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TRUNKING IN SHIP

Posted by: Engineers Zone Posted date: Wednesday, December 08, 2010 / comment : 0 ,


The method of ventilation and output are decided as discussed above, but to decide the size of fan to handle the total amount of air required it is necessary to investigate by calculation the resistance which will have to be overcome by the air in passing through the trunks and out through the louvres or terminals. Considering any one system, losses of head occur through friction of the air passing through the trunk, changes of section, bends in trunking, fittings such as coolers, heaters, valves, gas flaps, and discharges. Each of these losses is proportional to the square of the velocity of flow. The loss of head due to friction in straight trunking is proportional to the length of trunking and inversely proportional to the linear dimension of the trunk section. Apart from the obvious desirability of minimizing the length of all trunks, the principal points requiring consideration from the practical point of view are as follows:

·         ADOPTION OF EFFICIENT SHAPE OF SECTION
The rate of air supply to, or exhaust from, a compartment having been decided from consideration of its nature and size, comfort of the personnel there, or technical reasons, the sectional area of trunking will depend on the available fan pressure after deduction of 'fixed' resistance items such as coolers, heaters, etc. To simplify calculations the 'Equal Friction per Foot' method is used, the basis of which is to convert all bends and changes of section to an equivalent length of straight trunking to which is added the actual straight length of trunking in feet to give the total equivalent length of straight trunking. Knowing the total equivalent length of straight trunking, the volume of air passing through the trunking and the pressure available, the diameter of circular trunking for each section of the system can be obtained from standard trunk friction charts. Having determined the diameter of circular trunking it remains to decide the best practicable shape of section. Circular trunking is more efficient than rectangular but saving in time, space and cost of manufacture results in the adoption of the latter in the general case. Small trunks are more easily constructed circular. A rectangular trunk can be made in a variety of ratios of sides for the same equivalent diameter of section. Undue 'flattening' of the section for such reasons as increased head room or clearance of unimportant obstructions must be avoided. In warships the ratio of the sides of any trunk must not exceed 4 to 1.

·         Minimization of the number of bends and changes of section. Complete avoidance of change of direction is impossible, but by adopting easy bends with large radius, a material improvement is effected. Sudden changes of section must be avoided and in cases where a local enlargement is necessary, the change in sectional area must be carried out gradually to minimize loss of head. In cases where these preventive measures cannot be taken a splitter must be fitted inside bends or changes of section to reduce the resistance as much as possible by preventing the formation of eddies.

·         Layout of trunking 
 The trunking must be led in such direction and be of such length as to ensure a good distribution of air within the compartment. The layout should be such that the air leaving supply louvres does not pass directly to the exhaust trunk or other opening acting as exhaust in the compartment ('short circuiting'). Similarly in compartments exhausted by fan, the exhaust inlet in the compartment must be situated remote from doorways or hatches so that the fresh air travels some distance before being drawn into the exhaust inlet. The vertical position of the exhaust inlet will depend upon the nature of the noxious gases in the compartment. If these are lighter than cold air, the inlet is placed near the crown of the compartment, and if heavier than cold air, near the bottom.

PIPING IN SHIPS

Posted by: Engineers Zone Posted date: Wednesday, December 08, 2010 / comment : 3 ,


Copper-Nickel-Iron Alloy Piping (90/10) is used for the piping of the saltwater main and its branches, sanitary system, hangar and magazine spraying systems, hull and fire, fire and bilge, and hangar spray pump sea suctions, risers, overboard discharges and pump leak-offs, overboard discharges from water/fuel displacement systems, salt water supply to and overboard discharges from bathroom ejectors, bathroom drains within the bathroom (except as stated below) and urinal drains.

Galvanized Mild Steel Piping is used for all suctions to compartments, overboard discharges of salvage pump systems, suction stand pipes, ejector suctions to bathroom sumps, portable diesel pump sea suction, scuppers and drains, soil pipes, suction pipes within fresh water storage tanks, oil fuel residue suctions, air escape pipes, and all pipes within FFO tanks where the tank structure is galvanized. Steel pipes below 11/2-inch bore are connected by screwed sleeves and large pipes by flanges welded to the pipes, the flanges being welded prior to galvanizing.

Copper-Nickel-Iron Alloy Piping is connected as follows by flanges if the bore is 2 inches or more and smaller pipes by brazed core unions, capillary type fittings or compression fittings of the non-manipulative type. Ferrous fittings are not fitted in copper-nickel-iron systems. Copper and coppernickel- iron pipes are kept clear of aluminium alloy structure. At least 1/2 inch is provided between the pipes or the pipe insulation and the structure. Corrosion pieces, consisting of renewable 12-inch lengths of galvanized mild steel, are fitted between dissimilar metals in such piping normally dealing with salt water such as suctions to gland and plummer block compartments but are not normally required in dry suction systems.

No non-ferrous piping is introduced into any part of the ship so low as to come into contact with bilge water. Unless unavoidable, pipes containing liquid are kept out of compartments accommodating important electrical apparatus. The seating of pipe joints directly above electrical equipment is prohibited. No electrical installation is earthed to any water system and electric cables or associated fittings are not attached to such piping. Pipes connected to the outer bottom or to protective bulkheads are worked with generous bends, or alternatively with weakened shearing joints, so that they cannot act as struts in the vent of deflection of the structure.

