Microbial Contamination of Turbine Lube Oil Systems - Preventing Growth of Bacteria Colonies

Richard Winslow, Pacificorp - Naughton Plant
Tags: turbine lubrication, contamination control

Bacterial contamination of large circulating oil systems in steam turbines, paper machines and similar systems is a growing and expensive problem. The bacterial colonies, once established, will clog control systems, quickly degrade oil quality and oil performance and produce corrosive byproducts. If not detected early, the problem will manifest itself into expensive repairs, extended downtime and a significant expenditure of scarce resources. As the United States steam turbine fleet ages and maintenance intervals are stretched, the problem is growing. Because steam turbines generate approximately 60 percent of the United States’ electrical power, the potential problem is significant.

This article presents a brief description of circulating oil bacterial contamination, problem recognition, types of lubricants and conditions that appear to be most susceptible, methods of treatment, and known methods to clean up and prevent bacterial contamination.


Figure 1. Naughton Plant

Bacterial Contamination in Oil - What Is It?
Bacterial contamination in crude oil and refined petroleum products has been well documented since the 1970s. The problem of bacterial contamination in steam turbine lube oil systems is increasing. Three classifications or types of bacteria typically encountered in petroleum products include:

These three classifications of bacteria are prevalent in nature. With respect to human health and safety, none of these three bacteria classifications has been tested for human health hazards. It is possible, but not likely, that the GAB classification of bacterium could produce a bacteria strain that might prove pathogenic to a portion of the human population.

These bacteria need the following to survive and grow:

If any of the above is disrupted, the bacteria will not grow or survive. Obviously, the easiest environmental factor to influence is the water concentration in the oil.

Operational Signs and Symptoms
Bacterial contamination can be positively confirmed only by laboratory analysis. However, multiple signs and symptoms for operations and maintenance personnel are available. As the bacteria grow, they produce visible colonies of biomass. This biomass has approximately the same color as clear silicone room temperature vulcanizing (RTV) and looks like gelatin. One location where this biomass can be easily viewed is in the sight glass of a Turbo-tac or Kaydon Oil filtration skid.


Figure 2. Skid Sight Glass Bacteria Colony

Figure 2. Note the following in the sight glass from top to bottom: emulsified oil, oil water boundary, water layer and bacteria colony. This colony is primarily GAB.

Additional symptoms of bacterial contamination in a turbine lube oil system include:

The bacterial contamination may also be visible in the bottom of drained turbine oil reservoirs (Figure 3).


Figure 3. Oil Reservoir

Figure 3. A boiler feed pump turbine lube oil reservoir. Note the rust production from the bacteria colonies. The lower right-hand corner of the picture shows a cleaned tank surface.


Figure 4. Filter Housing

Figure 4. The interior of a Kaydon unit that has been subject to heavy bacterial contamination and turbine oil with prolonged elevated water content.

Reynolds Number

The author presents 2,000 gpm as a target flow rate for the system flush. This represents a Reynolds number of 10,000+. It is customary to calculate the flow required to create turbulence to enhance a system flush by using the Reynolds number for turbulent flow. The Reynolds formula is: R= pVD/µ where:

p = fluid density
µ = fluid viscosity in centipoise
V = free stream fluid velocity
D = pipe diameter

The Reynolds number must be above 2,000 for system operating conditions to achieve a state of turbulence. As the distance from the centerline to the sidewall of the pipe increases, the relative fluid turbulence decreases, reaching roughly 50 percent fluid turbulence at 5 percent of the distance from sidewall to centerline. As the fluid reaches the pipe sidewall, fluid turbulence decreases to a point of laminar flow.

Particle removal performance depends on the lift and drag forces of the flush media, and the depth of the laminar sub layer in the stagnant film at the conduit wall. A particle completely submerged in the laminar sub-layer is impossible to flush from the system.”1

If the intention is to remove the remains of the dead biological material strictly through flushing, the Reynolds number would need to be high.

Confirm with Laboratory Analysis
Turbine lube oil bacterial contamination may be positively confirmed only through laboratory analysis. This type of analysis is usually performed by a microbiologist. Most oil analysis laboratories do not have this capability or technical expertise. However, several of the major petroleum producers and refiners do have this type of technical expertise on staff. (Bacterial contamination is a major concern in the pipelines, tank farms and upstream side. PacifiCorp’s lubricant supplier was extremely helpful in this matter.)

The microbiological analysis will consist of incubating and growing colonies in different types of media and then analyzing for biomass activity/RNA/DNA. The laboratory will also attempt to perform activity level counts by the three different classifications of bacteria. It is important to communicate with your lubricant supplier/microbiological laboratory to determine factors such as the types of samples, preservation requirements and sample locations. This is imperative for optimal analytical results.

Conditions that Favor Bacterial Colony Growth
The following conditions appear to provide optimal conditions for bacterial colony growth:

Clean-up and Treatment Methodology
Once bacterial contamination has established itself in the system, eradication can be difficult. The most effective method involves the following steps:

  1. Remove the bacterially contaminated oil and send it off-site for use as fuel oil. It is important that in-plant transfer piping not be used during the removal of the contaminated oil. This will minimize the potential for spreading the problem to other components.
  2. Inspect and clean the lube oil system. This involves disassembly and cleaning of major components. Specific attention should be paid to the reservoirs and filter housings.
  3. High-volume /high-temperature oil flush. Utilize a new flush oil (do not use the old oil as flush oil) recommended by your lubricant supplier and perform a high-volume/ high-temperature flush. The oil temperature should be at 160°F minimum and flow rate should exceed 2,000 gpm for a 4,000-gallon turbine lube oil system (achieving a Reynolds number greater than 10,000 is desired). After the flush is complete, dispose of the flush oil as in step 1 above. This flush should also use high-quality, full-flow filtration capable of at least 3 micron at Beta>200. A before-and-after photo of the flush oil illustrates the cleaning that can be accomplished (Figure 5).
  4. Perform a final inspection and cleaning of the reservoir and any other accessible components. Cleaning should follow the lubricant manufacturer’s instructions. The use of cleaning solvents or detergents is generally not recommended.
  5. Refill with new oil. When refilling, utilize new hoses and fill directly to the reservoir from the transport unless the plant transfer piping is known to be clean and free of bacteria. Install new filter elements and place all filtration components in service. Ensure that the new oil is dry - less than 500 ppm water content.
  6. Add a recommended biocide to the lube oil. Consult with your lubricant manufacturer for the best application. Test the new oil doped to the recommended biocide level to determine baseline data for: RPVOT, acid number, demulsification, foaming, viscosity, ASTM and color.2
Flush Oil from the Transport
Flush Oil After
Three Days
High-Volume/ High-Temperature Flush
Figure 5. Turbine Flush Oil, Before and After

Keep in mind some important “dos and don’ts”:

Prevention of Bacterial Contamination
Prevention of lube oil bacterial contamination is relatively simple and straightforward: Keep the water in the oil to less than 500 ppm.


Other factors that will help prevent bacterial contamination include:

References

  1. Fitch, E.C. Fluid Contamination Control, 1988 p. 113.
  2. Author’s professional experience on multiple steam turbines over the past 20 years.

Acknowledgments
The author would like to thank Gary Jenneman and Ted Naman of ConocoPhillips for their contribution to this article.