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Keeping Contaminants Out of Your Vacuum Pumps

Vacuum pumps take in any and all contaminants that come in from the pump inlet. There are four relevant forms of contaminant:

  • Vapors
  • Liquids
  • Solids
  • Biological Agents

Vapors
There are some vapors that can pass through a vacuum pump without harming it; all others should be passed through a condenser to convert from vapor form into liquid, and then drained off before they can make it into the pump. That’s because even small quantities of vapor can take up large volumes, distorting the effects of a vacuum pump or air compressor.

If the pump casing fills with vapor, it can condense inside the pump, which can overload the motor and damage the pump. Also, if the vapors condense within the water ring, the vapor pressure of the water seal will rise, which can encourage cavitation.

A good recommendation is that a ‘cold wall’ condenser be put in line before the liquid separator (see below) unless the vacuum pump uses a steam ejector to produce the vacuum, in which case no condenser is needed except where extreme efficiency is mandated.

Liquids
Liquid condensing inside the vacuum pipeline — or worse yet, in the pump itself — can require a complete disassembly of the pump. Depending on the kind of water suspended in the vacuum stream, different tools may be necessary:

    A water trap or knockout pot for visible ‘slugs’ of water and suspended aerosols

  • Or a cyclonic separator for water vapors and other particles that are not particularly heavier than the air itself.

Either one of these should be installed after the condenser but prior to the vacuum pump intake.

Solids
Solid particles are filtered out with exactly that — filters. Filters come in paper, hydrophobic, synthetic fiber, fiberglass, metal mesh, or other more exotic types, but they all have the same basic purpose: to let air through while keeping solid particles suspended in the air trapped in the filter. Most industrial purposes require either high-efficiency particulate air (HEPA) or ultra-low penetration air (ULPA) filters. Such filters should be installed prior to the condenser.

Biological Agents
In most cases, a HEPA filter will take care of 99.9% of biological agents. Even given that fact, every wet vacuum pump should be configured in such a way that it can be quickly and safely sanitized — and it should be sanitized regularly, at least twice a year, even if no sign of biological contamination has been noted.

Keep Your Hydraulic Manifolds Working Longer By Reducing Cavitation

Cavitation is a deeply interesting physical phenomenon that occurs when liquids are subjects to sudden massive changes in speed. For example, if fluid moving under 40 megapascals (MPa) of pressure suddenly hits a hydraulic valve and the pressure drops to 20MPa, the sudden increase in speed (described by Bernoulli’s Theroem) would lead us to the conclusion that a small bubble of nearly empty space — a hard vacuum — would open up within the fluid at the point of pressure drop.

This does in fact happen, and the bubble both forms and then collapses at supersonic speeds, creating a tiny sonic boom within the hydraulic fluid itself. The shockwave of the bubble’s collapse generates enough energy that it can literally tear apart nearby molecules of hydraulic fluid — or, if the bubble collapses near the inner walls of the manifold, it can rend the molecules of the manifold itself, causing “erosion”.

Reducing Cavitation
That’s perhaps a misnomer — there’s no realistic way to reduce cavitation within a given system short of reworking it to avoid sudden pressure changes. But the critical phrase above is “if the bubble collapses near the inner walls of the manifold.” It may not be possible to realistically prevent cavitation bubbles, but it is possible to more carefully control where those bubbles collapse.

The simple rule of thumb is this: increase backpressure by 5% of the total drop in pressure across the valve or metering edge (or whatever else is causing the change in pressure.) In the above example, where pressure drops by 20 MPa, an increase in backpressure of just 1 MPa should force the cavitation bubbles toward the middle of the fluid stream, thus causing them to collapse where they will do as little damage as possible to the hydraulic manifold.

If that doesn’t work, switching from an aluminum manifold to a ductile iron manifold will reduce the rate of “erosion” by 90%. Alternately, if possible, increasing backpressure to 10% of the total pressure drop will have a similar effect, though often switching manifold materials is the easier solution.

How do you know if it worked? Listen carefully at the point of pressure change. White noise coming from within the system is the sound of cavitation bubbles collapsing; if the noise is reduced or gone, your attempt was successful.