First presented at the Fort Wayne Technical Seminar
on Sept. 28, 2010 by Benjamin Adams, P.E. of
Commonwealth Engineers, Inc.
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Why Do We Need a Pressure Sewer?
Why Do We Need a Pressure Sewer?
Pressure Sewer Systems are one form of sanitary sewage
collection facilities that allow for the transfer of wastewater
from the end user (residential or commercial structures) through
small diameter pipes which discharge to a municipal wastewater
treatment facility or another municipal collection system in the
case of regionalization.
Pressure Sewer Systems involve the installation of low pressure,
sanitary sewer conduits constructed at a depth of 4'-5'. A
simplex grinder pump station would normally serve one structure and
could possibly serve up to four structures. These grinder
pump stations discharge the raw sewage into pressure sewer
mains. Each structure would be connected to the grinder pump
station via a gravity service lateral. A typical Pressure
Sewer System includes a grinder pump, basin, controls, piping and
valves.
Pressure Sewer Systems are useful and economical when
- Significant Uphill Topography
- High Groundwater Tables
- Waterfront Locations
- Very Flat Land
- or Other Constraints on Excavation
Advantages of a Pressure Sewer System
- Sewage is collected & removed from the property for
Treatment
- Expanded use of property with no restrictions due to location
of septic system
- Initial Depth of Installation is minimized
- Infiltration is minimized or negated
- Removal of threat of contamination of the potable water
well
- Initial Costs for construction are minimized
Disadvantages of a Pressure Sewer System
- Pump Failure can disrupt service
- Main Break can lead to raw sewage being discharged to the
ground or nearby waterway
- Electrical consumption would be higher than with a conventional
Gravity Collection System
- Associated problems of operation and maintenance and
replacement of grinder pumps and grinder pump station controls
Typical
Grinder Pump Station Package complete with Control Panel and
Environmentally Sealed Pressure Switch >
The primary component of any Pressure Sewer System is the
grinder pump. There are two primary pumping technologies
utilized for grinder pumps for use in residential Pressure Sewer
applications.
The two types of pumps are Centrifugal and
Progressive Cavity.
Both styles of pumps have been implemented in basins for
Municipal residential sewage disposal for over thirty (30)
years.
Progressive Cavity Grinder Pumps (Positive Displacement)
Progressive Cavity (PC) grinder pumps utilize two
primary components, a metal rotor and a matching elastomeric
stator. The rotor turns within the stator creating a
sequence of sealed chambers. This interaction depends on
friction between each other in order to seal
against the developed pressure. The higher the operating
pressure, the higher the friction and the rate of wear. Even when
conditions are ideal, the friction interface of a PC pump is
continually wearing.
Like all
positive displacement pumps, a Progressive Cavity pump develops
whatever pressure, up to the point of stall (or stator rupture!),
is needed to overcome system pressure. If a Progressive
Cavity pump is operated against a closed valve or blocked line, the
result is pressure much higher than design limits and accelerated
stator wear and potential motor failure.
One perceived advantage of a Progressive Cavity or PD Pump is
that it pumps a fairly narrow range of flows depending on system
pressure and that it can handle these same flows in high head
(greater than 100') conditions/applications. As a result, many
engineers specify PC pumps when higher heads are expected.
Progressive Cavity pumps require a high torque
motor for starting. The high torque capability of the motor allows
the pump to generate a series of short duration, high-pressure
pulses that vary with system conditions. Since stator life
exponentially decreases with increasing pressure, these pulses can
be a large factor in cumulative stator fatigue
failures.
Progressive Cavity pumps are also susceptible to accelerated
wear due to abrasion from grit, sand and other
hard particles. Progressive Cavity pumps (not grinders) are
frequently used to pump highly viscous and abrasive fluids, but the
pumps are limited in speed and still exhibit
considerable erosion.
Centrifugal Grinder Pumps
Centrifugal grinder pumps utilize a grinder mechanism coupled
with a vortex-style centrifugal impeller that pumps the sewage
slurry into the piping system. Centrifugals have the ability to
provide high flow rates to ensure scouring of the
discharge line with high velocity. This self-cleaning action has
typically been limited to low or medium head applications.
Until only recently, Progressive Cavity pumps were the only way
to handle heads higher than 115-120 feet of head without resorting
to large diameter 5-7.5 horsepower pumps. Today, several
manufacturer's have high head centrifugal grinder pumps capable of
providing for heads over 180 feet at 8-9 GPM and
shut-off heads in excess of 200 feet with 2 HP pumps. Many of
these centrifugal models can pump nearly 30 GPM at
lower heads.
Most high head centrifugals utilize vortex impellers that easily
handle solids and tolerate continuous operation on
any part of their respective pump curves. Centrifugals can
run continuously at their shutoff head, a condition that can
conceivably create a catastrophic consequence for a PC pump.
No over-pressure cut-out devices are required.
A
Centrifugal grinder pump requires far less torque
than a Progressive Cavity pump at startup. This means that,
with a Centrifugal design, there is significantly more torque
available on starting to dedicate to the grinder compared to a
PC. Most vortex -style impeller designs also feature a
shredding device that shreds solid and stringy material much
more effectively than "garbage-disposal" style cutters.
Lower torque results in increased life expectancy of the cutter
components.
A
"vortex" impeller is so named because the swirling
action of its blades creates a vortex within the casing. This
vortex captures any solid particles and moves them out of the
casing discharge. During this process, less than 10% of the
particles come into contact with the impeller or casing,
dramatically reducing the potential for abrasion
and wear. Typical life expectancy of the impeller is that of
the pump itself.
