Excerpt from the December 2021 Pumps & Systems Article by Heinz P. Bloch
Understand cavitation, suction energy, specific gravity and more.
Process pumps are machines that take liquid from a suction condition to a discharge condition. In its typical form, there usually is a shaft-mounted impeller (or impellers) that, together, make up a rotor. Impellers impart velocity to the fluid as it passes from pump suction to pump discharge. Whereas in the pump casing’s internal passageways, the fluid velocity decreases and is converted to an elevated discharge head.
However, the resulting pressure is a function of the specific gravity of the product being pumped and the expression “head” is a universal indicator of the pump’s output. The height to which a droplet of water can be lifted as it leaves the tip of an impeller is determined strictly by the circumferential velocity with which the droplet leaves the tip. That velocity is a function of impeller diameter and shaft rotations per minute (rpm). Translation: At a certain speed and for a certain impeller diameter, a droplet of water with a specific gravity (SG) of 1.0 will be propelled to a height of, say, 100 feet (ft) and so would a droplet of mercury, which has an SG of 13.6. Comparing the pressure exerted by a column of mercury Y feet high with that of an equal-height column of water, the former would be 13.6 times greater than the latter, as would the power demanded to produce equal heads.
A certain pressure is needed to force a quantity of liquid into the eye of an impeller. The force (or pressure) also must overcome casing and pipe friction while creating a head of a certain magnitude. To design and manufacture a pump defined by needed flow and pressure, the OEM will propose a pump with an impeller requiring a designed-in net positive suction head (NPSH). The plant design needs to provide an available head or driving force (NPSHa) in excess of NPSHr (required). If there is no reasonable increment of NPSHa over NPSHr, the pump will cavitate and, other parameters remaining unchanged, the possible operating hours of the impeller(s), bearings and seals will be reduced. If a plant designer insists that a low NPSHa must be accommodated, the manufacturer’s offer will likely contain compromises or constraints that are of consequence to the buyer.
For decades, it had been incorrectly assumed and taught that a 1 foot (ft) incremental head difference was all that was needed. With evidence mounting that, for ammonia and other aqueous liquids, even 20 ft was barely enough and a multifaceted problem had to be faced nearly 100 years ago. It may require an NPSHa from two to 20 times NPSHr to fully suppress cavitation within a pump. Much depends on pump design, flow ratio (percentage of best efficiency point [BEP] flow) and how close the liquid’s temperature is to its vaporization temperature. Whatever fraction of driver power is converted into heat rise must not elevate temperatures to cause portions of the liquid to vaporize.
Cavitation is the effect of vapor bubbles collapsing and giving way to the surrounding liquid impinging on portions of the pump impeller. Collapsing bubbles cause variations in pump discharge pressure, which prompted interested parties to agree that a 3% fluctuation of discharge pressure indicated the onset of cavitation. However, by the time it becomes clear from a discharge pressure gauge needle, which hovers or jitters nervously between 100% and 97% of normal discharge pressure achieved, it is best to pay attention. Although industry calls it the onset of cavitation, destructive vibratory forces and impingement by collapsing vapor bubbles may already be well underway.
The pressure gauge needle dance is caused by collapsing vapor bubbles, which are accompanied by tiny high-velocity fluid jets that strike at points where collapsing bubbles contact impeller vanes or other internal surfaces. The struck material will erode, leaving behind millions of cavitation pockets. The extent of cavitation-induced damage differs, thus materials are classified in accordance with life factors. Mild steel is ranked lowest with an assigned number of 1. Aluminum-bronze tops the list of cavitation-tolerant materials with an 8, while stainless steels (4) are near the middle.
Next came the belated emphasis on where NPSH differential head plot lines should intersect the vertical (Y-axis) of such plots. Since about 1960, many researchers found this tends to vary greatly and none intersected the vertical axis (Image 1). Curve trends varied with pump suction specific speed (Nss), which is calculated by taking the quantity (rpm [gpm per impeller eye])^1/2 and dividing it by (NPSHr)^3/4. Still, many phantom plots created the impression that operation at almost zero flow would be allowed and would result in a low delta NPSH requirement. But low flows cause pump-internal recirculation, which also creates vapor bubbles and curtails pump life.
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