31 Mar 97
Discharge From Closed Systems
Containment capacity for closed systems is based upon testing
requirements only. The design and sizing of the containment
system will be affected by a number of factors, including the
system design, and number, size and location of AFFF
proportioners. Each proportioner must be tested individually.
As a minimum, the containment system should be sized to contain
the test flow of foam solution from the system proportioner with
the greatest design flow rate for a minimum of three minutes.
For example, assume that an aircraft hangar has four closed-head
sprinkler systems (no nozzles), each with a separate
proportioner. Assume also that the greatest flow rate is 2,500
gpm. A 3-minute test should produce at least 7,500 gallons of
foam solution, which would be the minimum capacity of the
containment system. In this example, the 7,500 gallon
containment system would have to be emptied between each test.
Designing to the minimum in this case does not facilitate system
testing. It is preferable to size the containment to handle test
flows for all four systems, or for at least half the systems.
Designing for only one system being tested would greatly lengthen
the testing period.
Floor Drainage Systems
Applicable design criteria for aircraft hangars require floor
drainage systems to restrict the spread of fuel in the event of a
spill. System configuration and size of drainage piping must
also take into consideration the hydraulic demands placed on the
system throughout it entire length. This includes the AFFF
discharges that could occur in the event of an inadvertent
activation of an open fire extinguishing system.
Oil-water separators are an integral part of hangar drainage
systems. They are installed in the hangar drainage system to
intercept oil or fuel spilled on the hangar floor before it
enters the influent piping to the wastewater treatment plant.
Oil in the influent to treatment plants inhibits the treatment
process and is never acceptable by the treatment plant
authorities above small threshold limits. An oil-water separator
is sized for a designated flow rate which is generally based upon
the maximum anticipated spill. Flow above the design rate would
have the effect of diminishing the effectiveness of the
separation process. Separation is based upon providing
sufficient detention time to allow the oil, which is lighter than
water, to rise to the top of the separator for removal.