Machinery Matters: Filters are more than just ‘a sock in a tin’

Dec 8, 2020 by Larry Rumbol

“Filters are just a sock in a tin.” That was a scornful remark made by an engineer who had suffered the ignominy of a dress-down by the filter company representative after he had foolishly claimed their filters were good for nothing as they were always blocking. He could get the same filter for a fraction of the price, he said, and it did not block. “After all, it’s just a sock in a tin.”

Luckily for him, the representative had the time and wit to re-educate this fellow and take him on as a subject for conversion rather than as an adversary. Needless to say, he became a disciple and covert salesman. I doubt he made friends with the discount store of marine parts but he certainly did to those who would listen. Just in case you are not with me here: A filter (of the correct size) is meant to block what is coming through if it is too big and needs to be blocked.

The origin of the word filter is from the Old French felt, the soft material. Add in a little Medieval Latin – filtrum, “to strain through a felt cloth” – and there we are.

So why filter? Why indeed. Water, the most basic need of man, started the process. Nobody likes to drink muddy water, or water with anything in it, so the Greeks started filtering water in 2000BC. Around 500BC, the Greek scientist Hippocrates invented the Hippocratic Sleeve, a device for domestic use where boiled water (smart fellow) was poured through a cloth bag, which removed impurities. The Egyptians in 400AD were also known to boil water by using the sun and red-hot iron ingots immersed in the water, and then sifting this through sand and gravel.

This was remarkably effective (especially the boiling) and little changed until 1627 when Sir Robert Bacon advanced science significantly with his work extracting salt from seawater. 

By now the microscope was in relatively common use and the various microorganisms in water were seen for the first time, spurring scientists to refine the “filter” substance ever more. Charcoal, natural sponges and wool began to be used in sequence with a realization that each could remove different-sized particles. 

Move on to 1854, again with little change but incremental improvements until British scientist John Snow discovered that Cholera was water-borne and that it could be purified with the use of chlorine. This discovery and advances in filtration methods meant that by the 20th century, most developed countries had set minimum standards for water quality, and the filter industry was truly born.

The filter’s migration into other liquids beyond the use of sieves and strainers was an obvious and rapidly spawning offshoot to where we are today. The concept is as old as it ever was and the improvements are all about the internal media, the process and the application.

The advent and refinement of the internal combustion engine and subsequently hydraulic engineering quickly became so much more than a niche market for filtration as the benefits were self-evident, reduced failure and maintenance particularly. Unlike its predecessor the steam engine, the internal combustion engine had a sump full of oil that had to be re-circulated and provide the primary lubrication throughout the engine. Steam arguably started the industrial revolution, and the growth of the internal combustion engine mechanized the world rapidly migrating from land-based vehicles to smaller boats, then eventually usurping the steam engine as reliable motive power. That reliability stemmed from proper lubrication, as it does today, and filtration is still a vital element in that process.

In the world of superyachts, filtration plays a major role in the cleanliness, viscosity and general health of lubricants, hydraulic fluid, fuel and water, daily removing contaminants and unwanted substances without constant human intervention.

Filtering lubricating oil

The journey from water to oil filtration most likely dallied with wine and undoubtedly olive oil until the dawn of mechanization and mobilization with the advent of the combustion engine. The combustion engine oil filter as we know it was invented in 1923 by Ernest Sweetland and George Greenhalgh, and they were granted a patent in 1929. They called it the Purolator, a play on “Pure Oil Later”. 

Their system was a lot better than just a strainer but still poor by today’s standards. It actually filtered the oil after the pump and before it went into the pressure-fed bearings, but only filtering a proportion of the oil as it was, effectively, a bypass filter. Frequent oil changes were required at that time, creating an automotive “lube job” service industry that lasted well into the 1970s. However, it was effective in securing an acceptable level of reliability by protecting the integrity of the lubricant.

