Blog 138 - Piston Ring Failures, Dew Point, Scavenge Air - A Symbiotic Relationship

Dew Point, Relative Humidity, Main Engine Air Inlet Temperatures, Piston Ring Failures and Excessive Liner Wear -  The  Existence of a  Symbiotic Relationship if Closely Monitored and Controlled

The below narrative / findings have been one of several pet projects of mine from my Second Engineer days. One could call it an extended 34 year (private) study of the whys, whens and wherefores of a singular phenomenon - why do piston rings break - or get stuck - very often on some engines and why do liners wear excessively on some engines? What can I, as Chief Engineer, do to improve conditions, alleviate the problem? Can I get to the root of the problem? Only practical observations are dealt with here.

During the initial part of my career, I had read several articles about what I am about to discuss, but could not find the necessary graphs to interpolate. Must have been sometime around 1987 that I found an old set of graphs, which are given well below. These graphs are relevant even today.

This entire narrative is meant more for the sailing engineer than anybody else.

So started, from 1973, a venture that has taken me into the scavenge spaces countless times of a vast array of Main Engines to inspect pistons, their rings, the deposit behind the rings (to the extent visible), the condition of the rings, the liner lubrication, the percentage of choking of scavenge ports on loop scavenged engines, the checks on fuel injectors to find probable cause etc.

At every decarbonisation, I would find myself observing each detail closely - how much of residue is accumulated on top of the liner (which needs to be cleaned and removed before lifting a piston), what is its composition (roughly, to my limited knowledge), how many rings are stuck or broken, how much of deposit behind the rings in the grooves, how does it compare to what I had seen through a scavenge space inspection, how had the unit performed during operation and a myriad  other observations.

Over a period of years, with a lot of practical observations and studying articles written by experts in the field, I was able to list, in my own mind, some of the factors that caused piston rings to deteriorate - break or get stuck - and liners to wear faster than the norm. Reading several scholarly articles, I could piece together the probable causes of ring breakage and liner wear, but there was no clarity nor definitiveness.

Below, I am taking up one of the factors, an ingredient that has a decisive effect on ring failures and liners’ excessive wear.

One of the very serious repercussions of uncontrolled quality / temperature of air into the Main Engine, is the steady build up of thick, sludge-like compounds behind piston rings, which inhibits the expansion and contraction of the piston rings in the cylinder, causing them to either break or get stuck. Thus, they become ineffective.

One of the keen observations that a Second Engineer should be interested in, is at the time of removal of the piston. As the piston is lifted and the first ring exits the liner proper, the spring action of the ring should be observed closely. If in good condition, the ring will expand fast. If the spring action is sluggish, it will move - spring out - very little. If stuck or broken, there will be no movement. The same observation goes for each succeeding ring.

This will, necessarily, need to turn your focus on to their causes.

Stuck piston rings

Broken or stuck piston rings mean that, as the rings are not performing their jobs, a series of cumulatively deteriorating events take place in the particular cylinder of the Main Engine.

1. The Compression Pressure inside the cylinder drops sharply, as the piston rings are not sealing the combustion spaces well enough, causing the pressure built up during combustion to leak past the rings.

2. ‘Indicator diagrams’, especially out-of-phase “Draw Cards” show the actual compression in each cylinder, the point of injection and the boost that the burning fuel gives to the combustion.

3. As Compression Pressure drops, (due to stuck or broken rings), the ignition temperature of the injected fuel oil may not have been reached. Although the fuel may be nearly atomised and injected, all the micro droplets may not get ignited. Incomplete combustion results. If this condition is allowed to deteriorate, the exhaust gas blackens, combustion pressure fluctuations take place between cylinders, the engine rpm fluctuates and the turbochargers start surging. 

4. (This effect is less evident - or takes a longer time to become evident - in Constant Pressure Exhausting - which is closely associated with ‘Uniflow’ Scavenging. The effect is more quickly discernible with ‘Pulse System of Exhausting’ - which is closely associated with ‘Loop Scavenging’)

5. Another consequence is the leakage of hot exhaust gases past the rings, causing what is termed a ‘blowpast’, causing a scavenge fire. The accumulated oil and sludge in the under piston spaces start burning. If this is not immediately brought under control, the ‘scavenge fire’ can spread to other under piston spaces.

