Environmental Engineering Reference
In-Depth Information
a contact tank, aeration basin, clarifier, pumping station wet well, bar
screen trunk, pipe, sewer, manhole, storage vessel, tunnel, boiler, reac-
tor, tank, or a large excavation where repairs to a damaged interceptor or
collection line are undertaken, proper confined space entry procedures
are essential to protect the worker. Several thousand injuries occur
each year in confined spaces. OSHA (1993) defined a confined space as
any space that:
•
Has limited or restricted means of entry or exit
•
Is large enough for an employee to enter and perform assigned
work
•
Is not designed for continuous worker occupancy
Hazards common to confined spaces include:
•
Atmospheric conditions
•
Engulfment
•
Internal configuration•
•
Physical
hazards
The primary consideration for confined spaces is oxygen deficiency.
Normal air contains about 20.8% oxygen by volume. OSHA indicates that
the minimum safe level is 19.5% and defines the maximum safe level as
25%. (Remember that an atmosphere too rich in oxygen can cause fires
and explosions.)
The potential for combustibility also makes confined space entry
hazardous. Gases and vapors are often trapped in confined spaces. These
built-up gases or vapors can be easily ignited by friction, cigarettes, or
sparks from hot work equipment.
In addition to a lack of oxygen and combustibility, another factor
that can make a confined space extremely dangerous is toxic air con-
taminants. Toxic air contaminants such as methane, hydrogen sulfide,
cleaning solvents, and coating vapors are a constant threat to those who
work in wastewater treatment and collection. Some toxic air contami-
nants can cause problems due to irritation of the respiratory system;
others can be much more serious, as they can cut off the person's oxy-
gen supply or enter the lungs and asphyxiate that person.
Engulfment is a problem, too. Engulfment is defined as the sur-
rounding and effective capture of a person by finely divided particulate
matter or a liquid. To gain a better understanding of engulfment and
the potential hazard, consider the following example. If you have ever
worked on a farm where grains such as wheat and corn are grown, you
are probably quite familiar with grain silos. Grain silos can be extremely
dangerous for a number of reasons. Fine grain dust is always an explo-
sion hazard, and grain silos are also engulfment hazards due to the
grain being stored inside the silo. Personnel can enter the compartment
below the storage area, but grain that is unexpectedly released from the