Automotive technology quickly permeated into the marine engine world and oil filtration was very much part of that, especially as both marine and locomotive engines became very large indeed by the 1940s and the masses involved were considerable. So much so that a breakdown in lubrication generally resulted in total and catastrophic failure. This is why classification societies and insurance companies are adamant about oil analysis and condition monitoring.

Every engine, be it diesel or gasoline, has set clearances between vital mating and rotating parts that an oil film preserves. Therefore, any foreign particle that is greater in size than that clearance will exceed that “gap” and cause damage as it contacts both previously separated surfaces, leaving scars as an aftermath. Should metal-to-metal contact occur, failure follows close behind. With correct filtration, that should not occur. How is that achieved?

In the beginning, engines only had a sieve strategically placed to catch these larger foreign substances. To some extent, that still exists as we have strainers today that perform a similar primary function in the lubrication sequence. However, today they are followed very closely by the filter, which is in the full flow of the oil “after and to” the oil pump. Filtration “to the pump” is obvious, as contaminants will be removed before damage occurs both to the engine and, of course, to the pump itself. 

Yet filtration “after the pump” may seem obscure. Imagine the oil was very dirty and full of contaminates. In a worst-case scenario, the filter would be overcome and reach capacity (unless it has a back-flush/cleaning facility). The filter is full and, as this is in the flow of oil to the engine, it would reduce the oil pressure to the engine, which could have disastrous results. To prevent this, a pressure bypass valve is installed to divert the oil around the filter at a set pressure to avoid this pressure loss. 

At this point, a secondary filter can be employed on the other side of the pump. Equally, the filter can be in tandem on the suction (to) side of the pump so that as one fills with debris and is taken offline, the other fulfils the same role without interruption. This is a typical application, and there are adaptations and hybrids of this, which are equally as effective.

Filter construction

Early filter designs used steel wool, wire meshes of different sizes and later cotton and various woven fabrics. When the idea of disposable filters became popular, cellulose and specially produced papers were employed to reduce production costs. Eventually man-made fibers were used, including fiberglass in certain applications.

Many superyacht engines today will employ spin-on type disposable filter canisters with removable elements and these will likely be a cellulose derivative with some more elaborate filters using microglass or extremely fine metal mesh. Choosing the right filter is vital to ensuring correct particle separation (thereby avoiding clogging) yet retaining the smallest particulates. By limiting a clogging concentration of solids, oil consistency is retained and changes in viscosity and oil efficacy are avoided.

There are, of course, alternative manufacturers to OEM-supplied filters. It need not be said that some trade on a low price point. I use one of my favorite maxims here: Would you buy a cut-price parachute? The consequences of saving $30 could well be a failure costing 5,000 times that. 

That said, few, if any, OEMs actually manufacture filters so alternatives are available from reputable manufacturers. If I were to go that route, I would sacrifice one of each, cut them open and scrutinize the contents carefully.

Filter media can be made of cellulose or fibrous materials, or synthetic materials designed especially for this purpose. Media in an engine’s primary filters will trap particles as small as 25-30 microns (µ). Secondary filters can do better – down to 5 or 10 microns – however this also adds restriction to the flow, which is an important consideration in engine design as oil not only lubricates but removes heat. A reduction in flow can result in an increase in oil/engine temperature.

A 56 micron-sized particle from a filter magnified 100,000 times on a Scanning Electron Microscope (SEM). Photo courtesy of Spectro / Jet-Care

A typical low-cost cellulose media oil filter will remove about 40% of particles in the 8-10 micron range. A typical medium-priced synthetic media oil filter will remove about 50% of particles in the 20-40 micron range but only 24% in the 8-10 micron range.

Keep in mind that fine sand is typically 90 microns, human hair is about 70 microns in diameter.

Secondary filtration

In addition to canister type and spin-on filters, there is another type of secondary filter commonly seen on superyacht engines and that is the bypass centrifugal filter, originally developed by the Glacier bearing company (founded in 1899 in the UK) as a way to protect their bearings from the bad press of failure as it was never the actual failure of the bearing but the result of poor quality and dirty lubricant. There are a number of manufacturers today supplying these as a retrofit, and many engines have them now as original equipment.

