Q: Although not typically entered, are the pump stations still typically provided with a ladder?
Jim Vukich (Xylem – Flygt Products): Yes, a ladder is advisable if there is room to accommodate it. This may involve confined-space entry issues.
Q: Does water hammer have any effect on the submersible pumps?
JV: Yes, submersible pumps are susceptible to water hammer damage just like any other pump type.
Q: What software program is used for TDH?
JV: In the presentation, screenshots were taken from the Flygt FLYPS program. Other programs are available, for example SewerCAD, RioGL, KY Pipe, etc.
Q: Regarding the power supplied to pumps, can you discuss more the "high leg delta" and "unbalanced pumping"? I've heard of such problems associated with failures of 3-phase motors. How can similar problems result from design decisions?
JV: High-leg delta is not common and is typically limited to certain localities. It does not have any real effect on pump performance, but it can impact the motor's service life.
Q: I highly disagree with sizing the force main with a 6 fps velocity. This will allow virtually no future growth. It is much better to choose a force main size that is as close to 2 fps as possible, but the flow should not be less than 2 fps to keep solids moving through the pipe.
JV: Agreed that 2 fps is the recommended minimum for preventing sedimentation within the pipe. Sizing the pipe for 6 fps maximum would be based on the peak anticipated flow rate, including any future flow rates. Keeping velocities 2 to 6 fps while handling a wide range of flows might require specific design solutions, such as dual force mains of equal or different size.
Q: Can you address air in the fluid?
JV: Centrifugal pumps can generally handle only a few percent of entrained gas by volume before they will become airbound. Entrained gas can also result in cavitation-like damage to the pump.
Q: With a duplex pump station, can the calculated minimum wet well volume be reduced by half because the pumps alternate with each cycle?
JV: In theory yes; however, in practice, the need to have firm pumping capacity means that you must size the wet well on the assumption that only one of the two pumps is in service.
Q: What are the symptoms of oversized pump?
JV: If the pumps are oversized, they will short-cycle (i.e. they will drain the wet well quickly), resulting in excessive starts per hour. This can result in higher electrical usage and shortened motor service life.
Q: Can you comment on the use of dual forcemains in a buildout scenario?
JV: This is a common solution to handling a wide range of flow rates between current and future conditions. The dual force mains can be of equal or different diameters. Valves can be employed to direct the flow from the pump station through one or both force mains as necessary.