To control this before it spreads, the engine rpm is brought down, the fuel for that particular unit is cut off and the cylinder lubrication is increased (to offset the dryness of the cylinder liner, as the scavenge fire would have burnt the light coating of oil). The dryness of the cylinder at the time of the ‘blowpast’, unless additionally lubricated, can start with ‘micro seizures’ of rings to liners. The worst case scenario is when the piston is unable to move because it has seized.

It is best to rectify this condition before proceeding.

So where does air quality come in to minimise or prevent the above condition of rings from prevailing?

Basics

Uniflow Scavenging

The left and centre images apply for 2 stroke engines. Loop scavenging is shown.

Let us take a quick look at the step by step change in air parameters:

Engine Room air at an average temperature of 45 degrees C. Humidity is dependent on what it is outside the ship on that day, mostly high humidity. Is sucked in by the turbocharger air side compressor.

Engine Room Air – Sucked in by the Turbocharger Air Side Compressor - Average 45 deg C - Mostly High Humidity - Slightly above atmospheric pressure of 1.01325 bar. (Slightly above atmospheric, as the Engine Room Blowers have pressurised the air - to a small extent - that is sucked in by the Turbochargers).

In Turbocharger Air Side Compressor - Pressure increases to around 2.0 to 2.5 bar on average. Modern engines increase it much more, to maybe 3 or 4 bar - Temperature increases to around 140 ~ 150 deg C - velocity has increased tremendously - flows to the Air Cooler.

A New Air Cooler - Air Side is Red Coloured (Fins)

Air Flows Top to Bottom

Flanges on the Left are for Cooling Water (Tubes)

Cooling Water Flows from the lower right SW inlet to the upper left SW outlet in this cooler

Image (forefront) shows the tubes through which cooling water flows

A clear division can be seen between the upper and lower bank of tubes, indicating a separation plate in the cooler cover and the evidence of a two-pass cooler.

A Badly Maintained Air Cooler After a Few Years of Use -

Note Deterioration of Fins due to uncontrolled Water Vapour.

Fins of Aluminium or Copper Finally Turn to Powder at the touch of a finger, if Not Looked After or Maintained

In the Air Cooler - Cooling Water flow through tubes - In old ships sea water is the cooling medium - in most ships built after 1985, fresh water is the cooling medium - maybe a single pass cooler but, mostly, double pass coolers. Some ships have four pass coolers.

Assuming a two pass horizontal, finned cooler, the cooling water enters the bottom half of the air cooler, cools the air surrounding those tubes, reaches the other end of the air cooler, reaches the top half section of the tubes, changes direction and flows backwards to the outlet connection.

After changing direction, the cooling water - which has increased in temperature - flows through the top bank of tubes, which are surrounded by the hot air (140 ~ 150 deg C).

The top bank of the air cooler is cooled by the slightly heated up cooling water, flowing at a lesser velocity and pressure (which is lost in its transit through the bottom half of the cooler, filling the back cover of the cooler and reversing direction to flow through the top tube bank). The lesser velocity of the water also allows more heat to be absorbed from the hot air surrounding the tubes. By the time this hot air reaches mid way of the air cooler, it has dropped from 140 deg C to around 60 deg C.

The bottom bank of the air cooler now contains air that has cooled to around 60 deg C, being cooled by the colder water (assume to be around 28 deg C), which has just entered  the air cooler. If effective cooling has taken place, the outlet temperature of air, on exiting the air cooler and entering the scavenge space will be around 35 deg C, if the Cooling Water valves were unregulated.

This outlet temperature of air (from the cooler) can be controlled to maintain any temperature above 35 deg C, by throttling the Cooling Water outlet valve enough to give the desired temperature.

Water flows through the tubes and absorbs the heat from the air that surrounds the cooling tubes.

The hot and humid air at a (relatively) high pressure passes through the Air Cooler and cools to (anywhere between) about 35 deg C to 55 deg C, depending on the cooling water valve settings that can regulate the temperatures. The pressure drops slightly (because of passing through a rather large sized air cooler. The velocity also drops slightly for the same reason.

