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The industry standard for determining when to change your turbine oil is when the antioxidants deplete to 25% of their original level. An accepted practice to extend the life of your turbine oil has been “bleed and feed”. Bleed and Feed is the practice of discarding a fraction of old oil and replacing it with new oil. Often 20-30% of the oil reservoir volume will be exchanged with new oil. This practice has become so widely adopted that few plants even pause to determine the economics of bleed and feed. In essence, this is the most expensive way of replenishing your antioxidants and is one of the most expensive practices for managing your turbine oils. Let’s look at why.
Antioxidants are critical to the life and performance of turbine oils. They make up approximately 1% of the turbine oil formulation. Antioxidants are reactive molecules designed to sacrifice themselves to protect the base oil. We have studied dozens of turbine oil deposits and have found, as one would expect, a significant portion of deposits are composed of depleted additives. In most cases, when turbine oils have reached the end of their life, the base oil is still healthy.
Bleed and Feed is extremely expensive for two reasons. First, almost all of the oil molecules that are being exchanged are still in acceptable condition. So you are replacing good lube molecules with additional good lubricant molecules. Second, the fresh antioxidants often react very quickly with the degradation products in the system. Several days after the bleed and feed, the effects are hardly noticeable through routine analysis. We have seen many cases where this happens, especially if there are high amounts of sludge and varnish in the system.
Let’s say you have a 5,000 gallon reservoir with 25% remaining antioxidants. To extend the life of the oil, you perform a 25% bleed and feed. At $14/gallon, this costs you $17,500. This should increase your antioxidants to almost 44%. However, the fresh antioxidants immediately react with oil degradation products resulting in only half of the antioxidants effectively being added to the system. Your resulting antioxidants have been increased from 25% to 31%. This is the equivalent of paying 4X more for your oil or $56/gallon!
There are times when bleed and feed makes sense. If a shutdown is not possible making this the only option to allowing the plant to limp by until a planned outage, even a 4X increase in oil price is cheap compared to shutting down. However, bleed and feed as a maintenance strategy or a practice to maintain the antioxidant levels is very expensive.
A better practice is to keep your oil clean and dry until you’ve reached its remaining useful life and then change the entire reservoir with new oil. Many plants have also considered replenishing the antioxidants as an option - a much less expensive option. If done correctly, replenishing spent antioxidants can double the life of your turbine oil.
It’s always healthy to question the status quo, especially if the status quo doesn’t make financial sense. As is often said, the seven most expensive words in business are, “we have always done it this way.” Keep this in mind the next time your oil analysis report indicates a depleted level of antioxidants.