Engineering Design Manual - Loudoun Water

Chapter 9: Sewage Pumping Stations
9.7 Design Criteria for Pumping Stations
All sewage pumping stations will be designed in accordance with the following criteria.
Additional requirements of SCAT Regulations shall also apply.
A. Pump Selection
1. All pumps shall be warranted against defects in workmanship and material for a period of
5 years or 10,000 hours of operation under normal use, operation and service. The
warranty period must start at time of final acceptance of the station by Loudoun Water,
and shall be non-prorated. All pumps using a center shaft shall be equipped with
oversized bearings (Class B).
2. All pump openings and passages shall be large enough to permit the passage of a sphere
3-inch in diameter and any trash that can pass through a 4-inch house collection system.
All pumps will have cleanout ports on the volute.
3. For new pumping stations, prepare Total Dynamic Head (TDH) calculations based on the
initial new pipe C factor and the anticipated C factor after the force main has been in
service for several years. When expanding the capacity of an existing pumping station
which has been in service for many years, the actual C factor of the force main should be
determined by field measurements.
4. Typically the TDH calculations are based on the entire length of force main pipe from the
pumping discharge to the discharge manhole of the gravity system. However, when
pumping from one valley over a hill to a gravity sewer on the other side of the hill, it is
important to consider whether or not there is free fall in the down hill segment of the
force main such that the downhill length of pipe does not need to be considered in the
TDH calculations. If the static head from the force main’s high point to the discharge
point exceeds the full flow head loss in the downhill segment of force main, sewage
should flow by gravity through the force main similar to a gravity sewer instead of as a full
pipe under pressure.
5. Consideration must be given during design to include a small submersible jockey pump in
the wet well, which will operate almost continuously through the day at a lower flow rate.
This lower flow rate will significantly reduce the friction loss and energy requirements
while providing reliable service. The larger pumps should only operate a few times a day,
(during peak flow periods in the morning and evening) which is sufficient to scour the
force main.
B. Protection Against Clogging
1. All pumping stations shall have a comminutor vault at the influent portion of the station.
The comminutor will be sized for the estimated peak hourly flow into the station.
2. The comminutor will be designed to be easily removed from the flow channel without
disturbing any piping connections. The comminutor channel shall be designed to settle
Engineering Design Manual
September 2010 141