Going slightly out of the narrative, but having everything to do with air coolers, is the subject of the manometer fitted on air coolers. It should always be working. In the (engine) stopped condition, the levels should be equal on both sides. The air or gas cock should be full open when running. The difference in levels (h) - at full speed running of the engine - in the manometer will determine the condition or quantity of choking of the air cooler. Good condition Air Coolers - with a Scavenge Pressure of around 2 bar - will have a manometer difference of around 140 mm or as low as 80 mm. The benchmark should be noted when the Air Cooler is fitted after a through chemical cleaning ashore. The higher the turbocharger output, the higher the manometer level difference. 

The lower the manometer difference (h), the cleaner the air cooler.

The higher the manometer level difference (h), the more the chances of the Air Cooler being choked. May require in situ cleaning or change.

Choked air coolers reduces the quantity of air that is sent into the engine. At one stage, turbochargers will start to surge, because of the back pressure on the compressor.

To get back to the main narrative.

The cooled air enters the scavenge space, which is constantly at around 2 bar pressure. Scavenge spaces of Newer types of engines are known to have between 3 and 4 bar.

But the humidity in the air fluctuates wildly throughout if unregulated, being totally dependent on the air temperature achieved by the cooling water valves’ settings.

Something known as “Dew Point” becomes very important from this point on.

Two definitions of “Dew Point”:

“The temperature at which air can hold no more water vapour. Below this temperature the water comes out of the air in the form of drops.”

“The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to achieve a relative humidity (RH) of 100%. At this point the air cannot hold more water in the gas form.”

What does ‘Dew Point’ have to do with scavenge air being sent into the engine?

Importance of Dew Point at Different Scavenge Pressures & The +4 Deg C Method

"Dew Point" of air changes with every bit of change of Scavenge Air Pressure. The higher the Scavenge Air Pressure, the higher the "Dew Point".

The temperature of the outlet air from the the Air Cooler - or the temperature of air at the inlet to the Scavenge Space - can be controlled to maintain any temperature above 35 deg C, by throttling the outlet valve enough to give the desired temperature.

(In colder climates, far lesser temperatures are achieved.)

Water flows through the tubes and absorbs the heat from the air that surrounds the cooling tubes.

If the temperature of air is below the ‘dew point’, there is a high possibility of a heavy concentration of water droplets, in the form of anything between micro droplets to larger sized droplets, being carried over into the Main Engine.

This air - whatever be its temperature - enters each cylinder of the engine (depending on its timing cycle) through the scavenge ports and performs two functions. It drives away the exhaust gases of the previous stroke (let’s call it ‘the exhaust period’) and - as per the timing - fills the cylinder with clean, fresh, pressurised air (let’s call it ‘the scavenging period’).

With the piston moving up, the scavenge ports close and the air inside gets compressed fast. Temperatures rise quickly to 350 to 400 deg C.

Fuel is injected, combustion takes place.

Meanwhile, the engine’s cylinder liner’s outer surface - which is in contact with the fresh water being circulated - enters the jacket spaces at around 65 deg C and exits the liner at temperatures between 80 deg C to 90 deg C, because of the heat being carried away by the flowing cooling fresh water. This CW temperature should not be allowed to go beyond 90 deg C, as it is possible for this water to turn to steam and ‘air’ lock the flow of CW.

The engine’s cylinder liner inner surface can be anywhere between 300> deg at the top and around 100> deg C near the scavenge ports.

Cylinder lubrication is in full flow, the entire liner is coated again and again with cylinder oil, the motion of the piston and the piston rings spreading the oil across the surface of the liner and scraping it downwards. Cylinder lubrication is supposed to be kept at the levels suggested by the manufacturer - normally between 0.8 to 0.9 grams / bhp hr. But most Chief Engineers keep it slightly higher.

Over lubrication, in combination with moisture in the air, increases the chances of piston ring failures.

Over lubrication

Under lubrication first causes micro seizures, definitely leading to a cracked liner, cracked piston or both or a piston seizure - maybe even a ‘twisted’ crank shaft.

Thankfully, nowadays, modern day cylinder lubrication systems - like ‘Alpha Lubricators’ - precalculate the quantity of cylinder oil to be delivered at each stroke using mocroprocessors, taking the responsibility out of the hands of the Chief Engineer. But, they can be tampered with. Cylinder oil leaks on the line may also deteriorate the lubrication process.

