Instrumentation

Bernoulli’s theorem states that the ‘total energy of a liquid flowing from one point to another remains constant’. It is applicable for non-compressible liquids. For different types of liquid flow Bernoulli’s equation changes. There is direct proportion between speed of fluid and its dynamic pressure and its kinetic energy. It can be used in various real life situations like measuring pressure on aircraft wing and calibrating the airspeed indicator. It can also be used to low pressure in the venturi tubes present in carburetor.

D.P. transmitter can be calibrated using following steps:

1. Adjust zero of Xmtrs.

2. Perform static pressure test: Give equal pressure on both sides of transmitter. Zero should not shift either side. If the zero shifts then carry out static alignment.

3. Perform vacuum test: Apply equal vacuum to both the sides. Zero should not shift.

4. Calibration procedure: Give 20 psi air supply to the transmitter and vent L.P. side to atmosphere. Connect output of the instrument to the standard test gauge. Adjust zero. Apply required pressure to the high pressure side and adjust the span. Adjust zero gain if necessary.

Flow varies directly as the square root of pressure. Thus, F=K of square root of applied pressure. Since this flow varies as the square root of differential pressure. The pressure pen does not directly indicate flow. Thus flow can be determined by taking the square root of the pen. Assume the pen reads 50% of the chart. So, flow can be calculated using the pen measure in the chart.

Pressure gauge includes following components:

a. ‘C’ type bourdon tube.

b. Connecting link

c. Sector gear

d. Pinion Gear

e. Hair spring

f. Pointer

g. Dial

Use of hair spring: Hair spring is responsible for controlling torque. It is also used to eliminate any play into linkages.

The single seated valve is used on smaller sizes where an absolute shut off is required. The use of single seated valve is limited by pressure drop across the valve in the closed or almost closed position.

In double seated valves the upward and downward forces on the plug due to reduction of fluid pressure are nearly equalized. It is generally used on bigger size valves and high pressure systems. Actuator forces required are less.

Valve positioner can be used for following reasons:

a. Quick action

b. Valve hysterisis

c. Viscous liquids

d. Split range.

e. Line pressure changes on valve

f. Bench set not standard

g. Reverse valve operations

Primary elements of measuring pressure are:

a. Bourdon Tube

b. Diaphragm

c. Capsule

d. Bellows

e. Pressure springs

These elements are known as elastic deformation pressure elements.

Types of bourdon tubes:

1. C type

2. Spiral

3. Helix

The commonly used control valves can be defined as follows:

a. Depending on Action:

Depending on action there are two types of control valves 1. Air to close 2. Air to close

b. Depending on body:

Depending on body there are 4 types of control valves

1. Globe valves single or double seated

2. Angle valves

3. Butterfly valves

4. Three way valves

Balanced draft boilers are generally used negative furnace pressure. When both forced draft and induced draft are used together, at some point in the system the pressure will be same as that of atmosphere. Therefore the furnace pressure must be negative to prevent hot gas leakage. Excessive vacuum in the furnace however produces heat losses through air infiltration. The most desirable condition is that the one have a very slight negative pressure of the top of furnace.

Intrinsic safety is a technique for designing electrical equipment for safe use in locations made hazardous by the presence of flammable gas or vapours in the air. Intrinsically safe circuit is one in which any spark or thermal effect produce either normally or under specified fault conditions is incapable of causing ignition of a specified gas or vapour in air mixture at the most ignited concentration.

The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost vertical breakdown region are known a s zener diodes. A zener diode operating in the breakdown region is equivalent to a battery. Because of this current through zener diode can change but the voltage remains constant. It is this constant voltage that has made the zener diode an important device in voltage regulation.

Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.

Force balance principle: A controller which generates an output signal by opposing torque. The input force is applied on the input bellows which moves the beam. This crackles nozzle back pressure. The nozzle back pressure is sensed by the balancing bellows which brings the beam to balance. The baffle movement is very less about 0.002 for full scale output.

Advantages:

a. Moving parts are fewer.

b. Baffle movement is negligible

c. Frictional losses are less

A controller which generates an output signal by motion of its parts. The increase in the baffle is to move towards the nozzle. The nozzle back pressure will increase. This increase in the back pressure acting on the balancing bellows, will expand the bellows. The nozzle is moved upward due to this. The nozzle will move until motion almost equals the input baffle motion.

A ratio control system is characterized by the fact that variations in the secondary variable don’t reflect back on the primary variable. A ratio control system is the system where secondary flow is hold in some proportion to a primary uncontrollable flow.

If we assume that the output of a primary transmitter is A. and the output of the secondary transmitter is B, and that the multiplication factor of the ratio relay is K, then for equilibrium conditions which means set valve is equal to measured valve, we find the following relation:

KA-B=0 or B/A = K where ‘K’ is the ratio setting off the relay.

1. Emitter +ve of meter and base -ve output =Low resistance

2. Emitter -ve of meter and base +ve output =High resistance

3. Collector +ve and base -ve output =Low

4. Collector -ve and base +ve output =Low

Emitter: Collector = High resistance

PNP: Opposite Results

A solenoid is electrically operated valve. It consists of solenoid coil in which magnetic plunger moves. This plunger is connected to the plug and tends to open or close the valve. There are two types of solenoid valves:

1. Normally Open

2. Normally closed

Use: It is used for safety purpose in different electric work

In automatic reference junction compensation, variable nickel resistor is used. As the temperature changes, so does its resistance. This reference junction compensator is located, so that it will be at the temperature of the reference junction. The reference junction is at the poset where the dissimilar wire of the thermocouple is rejoined. This joint is invariably at the terminal strip of the instrument.

When, in some processes, e.g. batch processes, long transient responses are expected during which a sustained deviation is present the controller integral action continuously drives the output to a minimum or maximum value. This phenomenon is called ‘integral saturation of the control unit’. When this condition is met, then this unit is de-saturated.