Industrial/commercial sewage pumping
stations are designed to serve a given industry and, generally, pump to the public
sanitary sewers. They are usually owned and operated by the industrial or commercial
facility.
This
article will provide guidelines for designing industrial/commercial sewage pumping
systems.
Throughout
the country, several standards are used to design sewage pumping stations. All of them,
generally speaking, use the same principal but have differences that need to be addressed.
The engineer should become familiar with the standard on which the local/state plumbing
code is based. The guidelines which follow describe a total system that is reliable and
requires little maintenance.
The
location of the pumping station will be a function of its size, but even medium to small
stations require access by maintenance crews and equipment. Ease of access should always
be considered.DETERMINING FLOW
For
industrial/commercial applications, the fixture units method is the most commonly used
method to determine the peak flow rates. A fixture unit (FU) is an arbitrarily chosen
scale that allows all of types of plumbing fixtures to be expressed in common terms. The
sole purpose of the fixture unit concept is to make it possible to calculate the design
load on the system when the system is composed of different kinds of fixtures, each having
a loading characteristic different from the others. Fixture unit values are designed for
application in conjunction with the probability of simultaneous use of fixtures so as to
establish the maximum permissible water supply and drainage load.
Most
local plumbing codes have design requirements on how to determine the fixture load for a
commercial application. Once the total fixture unit valve has been calculated, Figure 1
can be used to determine the peak flow rate into the sewage lift station. Past experience
has shown that the fixture unit method is a conservative method of estimating the peak
flow rate for a pumping station.
Once
the peak flow rate of the drain line into the sewage lift station has been determined,
pump capacity should be equal to that flow. If it is critical that fixtures be kept in
operations at all times, a duplex pump station should be specified in lieu of a simplex
station. In case of a pump failure, the other pump could carry the load while the failed
pump is being repaired.
FORCE MAIN SIZING, MATERIAL
Once
the peak flow rate has been determined, the force main can be sized. The velocity in the
force main should be a minimum of 2 feet per second (fps) and a maximum of 5 fps. This
will keep solids in suspension without generating a large had loss.
Minimum
pipe size should be 4 inches when wastewater pumps are used that have at least a 2-1/2
inch solids passing capacity in order to minimize clogging of the force main.
Force
mains can be constructed from several materials. PVC and polyethylene are being used more
often in both buried and building force mains. The construction of force mains should be
similar to water lines in that thrust restraints and blocks should be provided at bends
and tees. Air release valves should be provided at high points to prevent air locking and
siphoning. Vacuum valves shall be provided, as needed. Consideration should also be given
to clean outs so that locations at which clogs may develop can be cleaned; typically, low
spots.
SYSTEM HEAD CURVE ANALYSIS
Once
the force main has been sized, the system head curve can be determined. All elbows, pipe
fittings, and lengths of pipe run should be used to determine a total equivalent pipe
length.
The
two elements of the system head curve are: 1) the static head and 2) the friction head.
Static
head is the vertical lift of the fluid that the pump has to overcome. It is assumed to be
a constant head after the station is put into operation for a baseline of the system head
curve. It is defined as the following:
Static
Head = Highest elevation opened to the atmosphere* minus the system’s low point**
*This
will typically be the pipe outlet.
**All pumps off elevation (Suggestion: Use the average elevation between the “lead
pump on” and “all pumps off”. This will give the mid point of the pump
operation range.)
Static
head will vary during the pump down of the wet well as noted above because it changes
during the pumping cycle.
Force
main friction losses can be based on the hazen-Williams equation. Knowing the force main
size, material and equivalent length, the system head curve can be determined.
In
a given system, the friction head will vary with the flow rate, as defined by the
following equation:
where, HL= Total friction head loss, feet of water
L=Length
of equivalent
pipe
length of diameter di ft
C
= Hazen-Williams flow coefficient (see Table 2)
Q=
Flow rate, gallons per minute (gpm)
d
= Internal pipe diameter, inches (in)
The head loss through the system should be determined for each
section of the system separately based on pipe material, pipe diameter, and amount of
flow. If multiple pumps of the same size are to be operating at the same time, then the
flow rate from the pump to the common force main is assumed to be equal to one divided by
the number of pumps running.
A
general rule of thumb is to generate the system head curve with approximately 10 points
from 50 percent to 150 percent of the design flow. A separate system head curve is
generally required to determine the total capacity of a multiple operating pump station.
The system's head curve can then be plotted on the pump curve to determine the operating
point of the pump.
