Mechanical Engineering Dictionary-Steam Engines and Steam Turbines

Important Terms and their Meaning of Mechanical Engineering-Steam Engines and Steam Turbines:

ACCELERATED FLOW – Type of flow that takes place in nozzles. The flow accelerates and pressure reduces.

ADMISSION – The point in the working cycles of a steam or IC engine at which the inlet valve allows entry of the working fluid into the cylinder.

AIR PUMP – A reciprocating or centrifugal pump used to remove air, and sometimes the condensate, from the condenser of a steam plant.

ANGLE OF ADVANCE – The angle in excess of 90 degree by which the eccentric throw of a steam engine valve gear is in advance of the crank.

ALLAN VALVE – A steam engine slide valve, in which a supplementary passage increases the steam supply to the port during admission to reduce wire drawing.

ATMOSPHERIC ENGINE – An early form of steam engine in which a partial vacuum created by steam condensation allowed atmospheric pressure to drive down the piston.

AXIAL DISCHARGE TURBINE – A steam turbine in which the absolute velocity of steam flow at exit is a minimum i.e., the steam discharges in the axial direction.

AXIAL FLOW TURBINE – Steam turbine in which the general direction of steam flow has been roughly parallel to the turbine axis.

BACK PRESSURE TURBINE – A steam turbine from which the whole of the exhaust steam, at a suitable pressure, is taken for heating purposes.

BAROMETRIC CONDENSER – A high level jet condenser.

BASTERED CONDENSER – It is an atmospheric keel condenser, which are sometimes fitted to canal boats or other sea vessels.

BINARY VAPOUR ENGINE – A heat engine using two separate working fluids, generally mercury vapour and steam, for the high and low temperature portions of the cycle respectively, thus enabling a large temperature range to be used, with improved thermal efficiency.

BLADE – Part attached to the rotating element of the machine or rotor, in which the stream of steam particles has its direction and hence its momentum changed. Also called DEFLECTOR.

BLADE VELOCITY COEFFICIENT – The ratio of the relative velocity of steam at outlet to the relative velocity at inlet of the blade.

BLANK FLANGE – A disc, or solid flange, used to blank off the end of a pipe.

BLAST PIPE – The exhaust steam pipe in the smoke box of a locomotive, which terminates in a nozzle to provide draft by entraining the flue gases in the steam jet and exhausting them through the chimney.

BLEEDING – A method of improving the thermal efficiency of steam plant by withdrawing a small part of the steam from the higher pressure stages of a turbine to heat the boiler feed water.

BLEEDER TURBINE – A steam turbine in which the steam is extracted at one or more intermediate stages for industrial use, often at comparatively high pressure.

BYPASS GOVERNING – Governing arrangement in which part of the steam that enters the turbine is bypassed depending upon the extent of load reduction.

CARRY-OVER LOSS – Loss of kinetic energy at the exit of the turbine. Also called LEAVING LOSS.

CHOKED FLOW – When a nozzle operates with the maximum mass flow it is said to the choked.

COMPOUND TURBINE – A multistage steam turbine in which the pressure energy of the steam is progressively transformed into kinetic energy in two or more stages with or without velocity compounding in each stage.

COMPOUND STEAM ENGINE – An engine which has two or more cylinders of successively increasing diameters, so arranged that the exhaust steam from the first cylinder (high pressure cylinder) is passed on to do work in the second cylinder (low pressure cylinder), and to a third cylinder in triple expansion engine, before being finally exhausted into a condenser.

CONDENSER – A vessel into which steam is exhausted and condensed instead of being rejected into the atmosphere after doing work in an engine cylinder or turbine. This is primarily for removing the back pressure upon an engine or turbine and thereby improve the plant efficiency.

CONDENSING CYCLE – A steam power plant cycle in which the exhaust steam is discharged into a condenser having a low back pressure, so that more energy can be extracted per unit weight of steam.

CONSTANT VELOCITY FLOW – Type of flow that takes place in parallel ducts.

COOLING POND – A shallow reservoir having a large surface area for removing heat from the cooling water used to condense steam in condensers.

COOLING TOWER – An apparatus designed to remove from the cooling water, used in a condenser, as much heat as can possibly be abstracted per unit space occupied by the apparatus.

