head is about 10% which is much lower compared to orifice.
The coefficient of discharge is low (0.6).	They have higher discharge coefficient of about 0.95 – 0.98.
Turn down ratio* is generally less than 5:1.	Turn down ratio is 10:1.
Flow range is 10-3 to 5.5*103 m3/hr.	Flow range is 1.0 to 5.5*103 m3/hr.
Lower flow rate reduces the accuracy of orifice plates. Higher flow rates in high quality orifice plates gives increased accuracy.	Venturimeters are widely used for high flow rates. They can handle 25-50% more flow rate than an orifice.
Orifice plates are smaller in size when compared to venturimeters. It can be used in a wide range of pipe sizes from 3.175 mm to 18211.8 mm.	The size of the venturimeter is specified in terms of inlet pipe diameter and the throat diameter. Example: 300 x 150 mm venturimeter. The inlet pipe diameter is 300 mm and throat diameter is 150 mm. The throat diameter is usually between 1/3 and 3/4 of the inlet pipe diameter. They have a large size. So, it is generally not used below 76.2 mm pipe size.
The pipe design needs to be straight for an orifice.	The pipe design need not be straight as they are insensitive to the changes in the velocity profile of flow.
Clogging is a problem as time progresses and so it is not suitable for slurries.	It can be used for slurries and dirty liquids, if the pressure taps are protected from plugging.
Orifice plates have changing characteristics due to erosion, corrosion and scaling. This reduces its accuracy.	Venturimeters are used where pressure recovery is important and where accuracy is desired in the measurement of high viscous fluids.
Orifice plates are inexpensive. The initial cost is low.	Venturimeters are relatively expensive than orifice. The installation cost is high but lower maintenance and operational cost.

* The turndown ratio gives an idea about the range in which the particular flow meter can be operated with desired accuracy.

Turndown ratio can be expressed as:            

TR = qmax / qmin

Where,

TR = Turndown Ratio

qmax = maximum flow

qmin = minimum flow


Try this quiz on sensors and transducers. It will be fun test!

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