Compressor Over Lubrication: Is It a Problem?
Gulf South Rotating Machinery Symposium John P. Miguez March 20, 2007
Sealing Solution S olutions s
We Will Discuss #
Costs of the problem
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History of compressors
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Review of how packing works
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Why are we now having a problem?
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Types of over lubrication
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How to determine if you are over lubricating
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What to do about over lubrication
We Will Discuss #
Costs of the problem
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History of compressors
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Review of how packing works
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Why are we now having a problem?
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Types of over lubrication
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How to determine if you are over lubricating
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What to do about over lubrication
Costs of Over Oiling We often don’t think of the additional costs involved with over lubrication. The chart below lists lists several average compres compressors sors and the cost of oil, along with the cost c ost of over lubrication by twice as much. It does not not include include hidden costs, such as as disposal costs, packing, downtime and mechanic’s labor.
A Brief History of Compressors P
First compressors were steam engines
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Early packing rope and axle grease
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Late 1800s - Charles Cook: Mechanical Packing
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1900 - A. O. France: Modern Packing Design
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1960 - Ray Farrell: Liquid Cooled Packing Case
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1980s - Ariel Compressors dominates market
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1990s - Faster speeds and shorter strokes
How Packing Works Mechanical packing uses the cylinder’s internal pressure to collapse the three ring segments around the rod. As the pressure rises in the cylinder, the three segments are squeezed tighter and tighter around the rod’s OD, keeping the gas from escaping along the rod and into the distance piece and atmosphere.
How Packing Works - 2
How Packing Works - 3 As the pressure is released with the opening of the discharge valve(s), pressure behind the rings will cause the ring segments to move forward toward the piston end of the cylinder. At the same time, the rod reverses direction with the start of the suction cycle. The resulting drag of the ring on the surface of the rod will result in the ring’s OD moving faster than the ID, causing the ring to bow.
How Packing Works - 4
You can also get a torque effect on the ring, which causes it to spin around the rod. The end result is a three dimensional opening, closing, flexing and spinning of the rings before they close up again to seal on the next compression cycle.
How Packing Works - 5
Why Problem?
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Faster Speeds
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Shorter Strokes
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Type of Packing
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Rods
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Lack of Education on Oil System
Why Problem? Faster Speeds and Shorter Strokes
As compressor speeds have gotten shorter and the RPM increased, the time the packing rings have to seat themselves upon the start of the compression cycle has decreased. A quick example will help to illustrate.
Speed & Stroke
A compressor with a 12" stroke running at 400 RPM will take (60/400)/2 = 0.075 second or 5/64th of a second to complete the compression cycle, then reverse itself.
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At 1000 RPM, it takes 0.03 or 1/32nd of a second to travel the same 12".
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At 1800 RPM, we get 0.016 or 1/64th of a second.
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Stroke To further compound the rings problems, as the time to seal gets shorter, the stroke goes from 12" to 5-6" to 3-4".
So, not only do the rings need to collapse around the rod in 1/64th of a second, they have only 3" in which to do so.
It is a tribute to engineering and manufacturing that this happens more or less correctly 99% of the time.
Rod Condition Out of Roundness
Rod Condition - 2 RMS - Roughness of Surface We want rods to have some roughness to them. The roughness allows the packing to wear to the proportions of the rod and seal better. The roughness also provides something for the oil to grab and hold on to as the gas pressure attempts to blow it out of the back of the compressor cylinder.
Importance of Lubrication P
Serves as a coolant
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Washes away particle matter and helps sealing
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Prevents corrosion
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Reduces friction
The difference between perfect lubrication and no lubrication can be as high as a factor of 40. What this means is, the friction generated between a ring and rod with no lubrication vs. a properly lubricated ring would be the same as having 40 times more gas pressure on the same ring and rod. As an example, at 100 psi internal pressure, a frictional factor of 40 would generate the same amount of heat as having 4000 psi of pressure on the ring .
Importance of Oil - 2
Under normal circumstances, oil should be evenly applied to a film thickness of about 0.002 inches.
Oil Distribution System Divider Block Systems
Yesterday Drops per minute Drop size varies depending on pressure, temperature and viscosity of oil.
Today Pints per day 28.876 cu in. in a pint 0.002 cu in. in a drop Divider Blocks distribute so many “cu in.” of oil per stroke.
