Figure 1 compares the values of S with the percent of BEP flow at which recirculation can begin. Since Ns is less than 2,500 for both examples, we can use the blue curve for the comparisons. The lower, red arrow shows that Pump B could begin recirculation at about 43 percent of BEP flow. The upper, red arrow shows that Pump A could begin recirculation at about 52 percent of BEP flow.

Figure 1. Suction specific speed versus percent of BEP flow

The reason I disagree with the suction specific speed maximum of 11,000 is that no wastewater pump should ever be operated anywhere near the two recirculation points shown in Figure 1. The reasons that they should not be considered have nothing to do with recirculation. The first is the cost of pumping. Once the process drops below about 85 percent of BEP flow, the cost per 1,000 gallons pumped increases substantially for most pumps.

The second reason is pump life. Once the process drops below 75 percent of BEP flow, higher head wastewater pumps will generate increased radial forces that will cause vibration and shorten the life of seals, bearings and wear rings. Why debate this? Why not just follow the engineer’s recommendation, and select Pump B? Well, you certainly can if you want to. The pump I used in my example is a high-quality unit, but so is Pump A.

The reason Pump A should be allowed is that its cost is about 40 percent lower because it has a higher rotational speed. In many applications, 1,750-rpm pumps will have a life span that is very similar to their lower speed cousins, making first cost a significant factor in the equation.

Let’s close with one more example. A higher head, higher flow pump is designed for 12,000 gallons per minute at 140 feet of total dynamic head. It has a 16-inch suction and a 22-inch impeller that rotates at 1,170 rpm. The Ns is 3,250 and the NPSHr is 21 feet. The calculated suction specific speed is 13,065, which is about the same as Pump A in the previous example.

Although the curve for Ns = 3,500 is not shown in Figure 1, the S value of 13,065 would intersect the curve at about 60 percent of BEP flow. Once again, this is well below where any wastewater pump should operate.

Remember also that NPSH margin plays a role in the suction specific speed’s predicted point at which recirculation can begin. Most wastewater applications are flooded suction and can take advantage of 100 percent of atmospheric pressure.

Also, suction submergence is typically high. At sea level, the pump in this example could experience an NPSHa to NPSHr margin of 2-to-1. In the previous examples, it could be almost 5-to-1.

In my opinion, there is nothing wrong with high S-value wastewater pumps as long as they are sized correctly for the application. After all, suction specific speed was developed to identify pumps that could undergo recirculation at significantly reduced flows when running at full speed. It should not be used as a fudge factor for poor pump selection or system design.