Current through Register Vol. 41, No. 3, September 23, 2024
A. Purpose.
Reliability provisions are based on a measurement of the ability of a component
or system to perform its designated function without failure or interruption of
service. Overflow criteria, such as a period of discharge, are utilized solely
for the establishment of reliability classification for design purposes and are
not to be construed as authorization for, or defense of, an unpermitted
discharge to state waters.
1. The objective
of achieving reliability protection is to prevent the discharge of raw or
partially treated sewage to any waters and to protect public health and welfare
by preventing backup of sewage and subsequent discharge to basements, streets
and other public and private property. Provisions for continuous operability of
pumping stations shall be evaluated in accordance with the appropriate
reliability classification.
2. For
Class I Reliability, alternate motive force sufficient to operate the station
at peak flow rates being received shall be operating the station prior to the
expiration of an allowable time period. The maximum allowable period will be
the time transpiring between the high liquid level alarm and the occurrence of
an overflow, or backup and subsequent discharge, at flow rates being received
(except when an emergency holding basin is provided to satisfy the requirement
for continuous operability). The transpired time to be considered allowable may
be the critical (shortest) transpired time (peak flow rates) or a spectrum of
transpired times keyed to the 24 individual hours of the day. Certain
Reliability Class I pump stations, for which it is feasible to shut down or
discontinue operation during periods of power failure without bypassing or
overflowing, may be exempted from the continuous operability requirement. Pump
stations which may qualify for the exemption can be broadly categorized as
those which serve facilities or institutions which would be closed during
periods of power failure, such as certain industrial plants, schools and
recreational and park areas.
3. For
Class II Reliability, alternate motive force sufficient to operate the station
at peak flow rates being received shall be operating the station prior to the
expiration of a 24-hour period commencing at the time an overflow or discharge
subsequent to a backup begins.
4.
Reliability Class III pump stations are not limited to a specific period of
overflow or discharge, and will be considered on a case-by-case
basis.
B. Continuous
operability. The owner shall demonstrate, to the satisfaction of the
department, that the time allowances for continuous operability will be met on
a 24-hour basis. This information shall accompany the plans and specifications
when submitted and shall be subsequently modified and resubmitted at any time
in the future that the actual allowable time (transpiring between the high
liquid level alarm and the time that an overflow or backup and subsequent
discharge will occur at flow rates being received) becomes less than the
allowable time claimed in the original submission. The demonstration shall
include provision of instructions indicating the essentiality of routinely
maintaining, and regularly starting and running, auxiliary and reserve units
under field conditions. The following means for provision of continuous
operability shall be acceptable:
1. Alternate
power sources or auxiliary stand-by generator that can operate sufficient pumps
to deliver the design peak flow.
2.
Alternate drive arrangements whereby all pumps are backed by internal
combustion motors with "Y" mechanical couplings to the pump drive shafts or to
permanently mounted reserve pumps capable of delivering total peak
flows.
3. Portable pump resources
in accordance with this chapter.
4.
An emergency overflow holding basin with capacity to retain a minimum of one
day of station design flow and having provisions for recycling flow to the pump
station.
C. Electrical
power. The sources of electrical power required to operate pump stations shall
be evaluated in accordance with the reliability classification of the pump
station.
1. For Class I Reliability, electric
power shall be provided by alternate feed from distribution lines which are
serviced by alternate feed from transmission lines (e.g., 115 KV) where
possible. The transmission lines shall have alternate feed from the generating
source or sources. The capacity of each power source shall be sufficient to
operate the pumps during peak wastewater flow conditions, together with
critical lighting and ventilation. The requirement for alternate feed can be
satisfied by either a loop circuit, a "tie" circuit, or two radial lines. Where
alternate feed lines terminate in the same substation, the circuit feeding the
pumping station shall be equipped with two or more in-place transformers. Where
alternate feed is not possible, provision of auxiliary power sources will be
considered.
2. External alternate
distribution lines shall be completely independent. The two sets of alternate
feed distribution lines should not be supported from the same utility pole and,
if used, should neither cross over, nor be located in an area where a single
plausible occurrence (e.g., fallen tree) could disrupt both lines. A minimum
separation of 25 feet for underground routes shall be maintained unless a
properly designed and protected conduit bank is utilized. This shall apply to
service connections into the pump station. Devices should be used to protect
the system from lightning.
3. For
Class II Reliability, a single source feed is acceptable. If alternate power
sources are provided for a Class II or III station, one in-place transformer
and capability for connection of a mobile transformer may be provided where the
alternate feed lines terminate in the same substation.
D. Power distribution. The design of power
distribution circuitry and equipment provided within pump stations shall be in
accordance with the reliability classification of the pump station.
1. Reliability Class I pump stations shall
have the following features:
a. Final stepdown
transformer on each electric feed line with adequate physical separation to
prevent a common mode failure.
b.
In addition, Reliability Class I pump stations shall be provided with separate
buses for each power source.
c.
Each power source shall remain separate and from separate distribution
substations up to the transfer switch to preclude a common mode failure of both
sources.
2. Reliability
Class II and Class III pump stations may be equipped with a single final
stepdown transformer, a single bus, a single motor control center, and a single
power distribution system.
3.
Breaker settings or fuse ratings shall be coordinated to effect sequential
tripping such that the breaker or fuse nearest the fault will clear the fault
prior to activation of other breakers or fuses, to the degree
practicable.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