Potential Problems with Divider Block Systems P
Blocks wear out
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Wrong size blocks
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Pump not adjusted correctly
Too much oil can lead to over lubrication problems!
Two Types of Over Lubrication Too much oil is used during assembly Can lead to failure to seal properly
Two Types of Over Lube - 2 Too much oil going into cylinder can lead to excessive heat, wear and premature failure of packing.
Symptoms of Over Lubrication
If rings don’t seal on start-up, look for little or no sign of wear marks on bore of packing rings. Packing that has been in a while will often be saturated with oil. Sometimes the radial and tangent rings will be stuck together. You may see forward extrusion and overheating.
Examples of Over Lubrication
Before anything else! Ensure system is operating according to manufacturer’s specification. P
Check pump size
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Check pump rate
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Check divider blocks
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Cigarette paper test
Checking Divider Block Sizing Assumes the blocks are working correctly
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Measure divider block cycle time.
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Calculate the recommended oil rate in pints per day.
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Calculate time needed to inject above pints of oil into your cylinder.
Divider Block Checking - Step 1 Measure divider block cycle time
Using a stopwatch or the second hand of a watch, measure the time for one complete cycle of the pin or LED flash. Record this time.
Divider Block Checking - Step 2 Calculate the recommended oil rate in pints per day. Each manufacturer has published formulas used to calculate the normal rate of oil for its compressors. Most manufacturers call for one pint of oil per every 2,000,000 ft 2 of cylinder surface area to provide a 0.002" oil film. Arial Compressors is the major exception to this formula. Example of CP/EI/Gemini compressor FE650 (D Series): (Bore (in) x Stroke (in) x RPM)/31,800
10"x 6.0" x1000 = 60,000 60,000/31,800 = 1.89 pints/day To this we must add the packing case needs. 2" rod uses 1.5 pints/day. Total oil needed is 3.38 pints per day for this cylinder.
Divider Block Checking - Step 3 Calculate time needed to inject 3.38 pints of oil into your cylinder .
1. Record the numbers stamped on the divider blocks. Add the numbers and multiply by 6. 2. Divide the result by the recorded cycle time and compare the two. Example of CP/EI/Gemini compressor: (Value of divider blocks) x 6/number of pints per day (9 + 6) = 15, 15 x 6 = 90 90/3.38 = 26.6 seconds
Divider Block Checking Your measured time and calculated times should be the same or close.
If the timing is way off, you should get a qualified person to inspect the system.
Checking an Ariel Compressor Example of an Ariel JGC 10" cylinder : Cylinder bore x factor of 0.5 (Factors are 0.3, 0.4, & 0.5 according to frame model) 10" x 0.5 = 5 For the rod, the OD is doubled and the same factor is used. 2.5 x 2 = 5
5 x 0.5 = 2.5
So the number of pints recommended by Ariel is 5 + 2.5 = 7.5 pt/day.
Solving the Problem Remember, packing is cheaper than rods & pistons.
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Warning: Oil usage should not be reduced without considering each operating factor. Gas composition, gas pressure and separation equipment must all be factored into the equation. It is best to get a lubrication system expert involved before reducing lubrication levels. Always follow the manufacturer’s recommended procedures for reducing lubrication.
Solving the Problem - Part 2 P
When you lower the pump rate, you lower the oil rate to each part of the system.
Consider resizing divider blocks.
Only reduce oil flow by about 5% at a time. Allow the compressor to run a week or two. Then inspect for excessive ring wear before lowering the rate once again.
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Too little oil will produce rapid wear of piston rings. Grit from the rings may plug up valves and be seen on packing rings.
Thank You! The following companies helped me with this presentation, Listing their names is not meant to imply endorsement of any kind. It is to thank them for helping. Any errors are mine, and mine alone. P
Ariel Compressor (arielcorp.com)
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AAVOLYN Corp. (aavolyn.com)
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Hanover Compression (hanover-co.com)
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M&J Valve Services (mjvalve.com)
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Pro-Gress (progressive-equipment.com)
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Hasu Gajjar & Assoc. (
[email protected])
Ariel’s Response Ariel engineers were asked why they recommend so much more oil than other compressor manufacturer’s?
Ariel’s Response - Part 2
Ariel’s Response - Part 3