COUNTERFLOW STEAM ENGINE – The engine in which the steam leaves the cylinder at the same end at which it entered.

CRITICAL PRESSURE OF NOZZLE – The pressure at which the velocity of the fluid equals the local sound velocity.

CRITICAL PRESSURE RATIO – Ratio of critical pressure of nozzle to the initial pressure.

CRITICAL SPEED OF A SHAFT – The speed at which the shaft displacement tends to be very large, and the shaft may become permanently bent.

CUSHION STEAM – The steam present in the cylinder during compression which occurs just after the exhausting of steam by the inward movement of the piston.

CUTOFF GOVERNING – Control of engine speed is accomplished by changing the volume of steam admitted to an engine cylinder as the load fluctuates. The points of steam cut off comes early in the stroke of the engine piston with light loads and later when they increase.

DECELERATED FLOW – Type of flow that takes place in the diffusers velocity decreases.

DEGREE OF REACTION – In an impulse reaction turbine, it is the ratio of the enthalpy drop that takes place in rotor blades to the total enthalpy drop that occurs in the stage.

DIAGRAM EFFICIENCY or BLADING EFFICIENCY – Ratio of the rate of doing work per kg of steam (diagram work) to the energy supplied to the rotor per kg of steam.

DIAGRAM FACTOR – Ratio of actual indicator diagram area to the hypothetical indicator diagram area.

DIAPHRAGMS – Partitions, which separate one wheel chamber from the next and in which nozzles are usually fitted in the case of pressure compounded impulse turbine.

DIFFUSER – The duct in which a fluid is decelerated causing a rise in pressure along the stream.

DISC FRICTION – Surface friction loss due to relative motion between the disc and steam particles when the disc is rotating in the viscous fluid i.e., steam.

DOUBLE ACTING ENGINE – A steam engine in which a power cycle is produced in each end of the cylinder during one revolution of the crankshaft.

DOUBLE DECK SPRAY POND – The pond having spray nozzles arranged at different elevations.

D SLIDE VALVE – A sliding valve which alternately admits steam to and releases the steam from each end of the steam engine cylinder.

DRY AIR PUMP – A pump designed to handle air and gases only, which will give a higher vacuum than possible with a wet air pump.

ECCENTRICITY – The distance between the centre of the crankshaft and the centre of the eccentric and is equal to the effective crank arm of the eccentric.

ENGINE RELATIVE EFFICIENCY – Ratio of actual thermal efficiency to that of the Rankine cycle for the same pressure and temperature conditions.

EVAPORATIVE CONDENSER – A type of surface condenser in which the steam is fed through the condenser tubes over which cooling water is sprayed and cooling is affected mainly by evaporation of the sprayed water.

EXHAUST LAP – The overlap of the release edge of the D slide valve over the release edge of the port when the valve is in the mid position which is the middle of its travel.

FULL ADMISSION TURBINE – A steam turbine in which steam is admitted over the entire circumference of the blade annulus. Reaction turbines are full admission turbines.

GLAND – Device provided to minimise the leakage of steam, or in the case of the low pressure end of the turbine the leakage of air through the clearance space which separates the rotor from the casing.

HEAT ENGINE – A device which transforms heat energy into mechanical energy and render the latter available for doing useful work. It is merely an energy transformer.

HELICAL FLOW TURBINE – A single pressure, multiple velocity stage machine designed for helical flow.

IMPULSE – The act of impelling or suddenly driving forward in the same direction as the applied force.

IMPULSE TURBINE – A steam turbine in which the steam is expanded causing pressure drop in nozzles only and the moving blades attached to the rotor merely deflect the steam through an angle. The pressure on the two sides of the blades remains constant.

IMPULSE REACTION TURBINE – A steam turbine in which the steam is expanded both in the fixed blade and the moving blade continuously as the steam passes over them. The pressure drops gradually and continuously over both moving and fixed blades. Often called as REACTION TURBINE.

INTERNAL TURBINE EFFICIENCY – Ratio of the total useful heat drop to the adiabatic heat drop corresponding to the whole pressure drop that occurs. This is the product of stage efficiency and reheat factor.

JET CONDENSER – A closed chamber within which exhaust steam comes in direct contact with a spray or jet of cold water and is condensed.

