Mechanical and Electrical Design of Pumping Stations - 07...
Chapter - 07 : Discharge System...
7-1. General...
The discharge system of a pumping station is used to
convey the pumped water from the pumps to the receiving
body of water. Additional discharge system
information is contained in EM 1110-2-3102. Two types
of systems are used, over the levee either with or without
siphon recovery and through the line of protection.
Alternative studies of different type discharges may be
required in order to select the one that is best when considering
the layout of the station, site requirements, and
the selection of the pumping equipment.
7-2. Discharge Types...
( a ) Over the levee.
Discharge lines over the protection
consist of individual lines for each pumping unit or
a manifold discharge with one discharge line running
from the station over the protection. The lines can terminate
in a common discharge structure or each individual
discharge line can be supported by piling with the surface
below riprapped to prevent erosion. If the discharge
system is to operate as a siphon, the design should be
such that it is self priming and the lowest discharge level
should not be lower than 8.5 meters (28 feet) from the
top of discharge pipe (this limit, as illustrated in Appendix
B, does not include piping losses, velocity head, etc.).
The 8.5 meters (28 feet) may be obtained by terminating
the discharge lines in a concrete box containing a seal
weir at the proper elevation. An upturned elbow may
also be used to obtain the seal elevation on individual
lines. The invert of the discharge line at its highest point
should be at or above the level of protection expected to
be provided by the protection works. Manifolding of
pump discharge lines into a single line is generally cost
effective only for small pumps and when siphon recovery
will not be used. When considering a manifold system, a
cost comparison should be made between the cost of
individual discharge lines and the extra shutoff and check
valves needed in a manifold system. Pumps that discharge
into an open discharge chamber are not considered
to be manifold-type discharge.
( b ) Through the protection.
Discharges through the
protection usually consist of individual pump discharge
lines terminating in a discharge chamber or wall of the
protection. Flow from the discharge chamber would then
be carried by conduit to the receiving body of water.
7-3. Selection Criteria...
The type of discharge is sometimes set by the location of
the station. For example, siphons may be used when the
station is located behind the levee or as an integral part
of the levee. In those cases where different types of
discharge arrangements could be used, a study of alternatives
should be made. Selection is based on a life-cycle
cost analysis. Some of the variables would be operating,
equipment, and structure costs. Operating costs would
include the costs of energy, manpower, and operating
supplies and maintenance costs. Energy and manpower
needs should be determined using available hydraulic/
hydrology data to determine amount of operation time
and discharge levels. Forecasts of future energy costs
can usually be obtained from the utility that furnishes the
energy. Equipment costs could vary due to the difference
in discharge head requirements of the various discharge
systems. In most cases, only the driver size
would change. Although over the levee discharges with
a siphon recovery make pump selection more difficult
due to the fact that the pump is required to operate over
a greater head range, it does reduce the operating costs of
the station. The static head for a siphon recovery discharge
is based on the pool-to-pool head, whereas the
static head for a non-siphon discharge is based on the
maximum elevation of the discharge line. A study
should be made of the first costs and operating power
costs to determine if the siphon assist is justifiable.
7-4. Design...
( a ) General.
The size of the pipe is usually determined
by a cost analysis of the energy used due to
friction loss versus pipe cost. Through the protection,
discharge lines are usually short lines and have the same
diameter as the pump discharge elbow. Where any piping
connects to a pump, it should be supported so that
the pump does not support any of the weight of the pipe
or its contents.
( b ) Through the protection.
In general, two means
shall be provided to prevent backflow when the discharge
is through the protection. Discharge lines through the
protection should terminate with a flap gate to prevent
back flow. In addition to the flap gate, provisions should
be made for emergency shutoff valves, emergency gates,
or individual stop log slots to place bulkheads in case of
flap gate failure. All discharge lines with flap gates
should be fitted with a vent pipe located just upstream of
the flap gate to prevent excessive vacuum bounce of the
flap gate. The vent should extend 600 millimeters (2 feet) above the highest discharge water level. Flap gates
shall be of the type suitable for pump discharges. This type
of gate is of heavier construction, and the arms that support
the flap are double hinged so that the flap will fully close.
Flap gates with a built-in hydraulic cushion effect are not
required where the head on the gaye is below 7.6 meters (25
feet). All non-cushion type flap gates used on pump
discharges should have a rubber seat which aids in sealing
and eliminating some of the closing shock. A station having
this type of discharge arrangement is shown on Plate 5.
( c ) Over the levee.