With each stroke, combustion takes place.

Depending on many important factors - type of scavenging, fuel injectors’ condition, atomisation of the fuel oil issuing forth from the fuel injector, penetration of the atomised fuel into all segments of the combustion chamber, the mixing of each microdrop of fuel with the hot air, efficiency of combustion is established.

The less efficient the combustion, the more the physical debris of the remnants of combustion. The less efficient the scavenging - example ‘loop’ scavenging - the more the debris left behind.

This debris is in the form of unburnt fuel, hydrocarbons, carbon residue, other chemicals like sodium, potassium, vanadium and the like. They are in very small quantities, to be sure, but each stroke brings that little bit more.

Initially, this debris gets soaked up by the cylinder oil and gets scraped down to the under piston spaces.  In the case of loop scavenged engines, they also accumulate in the scavenge ports and choke the ports, as well as get scraped down to the under piston spaces. 

The consequence of scavenge ports getting choked, mean less quantities of air enters the cylinder, which in turn, affects combustion and shows itself as higher exhaust temperatures (due to after burning) and reduced compression pressures, if cards are taken.

The presence of water droplets or microdroplets in this scavenge air and the byproducts of combustion complicates things. The cylinder oil, the water particles and the byproducts of combustion now form a paste, which the piston rings find a little more difficult to scrape down.

This paste then starts getting accumulated and migrates towards the space behind the piston ring gradually and finds a resting place inside the groove of the piston, behind the piston ring.

Air mixed with Water + Cylinder Oil + Byproducts = Paste

The basic function of the piston ring is to seal the cylinder - seal it when compression is taking place, seal it when combustion takes place.

The piston ring also expands outwards and contracts inwards in its groove during each stroke, due to its spring action and the gas pressure behind the piston ring when the groove is clean.

If one were to calibrate an 800 mm diameter cylinder liner after 5 years or so, the top three readings will - or should be - be close to 803 mm. The bottom two will be close to 800.30, 800.50mm. The piston rings expand and contract due to the diameter differences.

One can exaggerate and think of it as an inverted cone.

When the piston is at the bottom most part of the cylinder liner, the piston ring is compressed into its groove in the piston because of the lesser diameter of the cylinder liner at the bottom. If deposits have filled the ring groove behind the piston ring, the ring has no space to contract and either breaks or sticks. Maximum breakages of rings takes place at the bottom third of the cylinder liner. (Which is why butt clearances of new rings are checked in the bottom third of the liner, as far below as possible).

As the piston moves up in the cylinder liner, the diameter becomes larger and the piston ring expands to fit into this diameter.

Thus, the piston ring expands and contracts, keeping itself partially within the groove or returns back more into the piston groove.

Air Mixed with Water + Cylinder Oil + Byproducts = Paste

This paste gradually accumulates behind the piston ring, in the ‘Back Clearance’, within the piston groove.

With the heat of repeated combustion and the heat of the piston, it transforms into a hard crust which, in turn, prevents or reduces the in and out movement of the piston ring. 

In the event of over lubrication (cylinder oil), thee paste formed is likely to remain in the form of thick paste and accumulate behind the piston ring.

If the scavenge air had been laden with too much moisture, the paste formed in the groove behind the piston ring will harden fast and accumulate.

Eventually, the piston ring either gets stuck or breaks, thereby losing its function of why it was assembled there in the first place.

The more the number of stuck or broken piston rings on a piston, compression pressure reduces, combustion is compromised, hot combustion gases leak past the piston rings in what is termed a ‘blow past’, leading to scavenge fires, turbochargers surging. Worst case scenarios - piston sticks in the cylinder liner causing a ‘twisted’ crankshaft - scavenge fires become uncontrollable leading to crankcase explosions.

It takes a while of continuous up and down strokes of the engine for this paste to either get scraped down or move behind the rings into the groove.

While this paste (along with the debris) is on the liner, with the rings moving up and down over it, the effect is similar to grinding paste being used. Liner wear increases.

Many engines are prone to excessive liner wear, mostly because of the above factor. I have noted this on two different types of engines - Mitsubishi B&W and Sulzer RTA Flex engines.

