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Impacts of HFO Sulphur Content on the Engine Life

Saqaff Ahmed AlkaffMechanical Engineering Department

Faculty of Engineering; University of Aden; YEMEN

E-mail saqaff@yemen.net.ye

1 .IntroductionThe heavy fuel oil (HFO) is a mixture of hydrocarbons composed of residual fractions from crude oil distillation and processing. It is characterized by its black colour, high specific gravity (0.92 to 0.98) and high viscosity. HFO is composed mostly of carbon (86% wt.), hydrogen (11% wt.) and sulphur (currently averaging around 3% wt.). It also contains other impurities such as ash, metals and water.

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CombustionIt is a process of rapid oxidation or burning of a fuel with simultaneous evolution of heat. The process is a chemical combination of the fuel with atmospheric oxygen to produce as the principal products carbon dioxide, carbon monoxide and water vapor, together with products such as sulphur dioxide that may be generated by the minor constituents of the fuel.

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Products of Combustion Due sulphur Content

Efforts to reduce the pollutants released by combustion, since these products contribute greatly to the engine life and the environmental problems.

Burning heavy fuel oil releases to the environment, gaseous oxides of sulphur, nitrogen and some of the metals present in the fuel.

The review indicated that in oil-fired systems, most of the sulphur in the fuel could be expected to appear in the stack gas as SO2 with some fuel

sulphur that may be oxidized to SO3 depending on the firing conditions.

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Theme of the Paper

This paper discusses one of the most recent operational problems encountered at the Al-Rayan Power Plant; Al-Mukulla, Yemen. Heavy fuel oil (HFO) is used to run the engines. After 17,000 run hours, a serious leak was observed on one of the engines cylinder heads. Thereafter, all the other engines show the same wear pattern.

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AL-Rayan Power Plant, Al-Mukalla, Yemen

HFO 50 MW Power Plant 5 Engines of 18 V cylinders.

E 49.3 N 14.6

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Statement of the problem The cylinder head material under and

behind the valve seat was found heavily corroded with a maximum depth of 30 mm.

The corroded area was not completely circular, but is at its maximum at the side of the injector hole.

At one point the corrosion caused a hole to the cylinder head wall into the cooling water space.

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Position of the hole seen on the valve seat pocket

Deterioration of the cylinder head valve seat pocket of the exhaust valve was found due to the reaction of sulphur with the water vapour, forming sulphuric acid as a corrosive agent.

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Proceeding of the Corrosion 45 Cylinder heads experience a similar pattern of

failure. The corroded parts of the cylinder head grow and

widen until reaches the O-ring of the valve seat. As the exhaust valve opened (during the exhaust

stroke), the coolant rushes through the hole and vaporised due to the high temperature in the combustion space. This forming a high pressure gases, a mixture of the vaporised coolant and the exhaust gases. This mixture found its way through the deteriorated O-ring of the valve seat and weaken points of the packing, causing the leak on the cylinder head

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Impacts of HFO Sulphur Content in Engine Operation and Life

When the fuel oil is combusted, the principal products are CO2, CO, and H2O, together with products such as SO2 and sulphate particles SO4, due to the minor constituents of Sulphur in the fuel. Presence of these compounds is a primary cause of acidic evidence.In general there are two major effects on the engine, due to the sulphur content in the engine fuel. 1. Contamination of the engine oil2. The Cylinder Head Corrosion

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1. Contamination of the engine oil Contamination of combustion gases into the

engine oil sump due to the blow-by might increase the acidic action in the engine oil.

It is of importance to have a control of the oil quality to suit such HFO operated engines.

The lubricant oil used should be of a proper Total Base Number (TBN), where an additive (basic solution) added to the lubricating oil to neutralize acid products formed due to combination of combustion gases with engine oil.

The selection of the engine Oil TBN for an engine using HFO actually depends on sulphur content of the particular fuel according to the following empirical formula:

Where x is the % of sulphur content in fuel and f(x) is the value of TBN in fresh oil.

711xf(x)

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Engine Oil Total Base Number (TBN) The additive solution TBN is to neutralize the

acidic effect in the engine oil. The oil companies provide different qualities of

oil which contain 30, 40, etc..... TBN to cover heavy fuel sulphur content ranges 2%, 3% etc..... respectively.

The lubricating oil TBN was chosen based on the fuel test analysis made by fuel supplier (Aden Refinery Company).

In Al Rayan Power Plant the sulphur content in the HFO, was 3% by weight (as reported by the fuel supplier test). Hence the lubricating oil type of 40 TBN was then selected.

