Standard centrifugal pumps are not self-priming and require the fluid end to be primed prior to activation. This can be accomplished by installing the pump in a location that provides a flooded suction or by using a device to prime the pump. Once the pump casing is full of fluid, it can then be energized. Running a pump dry or restricting suction flow can severely damage the fluid end, mechanical seal, or packing. The designs of self-priming pumps result in turbulent flow patterns, which cause excessive wear during pumping of abrasive fluids and increase operating costs. The drilling industry avoids using self-priming pumps due to increased downtime and costs.
Once a centrifugal pump is primed and then activated, suction head at the eye of the impeller drops. Actual positive suction head required at the eye of the impeller to prevent cavitation varies by pump size and flow and is noted on pump curves as NPSHR. When this suction-head drop occurs, atmospheric pressure pushes on the liquid surface and forces it into the pump suction. As fluid enters the pump, the impeller accelerates it. The diameter of the impeller and the rpm at which the impeller is rotated directly affect the velocity of the fluid. The casing of the pump contains this velocity and converts it into head. Casing size and impeller width control the volume that the pump is able to produce.
Consider a pump suction located 5 feet above liquid level in the suction tank. Head at the suction flange will be less than atmospheric pressure. Absolute zero pressure, a perfect vacuum, is -14.7 psi, or -34 feet, of water at sea level. If the suction of the centrifugal pump is 35 feet above liquid level in the suction tank, a vacuum will exist and no fluid will enter the pump. Each centrifugal pump has a minimum suction head required above absolute zero pressure that must exist at the suction to keep the pump full of liquid. A system head that produces a suction pressure less than this value will cause cavitation. When a pump is severely cavitating, it sounds like it is pumping gravel. If a pump suction line is too small or too long or has too many valves or elbows, the friction loss in the suction line may reduce head to a value below NPSHR.
If the supply tank is at sea level and is vented, inlet pressure to the pump will be:
(34 feet – vapor pressure in feet)/(SG)±liquid level above/below pump centerline − suction head friction losses.
The sum of this calculation is the NPSHA. If NPSHA is greater than NPSHR, the pump will function as designed. If NPSHA is equal to or less than NPSHR, the pump will cavitate. It is advisable to maintain NPSHA at least 3 feet above NPSHR to allow for calculation errors or system changes. This formula is discussed in greater detail later in the chapter.
Cavitation severely reduces life of the pump. As fluid enters the pump, the pressure at the eye of the impeller drops. If insufficient inlet pressure (NPSHA) is present, fluid transforms from a liquid state to a gas (boils).
Gas forms low-pressure bubbles and as these bubbles travel from the ID to the OD of the impeller, pressure increases. Eventually the pressure increases enough to collapse the low-pressure bubbles. When this occurs, the bubbles implode, and space once occupied by the bubbles fill with fluid. Fluid fills this space with such force that it actually fractures adjacent metal. As this process repeats, it will knock out sections of the fluid end and can even knock a hole through the stuffing box, impeller, or casing. Cavitation can be caused by improper suction or discharge conditions.
Figures 1 and 2 show the result of severe cavitation in a pump that was transferring clear fluid. Notice that the fractured metal has sharp corners.
Figure 3 shows the result of cavitation in a pump that was handling abrasive fluids. After the fracture occurs, sharp corners are worn smooth by abrasive fluid. The damaged part will look as if a spoon were used to scoop sections of metal from the part.
Entrained air in transferred fluid causes excessive turbulent flow patterns and can vapor-lock the pump. Air bubbles do not collapse like low pressure bubbles. As an air bubble enters the pump, it moves from the ID toward the OD of the impeller. Increased pressure at the OD of the impeller pushes the bubble back into the ID, where it combines with other air bubbles to become a larger bubble. This process continues to occur until the bubble at the ID of the impeller becomes large enough to impede suction flow, which can cause cavitation, and/or it becomes as large as the suction inlet and prevents fluid from entering the pump, resulting in vapor lock. Once the pump is shut down, the bubble will normally escape through the discharge, but when the pump is restarted, the process repeats itself. Recirculation of air bubbles from the ID to the OD and back again also causes turbulent fluid flow patterns that will result in excessive pump wear.
Air, entrained in fluid, can enter a pump through a loose flanged or threaded connection, through the pump packing when the pump has a high lift requirement, or through an air vortex formed in the suction tank.