Section 390. Reliability

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.

Historical Notes

Former 12VAC5-581-450 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-390, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.