Top Fluid Mechanics & Fluid Machinery Interview Questions
What is the difference between streamline body and bluff body?
In streamline body the shape is such that separation in flow occurs past the near most part of the body so that wake formed is small and thus friction drag is much greater than pressure drag. In bluff body the flow gets separated much ahead of its rear resulting in large wake and thus pressure drag is much greater than the friction drag.
Define hydraulically efficient channel cross section.
The shape of such section is that which produces minimum wetted perimeter for a given area of flow and carries maximum flow.
What is the difference between Center of Mass and Centroid?
If the material composing a body is uniform or homogeneous, the density or specific weight will be constant throughout the body, and then the Centroid is the same as the center of gravity or center of mass.
Centroid: Centroid is the point, where the whole area of plane is going to be act. It is valid only for 2D problems like plane figures, square plate etc. The center of mass is a point that acts as if all the mass was centered there (the mass on one side of the point is equal to the mass on the opposite side). If supported at the center of mass, an object will be balanced under the influence of gravity.
On what factors does the pressure at a point as a static mass of liquid depends upon?
Specific weight of liquid and the depth below the free liquid surface.
When maximum discharge is obtained in nozzle?
At the critical pressure ratio.
Under what condition the work done in reciprocating compressor will be least?
It is least when compression process approaches isothermal. For this purpose, attempts are made to cool the air during compression.
What is the difference between stalling and surging in rotary compressions?
Stalling is a local phenomenon and it occurs when How breaks away from the blades. Surging causes complete breakdown of flow and as such it affects the whole machine.
State Archimedes principle.
Any weight, floating or immersed in a liquid, is acted upon by a buoyant force equal to the weight of the liquid displaced. This force acts through the center of buoyancy, i.e. the e.g. of the displaced liquid.
What do you understand by center of buoyancy?
Center of buoyancy is the center of gravity of the displaced liquid and buoyant force acts through it.
Why the Centrifugal Pump is called High Discharge pump?
Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formula:
Centrifugal force F= (MV²)/R.
Where, M-Mass, V-Velocity, R-Radius
Why the electric motor of a fan with backward curved blades is never got overloaded under any condition?
The maximum power is consumed at about 70% of maximum flow in case of fan with backward blades. For higher flow, power consumption gets lower.
Why the work per kg of air flow in axial flow compressor is less compared to centrifugal compressor for same pressure ratio?
Isentropic efficiency of axial flow compressor is higher.
What is pitting? How it is caused?
Non uniform corrosion over the entire metal surface, but occurring only in small pits is called pitting. It is caused by lack of uniformity in metal.
How Cavitation can be eliminated by Pump?
Cavitation means bubbles are forming in the liquid. To avoid Cavitation, we have to increase the Pump size to one or two inch to increase the pressure of the Suction Head, or decrease the Pump Speed.
Which impurities form hard scale and which impurities soft scale?
Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime and magnesium form soft scale.
What is the difference between hard water and soft water?
Hard water contains excess of scale forming impurities and soft water contains very little or no scale forming substances.
Which two elements in feed water can cause corrosion of tubes and plates in boiler?
Acid and oxygen in feed water lead to corrosion.
Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?
The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapor due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms Cavitation.
Why large boilers are water tube type?
Water tube boilers raise steam fast because of large heat transfer area and positive water circulation. Thus they respond faster to fluctuations in demand. Further single tube failure does not lead to catastrophic.
What type of boiler does not need a steam drum?
Supercritical pressure boiler.
Why manholes in vessels are usually elliptical in shape?
Elliptical shape has minimum area of opening and thus plate is weakened the least. Further it is very convenient to insert and take out the cover plate from elliptical opening.
Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?
Centrifugal pump. Because of flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.
Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.
One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.
Low water in boiler drum is unsafe because it may result in overheating of water tubes in furnace. Why it is unsafe to have high water condition in boiler drum?
High drum level does not allow steam separation to be effective and some water can be carried over with steam which is not desirable for steam turbine.
What is the difference between Critical Speed and Whirling Speed?
