Radiographic Tests on Metals ( NDT ) AND ITS PROCESS

These tests are based on absorption and dispersion of X-rays or Gamma Rays passing through the material. By means of a luminescent screen or photographic plate , points of varying radiation intensity that occur at faults can be detected.

As radiation sources, X-rays produced by Betatron devices ( electron centrifuge ) or Gamma rays produces by radioactive decay processes are used. In these methods the radiation is passed through the metal being examined and is then allowed to impinge upon sensitive film. Thus, if an object has an internal defect , more radiation will be passed through the defective area than through the sound region and the defect will show up as dark area on the film. Unfortunately , scattering tends to obscure the defect, so that small defects may not be defected . In general , X rays are preferred for laboratory testing since they offer greater control over intensity and they produce sharper pictures.

X-ray equipment is generally large and bulky , whereas gamma radiography depends upon the use of small, mobile sources of radioactivity, such as radioactive cobalt, iridium , samorium and other radio active elements. In addition gamma rays, though of the same nature as X-ray , have a generally shorter wave length, and therefore, more effective in the study of thick sections.

In radiographic tests, the inclusions appear darker , while blow - holes, cavities and porosity appear lighter, than the surrounding metal.

Comments

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

Search This Blog