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Q2. With reference to pneumatically operated control valves:

(a) state FOUR the reason for fitting valve positioners;                                             (4)

(b) explain valve hysteresis, stating how it affects the process;                                (2)

(c) state the immediate action to be taken to restore control upon failure of the positioner bellows;                                                                                                                          (2)

(d) state the difference between fail safe and fail set                                            (2)