VARIOUS UNDERWATER VALVES IN SHIPS

Posted by: Engineers Zone Posted date: Wednesday, December 08, 2010 / comment : 1 ,


Generally speaking the body, bottle, bridge and cover of all valves are made of gunmetal whereas the valve, disc, spindle, thrust collar, studs and nuts are of aluminium or bronze. Soft faces are made of vulcanized synthetic rubber, except in ball plug valves where they are of an approved low friction type material.


VARIOUS UNDERWATER VALVES FITTED INCLUDES:

SEA OR COCK VALVE
Sea valves, normally of the screw down type, metal seated, are fitted for admitting salt water into the ship for flooding compartments and to supply the various salt water pumps in the system. In way of the double bottom they are secured to the inner bottom by a flange connected to the inlet tube between inner and outer bottom. Outside the double bottom the seacock is fitted to a distance piece on the outer bottom. Gratings of galvanized mild steel are fitted to the sea inlets as near as possible to the outer bottom plating. 
Where the sea tube serves more than one valve the clear area of the opening in the grating is suitable for the total number of valves using that tube. Zinc protectors are fitted inside the tube to take the effect of galvanic action set up by the non-ferrous material of the seacock as described in the chapter on cathodic protection. To enable compartments containing explosives or inflammable material to be flooded in dry dock, flooding bonnets are provided which can be secured to the mouth of the inlet tube after removing the grating. A flooding bonnet consists of a number of hose connections welded to a base plate. As the water falls below the inlet, when docking, the grating is removed and the bonnet secured in place, hoses being led from the hose connections to the shore hydrant. The bonnet is removed and the grating replaced when the vessel is being undocked. Pump suctions connected to a seacock used for flooding magazines or other danger spaces must be taken from the sea side of the master valve of the seacock.

SCREW-DOWN VALVE  
Screw-down valves fitted with soft face and renewable seats are fitted for isolation and control purposes, e.g. on branches from the salt and fresh water mains, and on filling pipes discharging into fresh water filling funnels. Alternatively diaphragm valves are used. Where it is desired to run pipes at right angles and space is limited, screw-down right-angle valves are fitted.

SCREW-DOWN NON-RETURN VALVE
Screw-down non-return valves fitted with soft face and renewable seats are fitted to all suction leads to ensure that the pumps are kept flooded. Their design is similar to the SDV except that the' bottle' has a longer neck with an expansion chamber on the valve. When the spindle is in the open position, the valve slides up the neck of the bottle when the pump is drawing, and falls back on the seating when suction is lost.

SCREW-DOWN NON-RETURN AND FLOOD VALVE
Screw-down non-return flood valves fitted with soft face and renewable seats are fitted to all suction leads where it is required to flood the compartment via the suction pipe. This valve is similar in action to SDNRV for pumping, but has a still longer neck to allow the valve to be lifted clear of the seating for flooding purposes. The inlet to the pump is inclined in order to facilitate the flow of water when pumping. Such valves are fitted at the feet of  branches to bottom compartments.

SCREW-DOWN DIAPHRAGM VALVE
Screw-down diaphragm valves are fitted in main suction systems where a high vacuum is essential in preference to screw-down valves. In this type of valve a flexible diaphragm isolates the fluid controlled by the valve from the valve operating mechanism.

PLUG VALVE
This is a form of shut-off valve, used where a full on or full off control is required, having a 'plug' which may be tapered, parallel or spherical in shape. It incorporates design features which reduce friction between the plug face and the body seat during operation and seals them against leakage. This type of valve has the advantage that it offers low resistance to flow.

BUTTERFLY VALVE
The flow regulator in this type of valve is a metal disc rotating in axial trunnion bearings. The valve has a resilient lining in the body incorporating the seat and may be used to control the flow from full open to full off according to the service required. These valves are now fitted in preference to double faced sluice valves in fluid systems in sizes above 4-in nominal bore. For smaller sizes valves of the full bore plug type are preferable.


SLUICE VALVE
Sluice valves are used for local connection between two or more compartments, e.g. one double bottom compartment draining into another. In shape they may be square or circular and consist of a casting with machined faces over which slides the valve casting operated by gearing.

SCREW-DOWN AND NON-RETURN DRAIN VALVE
These are fitted in flats and auxiliary machinery spaces for draining into compartments below. The non-return is fitted in the pipe below the deck leading to the compartment below, and is necessary to prevent water entering the compartment above should the one below be flooded.

STORM VALVE
Storm valves are simple non-return valves and are fitted to all drain and soil pipes discharging overboard, to prevent flooding when the sea rises above the outlet. Positive closing type storm valves are fitted when required. All storm valves are faced with leather to minimize noise when the vessel is rolling.

NON-RETURN VALVE
Non-return valves are fitted to prevent reversal in direction of flow, e.g. on the return piping in hot water circulating systems. They are also fitted on the discharge side of pumps to prevent back pressure.

