AUGUST 2017

MARINE ENGINEERING KNOWLEDGE (GENERAL) TIME ALLOWED -3 HOURS

Instructions 1. Answer any SIX questions.2. All questions carry equal marks.

3. Neatness in handwriting and clarity in expression carries weightage

4. Illustration of an answer with clear sketches /diagrams carries weightage

Q1. With reference to centrifugal pumps and pumping systems. A) Under what conditions, a centrifugal pump require a priming device for pump to operate normally? B) Draw a neat graph and explain and the performance curves of a centrifugal pump.

Answer:- The centrifugal pump operates on a rotodynamic principle whereby the water entering the impeller is given sufficient kinetic energy , which is further converted into potential energy or pressure head by the gradually increasing volume ofthe volute casing.

The main draw back of the centrifugal pump is that it is not a positive displace ment pump and so it will start pumping only when the casing is full of fluid. In situations when the suction level is below the centre of the impeller or the centre of the inlet , the air which is in the suction pipe will not be completely evacuated as the clearance between the volute casing and the impeller at the commencement of the volute casing will allow the return of air into the suction side thereby preventing the complete evacuation of the

trapped air. To overcome this situation or drawback a priming device is necessary in those pumps whose suction head is negative. If the suction head is positive as in the case of all the centrifugal pumps on board a ship placed at the floor plate level of the machinery space . The level of the sea water is higher than the pump suction in this situation.

(B)

From a mathematical consideration of the action of a centrifugal pump it can be shown that the theoretical relationship between

Head ,H and throughput, Q is a straight line as shown in the sketch., with the minimum throughput occurring when the head is maximum. Because of shock and eddy losses caused by impeller blade thickness and other mechanical considerations there will be some head loss, increasing slightly with throughput.

These losses together with friction losses due to fluid contact with the pump casing and inlet and impact losses, result in the actual H/Q curve shown in the sketch.

With the discharge valve fully open and the pump run at different speeds the H/n curve is obtained. The intersection of the H/Q curve and the H/n cuve gives the steady state service values for head, throughput in litres , and the RPM of the pump.

Q2. A. List the various type of valves used on board ship relating to the function required. B. With the help of sketch, describe how a screw down non-return globe valve be made a return valve?

Answer:- (A)The various valves used on board a ship are :

Straight globe valves. Angle valves (right angle) Sluice valves or gate valves. Butterfly valves Diverter valves used in auto controls.

The operational difference between globe valves and sluice valves is that lift of the globe valve needs to be only D/4 for full flow, whereas the sluice valve lift has to be D for full flow. In the globe valve because of change of direction there is a reduction in flow velocity , whereas in the sluice valve there is no change in flow velocity because the path is straight without diversion.

(B) In The screw down return valve the spindle end is attached to the valve head by an arrangement which enables

the spindle end to enter a suitable depression in the valve lid. The depression in the valve lid is bigger in diameter than the spindle dia. The spindle has a collar which is slightly bigger in dia and this collar is at the end of the spindle. This enables the spindle to turn without rotating the valve lid , but keeping it pressed whilst shutting the valve. The spindle carries a loose fitting nut on it which tightens on the valve lid , thereby preventing the detatchment of the valve from the spindle. When the valve is to be opened the valve will lift up as the spindle unscrews and lifts up. This arrangement makes it a return valve.

To convert this valve into a NR valve, the spindle is to be detatched from the valve. So when the valve is open it will prevent return flow from the top side of the valve since the valve will shut by sitting on its seat due to its weight and aided by the downward pressuire of the fluid. Flow is allowed only from bottom side to the topside and out.

To detatch the spindle from the valve, the nut is removed from the spindle and attached to the valve lid without the spindle . The valve is assembled in this arrangement, and can be reconverted to a return valve whenever needed.

SD NR.VALVE SD R.VALVE

Q3. Explain the working principle of differential Pressure Transmitter with the help of diagram and explain the following parts with their usages. A. Zero and span calibration B. Negative feedback bellow C. Pilot amplifier functions D. Zero Elevation Concept

Answer:- This question will be answered after I gather information on the differential transmitter.

Q4. A) Explain the advantages of using hollow shaft in place of solid shaft for intermediate shaft in propulsion system. B) With the aid of a simple sketch describe the lubrication system of Plummer block bearing.

