Difference between venturi meter and orifice meter
Flow is one of the crucial process variables that is needed to be measured, transmitted, manipulated, and controlled in order to ensure the safety and success of a given process in process control industries. Orifice plates and venturi meters are the two most widely used flow meters in industries for flow measurement.
In this article, the orifice meters and centimeters are compared and their similarities and differences are listed.
Similarities
1. Orifice plates and venturi meters come under the category of Variable head flow meters Differential flow meters or Obstruction type meters. They are the Primary elements of a 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 venturi meters 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 venturi meters are called as volumetric flow detectors as they measure the volume of the flow rate. Mass flow rates can be calculated by measuring temperature and pressure.
4. Orifice plates and venturi meters 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.
Differencesbetween orifice plate and venturi meter
Orifice plate
Venturimeter
Orifice plates are the simplest element of flow measurement. The orifice has a precisely measured hole in the middle and is inserted in the pipeline to reduce pipe diameter and thereby 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 in the middle of the throat.
The different types of orifice plates are 1. Concentric2. Eccentric3. Segmental4. Quadrant edge.
The various types of Venturimeters are 1. Long-form or classic2. Short-form3. Eccentric4. Rectangular
It is made of stainless steel, nickel, Monel, and phosphor bronze.
It is usually made of cast iron or steel.
The orifice causes 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 the orifice.
The coefficient of discharge is low (0.6).
They have a 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.
The flow range is 1.0 to 5.5*103 m3/hr.
A lower flow rate reduces the accuracy of orifice plates. Higher flow rates in high-quality orifice plates give increased accuracy.
Venturi meters 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 venturi meters. It can be used in a wide range of pipe sizes from 3.175 mm to 18211.8 mm.
The size of the venturi meter is specified in terms of inlet pipe diameter and throat diameter. Example: 300 x 150 mm venturi meter. The inlet pipe diameter is 300 mm and the throat diameter is 150 mm. The throat diameter is usually between 1/3 and 3/4 of the inlet pipe diameter. They have large sizes. 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.
Venturi meters are used where pressure recovery is important and where accuracy is desired in the measurement of highly viscous fluids.
Orifice plates are inexpensive. The initial cost is low.
Venturi meters are relatively more expensive than orifice. The installation cost is high but has lower maintenance and operational cost.
* The turndown ratio gives an idea about the range in which the particular flow meter can be operated with the desired accuracy.
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