Some Ideas - by John Hasslen
Over sizing pumps tends to be the most common cause of premature failure.
A centrifugal pump and motor are happiest if they run all the time.
- Less heat is generated
- Less precipitation and build-up of scale
- Fewer solids will settle out
One obstacle in sizing leachate pumps close to generation rate is the 100-year storm event requirement of many regulators.
This very often leads to over sized pumps.
I would like to offer these suggestions as possible solutions to this sizing dilemma:
- Use 100-year generation to size pump with pump running 24 hours per day (with no service factor).
- Design in more than one sump with lower flow rate pumps.
- Design sump with two or more pumps installed in one or two risers into a common sump area. These pumps could alternate running at low generation and both run at high generation. The control scheme would be lead, lag, alternating.
- Design system using variable frequency drive to change flow rate of pump. If you double the pump's speed, you will get twice the flow rate and four times the discharge pressure. For example, a pump designed for 25 GPM at 50 TDH at 1,800 RPM will produce 83 GPM at 550 TDH at 6,000 RPM. This pump would require 1HP at 1,800 RPM and 25HP at 6,000 RPM.
- Design system with pump for 100-year generation flow capacity and a flow control valve on its discharge line. This pump is used when the cell is first opened and then replaced with a lower flow pump as the cell fills with trash and the generation rate falls. Large pumps can be moved to a new cell as the site expands.
- Design system at permitting time for recirculation; pumping leachate from new cell to old cell to help promote biodegradation.
The reality of sizing pumps based on 100-year storm generation rate quite possibly will not meet regulator intent of minimum head on liner if pump and/or sump are out of service caused by over sized pumps.
Over sized pumps (high flow rate) create problems.
- High velocity
- This pulls in fine particulate matter and increases incrustation and fouling.
- Rapid start/stop cycles
- Decreases motor and pump life and increases motor surface temperature, which increases plating out on motor surface.
The goal should be to size leachate collection pumps at just above average leachate generation rate. From the pumps point of view, the closer the better.
Company’s Experience – 25+ years and over 10,000 pumps in the field.
Personal Experience – More than 40 years in the pump business.