boiler purge requirements 1

[spud22]

I have a 50,000 lb/hr high pressure boiler. In setting up the controls I have a dilemma. NFPA 85 calls for - 8 air changes, with the air dampers driven to their full open position- in one area, but in another section calls for only a- 15 seccond purge at prior setting air flows-with a boiler relight. My question is: Which shall apply and when? The 15 second purge seems too short under almost any conditions, but the eight air change requirement delays the restart significantly. Any thoughts appreciated.[/indent]

 

[FacEngrPE]

The purge requirement of 8 air changes is found in the Prefiring Cycle (the prefiring cycle requirements must all be true for the permissive to fire to be true).

The post firing purge requirement of 15 seconds is found in Normal Shutdown Cycle, and the Safety Shutdown Cycle, which is always followed by the prefiring cycle on restart.

NFPA 85 is not a design handbook, it recognizes that there may be situations where the minimum purge (or any other requirement stated) is not sufficient by the following:

1.2.3 This code shall not be used as a design handbook.
1.2.3.1 A designer capable of applying more complete and rigorous analysis to special or unusual problems shall have latitude in the development of such designs.
1.2.3.2 In such cases, the designer shall be responsible for demonstrating and documenting
the validity of the proposed design.

This paper has a list of additional things you should consider if you have technical cognizance over a boiler safety system.
Industrial Combustion: Avoiding Unintentional Safety Standard Violations for Burner-Management Systems