If you’ve worked in a steam turbine driven power plant you know how difficult it is to keep water out of your machinery and lubrication systems. When water finds its way into your lube oil system, it increases the risks of corrosion, wear and premature failure. There are many articles available that discuss water’s impact on bearing life performance and system reliability including Water – The Forgotten Contaminant by Mark Barnes from Noria Corporation. It provides a good overview of the harmful effects water can have on a process.
So if you have water in your system, what can you do to remove it? In almost all cases, the answer is “it depends” as there are multiple solutions available and we recommend that you consult with manufacturers to understand the tradeoffs that you will make. Also, filtration technologies should not take the place of a complete system analysis to diagnose the root cause. In the rest of today’s post, we’ll focus on the Colfax PurLubeTM system comparing its features and benefits to alternative water filtration technologies.
The Colfax PurLube System
A PurLube system is commonly installed in a kidney loop off the main lube oil console. In a kidney loop, the PurLube is able to pull lubrication oil from the system and return it after treatment.
Once the oil enters the PurLube, it is heated to 180°F without causing thermal or oxidative stress to the oil. Next, a venturi draws air into the oil creating small air bubbles. When the air bubbles mix with the heated stream of lube oil, the volume and surface area of the bubbles expand causing any moisture that contacts the bubbles’ surface to evaporate into the heated air inside the bubbles. The oil stream is then passed through a settling tray allowing the bubbles to come to the surface and break, allowing the formerly entrained water to simply escape as evaporating water vapor.
The ‘water entrapment principal’ is comparable to relative humidity. The amount of water in air (grams H2O per kilogram of air) increases with temperature. For example, air will entrain 30 times more water at 180°F than it will at 80°F.
The PurLube removes all three forms of water contamination in lube oil - free, emulsified and dissolved – to lower than 100ppm. Additionally, the discharge does not contain any oil or emulsion and maintenance is relatively low because the system only has one moving part, the oil recirculation pump.
Finally, the PurLube system is an OEM component available in steam turbine lubrication systems built by a major US based turbine / generator manufacturer.
Machinery Lubrication provides a good summary of the technologies available for water filtration. There are three other types of water stripping systems available today - centrifuges, coalescing filters and vacuum dehydrators.
As outlined in the Machinery Lubrication article, the centrifugal separator is the earliest design of an oil water separation system and removes both free water and emulsified water. The principal of the centrifuge is to separate the oil’s heavier elements by spinning the oil to create “G’s”. The technology works well when you have a significant difference in specific gravity between the two fluids.
A centrifugal separator, however, does not remove dissolved water and the discharge must be treated to insure that hydrocarbons do not enter the ground water or municipal water treatment systems. Also, a centrifuge will lose on average 1 or 2 gallons of lube oil per day in the discharged emulsion. At an average price of $30.00 / gallon, this loss can represent from $10,000 to $20,000 per year.
Coalescing filtration systems represent an improvement in energy consumption and maintenance when compared to a centrifuge and there is less loss of the expensive lubricants. However, there is a significant loss of filtration and separation efficiency if there is any emulsion in the lube oil. The coalescing filter system also has no effect on the removal of dissolved water in the lube system. The most significant operational cost can be the periodic replacement of the coalescing cartridge elements. This annual cost can easily exceed the initial cost of the complete system.
A vacuum dehydrator system lowers the partial pressure and thus the temperature at which water boils. Like the PurLube, a vacuum dehydrator will remove free, emulsified and dissolved water. However, there are several factors that set these systems apart such as:
- The vacuum dehydrator is a much more complex system having a significantly higher initial cost.
- The annual energy consumption, maintenance and consumable costs for these systems can approach the initial cost of the installed system.
- The separation efficiency of these systems may be compromised with higher viscosity or synthetic oils.
Marketing Manager Power-Gen / Energy for Colfax Americas
I always enjoy presentations from Mark Korzec (Director of Sales for Colfax’s Oil and Gas Organization). The content is interesting and he is a bit of a showman. Mark put together a presentation that discussed when to use a centrifugal pump versus a positive displacement pump that offered a simple checklist that I find helpful. His rule when reviewing a fluid handling opportunity is to review the application with a F (Fluid) D (Discharge) S (Suction) O (Operation) approach.
F – From the fluid perspective, will your pump see viscosities in excess of 20 cst, entrained gas or pump a shear sensitive fluid?
** If so, consider a PD pump for its efficiencies of operation and its ability to pass a fluid with minimum change or impact.
D – Will your system have varying pressure requirements?
** If so, consider a PD pump for its versatility and efficiency.
S – Will your pump have varying supply conditions?
** If so, consider a PD pump for its versatility in handling a wide range of NPSHA, fluid characteristics and ability to vary speed efficiently.
O – Will your operating objectives require occasional or even frequently changing demands in flow and pressure?
** If yes, consider a PD pump for its ability to respond immediately and efficiently to pressure changes, and its ability to operate at varying speeds.
With Colfax’s broad line of products – two and three screw, progressive cavity, peristaltic, centrifugal, crescent internal gear, metering and engineered systems – we believe we’re one of the best companies to review your application and to provide you with a solution that has the lowest total cost of ownership.
If you’re focused on maintaining and improving equipment and appreciate the simple checklists such as the one Mark uses, you should check out Dr. Lev Nelik’s article in Pumps and Systems magazine on “Using Simple Pump Troubleshooting Methods Before Rushing to Fancy Analysis”.
Industrial Market Manager