How a Self Operated Pressure Reducing Regulator Works

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A self operated pressure reducing regulator is a mechanical device that is used to control and reduce pressure especially in natural gas plants. A pressure regulator is essentially a force balanced device that adjusts to changes in the system it is controlling. There are two types of pressure reducing regulators used in natural gas systems:

1. Self operated regulators

2. Pilot operated regulators

Both types of regulators are very common in the gas industry the self-operated regulators are general used in lower flow and lower pressure system, and are less expensive regulators. While the pilot operated regulators are generally use in higher flow situation, like city gates, large customers, industrial accounts etc and where you have higher pressure to control.

Basic Parts of a Self Operated Pressure Reducing Regulators

Self Operated regulators consist of three basic components:

1. A loading element. 

2. A measuring element and 

3. A restrictive element as shown below

Self Operated Pressure Reducing Regulator

As seen above, the loading element is typically a spring but it can also be a weight or pressure from some external source. When the spring is compressed, it exerts a loading force. The measuring element or diaphragm is connected to the process fluid (gas) that is being controlled and creates a force opposing the loading force. The restricting element or valve is connected to the spring and diaphragm assembly and regulates the flow through the regulator.

Operating Principle of Self Operated Pressure Reducing Regulators

In a self operated pressure regulator, as downstream system pressure decreases the spring force overcomes the force of the gas acting on the effective area of the diaphragm and the valve opens increasing flow into the system. When system pressure increases, the measuring force (the force of the system gas acting on the effective area of the diaphragm) overcomes the loading force (spring force) and closes the valve reducing flow into the system.

How An Air Pressure Regulator Works

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In pneumatic instrumentation systems, instrument air is required to power valve actuators and other instruments - transmitters, controllers, control valves etc. A key component of the instrument air supply system is an air pressure regulator. The air pressure regulator is a simple device.  It is used to lower the main instrument air supply of a plant to a pressure suitable for an air-operated instruments; eg, a transmitter, control valve, etc.  

Normally, each air operated instrument has its own regulator.  So an air regulator is one of the most common devices in the plant.  There are various manufacturers of air regulators, eg Masoneilan and Fisher.  However, they all work in much the same way.  The schematic of a Fisher air pressure regulator is shown below:

Principle of Operation of the Air Pressure Regulator

1. The main air supply is connected to the AIR INLET PORT.  Air passes into  the filtering chamber at the bottom of the regulator.
2. Air passes through the filter which removes dirt particles in the incoming air which may block nozzles etc.  It then goes into the valve assembly.
3. The valve assembly is moved by the range spring pressing on the diaphragm.
4. The range spring will hold the valve assembly down until the output pressure is high enough to lift the diaphragm (via the air passage shown).  At this point the small spring in the valve assembly closes the valve.
5. Air is allowed to pass through a hole at the center of the diaphragm and out of the vent.  This maintains balanced pressure across the diaphragm.
6. If the outlet pressure is above the pressure set by the range spring, the air will go out through the vent above the diaphragm.  When the outlet pressure is correct, the valve assembly opens to set the correct pressure. This pressure exits the regulator through the OUTLET AIR PORT
7. If the outlet pressure is below the pressure set by the range spring the valve assembly will stay open until the set pressure is reached.

How a Current to Pressure Transducer (I/P) Works

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A “current to pressure” transducer (I/P) converts an analog signal (4 to 20 mA) to a proportional linear pneumatic output (3 to 15 psig). Its purpose is to translate the analog output from a control system into a precise, repeatable pressure value to control pneumatic actuators/operators, pneumatic valves, dampers, vanes, etc.

The  I/P converter provides a reliable, repeatable, accurate means of converting an electrical signal into pneumatic pressure in many control systems. Models of this device are usually available in direct and reverse action and are field selectable with full or split range inputs or output as the case may be.

