OPERATION AND MAINTENANCE

BOILERS TYPES.

Fire-tube Boilers. In fire-tube boilers, combustion gases pass through the inside of the tubes with water surrounding the outside of the tubes. The advantages of a fire-tube boiler are its simple construction and less rigid water treatment requirements.

The disadvantages are the excessive weight-per-pound of steam generated, excessive time required to raise steam pressure because of the relatively large volume of water, and inability to respond quickly to load changes, again, due to the large water volume.

The most common fire-tube boilers used in facility heating applications are often referred to as ''scotch'' or ''scotch marine'' boilers, as this boiler type was commonly used for marine service because of its compact size (fire-box integral with boiler section).

The name "fire-tube" is very descriptive. The fire, or hot flue gases from the burner, is channeled through tubes that are surrounded by the fluid to be heated. The body of the boiler is the pressure vessel and contains the fluid. In most cases, this fluid is water that will be circulated for heating purposes or converted to steam for process use.

Every set of tubes that the flue gas travels through, before it makes a turn, is considered a "pass." So, a three-pass boiler will have three sets of tubes with the stack outlet located on the rear of the boiler. A four-pass boiler will have four sets and the stack outlet at the front.

Fire-tube boilers are:

Relatively inexpensive

Easy to clean

Compact in size

Available in sizes from 600,000 btu/hr to 50,000,000 btu/hr

Easy to replace tubes

Well suited for space heating and industrial process applications

Disadvantages of fire-tube boilers include:

Not suitable for high pressure applications 250 psig and above

Limitation for high capacity steam generation

Water-tube Boilers. In a water-tube boiler, the water is inside the tubes and combustion gases pass around the outside of the tubes. The advantages of a water-tube boiler are a lower unit weight-per-pound of steam generated, less time required to raise steam pressure, a greater flexibility for responding to load changes, and a greater ability to operate at high rates of steam generation.

A water-tube design is the exact opposite of a fire-tube. Here, the water flows through the tubes and is encased in a furnace in which the burner fires. These tubes are connected to a steam drum and a mud drum. The water is heated and steam is produced in the upper drum.

Large steam users are better suited for the water-tube design. The industrial water-tube boiler typically produces steam or hot water primarily for industrial process applications, and is used less frequently for heating applications. The best gauge of which design to consider can be found in the duty in which the boiler is to perform.

Water-tube boilers:

Are available in sizes far greater than a fire-tube design, up to several million pounds-per-hour of steam

Are able to handle higher pressures up to 5,000 psig

Recover faster than their fire-tube cousin

Have the ability to reach very high temperatures

Disadvantages of the water-tube design include:

High initial capital cost

Cleaning is more difficult due to the design

No commonality between tubes

OPERATION AND MAINTENANCE

Feed Water Treatment

Modern high - pressure high temperature boiler requires very pure feed water. Most рurе water contains some dissolved salts which come out of solution on boiling. These salts then adhere to the heating surfaces as a scale and reduce heat transfer which сал result in local overheating or failure of the tubes. Other salts remain in solution and may produce acids which will attack the metal of the boiler. Therefore, feed water treatment is employed. The actual treatment involves adding chemicals into the samples of boiler water with a test kit. This test kit is usually supplied by the treatment chemical manufacturer with simple instructions for its use. Regular and correct use of marine chemicals will cut corrosion to a minimum. Otherwise due to corrosion a number of tubes of a ship's boiler may start leaking and have to be plugged which results in a low working pressure of the boiler.

Preparations

The uptakes should be checked; any dampers should be operated and then correctly positioned. All vents, alarm, water and pressure gauge connections should be opened. The superheater circulating valve or drains should be opened. All the other boiler drains and blow-down valves should be checked. The boiler should then be filled to slightly below the working level with hot de-aerated water. The various header vents should be closed. The economiser, should be checked. The operation of the forced draught fan should be checked and exhaust gas air heaters if any should be bypassed. The fuel oil system should be checked for correct positioning of valves. The fuel oil should then be circulated and heated.

Raising Steam

The forced draught fan should be started and air passed through the furnace for several minutes. The air slides (checks) at every register except "the lighting up" burner should then be closed. The operating burner should then be lit and adjusted. The fuel oil pressure and forced draught pressure should be matched. The superheater header vents may be closed once steam issues from them. When the drum pressure is about 210 КPa (2:1 bar) the drum air vent may be closed. The boiler must be brought slowly up to the working pressure. The main and auxiliary steam lines should now be warmed through and then the drains closed. The water level gauges should be blown through and checked for correct reading. When the steam pressure is about 300 k Pa (3 bar) below the normal operating value the safety valves should be lifted and released using the easing gear,

Once at operating pressure the boiler may be put on load and the superheater circulating valves closed. All other vents, drains and bypasses should then be closed. The water level in the boiler should be carefully checked and the automatic water regulating arrangements observed for correct operation.