In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or gear. As the speed of rotation approaches the objects natural frequency, the object begins to resonate which dramatically increases system vibration. The resulting resonance occurs regardless of orientation. Whirling Speed is due to the unbalanced forces acting on a rotating shaft.
Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 0°C in I.C. engine? Why?
High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can’t withstand.
What is basic difference between impulse turbine and reaction turbine?
(A) In impulse turbine, jet is used to create impulse on blades which rotates the turbine and in reaction turbine, no jet is used pressure energy is converted into kinetic energy.
(B) In impulse turbine fluid enter& leave with same energy, but in reaction turbine fluid enter with pressure energy& leaves with kinetic energy
(C)In impulse turbine all the pressure drops in nozzle only & in reaction turbine pressure drops both fixed & moving blades. The difference is due to blade profiles.
What is the Difference between a Generator and Inverter?
An inverter is only effective if there is already a source of electrical energy. It cannot generate its own. It can simply convert electrical energy that is already there. On the other hand, a traditional generator cannot make AC current into DC current.
What are the causes of failure of boiler tubes?
Boiler tubes, usually are made from carbon steel and are subject to
(A) High rates of heat transfer
(B). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums,
(C) External erosion from burners and flue gas,
(D) Possible corrosion on the boiler side, and
(E) Occasional manufacturing defects.
Failure may occur due to following reasons:
(A) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterized by the bulging of the tube with the eventual development of a longitudinal split.
(B) Fatigue cracking due to bending stresses occurs. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.
(C) Failure may arise due to over-stressing of a reduced section of metal.
(D) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.
(E) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes give rise to premature failure and may promote corrosion at these regions.
Why is sound faster in warm air?
The speed of sound in air Cair = 331.3 + (0.66 x T) m/s, where T is the temperature in °C.
The speed of sound is proportional to gas temperature and inversely proportional to its molar mass.
Sound is transferred by collisions of molecules. Therefore sound waves will travel faster on warm air because collision of molecules of air in warm air is greater.
What is a Newtonian fluid?
A Newtonian fluid possesses a linear stress strain relationship curve and it passes through the origin. The fluid properties of a Newtonian fluid do not change when any force acts upon it.
What are the causes of failure of superheater tubes?
Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high temperature strength, resistance to corrosion is also important. For example, low alloy ferrite steel such as 1% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.
Failures in superheater tubes may arise from:
(A) Prior fabrication history
(B) Faulty heat treatment
(C) Consequences of welding
(D) Overheating of the tube metal
(E) Gas-side corrosion
(F) Stress corrosion (austenitic steels).
Why supercritical boilers use less amount of steel compared to non supercritical boilers?
Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.
What does a pump develops? Flow or Pressure?
A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.
What is a Hydrostatic system?
Hydrostatics is the study of fluid bodies that are
(A) At rest
(B) Moving sufficiently slowly so there is no relative motion between adjacent parts of the body
For hydrostatic situations
(A) There are no shear stresses.
(B) There are only pressure forces that act perpendicular to any surface.
It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies energy to motor and motor gives return energy to pump supply.
What is the difference between Blower and Fan?
Fan is an air pushing device. Either Axial or Centrifugal type systems are used to move the air in low pressure. It is rotated by a motor separately.
When the fan is a housing of blades and motor, then it called as Blower. It directs the air in a single path with high pressure.
Out of electric heater and heat pump, which is economical in operation?
Heat pump.
What is Hydrodynamic Cavitation?
Hydrodynamic Cavitation describes the process of vaporization in a constrained channel at a specific velocity. Bubble generation and Bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure.
Is gate valve used for Throttling?
Gate valves are not suitable for throttling because the control of flow is difficult for the valve’s design, and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve.
Where Multi-stage pump used?
Pressure washing of Aircraft, Trains, Boats and Road vehicles as well as Spray washing of industrial parts and Electronic components.
Which furnace burns low ash fusion coal and retains most of the coal ash in the slag?
Cyclone furnace.
How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related?
Ratio of their thickness = (Prandtl number) – (1/3).
What is the effect of friction on flow of steam through a nozzle?
To decrease both mass flow rate and wetness of steam.
What is the function of Scoop in BFP (Boiler Feed Water Pump) in Thermal Power Station?
