Historical Use of Chlorinated Solvents and Their Stabilizing Compounds - Environmental Investigation and Remediation - page 14

from an idling open-top vapor degreaser can reach 0.20 (lb ft 2 )/h for TCE, 0.26 (lb ft 2 )/h for dichlo-
romethane, and 0.29 (lb ft 2 )/h for perchloroethylene (Dow Chemical Company, 1999b). *
1.1.1.3.3 SolventLosses
Understanding the rate of solvent losses from degreasers gives an appreciation of the frequency at
which solvents were replenished. The rate of solvent handling provides information about the
volume of frequent minor losses from spills, drips, container waste, and other factors contributing
to solvent releases associated with topping off or changing out solvents.
Solvent loss occurred in open-top vapor degreasers when the vapor blanket was disrupted by
drafts, movement of the work through the degreaser on a conveyor system, drag-out by the work as it
exits the vapor blanket, incomplete drying of the work, and frequent opening of closed degreasers
during operation. Other causes of solvent losses included diffusion, discarded solvent in degreaser
and still sludges, and leaks in solvent pipes and valves in the degreaser. Operator guidance since the
1960s has advised that the work should move through the solvent vapor blanket no faster than 11 ft/
min (ASTM, 1962; Thomas, 1995). Where problematic solvent levels are measured in the operator
zone around the degreaser, operator guidance calls for slowing the conveyor rate to 3 ft/min (Howell
and Tarrer, 1994). Solvent may also be lost when the condensed solvent fails to drain because of “cup-
ping” attributes in the shape of the work, leading to liquid solvent carryout. Estimates for solvent
losses due to these combined factors are as high as 70% of the solvent added to a degreaser over the
course of a year (Mertens, 2000a). A description of degreasing practices from the early 1960s detailed
an approach to solvent vapor management in which no hood is needed; in this approach, a large por-
table blower and roof ventilator are sufi cient to vacate solvent vapors (Stoddard and Wells, 1962).
Solvent consumption rates due to emissions have been estimated at 1-2 gallons solvent per ton of
small parts cleaned when a covered, in-line conveyor system is used, whereas small parts cleaned at
a similar rate in manually operated open-top degreasers consumed 20% more (ASTM, 1962). The
solvent and manufacturing industries substantially decreased solvent emissions from degreasing
equipment over the decades during which chlorinated solvents were used for vapor degreasing.
Improvements were motivated i rst by cost savings and later by regulation and enforcement of emis-
sions standards. Today, equipment is designed with vapor locks, vapor-detection devices, and other
controls, so that solvent emissions are substantially reduced. Even so, vapor degreasers equipped
with emission-control devices can still lose 5-25% of the solvent through vapor escape. In enclosed
(non-air-interface) degreasers, almost all of the solvent is captured and condensed for reuse. Solvent
losses in enclosed degreasers are less than 5% (Reuter, 2002).
Table 1.8 summarizes annual solvent loss to emissions and annual generation of waste solvent in
degreasers studied at several Air Force bases in the southeastern states. The study shows that methyl
chloroform degreasing operations generally produced more liquid waste than perchloroethylene degreas-
ing operations. Differences in emissions and waste generation were also the result of differences in the
quantity of parts cleaned and in the types of cleaning operation conducted in each vapor degreaser.
Table 1.9 provides a manufacturer's estimate of solvent losses at a major printed circuit board
operation where Freon 113 was the primary solvent used. Although evaporative losses would be
higher for Freon 113 than for chlorinated solvents, the values provide a general estimate of quanti-
ties released because of spills and leaks.
1.1.1.3.4 WaterSeparators
A smaller but signii cant source of solvent losses and possible releases to soil and groundwater
involves the management of condensate water produced in vapor degreasers. Water enters degreasers
when atmospheric water vapor condenses on the degreaser cooling coils, from moisture present on
* Based on tests in an idling open-top degreaser having a 24 × 58 in. (~10 ft 2 ) opening; units are pounds of solvent per
square feet of exposed solvent per hour of idling.