Iowa Admin. Code r. 875-91.9
A watertube boiler shall have the firing doors of the inward opening type, unless such doors are provided with substantial and effective latching or fastening devices or are otherwise so constructed as to prevent closed doors from being blown open by pressure on the furnace side. These latches or fastenings shall be of the positive, self-locking type. Friction contacts, latches, and bolts actuated by springs shall not be used. The foregoing requirements for latches or fastenings shall not apply to coal openings on downdraft or similar furnaces.
All other doors, except explosion doors, not used in the firing of the boiler may be provided with bolts or fastenings in lieu of self-locking latching devices. Explosion doors, if used and located in the setting walls within seven feet of the firing floor or operating platform, shall be provided with substantial deflectors to divert the blast.
Iowa Admin. Code r. 875-91.9