Piston rod sealing systems to combat high-BN cylinder oil side effects

Marine Sealing Services (MarSeal) developed its RPDS alpha piston rod seal based upon failure analysis over many years looking at two stroke crosshead diesel engine piston rod seals, reciprocating applications and experience of rotary seal failure analysis. The company has built up a specialisation in fluid sealing technology, primarily in the marine industry.

Similar problems to those encountered at sea have been experienced in land-based power generation plants, despite their much more stable platform than marine applications. Having analysed many failures over the years it became obvious to MarSeal that the construction material of seal/scraper systems plays an important role in the final barrier between the combustion side and the crankcase side of large two-stroke engines. The method of compression to the piston rod would be important, as in the past the materials caused wear to piston rods, which prompted OEM decisions to use only hardened piston rods.

If man-made fibres were to be used for seals in such applications, the material should be able to function at up to 600°C and be chemically inert to counteract any acid formations in the under piston area caused by low temperatures, as experienced during part-load, such as during slow steaming, and exacerbated by burning high sulphur content fuel oil. To be resistant to all of the expected conditions in the under piston area and in the well area of the stuffing box housing, pure graphite was employed, capable of transferring a micro deposition of graphite to the piston rod and scraper/seal lips. The innovation is that MarSeal’s patented design uses standard marine materials like phosphor bronze and cast steel/iron, but incorporates a means of sealing and scraping off oil from the piston rod. The construction allows the piston rod to become lubricated with a micro-coating of graphite which helps prevent attachment of under piston detritus and creates a lubricated surface for the seal rings and the scraper rings. This micro coating helps to prevent temperature increase at the point of contact as the piston rod travels through the seal and scraper system.

The major oil companies’ more recent research has concentrated on developing the best possible cylinder oil for today’s operating conditions and the uncertain fuel oil sulphur content possible in bunkers worldwide. The result has been a range of cylinder lubricants with high base numbers to counter the acidity caused by comparatively cold temperatures and high sulphur fuels.

A point to remember with these higher BN cylinder oils is this: should high BN oils mix with the lower BN system oil, lubrication of the whole engine can be compromised, with expensive results for ship owners and fleet managers. Even changing to a zero-sulphur fuel like gas may not make a lot of difference as the engine operating speed is the determining factor in damage to cylinder liners.

As regulations increasingly put the emphasis on low sulphur content in fuel, it is questionable whether or not bunker stations will be able to supply the specified fuel oil in sufficient quantities, which will push prices way beyond what is currently accepted as costly.

As use of low sulphur fuel increases, there may be further problems associated with increasing viscosity and BN to counter lubrication problems with slow steaming, high sulphur and retention of cylinder oils in the ring sets envisaged by using SAE 40/50 viscosity oils with BN of up to100.

Regardless of all the new requirements for meeting IMO standards and the ability to comply with ECA requirements there is still the question of system oil contamination. Even as long ago as 1995, a paper presented by Lloyd’s Register’s N.E. Chell, CEng, FIMarE examined system oil contamination in detail. The problem is still in evidence even after many OEM design improvements and the now-total adoption of hardened piston rods. Many types of seal and scraper materials and designs have been tried, such as filled PTFE and polymer materials which failed to meet expectation. With hardened piston rods there are still leak paths through the stuffing box sets of seal/scraper rings. The conclusion reached at the time was contamination of the crankcase oil is primarily caused by leakage through the stuffing box system with system oil reaching the under piston area and a mix of cylinder oil, fuel oil, water and particulates passing through to the crankcase. A fleet manager recently stated: “Modern two-stroke engines seem to use less make-up oil to the system, however there now seems to be more contamination of the system oil even though all current modifications as required by the OEM are carried out.” The question is, why? The most likely answer is that the under piston sludge is finding its way into the system oil, increasing viscosity and rendering it less effective. Moreover, the cost of SAE 30 system oil has risen over the last 4/5 years.

