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Showing posts from July, 2022

Non reversing locking gear

  When two pumping units are fitted and only one is running, the idle pump might be driven in the reverse direction by fluid under pressure from the running pump, if non-reverse locking gear were not fitted. This gear is integral with the flexible coupling connecting motor and pump.  It consists of a number of steel pawls so mounted on the motor coupling that, when pumping units are running, they fly outward due to the centrifugal effect and remain clear of the stationary steel ratchet secured to the motor supporting structure.  The limit of this outward movement is reached when the pawls contact the surrounding casing, which revolves with the coupling. When the pumps stop, the pawls return to their normal, inward position and engage the ratchet teeth, so providing a positive lock against reverse rotation.  This action is automatic and permits instant selection and commissioning of either unit without needing to use the pump isolating valves, which are normally open - and are only clos

Difference Between Neutral, Ground and Earth?

Neutral Neutral is return path for an AC circuit which is supposed to carry current in normal condition. This current may be because of many reasons, primarily because of phase current imbalance and some time because 3rd and 5th harmonics also.  There may be others reasons too but the magnitude of this current is in fraction of phase current and in few case it can be even double of phase current. So Neutral wire is always assumed to be charged (in active circuit). This neutral wire is given to ground (by grounding) to make the second terminal of neutral wire at zero potential. Earth or Ground Earth or Ground is for safety concerns against leakage or residual currents on the system via least resistance path. While phase and neutral is connected to main power wiring, earth may be connected to body of equipment or to any system which in normal condition doesn’t carry current but in case of some insulation failure, is supposed to carry some minor current. This current is not directly comin

Air-conditions and Refrigeration are always rated in Tons.Why?

Air conditioners are designed on the basis of quantity of heat removal from room, hall or specific area.  Quantity of heat is termed in Tons means if an air conditioner is able to remove 1000 kilocalories of heat or 4120 kilojoules or 12000 BTU of heat in an hour that AC rated as 1 Ton of AC because 1000 Kilocalories or 4120 kilojoules or 12000 BTU ( BTU = British thermal unit. A measurement of heat, specifically, the amount of heat needed to raise the temperature of a pound of water by 1°F ) equal to one Ton of heat.  Also, this is the same case for freezer and refrigerator i.e. refrigeration system.    Ton of Refrigeration : A Ton of refrigeration  is a unit of power used to describe the heat extraction capacity of air conditioning and refrigeration equipments.  It is defined as the heat of fusion absorbed by melting 1 short ton of pure ice at 0 °C (32 °F) in 24 hours.

Why rudder is situated aft of the ship?

To make use of propeller outflow for thrust. The increased velocity of the propeller outflow results in a greater lift force. If a rudder is placed just forward of the propeller, it will have the same turning effect with respect to direction, but the magnitude won’t be the same. The pivoting point of ship is 1/6 to 1/3 rd of length of ship from bow, the greater the perpendicular distance between point of action of force and pivoting point, the better rudder movement. Better protected at astern from damage. Drag is reduced if rudder is situated aft

Use of Concentric Spring in Auxiliary Engine Valve Arrangement.

  If one would look carefully, it is noticed that the two springs have their coil in opposite direction.One is right handed and other left handed. This is important since when the springs compress and decompress at high speed , there is a side thrust acting on the spring retainer due to coils of spring. To cancel this side thrust, another spring with opposite coil direction is introduced. Since the side thrust from second spring is opposite to that of first spring, the net side thrust is zero.  There is the effect of resonance at high engine speed. To prevent the matching of natural frequency of the running engine to the natural frequency of the spring, additional spring is added. This addition increases the natural frequency of the spring combination much higher than the engine's natural frequency, thus preventing any damage to the engine by excessive amplitude of vibration.  Tw o spring do provides an additional safety in case of failure of one. But remember th

Boiler steam drum is curved. Why

Circular hoops can withstand internal pressure by pure tension in the tankmaterial instead of bending. Materials can take more force in tension than inbending.  If the shape were not circular, then there would be bending stresses in the tank wall and it would not be able to withstand as much pressure without breaking.  Spheres are more efficient than cylinders if you just care about minimizing the mass of the tank to hold a certain amount of pressurized gas. But cylinders are easier to pack together. So it’s a trade-off between efficiency and practicality.

RAKE and SKEW of propeller

  RAKE: When the propeller is viewed from sideways, we see that the blades of the propeller are not perpendicular to the surface of the hub. It is ‘tilted’ at an angle either towards the fore-end or the aft end of the ship. This is termed as Rake.  One of the biggest reasons for the rake is allowing higher clearance between the blades and the vicinal hull surface. If the blade tip happens to be very close to the hull plating (which is very prone in case of no or little raked ones), there is a chance of induced vibration due to ship propeller action.  When the blade is raked forward, that is in the direction of the shaft axis towards fore-end, it is called Negative Rake. Conversely, when it is raked aft, it is called Positive Rake  SKEW:  When we look at the ship propeller surface from behind). The blade appears to be ‘Skewed’, that is, bent or twisted sideways.  After years of experimentation, analysis and sea trials, it was observed that aptly skewing a ship propeller nullifies or con

Floodable length and Permissible length

  Floodable Length:  It is also the maximum length of the ship that can be flooded without submerging the margin line. Margin line is a line just below the top of the bulkhead deck.The floodable length of a vessel varies from point to point throughout her length and is usually the greatest amidships and the smallest near the quarter length.  Permissible Length:  The floodable length of each point along the ship’s length is multiplied by the permeability to obtain the Permissible Length. The permissible length curves for 85 percent and 60 percent permeability are now incorporated into the floodable subdivision diagram. Here, for machinery compartments (e.g. engine room compartment BC) the vertices for the triangle needs to be checked against the 85% curve, instead of the floodable length (100%) curve The general values of permeability used for different types of compartments are listed below, followed by a logical understanding of the variation in the values: Watertight Compartment – 95

Types of Rudders

  Spade or balanced rudder: A rudder has 20-40% of the area forward of the stock, similarly there is no torque on the rudder stock at certain angles and it is called balanced. The axis of the rudder is placed near to the center of gravity, so torque required to move the rudder will be very less. Semi-balanced rudder: A rudder with 20% of its area forward of its stock is called semi-balanced rudder. It is often found in twin screw ships. In this at no angle, rudder is balanced. Unbalanced rudder: A rudder with whole of its area aft of its stock is called unbalanced. In this at no angle, rudder is balanced

How to order welding machine

Single phase or 3 phase transformer. System current requirement to be mentioned. (10 A – 200 A) Cable size and insulation requirements. Holder size (3.5 mm to 18 mm) Physical size of transformer Only for welding transformer rating in KW, (5 to 40 KW)

Working of welding transformer

More number of turns on primary and less number of turns on secondary, which produces high current and low voltage.  One end connected to the secondary electrode and other to work piece and other to work piece to be welded.  Due to high contact resistance, very high current flows. Therefore more I2R heat produces and electrode melts and fills the gap between the pieces.