Answer:- (A) The main advantage of a hollow shaft is that for the same torque transmitted the hollow shaft is lighter in weight with the same torsional strength. This reduces the material mass and its inertia.

(B)

The intermediate shafting (Figure 8.2) between the tailshaft and main engine, gearbox or thrustblock may be supported in plain, tilting pad or roller bearings. The two former types usually have individual oil sumps, the oil being circulated by a collar and scraper device ; roller bearings are grease lubricated. The individual oil sumps usually have cooling water coils or a simple cooling water chamber fitted. Cooling water is provided from a service main connected to the sea-water circulating system. The cooling water passes directly overboard.

Usually for plain and tilting pad bearings, only a bottom bearing half is provided, the top acting purely as a cover. The aftermost plumber block however, always has a full bearing. This bearing and any bearing in the forward end of the stern tube, may be subject to negative loading.

Q5. With respect to heat exchanges used on board ship A) Explain parallel and contra flow B) Draw a neat diagram showing the expansion arrangement of tubes in a tube and shell type heat exchanges C) What are the factors taken into account while deciding the number of phases for fluid at the design stage.

(A) The sketch shows a heat exchanger of the shell and tubes type with the coolant(water) flowing through the tubes within the shell. The hot fluid (oil) enters from the bottom and leaves from the top. The path of the hot fluid is shown in full dark line zig-zagging through the baffles. The hot fluid (oil) encounters the hotter water at the entry point of the oil . The exit of the coolant and entry of the oil are both at their top temperatures and the exit of the oil meets the entry of the coolant , both are at their low temperatures and their flow is in opposite direction . This is called contra- flow arrangement. If now the direction of flow of the hot fluid is changed as shown by the dotted line in the shell it is called parallel flow . In counter flow, by proper design it can be arranged to have the same temperature difference at every corresponding location in the flow. This results in constant temperature drop at all corresponding locations of their transit through the cooler, which eliminates thermal stressing of the tubes . In the parallel flow the

temperature difference at entry of both fluids is maximum and reduces to minimum at exit, the temperature difference being different at every location of their transit through the cooler will give rise to thermal stressing. It can be stated that the heat extracted or transferred in contraflow method is morethan heat transferred in the parallel flow method as can be observed from the shape of their diagrams.

(B)

Naval brass tube plates are used with aluminium brass tubes . the tube stacks are made up to have a fixed tube plate at one end and a tube plate at the other end , which is free to move when the tubes expand or contract according to the temperature .The fixed end tube plate is sand wiched between the shell and the water box, with jointing material. Synthetic

rubber O rings for the sliding tube plate permit free expansion

(C)In a practical heat exchanger, the thermal performance is described by the equation

Q=UƟA

Where:

Q=rate of heat transfer.

Ɵ=logarithmic mean temperature difference at the inlet and outlet of the heat exchanger; this is a maximum if the fluids flow in counter direction.

A=surface area of heat transfer wall

1/U=1/h1+1/h2+y/k

Where:

h1 =co-efficient of heat transfer on the hot fluid side..

h2=co-efficient of heat transfer on the cold fluid side.

K=thermal conductivity of the wall material

Y=thickness of the wall.

U = the combined coefficient of the phases or mode of heat transfer.

Q6. With regards to process control system explain following A. Proportional Control B. Integral Control C. Derivative Control D. The necessity of Derivative control .

Answer:- (A)Proportional control:-The action of a controllerose output signal V is proportional to the deviation Ɵ.

Ɵ is the differencebetween the measured value of the controlled condition (Ɵ0) and the command signal Ɵ1

V ἀ --Ɵ.

V=--k1 Ɵ

The negative sign denotes that the correction signal is opposite in direction to the deviation. k1 , is a constant depending on the controller characteristic , is called the Proportional action factor.

Potential correction( change of actual controlled condition Φ) is proportional to the movement of the correcting unit (which depends on V)

Φ ἀ V.

Φ=C1V

Where C1 is a constant depending on the correction unit characteristic

V= -K1Ɵ

Φ =-K1C1 Ɵ

Φ =-µ Ɵ

µ = Φ/ Ɵ and is numerically the proportional control factor , or simply the controller gain..