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Recommended Total Base Number The engine oil TBN, should not be allowed to go less

than 20 (by Engine Manufacturer) of the used oil. In engines, where the oil consumption is high the

TBN is to be improved by adding new oil frequently to assure that TBN of the oil is more than 20. The oil resumes this value for longer running hours of engine operating life.

Nowadays modern engines are equipped with anti polishing rings into the cylinder liner. The engines in Al- Rayan Power Plant are of this kind. The oil changes are usually made in 3000 running hour’s interval to get rid of acid compounds in oil.

Therefore, contamination of the engine oil for such engines due to sulphur content in the fuel could be controlled

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2 .The Cylinder Head Corrosion

The review indicated that in oil-fired systems, most of the sulphur in the fuel could be expected to appear in the exhaust gas.

It was believed that, flow of the entrained exhaust with sulphur compounds, might lead to their precipitation on the engine combustion chamber, especially in the area near the exhaust valve.

At engine part load, the activated high temperature sulphur combines with water to form sulphuric acid according to the following chemical reactions:

2 2 23 2 2S H O H S SO

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The sulphur dioxide dissolves in the water to produce H2SO3 according to the equation:

Where then H2SO3 oxidize to give sulphuric acid H2SO4

The sulphuric acid H2SO4 is a corrosive liquid agent. In such favourable condition the acid attacks the surrounding cast iron material of the cylinder head to form FeSO4 salt and rust Fe2O3. H2O is formed.

The corrosive parts of the cylinder head grow and widen until reaches the O-ring of the valve seat. The hole formed due to the severe corrosion of the material in the valve seat pocket.

2 2 2 3SO H O H SO

2 3 2 2 422 SO O H SOH

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Repairing Method of the Defective Engine Cylinder Head

Replacing of the cylinder head is the simplest option to make the engine available for work. However, the high expenses of installing a new cylinder head, directed the fixation of the problem towards repairing the defective cylinder head.

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The defected engine cylinder head was opened. To trace the problem, the exhaust valve seat should be removed. The cylinder head was mounted on the stand to remove the valve seat.

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Removal of the valve seat

Welding of an old valve to the valve seat, while hammering valve from the backside, this will push out the valve seat.

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Once the valve seat is removed, the corroded area is accessible. The area is to be made very clean by removing all rust and free metal pieces by a proper polishing and grinding of the corroded area. For the welding, a nickel iron electrodes as used to fill the cavity.

the valve seat pocket after welding and filling the cavity. Then the pocket area is probably further smoothened. When sufficient material is applied, the pocket is machined to the valve seat dimensions.

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The corroded area is accessible and is to be made very clean by removing all rust before welding.

The defective part of valve seat pocket after welding

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The valve seat fitting process, and the tools used for this purpose

The fitting of the valve seat should be done with sufficient care. Heating the region around the pocket is to be done, while the new valve seat is to be immersed in liquefied nitrogen to facilitate the fitting process

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The cylinder head after repairing and fitting of the new valve seat in the pocket.

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Tracing and identifying the impacts of HFO sulphur content on the engine.

Justifying the oil TBN, to neutralize the effect of sulphur content in the engine oil, could control the oil contamination.

The causes and formation of corrosion on the cylinder head were specified. The HFO sulphur content is the major cause of severe corrosion.

Repairing of the cylinder head was one of the most likely less cost option to make the engine available.

Conclusions

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Conclusions• 45 cylinder heads was repaired by the

above method. However after using all the repaired cylinder heads, only 7 failed.

• Elimination of the corrosion, could be possible by Improving the fuel quality or fuel replacement, which is the most favourable Solution. However this solution should be studied thoroughly, due to the cost of the LFO and other national constraints.

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Recommendations Cooling water treatment has to be improved.

Dosing and testing interval must be followed, to assure the coolant quality.

In the case of limited corrosion detected at the O-ring landing at the valve seat pocket. Repair must be carried out, with the use of an oversized valve seat.

Reduction of the engine thermal load, should be done by cleaning, repairing and proper scheduled maintenance programs.

Updating knowledge and skills in welding new techniques for the maintenance team, towards reduction of the maintenance cost.

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Providing well planned and oriented training for the maintenance team and building the attitude of taking their responsibilities in tackling the problems efficiently.

Providing the required facilities for inspections and improving the limited experience in weld inspection.

The repairing method prove its success, avoiding the costly replacement of the defected cylinder heads, a weld approval is required to assure its quality.

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