Classification of Commonly Used Boiler Doors Matched with Boiler Grates

I. Overall Classification Overview
Standard supporting doors for biomass chain grates and reciprocating grates are mainly divided into five categories: manhole doors, fire observation doors, ash cleaning doors, fuel charging doors and explosion relief doors. Fire observation doors, ash cleaning doors and manhole doors are the most frequently purchased items by field engineers.
Standard Materials

Grey cast iron HT200, heat-resistant ductile cast iron for general service conditions; RTCr series heat-resistant cast iron is adopted for high-temperature working environments.
1. Fire Observation Door
Functions
To inspect furnace combustion conditions, fuel bed thickness, flame status and grate operating performance; the door is embedded with high-temperature resistant quartz sight glass.
Structural Types
Two mainstream shapes: round and square
Round observation door: Widely applied on small biomass boilers
Square observation door: Standard fitting for medium & large-sized chain grate boilers and reciprocating grate boilers with a capacity above 6 tons per hour
✅ Advantages: Compact structure, flexible installation, favorable sealing performance; enables quick visual inspection of internal combustion conditions
❌ Limitations: Small bore diameter, inaccessible for personnel entry, incapable of removing large coke clinkers
Installation Positions
Side walls of the furnace chamber, front and rear arches, upper side walls above the grate
2. Ash Cleaning Door / Clinker Cleaning Door
Functions
Remove leaked slag, coke lumps and accumulated ash falling beneath the grate; most commonly installed on both sides of reciprocating grates.
Features
Oversized door plate with thickened sealing flanges; some models come with a miniature ash shovel for auxiliary cleaning.
✅ Advantages: Convenient slag removal and maintenance without shutting down the boiler and opening large manholes
❌ Limitations: Only applicable for cleaning loose ash and small coke fragments
Installation Positions
Lower left and right sides of the grate, side walls of air chambers
3. Manhole Door
Functions
Allow personnel access for maintenance during boiler shutdown; used for grate bar replacement, chain overhaul and removal of extensive coking deposits.
Specifications
Standard large-caliber square design, sealed via hinged compression bolts, fitted with high-temperature resistant asbestos or ceramic fiber sealing gaskets.
✅ Advantages: Spacious access for comprehensive maintenance of the grate assembly
❌ Limitations: Bulky structure occupying furnace space; strictly prohibited to open during boiler operation
Application Scope & Positions
Designed for boilers rated ≥ 4 tons per hour; mounted at maintenance openings on the front, rear and side walls of the grate section
4. Fuel Charging Door
Functions
For manual fuel feeding, mostly equipped on small biomass boilers with manual feeding systems; generally omitted on fully automatic feeding boilers.
✅ Advantages: Allows emergency manual fuel replenishment
❌ Limitations: Obsolete for automated boilers; opening the door may cause flame ejection and flue gas leakage
Current Market Trend
Seldom ordered separately for medium and large chain grate & reciprocating grate boilers
5. Explosion Relief Door (Explosion Vent Door)
Functions
Biomass fuels contain high volatile matter content. This door releases pressure instantly in case of combustible gas deflagration inside the furnace to protect the boiler body from damage.
Two Main Types
Weight-loaded gravity explosion doors, rupture disc type explosion doors
⚠️ Important Note: It cannot replace conventional maintenance doors and serves solely as a safety protection component.
Installation Positions
Top of the furnace chamber, inlet section of flue gas ducts
6. Air Chamber Inspection Door / Secondary Air Damper Small Door
Exclusive accessory for grate air chambers, one of the high-demand supporting parts for all grates
Used for air chamber maintenance, air leakage inspection and auxiliary primary air regulation; massively matched with air boxes of travelling chain grates and reciprocating grates.