Example of the proportional controller is the ordinary centrifugal governor . The main drawback of this system of control ois that its offset is variable and so is suitable for only small variation of the deviation.

(B)Integral control:- The main object of integral action is to reduce offset to zero.

The action of a controller whose output signal changes at a rate which is proportional to the deviation. And mathematically it is expressed as

dV/dt ἀ -

by the definition given above

Dv/dt = -K2 Ɵ

V =-k2∫Ɵ dt

k2 is called the integral action factor.

Φ=C1V

Φ= - C1K2∫Ɵ dt

Φ=-ρ ∫Ɵ dt

Ρ is called the integral control factor. Thus the potential correction Φ at a given time t is is proportional to the area between the desired and recorded values . Rate of change of potential correction with respect to time is proportional to the deviation

Now µ=C1K1

and ρ=C1K2

Therefore K1/K2 = µ/ ρ =s

S is called the integral action time .

S is a factor which combines the proportional controller with the integral controller.. The integral controller cannot exist on its own , and if it so exists it will degenerate to the basic two step control or commonly called on-off control

In a controller having proportional plus integral control, the time interval in which the part of the output signal due to integral action increases by an amount equal to the part of the output signal due to proportional action, when the deviation is unchanging.

in perfect balanced proportional plus integral controller VP =VI and the controller output change equals 2vp( because of the negative sign of VP). S= µ/ ρ . Integral action reduces controller gain and in the perfect balanced control offset reduces to zero.. µ=ρ and because of the negative sign of µ their actions cancel each other and this results in zero off set.

The perfect example of a balanced proportional plus integral control is the floating lever operation of the hunting action in the telemotor control of the steering

gear . Whereas in the mechanical isochronous governor of the alternator a small imbalance between µ and ρ results in a small offset necessary for stabilising the governor which is also called droop.

(C)Differential control:- In a controller having proportional plus derivative action, the timre interval in which the part of the output signal due to proportional action increases by an amount equal to the part of the signal due to derivative action, when the deviation is changing at a constant rate.

The object of derivative control action is to give quicker response and supplement inadequate proportional control damping.

Derivative control action output signal is proportional to the rate at which the deviation is changing

By definition Vἀ -dƟ/dt

V=-K3*dƟ/dt

K3 is called the derivative action factor.

C=C1V

Φ= -C1K3 dƟ/dt

Φ= -  η dƟ/dt

 Η is called the derivative control factor

K1/K3 = µ/ η = 1/t

 

Necessity of Derivative control:-

After time T V0 = VP and total controller output signal change equals 2 VP. . Use of derivative action increases the proportional control factor and reduces hunting by introducing damping. By the use of critical damping which is obtained when V0= VP The control action is critical and subsidence ratio is zero.

Q7. With respect to Main Air compressors explain the effect of the following on the running of the compressor A. Improper bumping clearance B. Improper connecting rod bearing clearance C. Fouled air intake filters D. Fouled after cooler

Answer:- (A) if the bumping clearance is more than the minimum required, the clearance volume is increased and this reduces the volumetric efficiency of the compressor.

If the bumping clearance is less contact between piston and cylinder head occurs which can damage the piston and the cylinder head.

The bumping clearance should be bare minimum at working temperature so that the volumetric efficiency is maximum and no contact damage occurs to the piston or the head. The makers instructions must be followed in deciding the bumping clearance.

(B)Improper con-rod bearing clearance :

*If clearance is less the lubrication will fail and will lead to bearing failure by over heating of the bearing and swiping of the white metal

* If the clearance is excessive it will lead to bearing failure by impact and destruction of the hydrodynamic fluid

film. In addition it will increase the clearance volume and reduce the volumetric efficiency.

(C)Fouled air inlet filter will reduce the mass inflow of air into the compressor which will reduce the volumetric efficiency of the compressor. The compressor will take more time to fill the bottle from zero gauge pressure to full pressure.