The Function of Scoop tube is regulating the varying amount of oil level in the coupling during operation of infinite variable speed.
In the Thermal Power Plant why Deaerator (D/A) placed on height?
To build a Very high pressure and the temperature for a boiler feed water pump and it discharge high pressure water to the boiler. And to provide the required Net Positive Suction Head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feed water during rapid changes in BFP.
In orifice why the Pressure and Temperature are decreases?
Orifice is a small hole like a nozzle. When a high pressure fluid passes through the orifice,
Pressure gets reduced suddenly and the velocity of the fluid gets increased. Also the heat transfer rate increases. We know that Heat transfer rate is directly proportional to the difference in temperature, Area and the Heat transfer coefficient. Heat transfer Coefficient remains constant for a fluid at a particular temperature.
Q= KA (T1-T2)
If the heat transfer rate increases, it seems the difference in temperature gets increased. There is no way in reduction of inlet temperature of the orifice. As a result, the outlet temperature of the orifice gets reduced. Hence the pressure and temperature gets reduced when it passes through orifice
Why gas turbine power plant needs efficient compressor?
Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.
Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon?
Liquid hydrogen has higher burning velocity.
Why axial flow compressor is preferred for gas turbines for aeroplanes?
Because it has low frontal area.
What is the difference between gas turbine and a steam turbine?
Gas turbine works on Brayton cycle where as steam turbine works Rankine cycle. Construction, operation of a gas turbine is entirely different to steam turbine. Gas turbine has a compressor to compress the combustion air, a combustion chamber to burn the fuel and a turbine section to extract the work for burning fuel. Steam turbine is just has a turbine section to extract the work from steam.
What is operating pressure?
The amount of pressure nearest the point of performing work at the output end of a pneumatic system. The system operating pressure is used to specify the capability of valves and actuators.
What is the effect of inter cooling in gas turbines?
It decreases thermal efficiency but increases net output.
What are the safety valves? How many on each boiler?
A valve opening automatically to relieve excessive pressure, especially in a boiler.
There are normally two to six safety valves provided in the drum depending upon the capacity.
The super heater outlet will have one to three safety valves on either side of the boiler. There will be an electro metric relief valve on the super heater pipe in addition. This valve will be set at lower pressure than the lowest set safety valve on the super heater. The reheater pipes both at the inlet and outlet side will also have safety valves which can range from two to eight both in the inlet and outlet of the reheater put together.
What is a sentinel valve?
Sentinel valves are simply small relief valves installed in some systems to warn of impending over pressurization. Sentinel valves do not relieve the pressure of the system. If the situation causing the sentinel valve to lift is not corrected, a relief valve (if installed) will lift to protect the system or component. If a relief valve is not installed, action must be taken quickly to secure the piece of equipment or system to reduce the pressure.
In which reactor the coolant and moderator are the same?
Pressurized water reactor.
Which reactor has no moderator?
Fast breeder reactor.
What are thermal neutrons?
Thermal neutrons are slow neutrons (having energy below 1 eV) which are in thermal equilibrium with their surroundings.
What is the function of Hydrogen seals work on a generator?
Provide a seal between the generator housings and rotor shaft to maintain the pressurized hydrogen gas inside the generator. Also, provides a trap-vent system to prevent the release of hydrogen into the turbine generator lube oil system and building atmosphere.
How is the excess discharge pressure prevented?
Pressure relief valves on the discharge side of each seal oil pump relieve back to seal oil system.
Which two seal oil pumps are driven from the same motor?
Main seal oil pump and re-circular seal oil pump; both are driven by Main seal oil pump motor.
Which heating value is indicated by a calorimeter and why?
Gross heating value because steam is condensed and heat of vapour formed is recovered.
When does Emergency Seal Oil Pump automatically start?
When its pressure switch senses Main Seal Oil Pump discharge pressure reduced to 78 PSI.
What is the consequence of not maintaining hydrogen (or air) pressure in generator casing at a value above atmospheric pressure when seal oil system is in service?
Failure to do so will cause excessive seal oil to be drawn into the generator.
What is a radial-flow turbine?