According to industry studies, power consumption of
the 2 HP Barnes - OGPTM, on a power cost per year basis, is
comparable to competitive 1 HP Progressive Cavity
pumps. Most centrifugal, high head grinder pumps can be
selected for Pressure Sewer applications anywhere
along their respective pump curves. At lower heads, the
grinder pump basin is simply emptied much faster than with a PC
grinder pump due to the high flow nature of the centrifugal
design.
Grinder Pump Design Considerations
The Pressure Sewer System's force mains are
intended to accommodate some maximum flow rate based upon their
expected design flow from a certain percentage of their grinder
pump connections during some peak flow period.
A Pressure Sewer System is also oftentimes
designed to accommodate some future expansion of the initial
service area, but in lieu of increasing pipe sizes to
accommodate this future flow, it is not usually designed to handle
the maximum flow from all of the grinder pumps at one time.
This would be considered the exception to its operation, not normal
operating conditions.
One important concept to keep in mind is that velocity in a
Pressure Sewer main:
- Does not decrease as volume increases
- Velocity actually increases as more sewage enters the main
until we reach some maximum as defined by system volume and
pressure
Typically, system pressure on a Pressure Sewer
System is self limiting meaning maximum volume and
velocity can be maintained without adverse effects.
One key trait of the centrifugal grinder pump is that it is self
limiting with respect to both flow and pressure. When system
pressure is low, the centrifugal grinder will seek a higher flow
point on its performance curve thereby increasing its scouring
velocity. When system volume and therefore pressure increases,
the centrifugal grinder will climb back up its system curve to a
point where its flow is reduced to the corresponding pressure.
The progressive cavity grinder pump is NOT self
limiting with respect to either flow or pressure. It is
designed to maintain flow at its specified rate of
flow. Therefore as system volume increases, it will increase
its output pressure to whatever value is required to support its
pump rate. In many applications, this would be considered a
good thing, however, in select instances this can lead to damaging
over pressure.
In the case of system overload, (too many pumps running at once)
or system failure (closed valve or plugged main), the centrifugal
grinder will operate at its pump 'shut-off' head which means the
pump will idle and pump at zero flow. The progressive cavity
pump's inherent design will cause it to attempt to maintain flow at
all costs regardless of system volume or pressure.
If in the case of a system-wide power outage and too many pumps
attempt to start at the same time, the flowing can occur for the
progressive cavity grinder pump:
- friction due to increased flow creates extremely high head
pressures
- pump motors will overload as thermal circuits are tripped
- extreme instances may cause pipe ruptures or pump failures
If in the case of a system-wide power outage and too many pumps
attempt to start at the same time, the flowing can occur for the
centrifugal grinder pump:
- Friction due to increased flow creates extremely high head
pressures
- Pump approaches its shut-off head idling until system pressures
are reduced
- Automatically increases its flow and seeks the optimal point on
its system curve as conditions evolve within the Pressure
System
After effects from system over load include the following:
- Overloading and overheating damages the motor windings and can
lead to premature pump failure - Progressive Cavity pumps
- Idling, outside of power consumption which is estimated at ΒΌ
the normal power consumption when operating at normal flow
conditions, has no adverse effects to the Centrifugal pump
Velocity is desirable due to these facts:
- Pressure Sewers rely on small diameter, sanitary sewer force
mains and pressure sewer service laterals designed to convey the
sewage slurry to the appropriate point of connection
- Scouring velocity of 2 ft./sec. is required to maintain a
minimum scouring velocity per IDEM standards
- As minimal grinder pumps are in operation, flow and therefore
velocity are actively reduced within the system
Progressive Cavity pumps perform poorly when considered simply
for velocity due to the fact that their design has low flow, fixed
rate characteristics.
Centrifugal pumps seek the highest flow hey are capable of
producing and increase system velocity and in turn, the cleansing
action it creates within the Pressure System.
Fact:
- One (1) centrifugal grinder pump will create the same fluid
velocity at low head conditions as five (5) progressive cavity
grinder pumps
- One (1) centrifugal grinder pump will produce the adequate
scouring velocity, as required by IDEM, to maintain a 2" force
main
- For a Progressive Cavity grinder pump, we would need as many as
four (4) pumps to maintain the same velocity operating under normal
conditions
Finally, the high speed cutter assembly of the centrifugal
grinder creates an extremely fine slurry which is nearly entirely
water. This, coupled with the fact that the pumps have the
ability to vary flow and pressure based upon the existing operating
conditions all allows for reductions in electrical consumption in
proportion to reductions in flow.
Final
Thoughts
Unexpected high rates of inflow due to
unauthorized connections to sump pumps, roof drains or downspouts
generate more frequent pump starts and additional operating
time. Consequently, pump wear is accelerated
and pump life is shortened.
Similarly, ground water infiltration into the
basin package due to leaking connections or damaged piping also
results in reduced pump life due to higher start
frequency and higher operating time. This condition has a
greater effect on the PC pump due to the pump's inherent wearing
issues.
Summary of Pump Pros/Cons
Centrifugal Advantages
- Self Limiting w/Respect to Flow/Pressure
- Hydraulic Performance that Mets or Exceeds PC Grinder
Pumps
- Shut-Off Head - Flow to Zero
Progressive Cavity (PD) Disadvantages
- Not Self Limiting
- Stator Interface is Continually Wearing
- Motor Overloads at High System
Pressures
References include the following
"Centrifugal versus Positive Displacement Grinder Pump Systems"
- Joe Evans, PHD, Pacific Liquid & Air Systems"
"OGP Pumps in Residential Applications" - Crane Pump &
Systems
