What’s your real vacuum problem?

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I can’t count the number of times I’ve heard someone say, “The vacuum pump is bad.” Then after the pump has been “changed out,” the “original” problem persists. That’s when the search begins for the real problem.

If the vacuum pump has been changed, and the problem fails to be solved, look at what was the original problem? It could be a leak, a gauge problem, water vapor in the system, or something else.

chart
In addition to the two original graphs, the blue line represents pump-down of a system contaminated with water vapor. Unless the water is removed, the vacuum level cannot go lower. When the water is eliminated, the pump-down rate returns to normal
Many different types of vacuum pumps are offered in the marketplace—oilsealed pumps, oil-free roughing pumps, diffusion and turbomolecular pumps (like little jet engines), cryopumps, and ion pumps.

Regardless of the type of vacuum pump used, a systematic approach needs to be taken to solving any vacuum pumping problems you experience in your system.

The first question to ask is: Has any work been performed on the vacuum system lately—

  • Changed a gauge sensor head?
  • Changed an O-Ring?
  • Experienced a power outage?

 

Analogous Situations

Chart
the red pump-down curve represents pump-down of a vacuum system with a leak. Once the leak rate equals the pump's pumping speed, the vacuum gauge reading levels out and will not go lower
If you just had the brakes on your automobile replaced and during your normal route home, you notice your car now seems to steer to the right by itself. Are you going to plan on having the front alignment checked? While you might need that done, I would suspect you should have the brake adjustment checked first.

You should view your vacuum system in a similar fashion. Whenever you work on it, and then you notice that something seems different, begin your investigation by looking at what operation you performed on it most recently. Only after you have eliminated what you did last as the source of the problem, should you move on to something else.

This could include a look at another set of very important diagnostic tools— the pump-down and leak-back curves. By comparing how the system is performing now to how it has 

Chart
The green line is the pump-down curve of a good vacuum system. the pump-down indicated by the yellow line should also be considered to be that of a "good" vacuum system
performed in the past (when it was working properly) you will be able to accurately diagnose the problem and apply the appropriate corrective action.

Pump-Down Curve

A pump-down curve is constructed by recording (manually or electronically) the vacuum pressure reading at specific time intervals while the vacuum pump is pumping down the vacuum system.

The preceding curves have been created by plotting pressure (on the X axis vs time (on the Y axis). The pressures and times are for comparison purposes only.

Leak-Back Curve

A leak-back curve is constructed by recording (manually or electronically) the vacuum pressure reading at specific time intervals after the main vacuum valve is closed, isolating the vacuum system from the vacuum pump(s).

chart
The yellow line on this plot represents a system with a small leak. If the rate of rise is acceptable, you may or may not want to continue with your process
The following curves plot pressure rise (Y axis) vs time (X axis). The pressures and times shown are for comparison purposes only.

Gauges

Most operators are unaware of the fact that the zero point and the span of the vacuum gauge electronics are completely adjustable. Every time the thermocouple (piriani) or the capacitance manometer gauge head is changed, the zero point can be (and often is) adjusted. Since such adjustments are routine, this fact must be considered when the vacuum system’s integrity is in question.

chart
The red line on this leak-up plot, indicates a system that is experiencing excessive leakage. When this plot is compared with the one above, it is clear that this system is experiencing a much more serious leak, that is likely to prevent continued processing
Both of the two most common gauges, the thermocouple (piriani) and the capacitance manometer gauges, are also very sensitive to the local environment. The physical mechanisms both of these gauges are very sensitive to “taps,” “bumps,” or local temperature changes. A heat gun, will affect the system pressure gauge reading, not only because water vapor is being “driven” off internal surfaces, but also because the temperature will affect the gauge reading (without an actual change in the system’s pressure).

Conclusion

A logical approach to inspecting a vacuum system that requires maintenance can result in an increase in the system’s uptime, a decrease in wasted time, and the elimination of money spent fixing the wrong problem.

Start your investigation by reviewing the most recent activity performed on the system. That’s the most likely source of the “problem.”

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 table

Create and use both pump-down and leak-up curves to compare the current systems operation with known good curves.

Woodrow D Farrow, originally trained as an analytical chemist, has more than 30 years’ experience in specialty gases and industrial processes. During his long career, he has managed analytical labs, production facilities, and sales/marketing programs. He is currently a consultant in the field and can be reached at woodrowdfarrow@yahoo.com

Specialty Gas Report FIRST QUARTER 2007 //