147.What are the types of Instrument air compressors? What is loading and unloading?

Reciprocating and centrifugal type. Loading - Compressor running with suction valve open. Unloading - Compressor running with suction valve closed, ideal running

146.What are the fail-safe conditions of control valves, give typical example?

Fail safe conditions of Control valves are Fail-Open and Fail-Close (Depending on the end position reached by control valve on loss of control signal/air supply).Normally Fail open valves are used in Emergency applications like recirculation line or Bypass lines (HP bypass, Heater bypass etc). Valves used for fuel flow are fail close type.

145.What are the characters of Control valve and give example of their application?

The typical flow characteristics of a control valve are Linear, Equal percentage, Quick opening type.          The characteristic of valve depends of loop sense, differential pressure across the control valve etc.                  Fuel oil flow control - Controller is Linear characteristic. Error at various valve opening, from 0 to 100%, demands same flow change.The DP across the control valve is high at loads and decrease with respect to load. Hence the CV at low loads shall be small compared to at high loads. A Equal percentage valve will provide such CV. Fuel Oil Temperature Control - Controller is non linear characteristic. Error is temperature. Same error at small valve opening require less flow. At higher valve open demands large flow. The mass of steam required for 50% fuel can only change the temperature by 50% at 100% fuel flow. DP across the valve is constant. Hence an Equal % valve can perform better. However the valves characteristics can be matched with the process using adapting gain in DCS using valve position demand.

144.What is Constant Pressure mode and Sliding Pressure mode?

Large units will be operated at reduced steam pressure at loads less than 60% from 25%. The pressure will be maintained constant at rated value from 60 to 100% load. Typically for a 550 MW Unit, pressure will be 120 bar at 25% load and increased proportionally to 160 bar up to 60% load. This mode of unit operation is known as Sliding/Variable pressure mode. The governer valve will remain at a fixed value except for any emergency. From 60% to 100% the pressure will be maintained constant at 160 bar by modulating the governer valve, it is called constant pressure mode.In sliding pressure mode an appreciable amount energy is saved in the BFP.