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For water supply to the boiler, UP 4000-class Big Boy No. 4014 employs a Nathan Type 4000 Size C "Special" automatic restarting live steam injector on the engineer's side, and an Elesco Type T.P. 502 Exhaust Steam Injector on the fireman's side. The Nathan injector is sized for 12,500 gallons per hour while the Elesco injector used on Big Boy is sized for 14,076 gallons per hour maximum operation.

In this video, you will see the process of operation first for the Nathan 4000, which starts with priming the injector (note the overflow that is a result of this process). Once primed, the operating handle is moved into the regulation portion of the quadrant, where the fireman can control the amount of water being injected into the boiler. After the proper water level is reached in the boiler, the injector is shut off and any excess water remaining in the system drains from the overflow.

In the next section, you will see the Elesco type T.P. injector running on live steam (locomotive throttle closed, no exhaust steam available). You will see the process of operation, which starts with pulling the starting valve into the priming position. This supplies steam to the cold water turbine pump located under the cab on the fireman's side, which then pumps water from the tender up to the injector located next to the smokebox on the same side. The exhaust for the cold water pump can be seen as a plume of white smoke on top of the turret ahead of the cab. Once the injector is primed, the starting valve is moved into the full open position, live steam is supplied into the injector, and the water is injected into the boiler. Overflow is a normal part of the starting process, and its drainage can be seen just ahead of the cylinder for the rear engine unit. Water supply to the injector is regulated with an indexed dial control in the cab. After the proper water level is reached in the boiler, the operating valve is shut off. Any remaining water within the system that was not injected into the boiler drains from the overflow.

Elesco type T.P. exhaust steam injector deep dive:    • Elesco Type T.P. 502 Exhaust Steam In...  
Nathan 4000 deep dive:    • Nathan Type 4000 Injector & Worthingt...  

For more information on how injectors work (and more info on the Elesco type TP), here are some useful links:
https://en.wikipedia.org/wiki/Injector
   • how steam injectors work  
https://utahrails.net/flebbe-papers/U... (Official UP test report on the type TP injector, as tested on CSA Challenger 3911)

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