One problem with a non-OEM system is acceptance of new sealing designs focused on current problems – it is even more important now and in the future to create a total barrier to prevent crankcase oil contamination. As the newer cylinder oils are expected to be in the SAE40/50 with BN of up to 100 then it is essential that cylinder oils and crankcase oils do not coalesce, risking earlier engine malfunction due to increased viscosity. Should the piston rod stuffing box fail, so will the engine.

Current expected system oil usage per cylinder is around 5litre/day, however actual figures are nearer to 12litre/cylinder/day, so a 7-cylinder engine uses 84litre/day at US$1.92 – or US$161.28 every working day. A recent set of readings from a vessel with only 11,000 hours operation showed a daily usage of 140litre/day of system oil in a 12-cylinder engine.

To find how effective any seal/scraper system is a finite oil analysis should be taken prior to installing a new system (this includes OEM modified designs) which gives an oil footprint at the time of change. A further helpful sample is from the under-piston area as an analysis of the actual sludge will confirm the mix and what it includes, generally the mix is crankcase oil, cylinder oil, fuel oil and water, with occasionally catalytic fines plus detritus from combustion and engine wear.

Crankcase oil and cylinder oil are mutually incompatible, so when the two mix engine performance is affected. Asphaltenes will deposit on the hot parts of the engine and specifically in the piston cooling chamber, creating hot spots leading to piston crown failure. Contaminated oils have been found to have high oxidation levels which can lead to sludge deposits in high temperature zones. Increased viscosity can affect oil flow through bearings, resulting in increased temperatures and reduced flow. Less oil flowing around the bearings leads to higher temperature retention and still greater likelihood of oxidation. System oil contamination is likely to increase emissions to the atmosphere – no matter how often the oil is purified, the viscosity will remain at the contaminated level.

The RPDS alpha seal design aims to prevent cylinder oil and crankcase oil from mixing in either the under piston area or the crankcase sump. Dependent upon the engine make and model type the seal scraper ring system varies from either two or three top rings, with the scraper set in the lower box including a wiper ring if required. Compared to conventional systems there are no gaps in the top seal/scraper ring sets, which helps prevent cross contamination. The scraper rings are designed with a 45° scarfe that directs oil flow from the piston rod to the castellated sections of the carrier rings. The top seal rings can have either pure graphite rings held within a specially designed holder ring or they can accommodate a PEEK polymer material if necessary.

The scraper carrier rings incorporate a small oil reservoir behind the scraper ring, which allows oil to flow to the sump without any restriction such as tension springs blocking the flow to return drain holes. The scraper rings are set to constantly deflect the oil scraped off the piston rod through to the underside of the carrier rings. Each ring gap is opposed at 45°, unlike conventional segments which generally are gapped at 5-10 mm and parallel in position. This allows easier flow to the under-piston side and down to the crankcase sump.

By design the Holder rings are also scraper lips so the top ring has five scraper rings, ring 2 has two scraper lips. When using either exfoliated graphite or PEEK polymer there is a micro-transference of graphite to the lips and to the piston rods, helping to lubricate the piston rod and reduce friction. This micro-graphite coating should help prevent adherence of under piston detritus during the scavenge cycle. The uni-holder-scraper-seal arrangement reduces the number of component parts in the restricted space in stuffing box housings.

The cost involved in retro-fitting any two-stroke crosshead diesel engine with the RDPS alpha system is no greater than the cost of OEM parts. So the normal investment in overhauling and replacing the OEM seal/scraper system is returned in less than six months. The RPDS alpha system can save up to 70% of system oil, help maintain a clean crankcase, and help maintain piston crowns operating at the ‘new oil’ state. As engine temperatures will be maintained for a longer operating period, emissions to atmosphere should be kept at design levels. Even at higher scavenge pressures, the system will prevent cylinder oil and crankcase oils from mixing.

As emissions regulations tighten, system oils must be prevented from mixing with new type cylinder oils from SAE40 to SAE55 with BN from 70 up to 100. With this in mind MarSeal is developing its next stage in barrier engineering, the Wrap and Outsert system, to remove the well area from housing entry area.