Sewer BackupS Protecting Structures from F
or many years the model codes have incorporated requirements that attempt to protect the public by preventing water and waste from municipal sewer
systems from backing up into a private structure, Are the current standards correct? Well, not entirely. All model codes require that if a building is connected
to a municipal sewer, and the flood rim of a plumbing fix- ture within the building is lower than the rim of the next upstream sewer manhole to which the building sewer is connected, then a backwater valve must be installed on the line that connects the fixture to the sewer. The backwater valve acts as a check valve, which prevents flow from the sewer system from entering the building system. This could occur if the main sewer line is overloaded and sur- charged. Under these conditions, the water level within the sewer system will continue to rise and will discharge from the plumbing fixture within the building that has the lowest flood rim. Example 1: A typical sewer system. The pad elevation of the building is higher than the elevation of the next upstream manhole. If the municipal sewer is surcharged, then the pressure will build up in the sewer main and push the manhole cover off, thereby relieving pressure in the system, resulting in sewage discharge to the street. Example 2: A common hillside condition.
The upstream municipal manhole is located higher than the pad of the building and higher than the flood rim of the drains and plumbing fixtures within the building. In this case, during a surcharge condition, wastewater will build up in the piping system and will overflow out of the fixtures within the building. Example 3: The long-standing practice requires that a
backwater valve be installed when the upstream manhole cover elevation is equal to or higher than the lowest flood rim of a fixture within the building. The goal is to protect the inside of the building from sewer backups. During a surcharge condition, the backwater valve prevents sewage from entering the plumbing system within the structure. Sewage is discharged from the sewer manhole into the street, thereby minimizing any damage to private property. The established standards, however, do not go far
enough. In order to relieve pressure from the piping sys- tem, sufficient pressure must build up within the piping system to lift the sewer manhole cover and allow water to discharge from the sewer. This is ignored under the cur-
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rent standard of practice. Manhole covers vary in size and weight. A fairly typi-
cal manhole cover is 26 inches in diameter and weighs approximately 200 pounds. It has a surface area of approximately 530 sq inches. • If water rises within a manhole, it will push at the bot-
tom of the manhole cover. The manhole cover will resist movement by applying a force of 200 pounds/530 sq inch- es = 0.376 pounds per sq inch; • 0.376 psi = 0.376 psi/0.433 psi per ft of head pressure
= 0.868 ft of head pressure; and • 0.868 ft of head pressure x 12 inches per foot = 10.4
inches. Therefore, the sewer system must build up 10.4 inches
of water pressure below the manhole cover prior to dis- placing the cover and discharging water. This may not sound like much pressure, but, under this scenario, to pre- vent discharge from the sewer system into the building, the flood rim of the fixtures within the building would actually have to be higher than 10.4” above the eleva- tion of the manhole cover. It gets worse. Manhole covers are a fairly tight fit to their frame, and, in addi- tion, debris and dirt often fill the annular space between the cover and the frame. This debris, which is compacted into the space from frequent vehicle and foot traffic,
be factored in as well. To protect private property and to reduce the likelihood of sewer discharges into
buildings, the standards should be modified so that back- water valves be required if the flood rim of the lowest fix- ture within a building is two ft or less above the rim of the next upstream manhole. David Fruchtman, P.E., is a licensed professional engi-
neer in the states of California, Nevada and Arizona. In addition, he is a CPD (ASPE certified plumbing designer), CEM (certified energy manager) and is an IAPMO certi- fied plumbing inspector and an IAPMO certified mechan- ical inspector. His firm, Fruchtman and Associates, spe- cializes in consulting, design and forensic engineering for HVAC, plumbing and fire protection systems. n
David J. Fruchtman, P.E., CEM, CPD, is a Certified
Mechanical Inspector, Certified Plumbing Inspector, Fruchtman and Associates, Consulting Mechanical Engineers.
May 2011
further
increases friction between the cover and the frame. This needs to
By David J. Fruchtman, P.E.
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