They work as a secondary filter employing centrifugal force (from oil pressure) rather than gravity or a filter media to separate contaminants in the oil. Pressurized oil enters the center of the housing and passes into a drum-type rotor free to spin on a bearing and seal. The rotor has two jet nozzles arranged to direct a stream of oil at the inner housing to rotate the drum. The oil then slides to the bottom of the housing wall, leaving particulate oil contaminants stuck to a removable thick paper on the housing walls. The housing must periodically be cleaned by removing this paper and replacing it, or the particles will accumulate to such a thickness as to stop the drum rotating. In this condition, unfiltered oil will be recirculated.

Advantages of the centrifuge are: 1. that the cleaned oil may separate from any water that, being heavier than oil, settles at the bottom and can be drained off (provided any water has not emulsified the oil); and 2. they are much less likely to become blocked than a conventional filter.

Without filters, oil would eventually become saturated with contaminants with the resultant loss of lubricity and the consequence of accelerated wear and ultimate failure of the parts the oil was meant to protect. In an apocalyptic world, oil could, with huge effort and expense, be cleaned and returned to use. Filtration allows an acceptable use period without intervention. On very large commercial vessels, the sump oil remains throughout the life of vessel and is slowly consumed and replenished. These large engines employ a complex separate purification process using a stand-alone centrifugal separator plant similar to that on some larger superyachts. Very necessary as a large container vessel main engine would have a lube oil capacity of 46,000 liters (12,152 U.S. gallons) or 40 tons.

In a superyacht application with far smaller sump capacities than this, oil life and condition as well as filter life cycles are vital elements in maintaining the highest level of engine health.

Hydraulic filtration

Equally important and perhaps twice as critical is the filtration of hydraulic fluid, which is far less tolerant of contamination and is not designed to carry contamination in suspension like lubricating oil. Consequently, hydraulic fluid filtration and the utmost cleanliness is absolutely essential as there is a very narrow margin where hydraulic fluid fails to perform when not in optimum condition. A thorough understanding of hydraulic cleanliness standards and filter ratings is essential.

A filter’s rating is a measure of its efficacy. A common rating and one we use in our hydraulic fluid laboratory analysis reporting is ISO 4406. This is a standard measure of the contaminant level found in a given filtration system and a method of reporting same as part of the laboratory analysis, bearing in mind that ultimate cleanliness with little margin for contamination is the goal.

ISO 4406 uses cleanliness codes and these are composed of two or three numbers, each of which represents a logarithmic measure of the number of contaminants present at three set size ranges (4µ,6µ,14µ[c]) in 1 ml of fluid. For example, a new sample of oil might measure 18/15. This corresponds to 1,300-2,500 particles of 4 microns, and 160-320 particles greater than 14 microns. A filter with a lower ISO cleanliness rating will therefore more thoroughly remove contaminants and can help to prolong the life of the hydraulic system’s components by two to three times.

Another measure of filter efficiency is the filter’s Beta ratio, which is the ratio of the number of contaminant particles upstream of the filter divided by the number downstream. The Beta ratio can be subtracted by 1, divided by the Beta ratio and multiplied by 100 to obtain the filter’s percent efficiency for a given contaminant size. When choosing a hydraulic filter, a greater Beta ratio is desirable.

But as always, the caveat is to check with the engine or equipment manufacturer. As with so much in a marine environment, compromises have to be made. An increase in filter efficacy might seem desirable yet result in a loss of pressure and poorer performance or no performance at all.

Filtration is most definitely not just a sock in a tin.

Larry D. Rumbol has 40 years of expertise in marine condition monitoring and is marine business development manager with Spectro | Jet-Care in the United Kingdom, United States and Switzerland. Comments are welcome below.