Use of Orifice Plates in Flow Instrumentation

Differential Pressure Devices using a constriction in the pipeline have been the most common technique used for measuring fluid flow until the development of more advanced methods like use of non-contact Ultrasonic or Coriolis Technologies. Even with the development of these modern methods, differential pressure is still widely used technique. A constriction causes an increase in fluid velocity in the area of the constriction, which in turn will result, in a corresponding pressure drop across the constriction. This differential pressure (d.p) is a function of the flow velocity and density of the fluid.

A flowmeter in this category would normally comprise a primary element to develop a differential pressure and a secondary element to measure it. The secondary element is effectively a pressure transducer. There are various types of primary elements. The main types includes: Orifice plate, Venturi, Dall, Rotameter, Gate meter, V-Cone, Nozzle, Target Meter and Giflo Element.  In this article we look at Orifice Plate.

Orifice Plate
A orifice plate in its simplest form is  a thin steel plate with a circular orifice of known dimensions located centrally in the plate. This is termed a concentric orifice plate.The plate would normally be clamped between adjacent flange fittings in a pipeline, a vent hole and drain hole being provided to prevent solids building up and gas pockets developing in the system. 

The differential Pressure is measured by suitably located pressure tapping on the pipeline on either side of the orifice plate. These may be located in various positions depending on the applications

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Advantages of using Orifice Plates 

• Inherent Simple in 
• No moving parts
• Long-term reliability
• Inexpensive

Disadvantages of using Orifice Plates

• Poor turndown ratio
• Square root relationship
• Critical Installation requirements
• High irrecoverable pressure loss

You can also read:

• The Ultimate Guide to Industrial Flow Instruments
• Ultrasonic Level Measurements                                                             

Industrial Instrumentation Terms

Industrial Instrumentation and Control Systems have their own unique terms and standards that are used. Some of them include:

Process: This is the physical system we measure or control for example: Steam Boiler, Oil Refinery, Power Generating Unit etc. 

Process Variable or PV: The specific quantity we are measuring in a process e.g. Flow, PH, Temperature, Pressure, Level etc. 

Primary Sensing Element: This is a device that directly senses the process variable and translates the sensed quantity into an analog representation (electrical voltage, current, resistance, motion etc.) These elements maybe like Thermocouple, Thermistor, Accelerometer etc. 

Set point: This is the value at which we desire the process variable to be maintained at. 

Transducer: This is the device that converts one standardized instrumentation signal into another standardized instrumentation signal. It may also perform some processing of the signal. Some of these includes: I/P converter (converts 4-20 mA electric signal into 3-15 PSI pneumatic signal). 

Transmitter: A device that translates the signal produced by a primary sensing element into a standardized instrumentation signal such as 3-15 PSI air pressure, 4-20 mA DC electric current, etc. which can then be conveyed to an indicating device, a controlling device or both. 

Automatic Mode: When the controller generates an output signal based on the relationship of the process variable (PV) to the set point (SP). 

Manual Mode: When the controller's decision-making ability is bypassed to let a human operator directly determine the output signal sent to the final control element. 

Final Control Element: A device that receives the signal from a controller to directly influence the process e.g. a control valve. 

Manipulated Variable: This is the signal commanding (manipulating) the final control element to influence the process. 

Controller: A device that receives a process variable (PV) signal from a primary sensing element or transmitter, compares that signal to the desired value for the process variable (called the set-point), and calculates an appropriate output signal value to be sent to a final control element, such as a control valve.

 

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The above are some of the Key but very basic terms that any Instrumentation Professional should be well versed with.

You can also read: Basic Introduction to Industrial Instrumentation

Introduction to Industrial Instrumentation

Instrumentation deals with measurement and control in automated systems. Industrial Instrumentation covers areas like:

• Fluid Pressure
• Fluid Flow rate
• Temperature measurement
• Chemical concentration
• Machine position, motion or acceleration
• Fluid volume measurement
• Electrical voltage, current or resistance etc. 

Once a measurement of a given quantity is taken, it is then transmitted as a signal representing its quantity to an indicating or computing device where automated or human action takes place. If the controlling action is automated, the computer sends a signal to a final controlling device which then influences the quantity being measured. 

The final control device usually takes one of the following forms:

• Electric motor
• Control valve

Both the measurement device and the final control device connect to some physical system which we call the process. 

Let's take an example of a Home Thermostat as our measurement and control system. Our home's internal air temperature is our process under control. The thermostat serves two functions: 

• Sensing
• Control

The home's heater adds heat to the home to increase the temperature while the air conditioner extracts heat from the home to decrease temperature. The work of this control system is to maintain air temperature at some comfortable level, with the heater or air conditioner taking action to correct temperature if it strays too far from the desired value  (set point). 

From the above, you have an idea of what we mean by industrial instrumentation.

You can also read: 

• Basics of Industrial Instrumentation Systems
• Industrial Instrumentation Terms
• Introduction to Biomedical Instrumentation Systems