Cylinder Wear and Blow-By After Long Service
A double-acting engine has run 12 years on saturated steam at 7 bar. Recently the exhaust has become smoky, crankcase oil consumption has risen 30 percent, and the engine will not hold full load at rated speed. Compression tests by barring the engine show poor compression on both ends of the cylinder. The lubricator is delivering oil at the correct rate, and the oil is the specified compounded steam-cylinder oil. The piston rod packing is new. The operator proposes increasing lubricator feed and retarding the valve timing to compensate.
1. Diagnose the likely condition and explain why the proposed remedies will not work.
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The combination of smoky exhaust, rising oil consumption, loss of full-load capability, and poor compression on both ends points to piston ring and cylinder bore wear - blow-by past the rings. Saturated steam at 7 bar carries moisture, which washes oil from the bore and accelerates wear, and 12 years is a plausible life for rings and bore.
New piston rod packing rules out packing as the source of oil loss and blow-by.
Why the remedies fail:
- More lubricator feed cannot restore compression. Excess oil will burn and increase smoke, and may deposit in the exhaust. It treats a symptom, not the worn sealing surfaces.
- Retarding valve timing changes the point of admission and cut-off, but it cannot compensate for gas escaping past the rings. It will reduce power further and may cause other problems such as increased compression at the wrong point.
The real fix is mechanical: inspect and replace rings, measure bore wear and ovality, and re-hone or rebore as needed.
2. Outline the inspection and repair plan, including measurements and the decision points for ring replacement versus reboring.
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Inspection:
- Remove the cylinder head and piston; measure bore diameter at top, middle, and bottom, in two planes at 90 degrees, to find wear and ovality.
- Measure ring end gaps in the bore and ring side clearance in the grooves.
- Check piston ring grooves for wear and the piston for cracks.
- Inspect the bore for scoring, pitting, and ridges at the ends of ring travel.
- Check the valve faces and seats while the head is off.
Decision points:
- If bore wear and ovality are within the manufacturer's limit and the surface is sound, hone and fit new rings.
- If wear or ovality exceeds the limit, or scoring is deep, rebore or sleeve to the next oversize and fit matching rings.
- If ring grooves are worn beyond limit, replace the piston or re-groove.
After repair: bed in rings per the manual, run at reduced load initially, monitor exhaust smoke, oil consumption, and compression, and confirm full load is restored before returning to normal service. Also review steam quality and trapping to reduce future moisture carryover.