Pipes over the levee require an air
release and a siphon breaker at the crest. If the pipe
system does not operate as a siphon, a permanent vent
opening can be used. Discharge pipes 1,500 millimeters
(60 inches) and less can usually be operated as a self-priming
siphon if the flow velocity at the crest is 2.2
meters per second (7 feet per second) or greater when
priming is initiated. Model tests should be considered for
discharge pipes of greater diameter and those pipes having
an arrangement different from the standard type discharge
shown on Plate 4.
For pipes operating as a siphon, the use
of a remotely operated valve to break the siphon should be
used. This valve is air-operated except for small sizes in
which electric operation is possible. The valve is operated
from a signal initiated by the starting of the pumping unit.
A timer is placed in the circuitry to provide a time delay
of valve closing after start-up to vent the discharge pipe
system. There are flow-operated siphon breakers
available, but care should be taken that they are designed
for heavy duty operation and are adjusted correctly after
installation to prevent air leakage into the pipe. The siphon
valve is sized according to the formula provided at the end
of this paragraph. When the calculated result indicates a
non-standard size valve or piping, the next larger standard
sized valve and piping should be used. A manual valve of
the same size shall be used in addition to the siphon valve
for emergency breaking of the siphon. All of the siphon
breaker piping and valves should be located near the
descending leg in an enclosure. Problems in priming the
siphon can occur when the changes in discharge line
gradient occur on the discharge side of the protection. The
section of pipe after the down leg should be as flat as
possible to prevent these problems. Manufacturers of air
and vacuum valves should be consulted for proper valve
sizing. Vent size can be calculated using the following
formula.
D V = ( 0.25 ) ( D P ) ( 2 / h ) 0.25
Where ;
D V = Diameter of vent ( ft ),
D P = Diameter of discharge pipe ( ft ),
h = Minimum submergence over outlet ( ft ).
7-5. Pipe Construction and Material...
( a ) Construction.
Discharge piping to pump connections
is generally made by means of a flexible coupling with
harness bolts across the connection. Rigid or flanged
connections could be used for pullout design pumps and
for those pumps that may be cast into the structure. All
buried piping needs to be connected with flexible
couplings with harness bolts whenever the pipe runs into
or out of a concrete structure, at bends, and at other points
where differential settlement or normal expansion or
contraction of the pipe is anticipated. Where piping leaves
a structure and goes underground: the first flexible
coupling should be placed within 1.5 meters (5 feet) of the
wall with an additional flexible coupling placed 1.5 meters
(5 feet) farther downstream. An embedded wall flange
should be provided for all piping passing through concrete
walls. Pipe selected should be of the minimum wall
thickness that will satisfy the requirements of the
installation and, if possible, should be of a standard stock
wall thickness. Where corrosion may be a problem. an
increase in thickness of 25 percent may be considered for
steel pipe. In addition to the tensile circumferential
stresses resulting from the normal internal water pressure,
stresses due to the following may be a consideration in the
design of some pump discharge lines.
(1) Excess stress due to water hammer.
(2) Longitudinal stresses due to beam action of the pipe, when the pipe is exposed and supported by piers or suspended
supports.
(3) Stresses caused by external loading.
(4) Stresses caused by collapsing pressures due to formation of vacuum in the pipe.
(5) Stresses due to temperature changes.
(6) Stresses due to differential settlement.
The discharge pipe and its support system including
anchors, thrust blocks, and tie rods should be designed in
sufficient detail so that the above considerations can be
checked. American Water Works Association (AWWA)
Manual M-11 should be referenced for recommended
design and installation practices for steel pipe. Preparation
of detailed pipe shop fabrication drawings should be made
the responsibility of the Contractor subject to the approval
of the Contracting Officer.
( b ) Material.
Discharge piping should be constructed of steel or ductile iron. Ductile iron pipe would generally be used for piping of
300 millimeter (12-inch) diameter and under. Exposed steel piping inside of stations may be flanged or flexible coupling
connected. Whereas ductile iron pipe should be fitted with mechanical joints or flanged. Steel and ductile iron pipe should
conform to the applicable AWWA standard specifications. All discharge line pipe should be protected on the inside with a
smooth coating. The designer should consult with the paint laboratory at the Construction Engineering Research Laboratory,
Champaign II. Buried pipe should also be provided with an outer protective coal-tar coating and a felt wrapping. Shop
coatings should be used to the maximum extent possible due to the better quality control. Flanges should conform to AWWA
specifications for the pressure rating required. Tbe applicable AWWA standards are listed in Appendix A. Valves should be
selected that will be easily maintained. Unless larger than 1.200 millimeters (48 inches), gate and butterfly type valves
are preferred. Gate valves with cast iron bodies should be bronze fitted. Butterfly valves should be rubber seated. The
type of valve operator, either electric or manual, is the designer’s choice based on site-specific requirements, e.g.,
accessibility, frequency of use, and anticipated loads.
