Engineering Procurement and Construction contractors (EPCs) will sometimes ask about literature that discusses practical steps for switching back and forth from preservation to operating modes when using oil mist. Still others will occasionally relay requests from clients that have asked them to submit leakage calculations for dry-nitrogen-purge systems to be used with condensing steam turbines. It seems that some steam-turbine manufacturers advocate what they always have because, to them, it is the safe route.
In the process of educating himself on the various aspects of nitrogen-purge-preservation systems, a reader associated with an EPC came across one of my old articles on the use of oil mist for machinery preservation. Although not directly involved in meetings with his company’s client, he wanted to know what alternatives the client might have (or not have) considered and be prepared to discuss them. He wrote to us, as follows:
“So, right now I’m just trying to understand the practical aspects and relative merits of nitrogen-purge versus oil-mist-preservation systems, especially as regards condensing steam turbines. Since one of the applications will involve an installed turbine and the surface condenser, the issue of water removal prior to introducing the oil mist, and then the
removal or cleanup of the oil prior to starting the turbine was something I was wondering about.
As things proceed with this project, I expect to get the opportunity to ask some questions about whether the client
has considered oil mist as an alternative to nitrogen purge. At that time, it will be very helpful to be able to mention
you guys as having experience in this field, and also the fact that Exxon has found oil mist to be a more economical way to go. Although I won’t be making the decisions about which services or equipment to purchase, I will be sure
to mention your books as experience-based resources in this area.”
OUR FACT-BASED RESPONSE
Note that there is a cost involved in using oil-mist preservation for grass-root projects. Unfortunately, no matter what the cost is, it will always seem expensive and cause a judgment in favor of VPIs (vapor phase inhibitors) or requiring personnel to keep bearing housings filled with oil and rotate the shafts.
Regardless of a project’s environment, be it hot, dusty, damp, wind-blown, or cold, and with long-term storage needs (more than 6 months), oil-mist preservation is the right choice to ensure low or no infant mortality of new equipment. Not only does oil-mist protect 24/7, it helps to prevent flat spots on bearings caused by ground vibration. (The oil mist completely coats the bearings and provides an oil film to cushion them from vibration.)
To be clear, the timely completion and budget concern for all grass roots projects will be negatively impacted if equipment is not properly preserved. Improper preservation or the lack of such protection will result in infant mortality and a large number of equipment failures during the first three years after start-up of new equipment/units.
API-RP 686 3.2.1 states: “When more than 10 pieces of equipment are to be stored for a period of longer than 6 months from time of shipment, oil mist protection should be considered”.
API-RP 686 3.2.2 states: “Oil mist should be used to protect the bearing housings, seals areas, and process end of equipment.”
API- RP- 3.2.3 through 3.2.16 states the full API recommendations.
Detailed analyses involving the manpower needs for other types of storage and preservation show much greater needs for VPI application (prep for storage and prep for installation/startup) as opposed to oil-mist preservation which would leave the equipment ready for installation and start up. Even pure nitrogen costs more than oil mist.
Documented accounts are available where oil mist has provided reliable protection for idle and standby equipment at many refineries over the years. There is no reason not to provide that same protection for current projects and new equipment during storage.
As a general rule:
(a) Oil mist will “plate out” (actually, coalesce) when equipment operates. Operation causes a measure of turbulence; the atomized particles get knocked together and grow large and heavy. Turbulence is obviously not created in equipment that’s standing still. Therefore, the conversion of oil mist to liquid oil proceeds at a rather slow rate in non-running equipment.
(b) A low-point drain location should be identified and left open at all times. This low point is useful because it serves as a water drain for large condensers. Once oil mist is applied, the drain prevents oil accumulation and ensures through-flow of a small amount of mist. That’s really your make-up mist.
(c) With rare exceptions, a 3 mm drain orifice is all that’s needed.
(d) A machine in storage and ready for preservation can be slanted or skewed to create or locate a low-point drain.
(e) A rotor should be manually rotated two-and-a-half turns every 6 months.
(f) Prior to machine commissioning, one should blow steam through the equipment. This will remove the thin coating of oil that will have accumulated on the surfaces. Use oil with formulations that will not promote future stress-corrosion cracking.
(g) Collect some of the exiting blow-through steam and observe if it contains oil.
As a not-so-general rule, one could proceed with the academic exercise (h), below:
(h) One could calculate the total interior-surface areas that are being wetted on a particular machine and assume an oil film with a thickness of 0.0005 inches coating these surfaces. Knowing area and oil-film thickness, one would be able to calculate the total volume of oil that will exist in the equipment at any particular time period and convert this to milliliters of oil. Using conversion formulas from the Internet, this milliliter volume can be readily converted to gallons. One could then assume the oil costs $XX per gallon and figure out what it all costs, after subtracting oil that’s lost. That would require accounting for oil lost through the 3-mm orifice, based on an equipment-internal pressure of 0.3 psi above ambient. So, would I engage in this valueless exercise? Of course not.
SOME THOUGHTS ON SAFETY
Finally, I recall mentioning that people have suffocated in nitrogen—but nobody has ever suffocated in oil mist that’s applied to stored equipment. To me it’s important to say this again: Safety is an ethical imperative. Regrettably, and to a few hard-nosed folks, safety is a concept that’s mentioned in passing and brushed aside all too easily.
As so often has been the case in the 50 years since I obtained my Professional Engineer’s license, choosing oil mist is just another no-brainer for me.TRR
ABOUT THE AUTHOR
Heinz Bloch’s long professional career included assignments as Exxon Chemical’s Regional Machinery Specialist for the United States. A recognized subject-matter-expert on plant equipment and failure avoidance, he is the author of numerous books and articles, and continues to present at technical conferences around the world. Bloch holds B.S. and M.S. degrees in Mechanical Engineering and is an ASME Life Fellow. These days, he’s based near Houston, TX.
Tags: reliability, availability, maintenance, RAM, steam turbines, bearings, lubrication, oil mist, dry-nitrogen-purge systems, vapor-phase inhibitors, equipment storage