Table 1 - Values of
Hazen-Williams Coefficient C(1) |
| Pipe Material |
C |
Asbestos-Cement
Brass
Brick Sewers
Cast Iron:
New
5 Yrs. Old
10 Yrs. Old
Concrete (regardless of age)
Copper
Galvanized iron
Polyethylene
PVC
Riveted Steel, New
Vitrified Clay
Welded Steel, New
Wood Stave (regardless of age) |
140
130
100
130
120
100
130
130
120
140
150
110
110
120
120 |
(1) Louisville and Jefferson
County Metropolitan Sewer District (MSD) Design Manual, Table 5-2.
WET WELL SIZING
“Design
of Wastewater and Storm water Pumping Stations” Water Pollution Control Federation,
Manual of Practice No. FD-4, 1981, p. 18, indicates that the wet well shall be sized so
that the cycle time for each pump will not be less than five minutes or that the average
cycle time will not be more than 30 minutes. The shortest operating cycle occurs when the
inflow equals to one-half the pump discharge rate. Therefore, if
V
= drawndown volume, gal
q
= Pump discharge rate, gpm
Q
= Inflow rate into the wet well, gpm
t
= Minimum time of one pumping cycle in minutes, start to start
t
= (time to fill) + (run time)
When Q = q/2,
which is reduced to the operating volume where
With the operating volume, the vertical distance between the
lead pump on and all pumps off floats can be determined for various wet well sizes.
Between the operating volume and emergency storage requirement, the wet well size can be
determined. Emergency storage volume will be dependent on the required response time and
the average inflow.
After
the size of the wet well has been determined, then the distance between the floats for
lead pump on and all pumps off floats can be determined. This would be a function of wet
well size and the operating volume requirement. The vertical distance between the common
stop elevation and the bottom of the wet well is a function of the pump selected. The
common stop elevation shall not be less than the top of the pump housing or as the
manufacturer specifies, whichever is greater.
The
distance between the lead, lag, and high water levels are generally a function of the
local requirement. If mercury floats are utilized, then these should not be spaced less
than six inches apart, with the high water alarm level being at or lower than the lowest
incoming sewer line.
These
settings will determine the depth of the wet well which will allow the buoyancy
calculations to be completed. The buoyancy analysis on the wet well will determine whether
additional methods of restraint will be necessary. Mechanical equipment, water weight, and
other temporary loads should not be included in the analysis. The soil angle of repose
should be assumed to be zero degrees, unless soil analysis determines that another value
is warranted.
The
buoyancy force equals the displaced volume of the wet well and bottom slab multiplied by
the unit weight of water.
The
opposing force is equal to the weight of the wet well, bottom slab, top slab, and the soil
over the bottom slab extension, if applicable. The safety factor is equal to the opposing
force divided by the buoyancy force. The safety factor should be >1.5.
FORCE MAIN PRESSURE AND WATER HAMMER CALCULATIONS
Water
hammer is an increase in pressure in the pipe caused by a sudden change in the velocity.
The velocity change usually results from the closing of a check valve. Most valves and
fittings used in plumbing systems are designed for a normal maximum working pressure of
150 psi. Therefore, unless the shock pressures can be reduced to less than 250 psi.,
serious damage to the valves, fittings and other components of the piping system may
occur. If the operating head of the system is greater than 35 feet, the check valves
should be spring loaded, or lever and weight check valves used.
There
are several design considerations that have not been covered, but are critical in some
applications. They will be described briefly:
The
following are several design considerations that have not been covered, but are critical
in some applications.
1)
Odor Control - Generally speaking, if the detention in either the wet well or force
main based on the average flow is less than 30 minutes, then there should be very few
problems.
2)
Net Positive Suction Head (NPSH) - In small to medium submersible pump stations, if
the pump housing is submerged and the wet well is vented to the atmosphere, there should
be few problems. When there are high flows, cavitation could be a major consideration.
3)
Air/Vacuum Valves - Depending upon the profile and size of the force main, air or
vacuum pressure could be a major factor in the life cycle of the system. Air entrapment
can cause an excessive head requirement that the pump cannot overcome. Large down grade
profiles that open to the atmosphere can cause excessive negative head that could collapse
the force main or over run the pump, causing it to overheat and burn out.
4)
Safety - The design of a pumping station requires a review of the components of the
system to assure that the system is safe to operate. The access ladder for the wet well
and valve vault, a hoist for lifting out the pump, ventilation to remove dangerous gases
and security for the electrical system are the major safety items that need to be
considered in the design.
5)
Wet Well Dead Zones - In all wet wells, there are areas that will allow solids to drop
out of suspension. These areas need to be eliminated or a method provided to re-suspend
the solids so that they are moved along.