KEEL CONDENSER – A type of marine outboard single pass surface condenser attached to the side of a hull below the water line. It requires no circulating water pump.

LAP OF D SLIDE VALVE – It is that portion of the valve face which overlaps the ports when the valve is in its central or neutral position. The distance overlapping on the outside is called OUTSIDE LAP or
STEAM LAP, and the distance overlapping on the inside is called INSIDE LAP or EXHAUST LAP.

LAYBRINTH GLAND – A type of gland consisting of a series of intricate passages, labyrinths, which are designed to destroy the kinetic energy developed by the steam leaving through the small clearance space. In the clearance space, this energy is converted to thermal energy.

LEAD OF D SLIDE VALVE – The amount by which the steam port is opened by the valve when the piston in a steam engine cylinder is at the begining of its stroke.

LOW PRESSURE TURBINE – A turbine operating at approximately atmospheric pressure and expanding to condenser pressure.

MISSING QUANTITY – In a steam engine, this is the lack of volume due to the steam not remaining dry and saturated during expansion.

MIXED PRESSURE TURBINE – A turbine designed to work on two or more pressures.

NATURAL POND – A natural flow pond having no baffle walls or spray nozzle.

NET EFFICIENCY – Ratio of network delivered at shaft to the total energy supplied to the steam turbine/steam engine. In the case of steam turbine it is the product of nozzle efficiency, diagram efficiency and mechanical efficiency.

NOZZLE – A duct of smoothly varying cross-sectional area in which a steadily flowing fluid can be made to accelerate by a pressure drop along the duct.

NOZZLE CONTROL GOVERNING – Governing arrangement in which nozzles are grouped together in three or five or more groups and each group of nozzles is supplied with steam which is controlled by valves.

NOZZLE EFFICIENCY – Ratio of the actual enthalpy drop to the isentropic enthalpy drop between the same pressures.

NOZZLE THROAT – The section of the nozzle where the area is minimum.

OVER EXPANDING – A nozzle which operates with a back pressure above the designed value of pressure at the exit of the nozzle.

OVERALL THERMAL EFFICIENCY – Ratio of the useful heat drop to the heat supplied in a turbine.

PARTIAL ADMISSION TURBINE – An impulse turbine in which the nozzles occupy only a part of the circumference leading into blade annulus and therefore admit steam over part of the blade annulus.
Impulse turbines are partial admission turbines.

PORT – The entrance at the valve seat to either a steam passage leading to the cylinder or an exhaust passage leading to the exhaust pipe.

PRESSURE COMPOUNDED IMPULSE TURBINE – Steam turbine in which the total pressure drop of steam is divided into stages in a number of rings of fixed nozzles and the velocity produced by a ring
of nozzles is utilized almost fully in the immediately following row of moving blades attached to the rotor.

PRESSURE VELOCITY COMPOUNDED IMPULSE TURBINE – Steam turbine in which the total pressure drop of steam is divided into stages and the velocity thus obtained in each stage is also compounded i.e utilized in two or more rings of moving blades.

RADIAL FLOW TURBINE – A steam turbine in which the blades are so arranged to cause the flow of steam radially inwards or outwards.

RANKINE EFFICIENCY OF TURBINE – Ratio of the adiabatic heat drop to the heat supplied.

REAMED NOZZLE – A round nozzle used primarily in the high pressure impulse stage steam turbines. They have lower efficiency and somewhat greater in length.

REHEAT CYCLE – A system in which steam is initially expanded through a turbine and then reheated before further expansion in the turbine.

REHEAT FACTOR – In a multistage steam turbine it is the ratio of the sum of the individual heat drops (cumulative drop) in the different stages to the direct or adiabatic drop in a single step for the whole pressure drop that occurs.

RELATIVE EFFICIENCY – Ratio of the overall thermal efficiency of the actual plant to the thermal efficiency of the ideal Rankine cycle.

RIM HORSE POWER – Power developed by steam in passing over the blade as obtained from the velocity diagrams.

SCOOP CONDENSER – A type of marine condenser with a flow of circulating water induced through an enclosed chamber by the movement of the vessel, rather than externally as with a keel condenser.

SIMPLE IMPULSE TURBINE – A steam turbine in which the expansion of the steam takes place in one set of nozzles only.

