by Joe Evans, PumpTech, Inc.
December 17, 2011

The example in this column has an Ns of 2,450, so the upper curve will be used. The red horizontal line at Y = 17,337 intersects the curve at X = 80 percent. Therefore, this pump could potentially begin suction recirculation when the flow drops to just 80 percent of BEP flow. If the suction energy ratio (Pumps & Systems, February 2010) of this pump is examined, an NPSHa to NPSHr margin of 4.0 could be required to provide stable operation at or below 80 percent of BEP flow.

Many higher flow pumps with relatively low values of S can still exhibit an increase in NPSHr as flow is reduced. For example, the performance curve for an 18-inch by 16-inch wastewater pump (Ns= 2,735) with a BEP flow of 16,000 gallons per minute at 200 feet shows an NPSHr of 31 feet. The calculated value of S is 11,072, which predicts an MCSF of approximately 50 percent. However, at 13,000 gallons per minute (a flow reduction of just 19 percent), the NPSHr, as shown on the curve, increases to 44 feet.

Submersible Wastewater Pumps

Submersible wastewater pumps can be especially problematic. Although some undergo comprehensive NPSHr testing, many are not tested at all. Others are tested at BEP only, and NPSHr values are calculated at other flow points. Often it is assumed that the additional inlet pressure provided by submersion will provide adequate NPSH.

Actually, submersion offers no NPSH advantage since pumps installed in dry pits also have a similar level of submersion due to the water level in the wet well. Submersibles do have one advantage, and that is lower inlet losses due to no inlet piping and fittings. This, of course, goes away when they are installed in a dry pit submersible application.

If the manufacturer does not provide NPSHr data, how can end users identify potentially problematic submersible pumps? There is no perfect way, but what I do is compare untested pumps with similar ones from other manufacturers that have been tested at multiple flow points. To get a good comparison, end users need to compute the Ns of the untested pump and compare it to tested pumps with the same or similar Ns, rotational speed, hydraulic efficiency, eye diameter ratio, flow and head.

Here are some rules of thumb. S is directly proportional to rotational speed and the square root of flow. Therefore, higher speed and higher flow pumps will more likely have higher values of S. I have found that most 1,750-rpm pumps with 4-inch discharges and flows under 800 gallons per minute will have an S under 10,000, and 1,750-rpm pumps with 6-inch discharges and flows under 1,750 gallons per minute have similar values of S. However, as the eye diameter ratio of either approaches 0.6, S can exceed 13,000. If an end user plans to run a wastewater pump to the left of BEP and NPSHr is not part of the test curve, he should get the manufacturer to sign off on the application.

 

Pumps & Systems, November 2011

 

Click here for a Readers Response to this article.

Pages