Nowhere will you find a more concise overview of an engineered product's capability than on a centrifugal pump's "oak tree" performance curve. The infinite amount of information from a single graph is astounding. It can be used for everything from basic hydraulic capability to the establishment of several additional parameters essential to good pump application processes.
It's also important to note that a pump's particular speed is a combination of many factors including:Chemical reations occuring in the liquid
- The actual head requirements of the system at specific flow rates
- The fluid temperature
- The physical suction conditions present
- The physical characteristics of the liquid being pumped
- Chemical reactions occurring in the liquid being processed.
We'll be covering the basics of the pump curve and how you can begin to use this knowledge to better your business!
Pump Curve Development
A centrifugal pump is basically a kinetic machine. The pressure it develops is primarily a function of peripheral velocity established by the rotating speed and diameter of the impeller.
A pump doesn't directly develop pressure but, based on a velocity relationships, produces a "head" in feet. The head remains the same for all liquids with the same viscosity. The head can be transformed into a pressure term utilizing the formula:
Total Dynamic Feet of Head = PSI x 2.31
It becomes apparent that the pressure which a pump develops depends on the weight per unit volume of the liquid being pumped. The capacity delivered by a pump is directly proportional to the area of its passages and the velocity of the liquid through these passages.
The graph also indicates the pump's drive requirements relative to speed (revolutions per minute) and power (brake horsepower) based on handling water. In addition, a complete pump curve will indicate efficiency of the unit.
The most common shape of the total developed head versus capacity characteristics curve for a conventional design volute centrifugal pump is the constant rising characteristic shown below.
The maximum head is usually developed at zero capacity — commonly referred to as "shut-off." As the capacity increases, the head decreases. The factors responsible for this are:
1. The direct proportion of energy gradient drop necessary to produce flow
2. The friction loss through the pump passageways.
Another commonly provided characteristic is the power curve indicating the pump's actual brake horsepower (BHP) requirements relative to capacity.
From these two curves, we can determine the pump's efficiency using this formula:
This, in turn, allows us to develop an efficiency versus capacity curve:
After having graphically established the pump's performance for a maximum imeller diameter, the exercise is normally repeated for several smaller incremental impeller diameters, each utilizing a maximum brake horsepower requirement at 1.0 specific gravity (water) equivalent to nominal full-load motor horsepower. The resulting data is incorporated on a single graph to present a complete picture of a pump's hydraulic capability and power requirements.
We are now in a position to evalutate a pump's performance to meet specific hydraulic requirements!
To reduce the number of variables, each pump performance is corrected to its particular design speed. Characteristics are usually established based on handling cold, clear water. Other liquids with properties differing from 68 degrees Fahrenheit water, such as specific gravity, viscosity, temperature, and the presence of solids combined with abrasiveness, have varying effects on a pump's hydraulic performance and power requirements. Most effects can be reasonably predicted by theoretical or empirical means. Others must be sorted out by actual test of the fluid medium.
And that wraps up the pump curve introduction! You can use the pump curve for test specifications, helping you pick the right pump, testing your current pumps, and much more.