Chapter 11-BOILER FITTINGS AND CONTROLS 



UNLOADING LINE 

 i 



SUPERHEATER 

 VALVE 



SUPERHEATER 



SUPERHEATER OUTLET 



98.82 



Figure 11-21.— Crosby two-valve superheater outlet safety valve assembly. 



single-fxirnace boilers. The assembly consists 

 of a pilot valve, an actuating valve, and an un- 

 loading valve. 



The spring-loaded pilot valve is mounted on 

 the top centerline of the steam drum. The 

 actuating valve and the unloading valve are as- 

 sembled as a unit and mounted on the piping at 

 the superheater outlet; they are connected to 

 each other by a rocker arm. The actuating valve 

 has a cylinder with a piston inside it. The un- 

 loading valve has a piston-type disk, without 

 a stem which is held in line by the cylinder in 

 which it works. The unloading valve is pres- 

 sure loaded, not spring loaded. 



Steam from the superheater outlet enters 

 the unloading valve cylinder and gathers around 

 the valve disk above the seat. The steam bleeds 

 through small ports to the space above the 

 disk. When the actuating valve is closed, the 

 steam above the disk of the unloading valve 

 cannot escape, so the pressure above the 

 disk equalizes with the pressure below the 

 disk— that is, the pressure above the disk is 

 equal to superheater outlet pressure. 



The cause of safety valve lifting in this 

 assembly is excessive pressure in the steam 

 drum, not excessive pressure in the super- 

 heater. When the pilot valve on the steam drum 

 opens, pressure is transmitted from the pilot 

 valve to the cylinder of the actuating valve. 

 Pressure in the actuating valve cylinder is 

 applied under the piston, causing the spring 

 to compress. The rocker arm moves upward 

 at the end over the actuating valve and down- 

 ward at the end over the unloading valve, 

 thus opening the actuating valve. 



When the actuating valve opens, pressure 

 bleeds off to atmosphere. Since the space 

 above the unloading valve disk is connected 

 to the actuating valve, relief of pressure in 

 the actuating valve also causes relief of pres- 

 sure above the disk in the unloading valve. 

 The unloading valve therefore opens, allowing 

 steam to flow from the superheater to atmos- 

 phere. When the pilot valve reseats, the 

 actuating valve also reseats. As steam bleeds 

 through the ports to the space above the disk 

 in the unloading valve, pressure builds up and 

 rapidly equals the pressure below the disk. The 

 unloading valve closes. In summary, then, the 

 superheater unloading valve always opens im- 

 mediately after the steam drum pilot valve 

 opens and closes immediately after the pilot 

 valve closes. 



Soot Blowers 



Soot blowers are installed on each boiler 

 for the purpose of removing soot from the 

 firesides while the boiler is steaming. Soot 

 blowers are used only on steaming boilers, 

 not on idle boilers. Each steaming boiler utilizes 

 its own steam (superheated) to supply its own 

 soot blowers. 



The soot blowers must be used frequently, 

 regularly, and in proper sequence in order to 

 prevent the accumulation of heavy deposits of 

 soot which would interfere with heat transfer 

 and which would constitute a fire hazard. The 

 process of using the soot blowers is usually 

 called "blowing tubes." 



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