PUMP SYSTEM HYDRAULIC DESIGN
5.6 HYDRAULIC CONSIDERATIONS IN PUMP SELECTION
5.6.1 Flow Range of Centrifugal Pumps
The flow range over which a centrifugal pump can perform is limited, among other things, by the vibration levels to which it will be subjected. As discussed in API Standard 610 (American Petroleum Institute, n.d.), centrifugal pump vibration varies with flow, usually being a minimum in the vicinity of the flow at the BEP and increasing as flow is increased or decreased. The change in vibration as flow is varied from the BEP depends on the pump’s specific speed and other factors. A centrifugal pump’s operation flow range can be divided into two regions. One region is termed the best efficient or preferred operating region, over which the pump exhibits low vibration. The other region is termed the allowable operating range, with its limits defined as those capacities at which the pump’s vibration reaches a higher but still “acceptable” level.
ANSI/HI Standard 1.1–1.5 points out that vibration can be caused by the following typical sources (Hydraulics Institute, 1994):
1. Hydraulic forces produced between the impeller vanes and volute cutwater or diffuser at vane-passing frequency.
2. Recirculation and radial forces at low flows. This is one reason why there is a definite minimum capacity of a centrifugal pump. The pump components typically are not designed for continuous operation at flows below 60 or 70 percent of the flow that occurs at the BEP.
3. Fluid separation at high flows. This is one reason why there is also a definite maximum capacity of a centrifugal pump. The pump components typically are not designed for continuous operation of flows above about 120 to 130 percent of the flow that occurs at the BEP.
4. Cavitation due to net positive suction head (NPSH) problems. There is a common misconception that if the net positive suction head available (NPSHA) is equal to or greater than the net positive suction head required (NPSHR) as shown on a pump manufacturer’s pump curve, then there will be no cavitation. This is wrong! As discussed in ANSI/HI 1.1–1.5–1994 and also by Taylor (1987), it takes a suction head of 2 to 20 times the NPSHR value to eliminate cavitation completely.
5. Flow disturbances in the pump intake due to improper intake design.
6. Air entrainment or aeration of the liquid.
7. Hydraulic resonance in the piping.
8. Solids contained in the liquids, such as sewage impacting in the pump and causing momentary unbalance, or wedged in the impeller and causing continuous unbalance.
The HI standard then states: The pump manufacturer should provide for the first item in the pump design and establish limits for low flow. “The system designer is responsible for giving due consideration to the remaining items” (italics added).
The practical applications of the above discussion can be accomplished by observing what can happen in a plot of a pump curve-system head curve, as discussed in Fig. 5.6. If the intersection of the system curve with the pump H-Q curve occurs too far to the left of the BEP (i.e., at less than about 60 percent of flow at the BEP) or too far to the right of the BEP (i.e., at more than about 130 percent of the flow at the BEP), then the pump will eventually fail as a result of hydraulically induced mechanical damage.
5.6.2 Causes and Effects of Centrifugal Pumps Operating Outside Allowable Flow Ranges
As can be seen in Fig. 5.6, a pump always operates at the point of intersection of the system curve with the pump H-Q curve. Consequently, if too conservative a friction factor is used in determining the system curve, the pump may actually operate much further to the right of the assumed intersection point so that the pump will operate beyond its allowable operating range, Similarly, overly conservative assumptions concerning the static head in the system curve can lead to the pump operating beyond its allowable range. See Fig. 5.13 for an illustration of these effects. The following commentary discusses the significance of the indicated operating points 1 through 6 and the associated flows Q1 through Q6:
• Q1 is the theoretical flow that would occur, ignoring the effects of the minor headlosses in the pump suction and discharge piping. See Fig. 5.12 for an example. Q1 is slightly to the right of the most efficient flow, indicated as 100 units.
• Q2 is the actual flow that would occur in this system, with the effects of the pump suction and discharge piping minor losses included in the analysis. Q2 is less than Q1, and Q2 is also to the left of the point of most efficient flow. As shown in Fig. 5.7, as the impeller wears, this operating point will move even further to the left and the pump will become steadily less efficient.
• Q1 and Q2 are the flows that would occur assuming that the system head curve that is depicted is “reasonable”: that is, not unrealistically conservative. If, in fact, the system head curve is flatter (less friction in the system than was assumed), then the operating point will be Q3 (ignoring the effects of minor losses in the pump suction and discharge piping). If these minor losses are included in the analysis, then the true operating point is Q4. At Q3, the pump discharge flow in this example is 130 percent of the flow that occurs at the BEP. A flow of 130 percent of flow at the BEP is just at edge of, and may even exceed, the maximum acceptable flow range for pumps (see discussion in Sec.
5.6.1). With most mortar-lined steel or ductile-iron piping systems, concrete pipe, or with plastic piping, reasonable C values should almost always be in the range of 120–
145 for water and wastewater pumping systems. Lower C usually would be used only when the pumping facility is connected to existing, old unlined piping that may be rougher.
• If the static head assumed was too conservative, then the actual operating points would be Q5 or Q6. Q5 is 150 percent of the flow at the BEP;. Q6 is 135 percent of the flow at the BEP. In both cases, it is most likely that these flows are outside the allowable range of the pump. Cavitation, inadequate NPSHA, and excessive hydraulic loads on the impeller and shaft bearings are likely to occur, with resulting poor pump performance and high maintenance costs.
5.6.3 Summary of Pump Selection
In selecting a pump, the following steps should be taken:
1. Plot the system head curves, using reasonable criteria for both the static head range and the friction factors in the piping. Consider all feasible hydraulic conditions that will occur: (a) Variations in static head (b) Variations in pipeline friction factor (C value) Variations in static head result from variations in the water surface elevations (WSE) in the supply reservoir to the pump and in the reservoir to which the pump is pumping. Both minimum and maximum static head conditions should be investigated:
FIGURE 5.13 Determining the operating point for a single-speed pump.
• Maximum static head. Minimum WSE in supply reservoir and maximum WSE in discharge reservoir.
• Minimum static head. Maximum WSE in supply reservoir and minimum WSE in discharge reservoir.
2. Be sure to develop a corrected pump curve or modified pump curve by subtracting the minor losses in the pump suction and discharge piping from the manufacturer’s pump curve (Table 5.5 and Fig. 5.13). The true operating points will be at the intersections of the corrected pump curve with the system curves.
3. Select a pump so that the initial operating point (intersection of the system head curve with the pump curve) occurs to the right of the BEP. As the impeller wears, the pump’s output flow will decrease (Fig. 5.7), but the pump efficiency will actually increase until the impeller has worn to the level that the operating point is to the left of the BEP.
For a system having a significant variation in static head, it may be necessary to select a pump curve so that at high static head conditions, the operating point is to the left of the BEP. However, the operating point for the flows that occur a majority of the time should be at or to the right of the BEP. Bear in mind that high static head conditions normally only occur a minority of the time: The supply reservoir must be at its low water level and the discharge reservoir must simultaneously be at its maximum water level-conditions that usually do not occur very often. Consequently, select a pump that can operate properly at this condition-but also select the pump that has a BEP which occurs at the flow that will occur most often. See Fig. 5.14 for an example.
4. In multiple-pump operations, check the operating point with each combination of pumps that may operate. For example, in a two-pump system, one pump operating alone will produce a flow that is greater than 50 percent of the flow that is produced with both pumps operating. This situation occurs because of the rising shape of the system head curve; see Fig. 5.8. Verify that the pump output flows are within the pump manufacturer’s recommended operating range; see Fig. 5.13.
5. Check that NPSHA exceeds the NPSHR for all the hydraulic considerations and operating points determined in steps 1 and 3.