Why are water tube boilers considered less dangerous than fire-tube boilers?

Answer 1

Advantages of fire tube boilers: - More residual heat in the water to allow for load swings - Simpler control systems due to larger volumes of water in shell - Water quality doesn't change as quickly due to larger volume of water

Disadvantages of fire tube boilers - Maximum Allowable Working Pressure is limited due to larger surfaces exposed to pressure, to about 250 psi - Danger of a pressure part explosion due to failure of the shell and resulting rapid expansion of the large volume of water in the shell - Longer warm-up times due to the large amount of water in the shell. - Larger footprint than a water tube boiler of the same rating. - Less efficient per sq. ft. of heating surface since less of the water is in contact with heating surface due to a larger volume of water in the shell.

Advantages of Water Tube Boilers - Rapid warming due to a much larger heating surface per pound of water in the boiler. - Easier to have a superheater if necessary - Virtually unlimited Maximum Allowable Working Pressure (within reason) - Smaller footprint than a fire-tube boiler of the same rating - Less working volume of water contained in the pressure parts - More water volume exposed to the heating surface than a fire tube boiler (more efficient) - Faster load change response due to less water - Less chance of a pressure part explosion due to less volume of water exposed to a rupture situation, plus, the rupture is usually into the furnace area which is safer.

Disadvantages of a Water Tube Boiler - Must be more careful of feed-water quality - More complex control systems for drum level necessary due to smaller water spaces, (although I have worked in plants with old marine three drum boilers with huge steam drums which only had thermo-hydraulic regulators for drum level) - Operators must be quicker on the draw when there is an upset because everything happens faster with a water tube boiler

Rahul Menon

Twitter- @menenrahul

Answer 2

1.High efficiency (generally greater than 85 %) hence reduced fuel consumption.

2.Flexibility of design - important space consideration.

3.Capable of high output (i.e. high evaporation rate).

4.High pressures and temperatures improve turbine plant efficiency.

5.Flexible in operation to meet fluctuating demands of the plant - superheat control rapidly responsive to changing demands.

6.Generally all surfaces are circular hence no supporting stays are required.

7.Steam can be raised rapidly from cold if the occasion demands (3 to 4 hours compared to 24 hours for smoke tube or fire tube boilers) because of the positive circulation.

8.Consider a fire tube or smoke tube boiler and a water tube boiler with similar evaporative rates the water tube boiler would be compact and relatively light by comparison and its water content would be about 7 tonnes or less compared with the smoke tube or fire tube boiler's 30 tonnes.

9.With double casing radiation loss can be cut to 1 % or less.

"Steam Engineering Knowledge for Engineers" by Thomas D. Morton

Answer 3

Less water in a water tube boiler