In a radial-flow turbine, steam flows outward from the shaft to the casing. The unit is usually a reaction unit, having both fixed and moving blades.
What are four types of turbine seals?
Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
Labyrinth mated with shaft serration’s or shaft seal strips.
Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.
What are two types of clearance in a turbine?
Radial – clearance at the tips of the rotor and casing.
Axial – the fore-and-aft clearance, at the sides of the rotor and the casing.
Which reactor uses natural uranium as fuel?
Gas cooled reactors.
Which reactor uses heavy water as moderator?
CANDU.
Which reactor requires no moderator?
Breeder reactor.
Which reactor uses primary coolant as fluoride salts of lithium, beryllium, thorium and uranium?
Molten salt breeder reactor.
Why an increase in area is required to produce an increase of velocity in case of supersonic flow?
Increase in area for increase in velocity for supersonic flow is required because the density decreases faster than velocity increases at supersonic speeds and to maintain continuity of mass, area must increase.
Under what circumstances would there be an increase in pressure in a divergent nozzle?
For subsonic flow at inlet section of a diffuser a lower velocity and higher pressure will exist at the exit section. For supersonic isentropic flow at the inlet section a higher velocity and lower pressure will exist at the exit but if a shock wave occurs in the diffuser then a higher pressure will exist at the exit.
What are some conditions that may prevent a turbine from developing full power?
The machine is overloaded.
The initial steam pressure and temperature are not up to design conditions.
The exhaust pressure is too high.
The governor is set too low.
The steam strainer is clogged.
Turbine nozzles are clogged with deposits.
Internal wear on nozzles and blades.
What is a stage in a steam turbine?
In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a “stage.” A single Curtis stage may consist of two or more rows of moving blades.
What is a diaphragm?
Partitions between pressure stages in a turbine’s casing are called diaphragms. They hold the vane-shaped nozzles and seals between the stages. Usually labyrinth-type seals are used. One-half of the diaphragms is fitted into the top of the casing, the other half into the bottom.
What are the two basic types of steam turbines?
Impulse type.
Reaction type.
What are topping and superposed turbines?
Topping and superposed turbines arc high pressure, non-condensing units that can be added to an older, moderate-pressure plant. Topping turbines receive high-pressure steam from new high-pressure boilers. The exhaust steam of the new turbine has the same pressure as the old boilers and is used to supply the old turbines.
What is a combination thrust and radial bearing?
This unit has the ends of the Babbitt bearing extended radically over the end of the shell.
Collars on the rotor face these thrust pads, and the journal is supported in the bearing between the thrust collars.
What is important to remember about radial bearings?
A turbine rotor is supported by two radial bearings, one on each end of the steam cylinder. These bearings must be accurately aligned to maintain the close clearance between the shaft and the shaft seals, and between the rotor and the casing. If excessive bearing wear lowers the he rotor, great harm can be done to the turbine.
How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation. One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed). The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
How is a fly-ball governor used with a hydraulic control?
As the turbine speeds up, the weights are moved outward by centrifugal force, causing linkage to open a pilot valve that admits and releases oil on either side of a piston or on one side of a spring-loaded piston. The movement of the piston controls the steam valves.
What is meant by critical speed?
It is the speed at which the machine vibrates most violently. It is due to many causes, such as imbalance or harmonic vibrations set up by the entire machine. To minimize damage, the turbine should be hurried through the known critical speed as rapidly as possible. (Caution, be sure the vibration is caused by critical speed and not by some other trouble).
How is oil pressure maintained when starting or stopping a medium-sized turbine?
An auxiliary pump is provided to maintain oil pressure. Some auxiliary pumps are turned by a hand crank; others are motor-driven. This pump is used when the integral pump is running too slowly to provide pressure, as when starting or securing a medium-sized turbine.
Besides lubrication, what are two functions of lubricating oil in some turbines?
In larger units, lube oil cools the bearings by carrying off heat to the oil coolers. Lube oil in some turbines also acts as a hydraulic fluid to operate the governor speed-control system.
Do you stop cooling-water flow through a steam condenser as soon as the turbine is slopped?
