Operating basis earthquake is a level of ground motion where the features of the nuclear power plant are required for continuing the operation. The component should function during OBE without undergoing any deformation. The level of ground motion of OBE is evaluated based on local geology, seismology and subsurface characteristics. The nuclear power plant is under operation during OBE. If this level of earthquake is exceeded, the plant should be shut down and inspected for any damages had occurred. The plant should restart only after obtaining the “fit for operation” certificate. The peak ground acceleration of this type of earthquake should not be less than 0.05 g in the horizontal direction. The estimated return period is 100 years.

Phase angle represents the relative position of a vibrating object with respect to a fixed point or with respect to another vibrating object. Consider the following example where object-1 is moving up and simultaneously object-2 is moving down from their mean equilibrium position. The motion of objects is plotted in the curve shown below. The horizontal gap between the peaks of the two curves is 180 degrees. Hence the phase angle between these two objects is 180 degrees which tells that the two objects are moving in opposite directions to each other.

Figure: Physical significance of phase angle.

Machinery vibration is measured in terms of displacement(microns), velocity(mm/s) and acceleration(). But velocity is the preferred parameter for measuring machinery vibration because most of the equipment failure during vibration is due to fatigue. (Fatigue failure occurs when an object is subjected to repetitive or cyclic loads). For example, consider a metal wire, when you try to break it by applying tension, it requires a good amount of load to fail it. But the same wire when subjected to and fro deflection or displacement, requires relatively less load to fail the wire. This is due to fatigue. Fatigue life depends on how much an object is deflected and its frequency of deflection. Metal wire fails early if the magnitude of deflection and its frequency is high. The vibration parameter that considers both displacement and frequency is velocity since velocity is the product of frequency and displacement.

It’s a medium carbon steel having composition as shown in figure below.

EN 24 is not a stainless steel because the chromium percent is less than 10 . It is a medium carbon steel since EN 24 consists of 0.4% carbon. EN 24 have good tensile strength due to adequate carbon presence but poor toughness. To get toughness, FCC (Face Centered Cubic) element i.e., Nickel is added as an alloying element. Nickel refines the grain size by blocking the grains from becoming larger and coarser during heating. Fine grains have higher toughness as compared to coarser grains. EN 24 is heat treatable and its hardenability is enhanced by adding Chromium as an alloying element. Manganese is used to remove impurities like oxygen and sulphur from the iron ore hence trace amounts will be left over in the final metal.