What is the logic behind tha id fan,fd fan,air preheater tripping ! If id fan trip what are the equipments will trip If fd fan trip what are the equipments will trip If air preheater trip what are the equipments will trip Pls explain logic in sequence?

Q.What is the logic behind tha id fan,fd fan,air preheater tripping ! 
If id fan trip what are the equipments will trip
If fd fan trip what are the equipments will trip
If air preheater trip what are the equipments will trip
Pls explain logic in sequence?

Ans.frst of all air preheater is an assesories not a mounting so this one ll not trip because of there is no any rotery equipment whill will trip its just a tube inside a shall from which the flue gas passes....

• if id trip it is nssry that all equipment should be trip like pa & fd coz if they didnt trip they may be reason of furnace blasting......just because of air pressure inside the furnace which will increse by pa & fd & if this pressuse didnt release than it is the reason of furnace blasting so this is important interlock of boiler....

•  Id trips consequent primery air, secondary air fan vl trip to aviode back firing. And even fuel feeding vl trip. In one word id trips boiler vl trip. In case only one id trips other is running then its your duty to control furnace pressure in healthy side.