SIMPLE MULTISTAGE TURBINE – A steam turbine in which the pressure energy of the steam is progressively transformed into kinetic energy in two or more pressure stages, there being one wheel to each stage.

SINGLE WHEEL IMPULSE TURBINE – A steam turbine having a set of nozzles and one row of moving blades fixed to the rotor. The moving blades deflect the steam and therefore cause a change in momentum and consequently produce force i.e., motive force.

SPRAY POND – A pond which is provided with spray apparatus so that the hot cooling water from the condenser is sprayed over the surface of the pond and cooled.

STAGE EFFICIENCY – Ratio of the rate of doing work per kg of steam (diagram work) to the energy supplied to the stage per kg of steam, in a steam turbine. Also called GROSS STAGE EFFICIENCY. This is product of nozzle efficiency and blade efficiency.

STEAM CONDENSER – Device in which the exhaust steam from an engine or a turbine is condensed and air and other non condensable gases are removed in a continuous process.

STEAM JET VACUUM PUMP – Pump system in which one or more steam jets moving at a high velocity (1000 m/s) entrain the saturated non condensable gases, and the mixture passes through an expanding tube in which the velocity is reduced with a resulting increase in pressure and finally the mixture is let out into the atmosphere. Also called STEAM JET AIR EJECTOR.

STEAM LAP – The distance by which the admission edge of the D slide valve overlaps the edge of the steam port when the valve is in the mid position, which is the middle of its travel.

STEAM RATE OF AN ENGINE – The weight of steam supplied to a steam engine per horse power hour.

STEAM TURBINE – A prime mover in which gradual changes in the momentum of a fluid are utilized to produce rotation of the movable member.

STEAM TURBINE EXTERNAL LOSSES – Losses which do not affect the steam conditions while it flows through the turbine, includes mechanical losses and losses due to the end leakages.

STEAM TURBINE INTERNAL LOSSES – Losses connected with the steam conditions while it flows through the turbine. These include losses in regulating valve, nozzles, moving blades, carry-over losses from one stage to another, rotor and guide blade clearance losses, loss due to steam wetness and exhaust losses.

SURFACE CONDENSER – A device for condensing steam, in which the steam and the cooling water do not come into contact with each other, but are separated by metal surfaces.

SURFACE CONDENSER SINGLE PASS TYPE – Type of surface condenser in which the cooling water flows in one direction only through all the tubes.

SURFACE CONDENSER TWO PASS TYPE – Type of surface condenser in which the cooling water flows in one direction through part of the tubes and returns through the remainder.

TAPERED BLADES – Blades tapered in width along the length of the blade so as to obtain an even more uniform centrifugal stress.

THERMAL EFFICIENCY OF RANKINE CYCLE – Ratio of the thermal equivalent of output to thermal equivalent of input of a unit working on Rankine cycle.

THERMOCOMPRESSOR or BOOSTER EJECTOR – Compression device designed to handle steam instead of other gases.

THROTTLE GOVERNING – Governing affected in a steam engine by varying the initial steam pressure within the engine cylinder. The governor is called THROTTLE GOVERNOR. Power output is varied by varying initial steam pressure.

TRANSONIC NOZZLE – A nozzle usually of the reamed type and this is used in small impulse steam turbines, or velocity compounded stages where large enthalpy drops are required.

UNDER EXPANDING NOZZLE – A nozzle which operates with a back pressure below the designed value of pressure at the exit of the nozzle.

VACUUM BREAKER – An automatic device used to protect the main engine or turbine from flooding when a jet condenser is used.

VACUUM KEEL CONDENSER – A type of keel condenser having tubular condensing surface, a return pipe for the condensate, the end of which connects with a wet air pump.

VELOCITY COMPOUNDED IMPULSE TURBINE – Steam turbine in which the total heat drop takes place in the nozzles, but the velocity (energy) generated is utilized in two or more rings of blades attached to the rotor.

WILLANS LINE – The line which represents the total weight of steam necessary per hour plotted as ordinates against the corresponding engine loads (either as IHP, BHP or KW) as abscissa.

WIRE DRAWING – In a steam engine, the fall in pressure caused by steam flowing through the restricted (valve) Passage immediately before the point of cut off.

WORKING STEAM – The steam that actually flows through the blades, this is different from the total steam flow through the turbine.

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