Answer MEP Question 31

Q. A marine diesel engine freshwater cooler end cover is badly corroded and holed. Other than replacing it, suggest some measures of repairing it

(a) While in operation.

(b) When not in operation.

Repair procedures must be effective. Practicable and innovative.

Corroded end cover of freshwater cooler

Straight tube type heat-exchanger is mainly composed of inlet & outlet end covers, return cover, shell, tube plate, cooling tubes, bafflers and accessories, etc.

End covers of shell-tube type heat-exchanger are made up of casting or welding pieces. There are inlet & outlet connecting pipes, both of which are equipped with thermometers for cold medium and vent & drain plugs in inlet and outlet covers. For exchangers with the medium of S.W., the end covers  must be equipped with zinc bars for the prevention from corrosion.

Various measures are being taken for repairing a marine diesel engine fresh cooler end cover which is badly corroded and holed. There are two general methods of repairing a hole: put something into it or put something over it. Choice of method depends upon the availability of repairing material and time to carryout the repair.

(A) While in operation,

When it is required to repair the heat exchanger end cover while it is in operation temporary cold repair is preferred. Cold repair methods such as using DEVCON require a curing period. But as the cooler is in operation the curing period is shall not be available. DEVCON emergency repair using underwater repair putty however is found to be an effective method to arrest the leakage and don't require any curing time.

Few other methods involve using a jubilee patch and clamp cannot be used because the shape of the end cover of cooler does not permit.

a.  Hook bolt

It is a long bolt having the head end so shaped that the bolt can be hooked to plating through which it has been inserted. The common types are the T, the J and the L, so called because they resemble those letters. The long shanks are threaded and provided with nuts and washers. Steel or wooden strongbacks are used with them; generally the latter. A hookbolt has no regular head.

The head end of the bolt is inserted through a hole, and the bolt is rotated or adjusted until it cannot be pulled back through the hole. A pad or gasket, backed by a plank or a strong back, is then slid over the bolt and the patch is secured in place by taking up on the nut. It is generally necessary to use these bolts in pairs.
A variation of the hook bolt is the folding T. It resembles the T hook bolt, but it has a hinge where the shank joins the cross-piece, so that it is much like the tumble toggle bolt. This bolt may be folded and inserted through a small hole; when pulled back, the cross-piece catches on the hull plating.

b. Wooden plug:-
The simplest method of repairing a fairly small hole is to insert some kind of a plug. Plugs made of soft wood have been found rather effective under battle conditions, especially in holes not over 3 inches x 3 inches. They also have held up well in much larger holes.

Every ship should have a large assortment of conical, square-ended and wedge-shaped wooden plugs at each repair station. The plugs should not be painted, for unpainted soft wood absorbs water and holds better. Plugs should be stowed in canvas bags secured to the overhead.
Combinations of conical, square-ended and wedge-shaped plugs may be used to get better conformation to the shape of the hole. It is best to wrap the plugs with lightweight cloth before inserting. The cloth will tend to keep the plugs in place, and also will fill some of the gaps between plugs. In most cases wooden plugs will not make a watertight fit, but by filling the remaining leaky area with rags, oakum and smaller wedges, the ingress of water can be greatly reduced. Square-ended plugs hold better than conical plugs in holes in plating one-fourth inch or less in thickness.

c. Welding a threaded pipe piece with pipe cap
A skilled welder can make an arrangement as shown in figure. This arrangement require a metal pipe piece with a threaded plug for blanking on one side; the other side should make a good surface contact with the end cover surface. 

The diameter of the pipe should be considerably larger than the hole diameter. The pipe piece should be welded on the end cover while keeping the plug removed, it will need the water to flow through the pipe, once the welding is completed the pipe can be plugged. This method is relatively time consuming and require a skilled person. But the repair is highly reliable.

B. When not is operation
Cooler when not in operation can be drained completely and permanent repairs can be carried out. Cold repairs like applying DEVCON putty can also be considered which in this case will have sufficient time for curing.

a. A quick method is fitting a nut bolt with washer and Gasket RTV. Drain the cooler and remove the end cover. Clean the area with wire brush, and remove the weak rusted metal. Inserting a bolt in the hole diameter must be as close as possible. A washer with considerable larger diameter on both side of cover, applying Gasket RTV on the surfaces of washer.  Tight the nut and left it for a curing period for few hours.
Now fit the cover back to the cooler and open the cooling water and check for any leakage.

b. Welding a patch over the damaged part. This require the surface preparation and heat treatment on the end cover. A patch of same material with a larger size than that of the hole is welded on the both side of hole. Time should be given for slow cooling. Check for any leakage, apply a coat of primer and then paint the cover. Now fit it back to the cooler, open the cooling water.