You should keep the cooling water circulating for about 15 miles or more so that the condenser has a chance to cool down gradually and evenly. Be sure to have cooling water flowing through the condenser before starting up in order to prevent live steam from entering the condenser unless it is cooled. Overheating can cause severe leaks and other headaches.
How can the deposits be removed?
Water soluble deposits may be washed off with condensate or wet steam.
Water insoluble deposits are removed mechanically after dismantling the turbine.
Experience shows that water soluble deposits are embedded in layers of water-insoluble deposits. And when the washing process is carried out, water soluble parts of the deposit dissolve away leaving a loose, friable skeleton of water insoluble deposits which then break loose and wash away.
How can the fatigue damage on high pressure blades be corrected?
Fatigue-damage on high-pressure blades arises due to vibration induced by partial-arc admission. This can be corrected by switching over to full arc admission technique.
How many governors are needed for safe turbine operation? Why?
Two independent governors are needed for safe turbine operation:
One is an over speed or emergency trip that shuts off the steam at 10 percent above running speed (maximum speed).
The second, or main governor, usually controls speed at a constant rate; however, many applications have variable speed control.
How will you detect that misalignment is the probable cause of excessive vibration?
Coupling to the driven machine is to be disconnected.
The turbine is to be run alone.
If the turbine runs smoothly, misalignment, worn coupling or the driven equipment is the cause of the trouble.
In which part of the steam turbine does corrosion fatigue occur?
In the wet stages of the LP cylinder.
In which part of the steam turbine does stress corrosion cracking (SCC) occur?
In the wet stages of the low-pressure turbine.
In which zone of steam turbines has temperature-creep rupture been observed?
Damage due to creep is encountered in high temperature (exceeding 455°C) zones. That is, it has been found to occur in the control stages of the high-pressure and intermediate-pressure turbines where steam temperature sometimes exceed 540°C. In the reheat stage, it has been observed that creep has caused complete lifting of the blade shroud bands.
What are the types of thrust bearings?
Babbitt-faced collar bearings
Tilting pivotal pads
Tapered land bearings
Rolling-contact (roller or ball) bearings
What are the types of turbine seals?
Carbon rings fitted in segments around the shaft and held together by garter or retainer springs.
Labyrinths mated with shaft serrations or shaft seal strips.
Water seals where a shaft runner acts as a pump to create a ring of water around the shaft. Use only treated water to avoid shaft pitting.
Stuffing box using woven or soft packing rings that are compressed with a gland to prevent leakage along the shaft.
What are the basic causes of the problem of rotor failure?
(A) Normal wear.
(B) Fatigue failure due to high stress.
(C) Design deficiency.
(D) Aggressive operating environment
What are the differences between impulse and reaction turbines?
The impulse turbine is characterized by the fact that it requires nozzles and that the pressures drop of steam takes place in the nozzles.
The reaction turbine, unlike the impulse turbines has no nozzles, as such. It consists of a row of blades mounted on a drum. The drum blades are separated by rows of fixed blades mounted in the turbine casing. These fixed blades serve as nozzles as well as the means of correcting the direction of steam onto the moving blades.
In the case of reaction turbines, the pressure drop of steam takes place over the blades. This pressure drop produces a reaction and hence causes the motion of the rotor.
What is the function of EGR VALVE?
EGR means Exhaust Gas Recirculation. The main function of EGR is to control NOx emission from the engine. At elevated temperature (during long run & full condition)if due to some reasons engine gets heated up beyond certain level… it produces NOx emission. In order to avoid this pollution, we have to control the temperature rise inside the cylinder. This can be achieved by mixing some amount of exhaust gas with intake air. By doing this the temperature inside the combustion chamber is reduced. And NOx is controlled.
What is the difference between Critical speed & Whirling speed?
In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or gear. As the speed of rotation approaches the objects natural frequency, the object begins to resonate which dramatically increases system vibration. The resulting resonance occurs regardless of orientation.
Whirling Speed is due to the unbalanced forces acting on a rotating shaft.
What is specific speed of turbine?
The specific speed is defined as the speed of the geometric turbine which can produce unit power at unit head
Material of Aircraft turbine blades
Answer: Ni, Cr, Al, Traces of C