|David W. Spitzer, P.E.|
Sometimes simple rules of thumb are good enough to solve the problem at hand. Suppose that an installed system consists of a water tank, a pump, and discharge piping to an atmospheric vessel located 60 feet above the pump. The pump is operating and all valves in the piping are completely open. The flowmeter installed between the pump and tank indicates a low flow and the tank fills slowly. What potential problems could exist and how would you address them?
A. Flowmeter is not operating properly
B. Pump impeller is too large
C. Pump impeller is too small
D. Pump discharge pressure is too low
E. Piping is obstructed
If the flowmeter is not operating properly (Answer A), the measurement could be low and be indicative of a problem that does not exist. In this case, the slow filling of the vessel is indicative of a real problem. Nonetheless, the flowmeter should be verified to be operating properly.
Increasing the impeller diameter increases the discharge pressure of the pump. In most applications, this condition would not cause the flow to be low. Answer B is not correct.
The accuracy of the pressure gauge on the pump discharge should be verified. This often entails installing a replacement gauge, or if no gauge exists, the installation of a new gauge. What could be the problem if an accurate pressure gauge measures 27 PSIG at the pump discharge? The “rule of thumb” is that two feet of water column corresponds to one PSI of pressure and 27 PSIG will support a water column 54 feet high. If this calculation were exact, there would be no flow because 60 feet must be overcome to flow into the tank. Yet, here there is flow because the exact factor is 2.31, and the pump is generating 62.4 feet of water column. With only about 2.4 feet of water column remaining for friction losses, it is certainly possible that the pump impeller could be undersized (Answer C) or the pump discharge pressure is low (Answer D).
The “rule of thumb” is easier to apply in the metric system because 10 meters of water column corresponds closely to one bar of pressure. If the tank was located 20 meters above the pump and the pump discharge pressure was measured at two bar, there would be little or no pressure available for friction losses. In this case, it is likely that there would be little or no flow to the tank.
If the flow was low and the pump discharge pressure in the first example was 45 PSIG (over 90 feet of water column), there is likely a restriction in the piping system (Answer E). The restriction could be an undersized piping system that exhibits a large pressure drop, debris that obstructs flow and is more likely in new installations, or perhaps a partially closed valve that restricts flow.
Additional Complicating Factors
There are a number of issues that can occur to complicate the issue. For example, the fluid could be non-Newtonian whereby its viscosity increases as pressure (pumping) increases, so its flowrate could decrease with a larger impeller. Pressurizing the vessel would increase the pump pressure requirements in order to overcome the static pressure in the vessel.
David W. Spitzer, P.E., is a regular contributor to Flow Control. He has more than 30 years of experience in specifying, building, installing, startup, and troubleshooting process control instrumentation. He has developed and taught seminars for over 20 years and is a member of ISA and belongs to the ASME MFC and ISO TC30 committees. Mr. Spitzer has written a number of books concerning the application and use of fluid handling technology, including the popular “Consumer Guide” series, which compares flowmeters by supplier. Mr. Spitzer is currently a principal in Spitzer and Boyes LLC, offering engineering, product development, marketing, and distribution consulting for manufacturing and automation companies. He can be reached 845 623-1830.