On-stream oil purification lowers operating expenses.

On-stream oil purification lowers operating expenses 

In November of 1988, a world-scale ethylene plant in the U.S. Gulf Coast area set an unusual record. For exactly 10 years, they have used the same 22,000 gallons of lube oil in their five major unspared turbomachinery trains and 25-odd general purpose pumps, fans and blowers with circulating lube oil systems. It was estimated that the plant had saved approximately $500,000 in this time period by not opting for the more frequent routine oil change intervals that are still common in the process industries. The emergence of reliable means of on-stream purification of premium grade lube and hydraulic oils now makes it possible to capture maintenance cost savings while at the same time reduces the risk of experiencing unexpected equipment outages.

But before we examine how this plant managed to achieve these desirable results, let's review why it is so important to operate machinery only with clean oil.

Lubrication experts have repeatedly pointed out that water destroys the base stock of lube and hydraulic oils and depletes the critically important additives. The result is degradation of equipment components and final effects which can range from frequent oil replacements to excessive unit downtime or even catastrophic damage to rotating equipment, hydraulics systems and other machines. The failure mechanism is rather well understood: Water combines with oxygen in the ever-present air and, in the presence of heat, oxidizes both base oil and additive package. Without anti-wear ingredients, the highly pressurized or loaded sliding components experience rapid and substantial wear. Silting, corrosion and hydrogen embrittlement take their toll and the damage is irrevocable.

Recognizing these potential liabilities, the ethylene plant investigated a number of on-stream purification options. Centrifuging was reviewed, but found to remove only free water. Dissolved and emulsified water and entrained gases would remain in the soil, a condition the plant found unacceptable. Coalescer and similar filtration methods showed the same constraints and, additionally, were considered maintenance-intensive due to the anticipated frequency of cartridge replacement.

After a thorough review of these available alternatives, the ethylene plant decided in favor of vacuum oil purification. Vacuum oil purification is a method whereby water is removed from the oil by the application of heat and vacuum. Wet oil is introduced into a vacuum vessel where it is distributed over a large surface or sprayed over tower packing or crinkled wire mesh at temperatures of approximately 70-80 [degrees] C (158-176 [degrees] F). The water is then removed in the form of vapor and condensed before rejection from the system.

A typical oil conditioner operating on the vacuum/heat principle is shown in figure 1. The fluid entering the vacuum vessel is controlled by a solenoid valve which allows a standing reservoir within the vessel. The fluid is exposed to heat and vacuum, while flowing in a thin film over baffled, slanted, aluminum trays or while being drawn through cartridges. Dehydration, deaeration and degasification are thus accomplished. The vapors are drawn from the vacuum vessel through a condenser. The condensed vapors settle in the distillate collection tank, which is drained periodically.

Temperature and vacuum are typically controlled to 70-75 [degrees] C (158-167 [degrees] F) and 750 mm (29.6 in. Hg). If an abnormal quantity of water or volatiles is encountered, temperature and vacuum sensors may signal a solenoid valve to divert the flow back into the system inlet for further processing, or the process may be slowed automatically by other means.

But, there are pitfalls for the potential purchaser of vacuum oil purification equipment. As John Ruskin, the 19th century critic and observer of the industrial scene expressed it: "There is hardly anything in the world that some man cannot make a little worse, and sell a little cheaper, and the man who buys on price alone is this man's lawful prey." For a vacuum oil purifier to give technically superior, long-term, low maintenance performance it has to satisfy three key conditions:

* Develop a high enough vacuum to remove water down to the 40 ppm residual dissolved - not free - level;

* Protect the vacuum pump from water vapor ingestion which would deprive it of reliable lubrication; and

* Use a method of surface extension for the contaminated oil which allows for the effective removal of contaminants without in itself becoming a difficult-to-maintain or failure-prone component.

The importance of creating a sufficient vacuum is best illustrated in figures 2 and 3. Figure 2 shows that with a good vacuum, even moderate processing temperatures will allow highly effective dewatering to take place. In contrast, vacuum dehydrators using less suitable vacuum pumps or eductors may achieve effective dewatering only at very high processing temperatures, or perhaps not at all.

Protecting a precision vacuum pump from water or condensate ingress is necessary to prevent internal corrosion and loss of oil film. This oil film, incidentally, may serve as a sealing medium and is generally needed to produce a high vacuum. Refrigerated condensing will provide the necessary protection and, in very many cases, will make it possible to select a smaller size vacuum pump than would be necessary for alternative condensing means. This is graphically illustrated in figure 4. An additional benefit of refrigerated condensing is its inherently better energy efficiency than would be obtainable with ambient temperature condensing systems.

Thus, while the initial cost of vacuum dehydrators with refrigeration units will be higher, their bottom line cost in terms of dollars per gallon of purified oil will be lower since both maintenance and utility-related costs are reduced.

Finally, we get to the issue of surface extension for the incoming, contaminated oil. An inexpensive vacuum oil purifier can certainly get by with a simple disperser cartridge mounted within the vacuum vessel, but experience shows that cartridge replacement costs and hence maintenance expenditures quickly become burdensome. The result is often maintenance neglect and serious malfunction of inexpensive vacuum oil purifiers. Evaluating the two state-of-the-art vacuum oil purifiers at the U.S. Gulf Coast ethylene plant we find that the vacuum vessels had never been opened in 10 years of successful operation.

Which gets us to the point we wanted to make in the first place: Reliable plants depend on reliable machines, and to be reliable, a machine must be operated with clean oil. You can get clean oil either by frequent oil replacement or by on-stream purification with state-of-art vacuum oil purifiers. And we all know which is the more cost-effective way of achieving this goal.

The Gulf Coast ethylene plant has proved it; remember: The same 22,000 gallons of lube oil for the past 10 years of continuous operation.