Pumps & Systems, December 2008
Editor's Note: This is part one of a two part series about the 'commandments' of increasing pump reliability and life. To read the second part, click here.
In Mel Brook's movie, History of the World, Part 1, Moses comes down from Mount Sinai carrying three tablets. "People of Israel", Moses exclaims, "I bring you these 15 [dropping one tablet]. . . 10 Commandments!"
Only a handful of people know the untold commandments. In the next two articles, we will review two of these and the reasoning behind them.
Commandment 11: Operation at BEP
The majority of the failure mechanisms that significantly reduce the reliability and life of a centrifugal pump are caused by operation away from the pump best efficiency point (BEP). To achieve best-in-class life, it is essential to either operate close to the pump BEP (which is always the preferred method) or to provide provisions that anticipate the various effects of off-BEP operation and attempt to mitigate these consequences.
The pump BEP is the operating condition at which the angle of the fluid entering the impeller is parallel to the impeller blade. The farther that one operates away from the BEP, the greater dissimilarity between the flow incidence angles and the inlet vane geometry. Similarly, as the pump operation becomes farther removed from BEP, the possibility of serious problems and the severity of existing problems will almost always increase.
The most detrimental effects of operation away from the best efficiency point are experienced due to:
- Suction recirculation
- Discharge recirculation
- Larger radial loads
- Opening of wear ring clearances
Each of these problems will induce other problems that negatively impact reliability and performance, such as separation-cavitation, axial shuttling or increased shaft deflection.
Suction Recirculation
Suction recirculation most often occurs when off-BEP operation is coupled with large impeller eye areas. The stalled area on the pressure side of the inlet vane, caused by the mismatch between the fluid angle and the blade angle, results in the formation of fluid swirl. The result is a separation of the fluid from the vane that has enough room (via the large inlet area) to recirculate.
When severe, the fluid circulates out of the impeller eye, interfering with normal suction flow. This fluid swirl can cause localized pressure drops, which reduce the total head generated and cause the suction pressure to fall below the fluid vapor pressure. The effect of this pressure drop is known as separation/cavitation.
The consequences of suction recirculation are increased rotor vibration from fluid instability and impeller damage due to cavitation. The design emphases to significantly reduce or eliminate these issues are:
- Reduce the impeller eye diameter to the extent allowed by suction (NPSHA) conditions
- Operate the pump as close to its BEP as possible to minimize separation effects
- Provide state-of-the-art air foil shapes to the impeller inlet vanes to be more permissive of off-peak operation
1. Reducing the Impeller Eye Diameter
Limiting the suction specific speed (NSS) of the impeller can reduce the impeller eye diameter. Suction specific speed (NSS) is a dimensionless parameter that helps define impeller geometry. When designing an impeller, lower values for suction specific speed lead to smaller eye inlet areas and less susceptibility to inlet fluid recirculation.
Empirical data has shown that the failure frequency of pump components significantly increases with an NSS > 11,000 based on a 3 percent ΔH. This analysis is based on J. L. Hallam's refinery industry study of pumps handling hydrocarbons.[af1] For water applications, a maximum NSS of 9,500 should be used to achieve reliable operation.
2. Operation Near the BEP
The affinity laws state that flow is proportional to the change in speed or diameter, and that total developed head is proportional to the square of the change in speed or diameter:
If the pump is operating away from its BEP flow for sustained amounts of time, the affinity laws can be used to modify impeller output to operate more closely to the BEP. This can be accomplished by changing the impeller diameter or by upgrading to a VFD.
Changing the pump speed is a more costly design change; however, it allows the pump to operate at its BEP for its whole range of operating requirements. Changing the impeller diameter only allows the pump to operate at its BEP for a single operating mode.
3. Bias-Wedge Impeller Design
It may not always be possible to run the pump near BEP if it is subjected to variable operating conditions. For these situations, the impeller inlet blade geometry can be redesigned as an air foil to further reduce separation effects at off-peak operation. This modification is typically referred to as a bias-wedge impeller.
With this modification, divergent flow angles follow the more rounded air foil blade shape more closely, reducing separation effects. This method is best applied if the pump is tested to obtain actual performance so that the flow approach angle can be determined.
Discharge Recirculation
Like suction recirculation, the generic root cause for discharge recirculation is a combination of large areas that allow the formation of fluid swirl and operation removed from the pump's best efficiency flow that causes a mismatch in the fluid and volute cutwater inlet vane angles.
The fluid swirl intermittently disrupts the pressure regions on both sides of the impeller, significantly reduces the damping characteristics of the wear rings and has an erosive nature. The consequences are:
- Rotor axial shuttling
- Increased vibration resulting from less damping
- Accelerated erosion of the volute cutwater, diffuser inlet blades, and/or impeller exit vanes
The design emphases to eliminate the detrimental effects of discharge recirculation are:
Gaps A & C
The fluid swirl can be straightened by forcing the fluid through a tight-fitting orifice of sufficient length. This orifice is formed through a re-design of the casing and impeller.
Gap A, the orifice clearance, is defined as the radial clearance between the impeller exit shroud and the volute inlet shroud:
Gap C, the orifice length, is the amount that the impeller exit shroud is overshadowed by the volute inlet shroud:
Referring to Figure 5, the figure on the left depicts the existing design, which has insufficient Gap A or Overlap. The figure on the right represents the recommended design.
The proper Gap A is approximately three times the design wear ring clearance. The optimum Gap C, which completes the flow straightening orifice, is four to six times Gap A.
Figure 6 shows the positive effect of the proper Gap A. The idea is to eliminate all thrust reversals for the full range of flow. Note that not only is the thrust uni-directional, but the magnitude is also significantly less.
Bias Ring Design (Double-Suction Impellers)
Both welding and the incorporation of Gaps A and C rings are costly modifications and may not be financially feasible for non-critical or low-energy applications. An alternate solution is to live with the fluid swirl but redesign the pump to obtain uni-directional thrust for any operating mode by creating an axial pre-load on the pump rotor. By arranging the bores at different diameters, a continuous unidirectional axial thrust is created:
Where:
The larger wear ring bore diameter should be designed on the inboard end of the pump to place the shaft in continuous tension.
Increased Radial Loads
Hydraulic radial loads are related to the energy output and the operation of the pump relative to its BEP. The radial load is expressed as a function of the impeller diameter, impeller width (including shroud thickness) and discharge pressure. These radial loads act perpendicularly to the axis of the pump shaft in the direction of the volute cutwater, causing the shaft to deflect.
Increased radial loads associated by operation away from BEP can be offset by incorporating either a dual volute or a diffuser into the design of the pump in lieu of a single volute. The placement of the diffuser blades or dual volute cutwaters more efficiently balances the pressure fields at any operating regime, thus reducing the unbalanced radial loads to acceptable limits.
Commandment 12: Premature Opening of Wear Ring Clearances
One of the most significant factors in increasing pump reliability and life is the maintenance of design clearances. This topic warrants a more in-depth discussion, and will be covered in the second part of this article series.