When inspecting the boiler, you may find cracks or holes in the furnace lining. To make necessary repairs, mix some of the fire clay you used for brick mortar into a thick mixture. Use more mortar than you used for the brick mortar mix. Use a trowel to apply this wash.

While standard firebrick generally is used for normal refractory work, plastic firebrick is recom- mended for emergency patches and for building up furnace openings. Plastic firebrick is unfired firebrick in a stiff plastic condition. It offers a particular advantage in that, because of its plastic nature, it can be pounded into places where otherwise a firebrick of special shape would be required. The fusion point of plastic firebrick is practically equal to that of standard firebrick. Because of the moisture in the plastic material, however, a greater degree of shrinkage takes place. This factor prevents its general use for sidewalls. It provides an excellent material, though, for repairing brickwork, topping off side and back walls, repairing and constructing the burner openings and, general, for any part of the furnace not exposed to temperatures in excess of 2000F. It is particularly adapted for use in place of specially formed brick of complicated shapes.

Plastic firebrick material, as received from the factory, ordinarily contains enough moisture for working. Avoid the addition of water or any foreign material. In laying up, chunks of plastic just as taken from the can should be rammed tightly into place (preferably in horizontal layers). In general, the more solidly the section of plastic is rammed up, the better it will be.

As the next step, the plastic section should be vented with 3/16-inch holes. Ensure that the holes extend clear through the plastic and are not more than 2 inches apart. This positioning allows deeper heat penetration during the baking-out process. It also permits ready escape of the steam formed from the moisture in the plastic. Do NOT trowel the surface of a new plastic section. This tends to prevent the escape of steam during baking out.

The plastic section should be held in place with as many anchor bolts as would have been provided had standard firebrick been used instead of plastic. The plastic section should be air-dried. This takes from 48 to 72 hours, depending upon the atmosphere. As soon as practicable after air drying, the furnace should be fired with a small fire and gradually brought up to operating temperature to complete baking out. Plastic requires a temperature of about 2900F to 3000F for baking out. If small shrinkage cracks open up, they should be filled with fire clay. If large cracks occur, they should be filled with plastic.

When used for patches, as in the case of brick falling out, the hole should be cleaned out to give at least 4 inches of body thickness to the plastic brick. In building up furnace openings, the use of a metal form is desirable. However, it is not absolutely necessary if care is exercised in making openings of the proper shape and concentric with the atomizer at every point. If furnace openings, as built, have a smooth surface, they should be roughened with a stiff wire brush before baking out.

The following ways to maintain newer boilers are recommended. The boiler is normally shipped with a completely installed refractory. This consists of the rear head (fig. 2-19), the inner door, and the furnace liner (fig. 2-20). Follow the instructions in the manufacturer's manual for the boiler you are maintaining. Where specific directions or requirements are furnished, follow them.

Normal maintenance requires little time and expense and prolongs the operating life of the refractory. Preventive maintenance through periodic inspection keeps the operator informed of the condition of the refractory and helps guard against unexpected downtime and major repairs.

Frequent wash coating of refractory surfaces is recommended. A high-temperature bonding air-dry type of mortar diluted with water to the consistency of light cream is used for this purpose. Recoating intervals vary with operating loads and are best determined by the operator when the heads are opened for inspection.

Maintenance consists of occasional wash coating of the entire liner. Face all joints or cracks by applying high-temperature bonding mortar with a trowel or use your fingertips. This should be done as soon as the cracks are detected. Should segments of the liner become burned out or broken, replace the entire refractory. Any refractory that may break out should be removed as soon as detected, so it will not fuse to the bottom of the furnace and obstruct the burner flame.

Remove the existing refractory and thoroughly clean that portion of the furnace covered by the liner to remove all old refractory cement or other foreign material to ensure the new liner seats firmly to the steel. Inspect all furnace metal for soundness. There may be metal clips welded in the furnace at the extreme end of the liner. These clips were installed to prevent shifting during original shipment and serve no other