Unfortunately, I did not stay long enough on the Sulzer RT Flex engine, long enough to investigate thoroughly.

But I did have a measure of success on the B&W engines, using methods I have discussed below. Despite the grumbling of the Second Engineer, we pulled out two units twice within an eight month period (they were only due after another three years or so, with the present running hours) to check liner wear and see if it had reduced. They had.

Air Mixed with Water + Cylinder Oil + Byproducts = Paste

(A very simplified equation)

Take out the ‘Water’ component.

Send in ‘dry air’, devoid of water particles.

This is where the correct settings of the air cooler cooling water valves come in. If controlled, 95% of the water present in the air can be eliminated from entering the engine spaces. The air cooler drains should be kept clear, so that the water drains out.

The outlet temperature of the air leaving the air cooler and entering the engine should be kept at a temperature slightly above the pressure dew point (about +4 deg C above), to ensure dry air is sent into the engine.

The pressure dew point means the temperature to which the compressed air can be cooled without condensate precipitating. The pressure dew point depends on the final compression pressure. If the pressure drops, the pressure dew point drops with it.

Example 1: Intake air

• relative atmospheric humidity j = 70 %

• inlet temperature T=35°C

Example 2

Intake air

• relative atmospheric humidity j = 80 %

• inlet temperature T=35°C

Compressed air

• Final compression pressure pop=8 bar

Þ The pressure dew point is approx. 73° C

Example 2 Intake air

• relative atmospheric humidity j = 80 %

• inlet temperature T=35°C

Compressed air

• Final compression pressure pop=8 bar

Þ The pressure dew point is approx. 73° C

• Final compression pressure pop=10 bar

Þ The pressure dew point is approx. 82° C

(Above calculations are only as an example, as the pressures and temperatures do not match the engines we are talking about. But relevant in the method of using the graph.)

The instruments needed to find the pressure dew point are a wet and dry bulb thermometer (hygrometer), and a pressure gauge fitted on the scavenge trunking and the relevant graphs. (The other various thermometers and pressure gauges help in the settings needed).

Wet and Dry Bulb Thermometer

or

Sling type psychrometer (obsolete?)

Any Number of Digital Hygrometers are Available Today

Graph 1 is on top.

Graph 2 is below.

Suppose ‘Dry Bulb’ Temperature ( on the ‘Y’ axis) shows 44 deg C 

(Graph 2)

Assume ‘Wet Bulb’ Temperature (as on the curved lines reaching the X axis) shows 30 deg C

Where the two lines meet, the ‘Relative Humidity’ is (approximately) 40%.

Actual Scavenge Air Pressure reads 2 Bar.

Draw a vertical (from the meeting point of 40% Relative Humidity in Graph 2) to meet the Scavenge Pressure lines in Graph 1.

A Horizontal extension to the Y axis shows 45 degrees C. This is the “Dew Point’.

Add 4 degrees more and maintain air inlet temperature at 49 ~ 50 degrees C and you are assured of dry air entering the engine.

It is often wrongly assumed that the cooler the air that is sent into the engine, all the better for the engine.

Even with cool air, the humidity is important and must be controlled.

Starting from around 1990 or so, I have used this graph and maintained air cooler temperatures as per the graphs. On ships where I have served for longer periods, I was physically able to confirm the efficacy of maintaining the air cooler temperatures.

One of the consequences of correctly maintaining the engine inlet air slightly above dew point will be directly seen with the 'Air Coolers' Drain Tank' filling up fast.

Dew Point + 4 degrees is sufficient to drastically reduce the quantity of water entering the engine. If this air is maintained at a higher temperature than this, engine exhaust temperatures shoot up, to the detriment of the engine.

On my ships, this adjustment to the air cooler cooling water outlet valve was done at 10 am and 10 pm daily, based on the observed 'dew point'.

In the shipping world of 2023, I am certain that the graphs shown above are anachronistic and nobody will even think of using it. More sophisticated calculators are available, where you input two parameters and get a dew point at a particular pressure.

Is this line of thought in use or even prevalent? I am not sure.

(This article / blog also appeared in the August 2024 edition of Institute of Marine Engineers' Newsletter iMe'lange)

A Ranganathan