By Allan R. Budris
Due to the desire to fully cover this topic, the subject has been divided into two successive Pump Tips & Techniques columns. This column spells out the NPSH margin issues, along with the "Suction Energy" concept. The January 2013, Part 2 column covers "NPSH Margin Reliability" and "Suction Energy vs. NPSH Margin Reliability" in significant detail, in addition to the writer's NPSH Margin recommendations.
Several of the writer's previous WaterWorld columns have dealt with the issue of pump cavitation and the desire to provide sufficient NPSH Margin for optimum pump reliability (Oct 2007, Jan. 2010, Apr. 2011 & Jan. 2012), where the message was generally "the larger the NPSH Margin the greater the pump reliability." However, it is acknowledged that these higher NPSH margin ratios (NPSHA/NPSHR) may very well come with an increased initial cost (such as larger slower pumps, or higher suction tanks), plus it is also understood that many pumps do not experience any notable damage, even when operating with appreciable cavitation. On the other hand, there are other applications which may experience significant cavitation damage under similar conditions, resulting in higher total pump life cycle costs, when including the price tags for repair and downtime (see Dec. 2009 WW column).
Cavitation starts to occur in a centrifugal pump at NPSH Margin Ratios from 2 to 20 (with the average being about 4), and it can take (on average) from 1.05 to 1.7 just to avoid any head loss from cavitation. Depending on a combination of factors, operation with pump cavitation will, or will not, cause:
- Reduced pump head
- No damage
- Increased vibration (with slightly reduced bearing and seal life) and little impeller material damage
- Significant impeller material damage and high vibration (with significant reduction in bearing and seal life)
NPSH Margin / Cavitation / Reliability Issues
There is no question that the interaction between pump cavitation, NPSH Margin and pump reliability is a complex issue that is affected by many variables, such as:
- Pump power level (above or below 45 to 75 kW/stage)
- Pump Suction Specific Speed* (above or below 11,000)
- Impeller inlet tip speed* (above or below 70 – 85 ft/sec)
- Pumped liquid thermodynamics (water, hydrocarbons, chemicals, specific gravity*)
- Impeller type* (axial flow, mixed flow, radial flow, vane over lap)
- Relative pump flow rate (Preferred Operating Region-POR, Allowable Operating Region-AOR, or low flow Suction Recirculation)
- Pump materials of construction (see Table 2)
- Pumped liquid air content (see June 2009 & Jan. 2010 WW columns)
- Abrasives in pumped liquid
- Corrosive nature of pumped liquid
- Amount of time spent in cavitating conditions
So what is the "best-of-class" pump user who values "total pump cost," not just initial cost, to do? This is why the writer spent many years and much effort, while working for several major pump manufacturers, to simplify the prediction of pump cavitation damage. The result has been a reduction in the number of relevant factors, by combining several of the key variables (indicted with an (*) in the above table) into one parameter. The method is called "Suction Energy" (see Oct. 2007 WW column), and it is reasonably effective in predicting when pumps are susceptible to cavitation noise, vibration, and/or damage, versus when they are free of these damaging effects. The method includes distinct "Suction Energy" gating values (Start of High Suction Energy) for five different pump types and three levels of suction energy (Low, High and Very High), as shown in Table 1.
