In early October DNV released a technical newsletter focusing on the immediate and consequential impact of oil contamination on auxiliary boilers. This kind of leakage in coil flanges or broken gaskets in plate heat exchangers frequently occurs. If the problem is not observed in time, the boiler could be completely destroyed due to overheating of the furnace because of reduced water flow and minimised heat transfer/cooling of the boiler tubes.
Notwithstanding the protective devices fitted on steam plants exposed to the risk of oil contamination, it is still not rare to experience various degrees of contamination of boilers and associated systems. Cracks and an associated loss of integrity at high heat transfer areas are also encountered in some cases. Most of them result in expensive and time-consuming repairs involving the replacement of pressure parts, chemical-mechanical cleaning and downtime.
The most common sources of oil contamination observed on boilers originate from leaking heating coils fitted in fuel oil tanks, fuel/lub oil heaters, cylinder lub oil from reciprocating steam engines for pumps and heating coils in double bottom tanks dedicated for sludge/waste oil tanks. It is also not rare for cargo tank heating coils and tank cleaning heaters fitted on the cargo side to contribute to the contamination in some cases. However, main propulsion boiler plants with a segregated saturated steam system as the main source of heating medium are least likely to be contaminated by oil.
The immediate effects of oil contamination range from foaming and carry over in oil-fired boilers due to increased tension at the water surface to the malfunction of boiler water level controls and even protective shutdown devices. In the worst cases, the carry-over of water and moisture with the steam may even reach the intensity of priming, causing havoc to consumers, eg: turbines, super heaters, steam piping and associated gaskets. Severe oil contamination leads to a collapse of the heat transfer rate through the boiler steel, contributing to a higher metal temperature than the design value.
Even an oil film or deposit as thin as 0.5 mm on the water side can easily increase the metal temperature on the furnace side from the design value of 250 deg C to well above 600 deg C under normal operating conditions on an auxiliary boiler rated at 7 bar. This has a domino effect of exponentially reducing the yield strength of the material until the pressure parts subjected to active heat transfer fail. In cases where the reduction in the strength does not lead to immediate failure, the boiler steel can still be subjected to a time-dependent creep zone that is hard to evaluate unless catered to at the design stage by alloying.
In the case of exhaust gas water tube boilers with an extended surface area that forms part of the steam generation system by forced circulation, this may in the worst cases lead to soot fires due to a lack of heat transfer from the gas side and rise in the metal temperatures due to the uncooled boundaries. Smoke tube exhaust gas boilers are prone to cracks on the tube terminations, due to differential expansion of the overheated tubes with respect to the shell. Some of the problems encountered due to boiler oil contamination are shown in the following images.
One of the first preventive actions to be taken when oil leakage into the feedwater system is suspected is to take a look in the cascade or hotwell tanks. If oil is found in the last compartment just before the water enters the feedwater piping, it is recommended to check whether a dark oily film contaminates the boiler water level glasses inside. If so, the boiler should not be blown down from the bottom, only surface blow for several times. If the boiler is bottom blown, it will become totally covered in oil. The boiler should be shut down and released from pressure and the venting valve opened. Afterwards, the boiler can be drained slowly until water stops flowing from the loosened, upper manhole door before opening up the manhole for inspection. The oil will now only cover the boiler in the normal water level range and can be manually removed. Hot or cold water high-pressure jet equipment together with oil dispersive additives would be efficient for removal of the oil. In case the boiler is completely oily inside, the cleaning work could be turned over to a cleaning company specialised in such work.
Furthermore a careful examination and hydrotest of all suspected heating coils, heaters or whatever is leaking in the steam/condensate system should be conducted. Even when one leakage is found, there could be several more remaining. When it is assured that no more leakage can be found, the piping system, coils and heaters have to be cleaned of all remaining oil. The oil-fired boiler must not be started up again until an oil-free feedwater supply is guaranteed.
Oil contamination on boilers should lead to the rectification of the heat-transfer surfaces on the water and steam side prior to the boiler being put back into service. Boiling out the water side of the boiler using recommended chemicals and/or mechanical cleaning are normal procedures undertaken to facilitate satisfactory cleaning. This may be additionally supported by hardness checks and a hydrostatic pressure test at 1.5 times the design working pressure to ensure the expected safety factor at the design temperature. In view of the oil deposits on the water side, it is also imperative that the impulse piping to the level transmitters is blown through and safety functions verified for satisfactory operation.
In exceptional cases and only in order to support main functions, the short-term use of oil contaminated boilers may be allowed and only after careful assessment of the heat-transfer surfaces on the water and steam side and satisfactory isolation of the source of the contamination. In order to balance the use of an oil contaminated boiler as precautionary measure the firing rate/heat input in conjunction with the design working pressure could be reduced. A reduction in the design pressure is deemed necessary when the stress levels are expected to exceed the acceptable limits at the design pressure and temperature conditions with the required safety factor. Some of the common reasons range from local/general corrosion, the adverse impact of contaminants on the boiler pressure parts and conditional temporary repairs. However, the reduction in the design pressure has an impact on the specific volume of steam and subsequently the relieving capacity of the safety valves, the attachment to the boiler shell and the waste steam piping.
The derating of design pressure must always be compensated by a corresponding reduction in the firing rate to limit the evaporation rates for the satisfactory operation of the safety valves at an increased specific volume of steam. This can be verified by either design calculations or an accumulation test at the revised evaporation rate and derated pressure testing.
In all cases consultation with the manufacturer should also be acquired in order to ensure proper repair of the boiler system.