As per classification regulations the auxiliary boiler and the Exhaust boiler or exhaust gas economiser are to be surveyed once every two years.
To carry out the survey the boiler has to be blown down completely according to the blowing down procedure. After blowing down and before commencing the cleaning operation the boiler must be safely isolated to prevent any accidents occurring and for this the steam outlet connection to the main steam line must be blanked using a strong spoon flange at the connection to the main steam manifold. Similarly the common feed line must be blanked to prevent accidental entry of feed water in the boiler under survey. These precautions are necessary to be observed in ships where there are two or three boilers connected to a common steam manifold.
Cleaning procedure:- The boiler’s fire side smoke side and water/steam side have to be thoroughly cleaned and made free of all deposits so that the bare metal surface is exposed. In water tube boilers rotary descaling heads mounted on a flexible shaft is very effectively used for descaling the internal surfaces of the water tubes where scaling occurs. In smoke tube boilers the external surfaces are descaled manually. In small package boilers where entry into the boiler is not possible ,chemical cleaning or descaling is done by warming up the boiler with a very weak acid solution which loosens the scale and breaks it. The scale collected is flushed out.
All the 10 listed mountings are dismantled , overhauled and surveyed. The furnace lining is suitably repaired by renewing broken bricks ,the fire cement wherever cracked is suitably excavated and fire cement mortar mix made and filled up.
Classification of defects:- The types of defects encountered during a survey are classified as follows:
- Wastage (external)
(f)Wastage caused by exhaust gas.
(a) External wastage: (1) Wastage of the shell plating around the mountings especially on the top of the boiler caused by water leakage falling and collecting around the mounting. The space around the mounting is made bare by the formation of a well by the thick boiler insulation and the steel cladding to protect the insulation. The leakage is due to loose glands or a leaking joint and the leaked water collects in the well causing air corrosion.
Repair :- The wasted shell plating around the mounting is suitably built up by welding using a compatible electrode, as recommended by the class surveyor. The welding has to be done by a class approved welder with the recommended preheat, and ground smooth. The corrective action suggested should be done when the residual thickness is not less than 75% of original thickness.
(2)Leakage from manhole door:- Leakage from manhole door on a continuous basis causes wastage of the plate flanging, which results in increased clearance between flange and sphigot resulting in the joint getting squeezed out and leakage increasing
Recommended repair:- The wasted portion of the flange is to be built up by welding and done by a class approved welder using compatible electrodes and under recommendation of class surveyor.
(3)Wastage of support stools.:- Water collecting in the coaming around the boiler and stagnating causes air corrosion of the boiler stool. This collection occurs when the funnel scupper is choked and the scupper overflows into the coaming space around the boiler, stagnating and causing corrosion.
Recommended repair:- The choked drain should be cleared and wasted portion of the stool should be suitably cropped and renewed. Since this location is beyond the pressure vessel there is no need for a class approved welder to carry out this repair.
(b)Internal wastage :- (4) Wasted and necked stays:- Necked and wasted stays which have wasted more than 25% of its cross section area are to be renewed completely and attaché in the same manner as they were originally attached, which means if they were screwed in and fitted with check nuts , the same method of fixture should be adopted for the renewed stays. If they are welded to the plating, the renewed stays should be similarly welded.
(5)Holed and plugged tubes:- These should be renewed. In low pressure boilers the tubes are placed in position and expanded using a roller set. The material of the tube is mild steel seam welded with a carbon content of not more than 1% , and a ductility corresponding to 23 % to 26 % elongation during a tensile test. Each tube before fitting and expanding in position is to be subjected to a collapsing test. This test is done on the work spot before fitting the tube as follows:
The tube is placed in position , its length adjusted and the extra length about 20 to 30mm long is cut out. This piece is then collapsed in a vice till its diameter is reduced to half and becomes elliptical in shape .The elongated end surfaces are examined for crack formations.. If cracked , the tube should be rejected and another tube tried out.
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