Pumps and Systems, July 2009
Q. Simplicity and low cost make rotary pumps a good choice for some applications. Is there a simple guide to the maximum flow rate and discharge pressure for which such pumps can be used?
A. The Hydraulic Institute has recently published a revised standard, ANSI/HI 3.1-3.5-2008, Rotary Pumps for Nomenclature, Definitions, Applications and Operation. Included in this standard are charts for discharge pressure versus flow rate for both metric and U.S. customary units. These charts include 11 different coverage lines in color for the 11 different rotary pump types. The chart in U.S. customary units is shown in Figure 1.
Figure 1
ANSI/HI 3.1-3.5-2008 includes the following changes and additions:
- The rotary pump Consolidated Range Chart in color shows the performance map of 11 different rotary pump types in metric and U.S. customary units
- The rotary pump Capability Table describes the operating envelope of 11 different rotary pump types in terms of solids handling, dry running, reversible operation, resistance to abrasives, allowable temperature range, shear sensitivity and self-priming ability
- Learn the basic methods of calculating rotary pump volumetric efficiency, discharge and suction pressure, Net Positive Inlet Pressure Available (NPIPA) and Required (NPIPR), and pump power and efficiency
- Understand the effects of entrained gas and dissolved gas in saturated solution on liquid flow rate of rotary pumps
Q. We are operating a large horizontal centrifugal pump lifting water from a lake, and the flow rate gradually reduces with time. However, when the pump is restarted, the rate of flow returns to its original, normal value. What is the cause of this problem?
A. The likely cause is air binding in the impeller. When operating on a lift, the impeller inlet is under a vacuum, and any leaks in the suction piping or shaft seal will allow air to enter the impeller. When this happens, the air can accumulate in the impeller eye due to the centrifuge effect, which forces the air to the impeller's center. The air bubble in the impeller eye increases in size and blocks the flow area, thereby reducing the flow rate. When the pump is shut down, the air rises in the discharge pipe and is expelled when the pump is restarted.
This problem is not common in horizontal pumps, but is more likely to occur at flow rates lower than the best efficiency flow rate. A centrifugal pump can normally operate successfully with up to 6 percent of entrained air, although the flow rate is reduced due to the effective reduction in eye area that the air bubble causes. However, as the flow rate is reduced, the bubble can increase in size, which further reduces flow and may even reduce the flow to zero.
Maintaining leak-free suction piping and a secure shaft seal are the obvious steps to avoid this problem.
Q. Some engineers refer to "ANSI pumps," but a web search on ANSI (American National Standards Institute) generates a list that is nearly 400 pages long. What does this designation represent?
A. The most common use of "ANSI pumps" in the pump industry refers to pumps built to ASME B73.1 Standards. An ANSI committee developed the standard during the 1960s and later gave it to ASME to administer.
The first such standard is ASME 73.1, End Suction Centrifugal Pumps for Chemical Process. This standard includes dimensions, performance, materials of construction and design specifications for about 15 different pump sizes. Although originally developed for the chemical industry, it is often specified by other pump purchasers because of the proven reliability, competitive availability and dimensional interchangeability of ANSI/ASME pumps.
There are three additional ASME B73 Pump Standards:
- B73.2, Vertical In Line Centrifugal Pumps for Chemical Process
- B73.3, Specifications for Sealless Horizontal End Suction Centrifugal Pumps for Chemical Service
- B73.5M, Thermoplastic and Thermoset Polymer Material Horizontal End Suction Centrifugal Pumps for Chemical Service
