Larry Bachus
(a.k.a. “Pump Guy”)

Pump Guy,

I started reading the Pump Guy 10 years ago when I was a student. You have a way with words. “Vapor lock” is a phenomenon that is hazy to me. The term is used with pumps, but not with compressors. Why is it forbidden for vapor to get into a pump? Centrifugal pumps and compressors both have impellers of similar design. Perhaps there is a fundamental principle that I’m missing. Can you please bring the haze into focus and explain this phenomenon to me?


Doug B.
Reliability Engineer


Hello Doug,

Thank you for writing. I see two issues in your email. One issue is “vapor lock.” Another issue deals with the differences between pumps and compressors. You are not alone in your haze. You are brave where others suffer in quiet desperation. Let’s bring the two issues together, beginning with inadequate terminology.

Our industry and profession teaches and promotes inadequate and sometimes incorrect terminology. The imprecise terminology leads us to misunderstand pumps, compressors, and engineering concepts.

Here are a few examples:

  • We confuse and interchange the terms “head” and “pressure.” These are different terms with different definitions.

  • We confuse and interchange the terms “energy,” “work,” and “power.” These terms have different definitions.

  • There is a family of pumps called the “end-suction pump” because the low-pressure (suction) flow is received on the extreme end of the pump shaft and impeller. Yet, other pumps, like multistage verticals, receive the fluid in the same fashion, but are not considered “end-suction pumps.”

  • We misuse the term “fluid.” We learn to misuse the term in school. (The word “fluid” is derived from Latin “fluere” meaning “to flow.”) The word “fluid” encompasses both liquids and gases. Liquids and gases have no fixed shape. Liquids and gases will both take the shape of the container they are in. Both will flow.

  • Engineering students take a course called “Fluid Mechanics.” However, most of the investigated properties and formulas in the course refer to liquids only. My old university text defines a pump as a device that increases the pressure of a “fluid.” This is imprecise because a pump increases the pressure of a “liquid.” Compressors increase gas pressure.

  • The illustrations in my college text show cartoon pumps transferring a colored substance identified as “fluid.” Shouldn’t the colored substance be identified as a “liquid”? If the text author really liked the word “fluid” for the colored substance, then the mechanical device should be labeled a “pump-or-compressor.”

  • The text investigates “viscosity” as a property of “fluids.” But the examples (water, motor oil, tomato sauce, and peanut butter) used in the viscosity chapter are all “liquids.”

Engineering is not alone in its use of imprecise terminology. It is grammatically impossible to ride down on an escalator or elevator. I mean, if it escalates or elevates, it only goes up. Correct?

I admit, it may be impractical to separate the college course into two courses, “Liquid Mechanics” and “Gas Mechanics,” but university texts should be specific. I mean, “What is a viscous gas (‘fluid’)?”

We say pumps “generate” or “develop” flow and head. A pump cannot “generate” or “develop” flow. Only a magician can start with three gallons of liquid (at the pump suction nozzle) and convert it into five gallons of liquid (at the discharge nozzle). If a pump can really generate flow, then the global energy crisis and the water shortage are resolved.

So, we use inadequate and incorrect terminology in our profession. It leads to confusion. Surgeons and trial lawyers could never do this and be successful in their professions. I will try to use specific, unambiguous terminology as I offer some thoughts on pumps, compressors, and vapor lock.

Classic pumps and classic compressors are different machines. Classic pumps are for liquids. Classic compressors are for gases. Consider the following:

  • A classic pump is a mechanical device that adds energy to a liquid.

  • A classic positive-displacement pump adds energy to improve or assure the liquid’s flow. Increased pressure is a secondary effect.

  • A classic centrifugal pump adds energy to increase the liquid’s pressure. Improved or assured flow is a secondary effect.

  • A classic compressor is a device that adds energy to a gas to increase the gas pressure. As a secondary effect, the added energy can improve (assure) the flow of the gas.

  • A fan (blower, ventilator or air extractor) is a device that adds energy to a gas (air) to increase or improve gas flow. A fan is not practical for gas pressure.

  • A classic pump can be modified to handle fluids (liquids with entrained gas or air bubbles). This is called a “bi-phase” pump. (And a slurry pump (a “multi-phase” pump) would include “solids” with the liquid and gas.)

  • A classic compressor can be modified to handle fluids (both gases and liquids). This is called a “bi-phase” compressor.

  • So, as you compare pumps, liquids and gases, you must know if the pump is a “classic” pump or “bi-phase” or “multi-phase” pump. You must know if a compressor is a “classic” compressor, or a “modified” compressor.

Now, I will say a few words regarding “vapor” and “vapor lock” in a pump.

Vapor is a gas. Air is a gas. A liquid will convert to a gas (vaporize) if the liquid’s vapor pressure (a property studied in “Fluid Mechanics”) rises above the available or atmospheric pressure on the liquid.

Low-pressure zones exist in functioning pumps (centrifugal and high-velocity PD pumps). A popular low-pressure zone is in the suction nozzle because the liquid velocity increases. (Pascal’s law states: “As velocity increases, the pressure decreases.”) If this reduced available pressure should be less than the liquid’s vapor pressure, then the liquid will vaporize (meaning the liquid will convert into a gas) in the eye of the impeller.

Many low-density liquids (liquid propane, for example) also have a low vapor pressure and are prone to vaporize (convert into a gas) easily in the low-pressure zones of a pump. Temperature (heat) usually facilitates the conversion.

Also, high-viscosity liquids (honey, for example) that experience high velocity in a pipe or pump can separate and form vacuum bubbles or voids. Temperature (heat or cold, depending on other liquid properties) can facilitate the void formation.

A gas (air, vapor or void) will not centrifuge, so the gas (vapor) bubble lodges into the eye of the impeller. Although a gas, a vapor bubble or air pocket in the impeller eye acts like a stainless steel ball blocking flow into the pump. The gas bubble prevents the entrance of more liquid into the pump.

When a pump is starved of liquid on the suction side, the pump’s output or production is reduced or even destroyed. When the pump’s production or output stops or suffers due to the vapor bubble lodged into the eye of the impeller, we say the pump is “vapor locked.” Some people say “vapor blocked.”

We discuss these and many other issues in the Flow Control Pump Guy Seminars. We’ll be in Los Angeles for a May 16-18. Come if you can.


The Pump Guy

Larry Bachus, founder of pump services firm Bachus Company Inc., is a regular contributor to Flow Control magazine. He is a pump consultant, lecturer, and inventor based in Nashville, Tenn. Mr. Bachus is a retired member of ASME and lectures in both English and Spanish. He can be reached at