(D) Fouled after cooler will not cool the pressurised air to its required temperature, thereby resulting in higher air temperature in the air bottle which results in reduced air mass intake in the air bottle

Q8. With regards to Boiler water level Control explain the following A. Shrink and swell phenomenon B. Cascade control C. Split control D. Condensing chamber - function and location

Answer:- (A) Robot feed regulators are proportional controllers (single term) working on a fairly sensitive proportional band. Due to drum contents swell and shrinkage during manoeuvring load changes, the action is temporarily in the wrong direction.The storage capacity of the steam and water drums of the water tube boilers is small because their evaporative rate is high which compensates for the need to have a large capacity. This results in swell and shrink. This wrong action is very severe due to the narrow bandwidth and hand operation of feed checks was often necessary. Proportional action is made less sensitive, this reduces the severity of the short term wrong way action but introduces offset. Offset is got rid of by the addition of integral action i.e. the control is tow term for the single element action.

(B)Cascade control:- A two element action is , the equivalent cascade control for the boiler level controller, is obtained by incorporating a steam flow measurement to reduce severe feed flow variations when manoeuvring . This signal would give an anticipatory action which is usually desirable in all control systems. The level signal would act as a trimmer and has a wide proportional band so as not to affect the system during swell and shrinkage.

(C)Split control:- The three element control which is the equivalent split control gives the best value of performance . Feed flow is compared to steam flow for the correct1:1 ratio.If the ratio is incorrect then an out of balance signal isgiven to the controller. Drum level again acts as a trimming device on a wide proportional band with integral action.

(D) The function of the condenser chamber at the top end of the fixed limb of the level sensor is to form a reservoir maintained at a constant level so that the variable level limb can be compared with it. Since the steam pressure acts equally on both limbs, only the levels or heads are compared.The waer level cannot cross the weir within the reservoir and that gives it the constant level datum.

Q9. With reference to electro hydraulic steering gear systems with four rams A. With the aid of a sketch describe the working principle of hydraulic pump. B. Explain the method adopted to prevent hydraulic oil leakage along the rams C. Discuss the methods adopted to prevent damage to the steering gear due to jumping of rudder in heavy seas.

Answer:-

The shaft is stationary and the cylinder body forming the cylinders

rotates around the shaft,being driven by a constant speed anddirection electric motor (or steam engine). The plungers are connectedto slippers which run in annular grooves inside two circularrings on each side of the plungers. When the centre of the ringscoincide with the shaft centre (0) the pump travel is at midposition. At this position the plungers rotate at a fixed radiusdistance from the shaft centre (distance OC), this means there isno relative motion between the plungers and the shaft and nopumping action takes place.If now the circular slipper rings are moved to the right by theoperating rod, through the casing from the telemotor rod, then thecentre of rotation of the slippers and plungers is at B which iseccentric to the centre of the shaft. 0. This means the greatestdistance the plunger gudgeons are from 0 is OG and the shortestdistance is OF. This means, with the direction of rotation asshown, in travelling round from G to Fthe plungers are moving inrelative to the fixed central shaft and ports hence the top port Tacts as a discharge. In completing the circular route from F backto G the plungers are moving out relative to the central shaft andports and the bottom port B acts as a suction. The path is shownon the sketch dotted, likewise the relative plunger movement atfour positions.If the circular slipper rings are moved left so the centre of plungerrotation is at A then the shortest distance is OD and thegreat~st distance is OE, i.e. plungers are moving out in top half ofrotation and Tis a suction and in during bottom half of rotationand Bis a discharge. The path and plunger movement are shownhere as dashed line.As the stroke of the plungers depends on the movement of theslipper path horizontally and hence the eccentricity, so the pumpis of the variable delivery type. Also direction of flow dependson movement left or right of central position so that forunidirectional rotation the direction of flow is reversible.

(b

(B) The rams are provided with lantern type packing rings made of Neoprene rubber hosed in a proper gland at the end of the hydraulic cylinder where the ram emerges out. The gland cover can be suitably tightened to gicve sufficient tightness to prevent leakage of the oil under pressure..

(C) The above sketch shows equal clearances on top and bottom of the trunnion ring which allows the tiller to jump up along with the rudder and rudder stock in a heavy sea. The upward motion is further prevented by the jumping bar fitted in the clearance between rudder top and skeg (stern frame) bottom. tHis plate or bar is welded to top of the rudder. Further motion on the bottom side is prevented by the rudder carrier bearing.