Imagine watering flowers in your yard. When your thumb is halfway over the opening of the hose, the water sprays out 10 feet or so onto your petunias. But when you move your thumb three-fourths of the way over the opening, does the water spray out a longer or shorter distance, and does more or less water come out of the hose? Of course, the water goes a longer distance, and less water comes out of the hose. The reverse is true when you cover less of the hose opening.
If you understand this very simple concept then you also understand how 99 percent of centrifugal pumps work.
A centrifugal pump uses a rotating impeller to convert input power (usually from a motor) into kinetic energy. Fluid enters the eye of the impeller and is transferred radially into, in most cases, an expanding volute. There is a conversion of velocity energy to pressure energy in the volute, and voila, pressure is generated and the fluid goes out of the pump and into the downstream piping. Centrifugal pumps can deliver high or low flows at low or high pressures, all depending on the application and what specific pump is applied.
Regardless of manufacturer or style, most centrifugal pumps have several common parts: impeller, casing, packing or seal, and bearings. The impeller is the component that adds energy to the fluid. Most impellers are either enclosed, which means that they have a front shroud covering the vanes of the impeller, or open, meaning no front shroud.
The casing is manufactured with either flanged or threaded connections. The casing transfers the liquid from the impeller through the volute to discharge. Most pumps use an expanding volute with a casing cutwater. At point A, the area within the volute is smallest, the velocity is highest and the pressure is lowest (see Figure 1). As the fluid travels through into the expanded area B, the area within the volute increases, the velocity decreases and the pressure increases. This is the velocity energy to pressure energy conversation.
The packing or mechanical seal is used to seal the pump and prevent fluid from leaking into the environment. The bearing housing is mounted to the back of the casing and carries the bearings that are mounted on the pump shaft, used to absorb radial and axial thrusts generated by the pump.
The Pump Curve
Ninety nine percent of centrifugal pump curves look similar, which also means that 99 percent of centrifugal pumps operate the same way. Flow, usually expressed in gallons per minute (gpm) is on the X-axis and pressure, expressed in total dynamic head (TDH) is on the Y-axis (see Figure 3). TDH is used instead of pounds per square inch (psi) because any given centrifugal pump will produce the same TDH, expressed in feet of liquid pumped, irrespective of specific gravity (SG). The calculation for TDH is: (PSI x 2.31)/SG). 2.31 is the constant for 1 pound of pressure being developed by a 2.31 foot high column of water.
Figure 2 shows that pumping water at 43.3 psi produces 100 TDH (think of a column of water 100 feet in the air). Even though the pressure gauge on the sulfuric acid application shows a much higher pressure, the resultant TDH is still 100 feet. The reverse it also true for the gasoline application; lower pressure but the same 100 TDH.
The formula provided above is probably the most important formula for the pump application engineer to know, especially when troubleshooting running pumps. In many cases, all you have in the field is a pressure gauge and a pump speed. Converting the pressure to TDH allows you to begin to plot points on a curve, “backing into” gpm, helping to understand where the pump is running on the curve. It is important to note that TDH is total dynamic, or developed, head. This takes into account only the actual work the pump does. So when calculating head, the suction pressure must be subtracted from the discharge pressure before the conversion to TDH. Also remember the horsepower required to do the same amount of work. It requires more horsepower to pump a heavier fluid the same distance.
Now, back to the pump curve. Centrifugal pumps are dynamic machines, or as I like to say it, they are dumb. They are slaves to the system that they are installed in. The centrifugal pump represented by this curve can run anywhere on the curve. Let’s go back to the garden watering example. So, we’re back to your thumb being half over the hose. You are “pumping” a certain flow, let’s call it 10 gpm at a TDH of 15 feet. The pump is running at point A on the curve (see Figure 3). You then put your finger three-fourths over the hose opening; less flow, more pressure. Now the pump is running at point B on the curve. Now you move your finger to cover only one-fourth of the opening; more flow, less pressure. Now the pump is running at point C on the curve.