REDUCING VALVE
Reducing valves are fitted where a low pressure service is supplied from another of higher working pressure. They are provided with renewable seats and discs of erosion resistant material. Where suitable, pressure reduction is achieved by the comparatively simple process of fitting an orifice plate of nylon or aluminium bronze. A diagrammatic sketch is given to illustrate the action of a reducing valve. When no pressure is on the spring and the inlet is open to water pressure, the valve A and piston B are in equilibrium, so that no water passes through the valve. When pressure is put on the spring to the required indicator setting, the valve A opens to let water through, until the pressure above A just exceeds that to which the spring is set: the valve A then closes against the spring. This action is repeated as the water is used.

GOVERNORS

Posted by: Engineers Zone Posted date: Monday, November 29, 2010 / comment : 4 , ,


  • The function of governor is to regulate the mean speed of an engine, when there are variations in the load.
  • Ex. When the load on an engine increases, it becomes necessary to increase the supply of working fluid.
  • When the load decreases, less working fluid is required.
  • It automatically controls the supply of working fluid to the engine with the varying load condition and keeps the mean speed within certain limits.
Note:
The function of a flywheel in an engine is entirely different from that of a governor. It controls the speed variation caused by the fluctuations of the engine turning moment during each cycle of operation. It does not control the speed variations caused by a varying load. The varying demand for power is met by the governor regulating the supply of working fluid.
Types of Governors:
  1. Centrifugal governors
  2. Inertia governors
Centrifugal Governor:
The centrifugal governors are based on the balancing of centrifugal force on the rotating balls by an equal and opposite radial force, known as the controlling force.
Watt Governor:
The simplest form of a centrifugal governor is a watt governor. It is basically a conical pendulum with links attached to a sleeve of negligible weight.
Porter Governor:
The porter governor is a modification of a watt governor, with a central load attached to the sleeve.
Proell Governor:
The Proell governor has the balls fixed at the end point to the extension of the links.
Hartnell Governor:
A Hartnell governor is a spring-loaded governor. It consists of two bell crank levers pivoted at the point to the frame. The frame is attached to the governor spindle and therefore rotates with it.
Hartung Governor:
In this type of governor, the vertical arms of the bell crank levers are fitted with spring balls which compress against the frame of the governor when the rollers at the horizontal press against the sleeve.
Wilson-Hartnell Governor:
A Wilson-Hartnell governor is a governor in which the balls are connected by a spring in tension. An auxiliary spring is attached to the sleeve mechanism through a lever by means of which the equilibrium speed for a given radius may be adjusted. The main spring may be considered of two equal parts each belonging to both the balls.
Pickering Governor:
A pickering governor is mostly used for driving a gramophone. It consists of three straight leaf springs arranged at equal angular intervals round the spindle. Each spring carries a weight at the center. The weights move outwards and the springs bend as they rotate about the spindle axis with increasing speed.
Sensitiveness of Governor:
The sensitiveness is defined as the ratio of the difference between the maximum and minimum equilibrium speeds to the mean equilibrium speed.
Stability of Governor:
- For a stable governor, if the equilibrium speed increases, the radius of governor balls must also increase.
- A governor is said to be unstable, if the radius of rotation decreases as the speed increases.
Isochronous Governors:
A governor is said to be isochronous, when the equilibrium speed is constant (i.e., range of speed is zero) for all radii of rotation of the balls within the working range, neglecting friction. The isochronism is the stage of infinite sensitivity.
Hunting:
A governor is said to be hunt if the speed of the engine fluctuates continuously above and below the mean speed.
Effect and Power of a Governor:
- The effect of a governor is the mean force exerted at the sleeve for a given percentage change of speed (or lift of the sleeve).
- The power of a governor is the workdone at the sleeve for a given percentage change of speed. It is the product of the mean value of the effort and the distance through which the sleeve moves.
Controlling force:
When a body rotates in a circular path, there is an inward radial force or centripetal force acting on it. In case of a governor running at a steady, the inward force acting on the rotating balls is known as controlling force.

Difference between Hardness and Toughness

Posted by: Engineers Zone Posted date: Sunday, November 28, 2010 / comment : 7

Hardness

i. It is the property of the material to resist scratching, abrasion, indentation or penetration.

ii. Hardness of a material is stated relative to the hardness of other material.

 

iii. Important in shafts, bearing whichever is having relative motion.

Toughness

i. Ability of a material to withstand both elastic and plastic deformation, shock and vibration.

ii. Toughness is measured in terms of the energy a material can absorb before a actual failure takes place.

iii. Important in structural members, machine parts which are subjected to shock and vibration. E.g. shafts, spring.

Moment of Momentum Equation

Posted by: Engineers Zone Posted date: Sunday, November 28, 2010 / comment : 0 , ,

Moment of momentum equation is derived from moment of momentum principle, which states that the resulting torque acting on a rotating fluid is equal to the rate of change of moment of momentum.

Moment of momentum per second = rQ x V x r

Where r = density of fluid

Q = rate of flow of fluid

V = velocity of fluid

r = radius of curvature

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