Why Pump Cavitation Matters In Your Pump System

05 Feb

Why Pump Cavitation Matters in Your Pump System

Last week, we talked about how understanding your pump curve and Best Efficiency Point (BEP) is crucial in finding profit in your pump system. Cavitation is another concern you need to think about.

Pump cavitation, the phenomenon where in a moving liquid the local pressure becomes lower than the vapor pressure of the liquid, can demolish pumps rapidly. If there is a low intake pressure, or the pump is operating at the far end of its curve, it pulls the fluid through so rapidly that the fluid pressure drops below its vapor pressure. When this occurs, there is no longer sufficient pressure on the fluid to keep it in a purely liquid state. The fluid appears to boil for a brief instant, and this is called cavitation.

At sea level, for example, water boils at 212°F. At The top of Mount Everest where the pressure is much lower, it boils at 160°F. In a pump, the pressure may drop low enough that water “boils” at 60°F, or whatever the ambient temperature may be. It may seem difficult for air to strip away steel in a pump, but it’s the millions of vapor bubble explosions and implosions that do the damage. This effect is clear once the pump is in operation. Cavitation can sound like gravel being pumped no matter the base fluid.

Pump Cavitation and Pump Selection

How does this tie in to pump selection? Some users try to avoid pump challenges by spending extra money on an oversized pump. However, this may cost more money in terms of maintenance and repairs as the operating point could be far from the BEP.

After all, the state of individual process piping is generally unknown. Thus engineers tend to oversize pumps in an attempt to build in a margin of safety. They have no real idea how a contractor will route pipes, and add a safety factor into their calculations. This safety factor, however, generally goes on top of the worst-case design scenario. Engineering firms are incentivized by the fact that they do not want to be blamed for an undersized pump that cannot meet process requirements. Imagine the outrage and damage to a firm’s reputation if clients did not receive adequate water pressure from a shower, cooling from the HVAC, or flow from the sewers.

The general rule of thumb for safety factors is 10%. A junior engineer will generally assume a certain amount of required energy based on the expected piping, process, and control elements. To that number will be added 10%. A senior engineer may then review this work and add an additional 10%, just in case. Things can continue to become even more complicated as end users may introduce unrealistic expectations for production capacity or may want the infrastructure being put in place as part of a potential expansion project that is still years down the road.

When the pump is finally ordered, the manufacturer’s representative will help select one that is large enough to handle these operations and then some. These safety factors end up compounding on each other and the actual process inherits something totally different than that which was originally required. This may leave the end user saddled with an inefficient pump and the possibility of hundreds of thousands of dollars in additional energy usage and maintenance work. Using software to model expected flow conditions at various operating points is recommended as a safeguard against such excesses.

The Research on Pump Efficiency

Pumping System Optimization is a course by the Hydraulic Institute which includes a study covering an evaluation of 1690 pumps at 20 process plants. The study discovered some alarming results. It found average pumping efficiency to be below 40%. Additionally, over 10% of pumps were less than 10% efficient. A major reason behind such poor numbers was improper pump selection.

These findings should be appreciated in the context of pump economics. The general rule is that a pump and motor combo will cost about $1 per day per horsepower of the motor. While energy costs vary by location, this is a good starting point to begin understanding the potential costs being facing. For larger horsepower pumps running inefficiently, the wasted capital can be staggering.

But energy costs alone are seldom cause for change, much less transformation of an industry. Once the pumps are installed and running, the energy costs can sometimes be out of sight and out of mind. On top of that, there are many other costs in industrial facilities. Discovering the true cost of the pump is hard when it is buried in an industrial energy bill alongside the high costs of heating, cooling, and running of the equipment.

Designing a fully optimized system without unnecessary safety margins can offer energy savings, better process quality and quantity. However, software evaluation of existing pumps can bring to light slight changes that can be made to the system that may result in sizeable increases in production throughput.

Reliability and reduced maintenance generates some of the biggest savings when the pumping system is running correctly. From the Barringer curve we can see that we must operate close to the pump’s BEP in order to maximize our efficiency and the MTBF. Reputable pump manufacturers design their pumps to operate for 20 or more years if done correctly. Some users are satisfied with five years of pump life. The opportunity from better maintenance and reliability alone, warrants a second look at pump and system efficiency. http://pumps.org

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