Ultrasonic Test on Metals (NDT) AND ITS PROCESS

It relies upon transmission and reflection of ultrasonic beams or waves of frequencies between 100 kHz and 25MHz. The ultrasonic waves are usually produced by the the piezoelectric effect within the crystal probe which is placed on the surface of the specimen. Discontinuities below the surface cause reflection of the ultrasonic waves which appear as peaks upon the cathode - ray oscilloscope receiver. The size of the peak seen on the receiving tube is some indication of the size of the defect. The crystal probe thus becomes the receiver as well as transmitter. Ultrasonic techniques are useful for detecting cracks , voids, and defects far below the surface as well as near the surface.

While radiation and ultrasonic tests are very sophisticated and yields excellent results under almost all circumstances, the older hammer test is still employed for the detection of internal defects. If a "sound" object , that is one free from large internal flaws, is struck sharply with a suitable hammer it emits a clear ringing note, where as a defective object emits a flat, unusual not. This test gives valuable information to a skilled operator about the quality of the object.

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ELECTRICAL COMPARATORS AND ITS WORKING

Electrical Comparators are used as a means of detecting and amplifying small movements of a work contacting elements. An electrical comparator consists essentially of a pick-up head or transducer for converting a displacement into a corresponding change in current and a meter or recorder connected in the circuit to indicate the electrical change, calibrated to show in terms of displacement. Generally, an amplifier is needed to provide the requisite sensitivity and to match the characteristics of different parts of the circuit. Electrical comparators can be classified according to the electrical principle used in the pick-up head. Most of the comparators use either a differential transformer, an inductance bridge, a strain gauge or a capacitor as a means of detecting movement of the gauging element. There are different types of electrical comparators. One kind called an electrolimit gauge is used to check or measure the outside diameter of a roll. The object to be checked is place

THROTTLE GOVERNING OF STEAM TURBINES

Steam Pressure at inlet to a steam turbine is reduced by throttling process to maintain the speed of the turbine constant at part load and hence this method of governing is called "Throttle Governing". Construction : Throttle governing system consists of a centrifugal governor, a lever , an oil pump , a pilot piston , control valve , a relay piston and a throttle valve. The throttle valve is moved by a relay piston. The relay piston is actuated by pilot piston control valve. There are two piston valves covering ports in the pilot piston control valve without any overlap. These piston valves are operated by lubricating oil supplied by a gear pump at 2 to 4 bar. The oil returns to the drain from this chamber. Working : When the turbine works on full rated load, the throttle valve will remain open. When the load is decreased, the energy output of the turbine becomes in excess and the turbine shaft speed increases. Hence, governor sleeve will lift. The upward movement of the

Advantages And Disadvantages Of Air Refrigeration System :

Advantages Of Air Refrigeration : 1. The refrigerant used namely air is cheap and easily available. 2. There is no danger of fire or toxic effects due to leakages. 3. The weight to ton of refrigeration ratio is less as compared to other systems. Disadvantages Of Air Refrigeration : 1. The quantity of Refrigerant used per ton of refrigeration is high as compared to other systems. 2. The COP of the system is very low. Therefore running cost is high. 3. The danger of frosting at the expander valves is more as the air contains moisture content.

Mechinopedia For Mechanical Engineers

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