Q.1(a): Describe any one method of calibrating horizontally installed cylindrical
fuel tank.
Answer: Volumetric calibration performed by adding liquid, generally cold water to
a fuel tank which is in small volumes. It totally depends on the volume of the tank and may vary from 50 to 400 litres. The liquid can either be metered in a calibrated flow meter or poured from a calibrated volumetric prover. Basically, in fuel tanks calibrated flow metering systems are used. After the new volume is added the level is measured and recorded by an electronic level gauge. All temperature corrections, provoked by the water temperature change must be indicated in the graduation table. If the fuel tank service conditions for
hydrostatic pressure are different from the calibrating conditions, the volume error will
occur. Thus corrections for hydrostatic pressure must be made. Those fuel tanks
which were installed at different angles from which they were calibrated (installation
angles may vary because of geological reasons or wrong installation) must be made one
more correction in order to obtain precise data. If the tank inclination is different from
the calibrated condition, there will be errors in the reported volume. The volumetric
calibration has limitations which with some corrections gives satisfactory results.
Therefore it is important to mention, that the volumetric calibration process requires a certain
amount of water subject to the fuel tank’s capacity and when the process is finished the
fuel tanks must be cleaned again to avoid further incorrect tank usage.
Q.1(b) Find the surface area and volume of a hexagonal tank with the given apothem length 2m, side 3m, and height 4 m.
Q.2(a): Describe the wet back three-pass package boiler in detail with a sketch. Explain why and where it is suitable to use.
Answer :
Silent features of wet back three pass package boiler :
- Low-cost heating, trouble-free operations & long-lasting
- Available with fixed grate and rocking grate furnace
- Available in shell & smoke tube types
- Works on natural proper water circulation
- provided with ID fan for maintaining to draught
- Safety valve, steam stop valve, water level gauge glass & control
- Switches mounted on the boiler
- Multi-fuel option – coal/lignite/wood/briquettes
Q2(b) : Calculate boiler pipe thickness for boiler having given the working pressure using following formula for IBR-350: WP= 2 f e(t-c)/(D-t+c) Where WP is working pressure 100 kg/sq cm(g) , t= minimum requried thickness , D= Pipe outside diameter 508 mm , f= pipe metal allowable stress 1200 kg/sqcm , e= joint efficiency 1.00 , c= 0.25
Answer : Working pressure WP = 2fe(t-c)/(D-t+c)
100 = 2x1200x1.00x(t-0.25)/(50.8-t+0.25)
(t-0.25)/(50.8-t+0.25)= 100/2400
2400(t-0.25)=100(50.8-t+0.25)
2400t-600=5080-100t+25
2400t+100t=5080+25+600
2500t=5705
Boiler pipe thickness t = 5705/2500=2.282 cm= 22.82mm
Q.3(a) : A circular beam of 105 mm diameter is subjected to a shear force of 500 kg. calculate the value of maximum shear stress and sketch the variation of shear stress along with the depth of the beam.
Q.3 (b): Find the torque which a shaft of 250 mm diameter can safely transmit if shear stress ( due to torsion ) does not exceed 460 kg/cm2
Q.4(a) : What is the function of a condenser in a steam power plant. List out the merits and demerits of surface condensers overjet condensers.
Answer: Function of the condenser in steam power plant: Low exhaust pressure is necessary to obtain low exhaust temperature. But the steam cannot be exhausted to the atmosphere if it is expanded in the engine or turbine to a pressure lower than the atmospheric pressure. Under this condition, the steam is exhausted into a vessel known as a condenser where the pressure is maintained below the atmosphere by continuously condensing the steam by means of circulating cold water at atmospheric temperature.
ADVANTAGES OF A SURFACE CONDENSER :
The various advantages of a surface condenser are as follows:
1. The condensate can be used as boiler feedwater.
2. Cooling water of even poor quality can be used because the cooling water does not come in direct contact with steam.
3. High vacuum (about 73.5 cm of Hg) can be obtained in the surface condenser. This increases the thermal efficiency of the plant.
DISADVANTAGES OF A SURFACE CONDENSER
The various disadvantages of the surface condenser are as follows:
1. The capital cost is more.
2. The maintenance cost and running cost of this condenser are high.
3. It is bulky and requires more space
Q.4(b) : In a small capacity thermal power plant, the condenser is provided with a separate air cooling section. The temperature of steam entering the condenser is 56*C and the temperature at the air pump suction is 46*C. The barometer reads 76cm of Hg.
Find [1] The vacuum in the condenser.
[2] If the discharge of the dry air pump is 90m3/min. Find the air leakage in the condenser kg/hr.
[3] Loss of condensate due to the water vapour carried with air through the air pump.
Solution : [1] Considering no ingress of air at the inlet, the pressure in the condenser will be the same as the pressure of the steam entering at that temperature. From the steam table at 56*C, the corresponding pressure = O. 1653 bar absolute = 0.1653 x 750 = 123.98 mm of Hg, Vacuum in the condenser = 760 — 123.98 = 636.02 mm of Hg
[2] Now at the outlet the partial pressure of the steam = 0.10101 bar abs. (from steam table at 46*C) Therefore the partial pressure of air = 0.1653 — 0.10101 = 0.0643 bar The discharge of air pump = 90 m3/ min = 5400m3/ hr. Here T = 46*C + 273 = 319 = 0.0643 bar = 0.0643 x 102 kPa, V = 5400 rn3, R of air =0.294 kJ/ K pv = mRT Therefore mass of air leakage in the condenser per hour m: x 102 x 5400)/ (0.294 x = 377.63 kg/hr
[3] Loss of condensate due to water vapour carried with air through air pump Suppose dryness fraction of exhaust steam is 0.85. It means 15% moisture content in existing air. = Total mass of air leakage x % moisture content. Mass of water vapour =377 -56.6445kg/hr