How a Dead Weight Tester Works

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A dead weight tester is a very handy piece of instrument for calibrating a pressure gauge or other pressure transducers in an industrial plant. But how does this device work? Lets find out:

A deadweight tester consists of a pumping piston with a screw that presses it into the reservoir containing a fluid like oil, a primary piston that carries the dead weight, W, and the pressure gauge or transducer to be calibrated as shown in the schematic above. It works by loading the primary piston (of cross sectional area A),

Linear Variable Differential Transformer (LVDT)

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A pressure sensor can be created using the motion of a high permeability core in a magnetic field created by the coils of a transformer. This principle is what is used in a Linear variable differential transformer. The movement of the core is transferred from the process medium to the core by the use of a diaphragm, bellows or bourdon tube.

The LVDT operates on the inductance ratio between the transformer coils.

Vibrating Wire Sensors

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It is a well known fact that the natural frequency of a tensioned string increases with tension. Mathematically, the relationship between the resonant frequency of a string and the tension applied on the string is given by:

Where,

F = Fundamental resonant frequency of string (Hertz)

L = String length (meters)

T = String tension (newtons)

μ = Unit mass of string (kilograms per meter).

This implies that a string can be used as a force sensor. In this type of sensor design, an electronic oscillator circuit, is used to keep a wire vibrating at its natural frequency when under tension. The principle is similar to that of a guitar string.

Differential Capacitance Pressure Sensors

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Like the strain gauge, differential capacitance sensors use a change in electrical characteristics to infer pressure. Here a change in capacitance is used to infer pressure measurement. The capacitor is a device that stores electrical charge. It consists of two metal plates separated by an electrical insulator. The metal plates are connected to an external electrical circuit through which electrical charge can be transferred from one metal plate to the other.

The capacitance of a capacitor is a measure of its ability to store charge. The capacitance of a capacitor is directly proportional to the area of the metal plates and inversely proportional to the distance between them. It also depends on a characteristic of the insulating material between them. This characteristic, called permittivity is a measure of how well the insulating material increases the ability of the capacitor to store charge. Mathematically this can be put as:

C = ε A/d, 

here C = capacitance, A = area of plates, d = distance between plates of capacitor. ε = is the permittivity of the insulator between capacitor plates.

How a Strain Gauge Works

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Several different technologies exist for the conversion of fluid pressure into an electrical signal response. These technologies form the basis of today’s electronic pressure transmitters. One of such technology is the strain gauge discussed here.

A strain gauge is what may be described as a ‘’piezoresistive element’’. This means its resistance changes with changes in applied pressure. Basically, a strain gauge uses the change of electrical resistance of a material (wire, foil or film), under strain to measure pressure.

The electrical resistance of any conductor is proportional to the ratio of length over cross-sectional area (R ∝ L/A), which means that tensile deformation (stretching) will increase electrical resistance by simultaneously increasing length and decreasing cross-sectional area while compressive deformation will decrease electrical resistance by simultaneously decreasing length and increasing cross-sectional area.

The complete strain gauge pressure-measuring device includes:

Mechanical Pressure Sensors

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Pressure:

Pressure is defined as a force per unit area, and can be measured in units such as psi (pounds per square inch), inches of water, millimeters of mercury, pascal (Pa, or N/m²) or bar. Until the introduction of SI units, the 'bar' was quite common.

The bar is equivalent to 100,000 N/m², which were the SI units for measurement. To simplify the units, the N/m² was adopted with the name of Pascal, abbreviated to Pa.

Pressure is quite commonly measured in kilo pascals (kPa), which is 1000 Pascal and equivalent to 0.145psi.

Absolute, Gauge and Differential Pressure:

Pressure varies depending on altitude above sea level, weather pressure fronts and other conditions. The measure of pressure is, therefore, relative and pressure measurements are stated as either gauge or absolute.

Gauge pressure is the unit we encounter in everyday work (e.g., tire ratings are in gauge pressure). A gauge pressure device will indicate zero pressure when bled down to atmospheric pressure (i.e., gauge pressure is referenced to atmospheric pressure). Gauge pressure is denoted by a (g) at the end of the pressure unit , e.g., kPa (g)