Friday, December 17, 2010

Waste Heat Boiler Problems

Problems in Exhaust Gas can be classified under categories:

a.      Low temperature cold end corrosion
b.      Fouling of the gas and the water side
c.       Tube failure due to vibration

Gas Side Fouling

Could be kept under control by wider tube pitching or in-line fitting of fins on to the tubes and use of regular soot-blowing at individual tube banks. Good quality combustion goes a long way in reducing the amount of soot deposits; although the quality of fuel is constantly deteriorating, proper centrifuging and treatment would be helpful in reducing the harmful combustion products promoting fouling. Many shipping companies use some fuel treatment chemicals to keep the harmful contaminants under control. Tube externals would still get fouled and periodic cleaning by water washing is the most effective way of keeping the tube externals clean. Cleaning is either done through fixed nozzles inside the boiler banks or done through spray nozzles and connecting pipes which are moved around as required. Water at about 60° C should be used for cleaning and precautions should be taken to ensure that the drained water with high acid content do not flow into the main engine exhaust duct.

Water-Side Scaling and Corrosion

Is mainly attributed to the poor quality of feed water used. If the water used is alkaline with low T.D.S. and has less contamination from oxygen, tubular boiler tubes should run for long time without giving any problem. But, the water used is normally the water from the open feed system hot well and scaling/corrosion problem would be impossible to overcome completely. Certain improvements and reasonable running period could however be achieved by keeping the T.D.S. content and chloride level low by regular blow down and chemical treatment and dosing of appropriate chemicals (e.g., NaOH) would maintain the water in an alkaline condition; oxygen level could be kept within acceptable margin by chemical treatment (hydrazine) and keeping the hot well temperature high. Certain operational precautions like venting while starting, draining the boiler or keeping it full up, as shutting down would help in reducing corrosion. Fouled water side could only be effectively cleaned by chemical cleaning.

Tube problem from vibration is sometimes a problem. Modern boilers are made considerably large that takes up a sizable part of the uptake. The supporting box-type casing which built on a system of beams are made fairly rigid. If the arrangement is too rigid, problem from fatigue failure is again possible. Heavily stiffened support steelwork and casings do reduce the effect of pulsating gas stream. There is no easy solution to a vibration problem if it starts, but this is a problem better considered at the design stage.


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