by Deborah Ince

Q. When purchasing rotodynamic (centrifugal or vertical) pumps, how can we estimate if the efficiency of the proposed design is the best to be expected for our application?

A. The Hydraulic Institute (HI) has recently published ANSI/HI 20.3 Rotodynamic (Centrifugal and Vertical) Pump Efficiency Prediction which includes figure 20.3a and c—Optimum normally attainable efficiency chart (U.S. customary units). The product types referred to are as follows:

A. Slurry pumps, end suction

B. Solids-handling, end suction

C. Submersible sewage, end suction

D. Paper stock, end suction

E. Horizontal multistage, axially split, segmented ring diffuser barrel

F. ASME B73, API end suction, and end suction-small

G. End suction-large, greater than 5,000 gallons per minute

H. API double suction

J. Other double suction

V. Vertical turbine bowl efficiency

Figure 20.3a — Optimum normally attainable efficiency chart (metric units)

Figure 20.3c — Optimum normally attainable efficiency chart (U.S. customary units)

In addition to efficiency variation by pump type and size, factors such as design specific speed, surface roughness of waterways, impeller wear ring clearance and other factors affect the pump efficiency. These factors are discussed in detail in the above publication.

Q. We are installing a vertical sump pump and need to know how far below the surface the inlet to the pump must be.

A. There are two considerations. The first is to satisfy the NPSH requirement of the impeller. Usually, this is not a problem when pumps are at sea level and handling cold water. However, it must be checked.

Another consideration is the possibility for the formation of a vortex at the inlet of the pump, which would allow the entrance of air and reduce pump performance. To prevent this, the distance from the liquid level to the pump inlet opening must be as shown in Figure B1.—Rate of flow versus minimum submergence.

Figure B1. — Rate of flow versus minimum submergence

Q. How is the rate of flow from rotary pumps affected by entrained gas within the liquid?

A. Most liquids are susceptible to air or gas entrainment. Air entrainment is prevalent in systems in which the liquid is circulated repeatedly and is exposed to air or mechanically agitated in air. It is also found in systems where the pump handles a quantity of air, either intentionally or unintentionally.

Many liquids are known to contain dissolved air or gas. The solubility of air or gas in liquids varies with the type of liquid, time, conditions of exposure and other factors. Lubricating oils at atmospheric pressure and temperature may contain up to 10 percent dissolved air by volume. Under similar conditions, gasoline may contain as much as 20 percent air in solution.

Entrained and dissolved air or gas in liquids handled by rotary pumps are important factors affecting pump performance, both mechanically and hydraulically, especially when negative inlet pressure exists. When the inlet pressure is below atmospheric pressure, entrained gas in the fluid expands, or dissolved gas is freed, and takes up a larger part of the pump displacement, thereby reducing its liquid flow rate. See Figures 3.3.4a and b. A saturated solution is one that contains as much dissolved gas as it can at a given temperature.

Figure 3.3.4a — Effect of entrained gas only on liquid rate of flow of rotary pumps (metric)

Figure 3.3.4b — Effect of entrained gas only on liquid rate of flow of rotary pumps (US units)

Pumps & Systems, June 2011