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I cannot begin to imagine how many downtime hours are attributed to hydraulic system or component failure, and yet almost all of this downtime is preventable by implementing just a few inexpensive ideas.
You are in charge of the plant’s hydraulic systems, and you dread going to work every day because of this portion of your job description. I have taught many people in hundreds of hydraulic classes over the past 4 decades, and here’s the short version of keeping your hydraulic systems running forever: “Keep your hydraulic fluid clean and keep it cool.” It sure is simple when you say it fast, but it is so true.
It has been documented over several decades that 70% of all hydraulic failures are due to contamination. But my oil looks clean! That doesn’t necessarily mean that it is clean enough, however. How hot is too hot for my hydraulic system? It depends on your hydraulic system and the type of fluid that is used.
This blog will stick with the industrial type of hydraulic systems, but let’s just say you had a bulldozer with a hydraulic system operating at 170° F. Do you panic and replace the fluid because you suspect it has been burned? No, for mobile systems 180° F is not a problem. Different seals is the main reason.
The two different types of industrial hydraulic fluid you’ll most run into is a petroleum oil (possibly food grade) or a water-glycol fluid (possibly food grade). A hydraulic fluid consists of a base fluid plus an additive package of anti-foaming agents, rust inhibitors, and anti-wear particles. In other words, your hydraulic fluid is only as good as the additive package that is a part of it. And a food grade fluid will have a food grade additive package. Trust me when I say that a non-food grade hydraulic oil with a good additive package is the best thing for all your components. But in food processing, we compromise. Water glycol is great if there is any potential of a high heat source being close to any hydraulic line, preventing potential fires.
For heat removal in your hydraulic system, we start with the fluid type. Water glycol should be limited to 110° F operating temperature, and petroleum oil should be limited to 125° operating temperature. You get much warmer than the 110° F on the water glycol, and you can see the water vapor coming out of the fluid, meaning that your 60/40 mix is no longer. If you do a Brix test on a water/glycol fluid that has been running at 140° F for awhile, you’ll be lucky to have an 85/15 mixture, as the water has been boiled off. I know, the fluid hasn’t reached the boiling point, but that vapor you see in the air is water vapor, and it is leaving that water glycol mixture a little bit at a time.
How do I remove the heat? First of all, you have to hope that you have a well-designed system, as a poorly designed system can create more heat than a normal sized heat exchanger can remove. You have two choices for heat exchangers: water/oil (shell and tube type exchanger) or an air/oil type (electric fan blade and radiator). In the south, it is almost impossible to get water glycol cool enough with an air/oil heat exchanger, as the ambient temperature may run to 105° F for a week or so. And that’s what we have to design for- the worst case scenario, not the average temperature for that area. If you want 110° F fluid coming out of your heat exchanger, and you have a cooling medium (fan driven ambient air) of 105° F, that means that your “approach” temperature is only 5° F. The higher the approach temperature, the more effective your heat exchanger is.
If you have a shell and tube heat exchanger with 80° F (ambient) water flowing through it, this gives you a 30° F approach temperature and your heat removal is much more effective.
But I use too much water already, I can’t add another water usage point in my plant! Think about re-purposing your water. If you are taking ambient water and heating it to 160° F for washing down equipment, run this water through a shell and tube heat exchanger to “pre-heat” the water for the 160° F requirement. Whatever BTU’s your hydraulic fluid adds to this water are BTU’s of natural gas that you aren’t paying for, since you are starting with a water temperature that’s greater than ambient.
Another thing to keep in mind about hydraulic fluid temperature. Many industrial hydraulic components are supplied with Buna N, or nitrile seals, which are “good” to over 200°. For every 10° over 140° F, the nitrile seal loses 10% of its life. If your systems run at 160°F most of the time, when you remove a nitrile O-ring, it’ll probably snap like a potato chip, instead of flexing and sealing like it should.
Keeping your fluid clean is a little more involved in doing it properly, but if you can see contamination, you are really in trouble. The human eye can only see down to 40 µm (micron, or micrometer), and that is too much contamination to run in any hydraulic system. That’s why you should filter new oil- it’s not clean enough.
There are ISO contamination target codes and lots of info on the web about it. Here are just a few guidelines:
There will be a future blog about what needs to be done to keep your oil clean. I have seen several installations where the oil could not be cleaned- contamination was being added to the system faster than the filters could remove it.