What is Control Valve?

 

Control valve

• A Control valve is a special type valve which is used to control fluid flow by changing the size of flow passage as controlled by a signal from a controller.
• Control valve is a power operated device that is used to regulate or manipulate the flow of fluid such as gas, oil, water, steam.

Figure: Typical Control Valve

• A valve with pneumatic, Hydraulic, Electric or other externally powered actuator that automatically opens or closes the valve fully or partially by the signals received from a controlling instrument is called control valve. 

What are the parts of a control valve?

Any control valve can be divided into two sections 

1. Actuator
2. Valve

Here we will learn the parts related to the actuator only. The valve part is completely mechanical item which will be discussed on another post.

Figure: Cross-section of Control Valve

1. Actuator
2. Positioner
3. Solenoid Valve
4. Limit Switch
5. Volume Booster
6. Hand wheel

Also read: What is droop speed control?

Actuator

• The actuator is the device connected to the valve through the valve-stem that provides the force required to change the size of flow passage of that valve. 
• Suppose a valve is now 30% open, you want to open it more like 50%, If you give the command to do this, The actuator will provide the driving force to the valve for opening it from 30% to 50%.
• In case of manual valve, there is not any actuator like this, wheel or handle is there to do the work of actuator.

There are many types of actuator available:

1. Electronic actuator
2. Motorized actuator
3. Hydraulic actuator
4. Pneumatic actuator

Hydraulic actuator:

• Hydraulic actuators are used on that places where large amount of force is required to operate the valve. 

For example, the main steam injection valve of any steam turbine.

• Although hydraulic actuators come in many designs, piston type is the most common. 
• A typical piston type hydraulic actuator is shown in below figure.

Figure: Piston type Hydraulic actuator

• It consists of a cylinder, spring, piston, valve-stem and a hydraulic fluid chamber. 
• Whenever the hydraulic fluid is allowed to go inside this chamber it forces the piston to move upward against the spring force on the top of piston.
• If the fluid force is greater than spring force, the piston  begins to move upward, spring compress as a result the valve begins to open. 
• Oppositely if we allow the fluid to go outside the spring will press the piston to go downward as a result the valve will be close.   
• We can operate the valve at fully open, fully close or partially open/close. 
• By regulating the amount of fluid inside the chamber, the valve can be operated at partially open/close condition.

Motorized actuator

• Normally if you want to operate any manual valve you have to rotate the hand-wheel of that valve, right?
• Now you are thinking......you will not rotate the wheel or handle, you will use an electric motor to rotate that wheel/handle, then it will be motorized actuator.

"The actuator in which motor is used for operating the valve is called motorized actuator"

More read: What happens when a motorized valve fails?

Figure: Motorized Control Valve

This types of valve used in such an application where the control of fluid temperature is desired.

What is Electrical Generator?

  Electrical Generator

• An electrical generator is a machine which converts the mechanical energy into electrical energy.
• The electrical energy, it could be DC power or AC power.
• If the generator produces AC power then usually it called alternator.

Cross section view of a typical Alternator

1. What is synchronous generator?
2. What is prime mover in an alternator?
3. What is Automatic Voltage regulator (AVR)?
4. Alternator armature on stator vs on rotor.
5. Why do turbo alternators use nonselient poles?
6. What is the most severe fault on an alternator?
7. The effect of prime mover failure in an alternator?
8. What happens if we connect a capacitor to a generator load?
9. How is the efficiency of an alternator affected by the load power factor?
10. Why do we use hydrogen for cooling large turbo-alternators instead of air?

Transformer

 What is Transformer?

• A transformer is a static electrical machine which is used to change the voltage level of the AC power supply.
• It can increase(step up) or decrease(step down) the level of voltage.
• It accomplishes this by electromagnetic induction.
• It does so without any change of the supply frequency.

Working Principle of Transformer

"The transformer works on the basic principle of electromagnetic induction and mutual induction between two coils/windings which are magnetically connected"

• Before going to the theory and laws, we will know the main function what it does.
• There are usually two coils primary coil and secondary coil on the transformer core if we give a power supply across the primary coil at any specific voltage level then the secondary coil will provide the same power with another different voltage level.  

 

Theory Explanation

• Suppose you have a coil or winding which is supplied by an alternating electrical power source. 
• Then the alternating current will passes through the coil and it will create a changing(Amplitude & Direction) or alternating magnetic flux that surrounds the coil. 
• Now if another winding is brought close to this winding some portion of this alternating flux will touch or link the second winding.
• Hence there will be an EMF or voltage induced in the second winding as per Faraday's law of electromagnetic induction.
• If you close the circuit of this second winding the an alternating current will flow through it. This is the basic working principle of a transformer. 

Motor

 Electric drives

• An electric motor or drive is an electro-mechanical machine which converts the electrical energy into mechanical energy.
• The working principle of electric motor is mainly depends on the interaction of magnetic and electric field.

Typical Motors. 

 Types of electric motor

There are at least a dozen different types of electric motor, but there are two main classifications.

1. Alternating Current(AC) Motor
2. Direct Current(DC) Motor

Motor Classifications

AC Motor

• The alternating current is converted to mechanical energy by the AC motor.
• The AC motor takes alternating current as a input.
• AC power is required to run this motor.

DC Motor

• A machine that converts the DC electrical power into mechanical power is known as DC motor.
• Works on the principle that "when a current carrying conductor is placed in a magnetic field, it will experience a mechanical force" 
• The direction of this mechanical force is as per Fleming's left hand rule.
• The DC motor takes direct current as a input.

Synchronous Motor

• The machine changes the electrical power to mechanical power at a desired frequency is called synchronous motor.
• The speed of motor is synchronized with the supply frequency.

Linear Motor

• The motor which produces linear force instead of rotational force is known as linear motor.

Asynchronous or Induction Motor

• This motor uses electromagnetic induction phenomenon for the conversion of electrical power into mechanical power.
• It never runs at synchronous speed.