enter the tank alone, you must use a safety harness
10. Have the gas-free engineer inspect the tank
and tending line. If more than one person enters
and issue a gas-free certificate as required in NSTM,
the tank, they should not use the tending line.
chapter 074. Pay particular attention to the
However, the OBA or air-line mask wearers should
presence of hydrogen sulfide, methane, carbon
keep in constant sight or touch with one another.
dioxide, and oxygen. If the tank is not gas-free,
Personnel should always be on hand outside the
reseal the tank and repeat the washdown procedure
tank to lend assistance if required.
outlined previously (steps 1 through 9) until the
tank can be certified gas-free and safe. The tanks
16. After the tank is entered, remove the
must be recertified periodically. In any event, the
remaining sludge and wash down.
recertification must be performed at least every 4
hours while the tank is open.
17. Have the tank recertified as gas-free and
safe by the gas-free engineer. If the tank is found
11. Even though a tank may be certified
unsafe, continue ventilation until the tank can be
gas-free, toxic gases may remain in the sludge
recertified gas-free and safe.
blanket and may be released when the blanket is
disturbed. Before opening the tank in any manner
18. Personnel may now continue work in the
(for example, by removal of manhole access covers
tank without air-line masks or OBAs, provided
or liquid-level sensor flanges), wear either an OBA
ventilation is continued.
or an air-line mask as described in NSTM, chapter
079, volume 4. You must have a second person on
19. Inspect the tank coating, level sensors,
hand to lend assistance as required. A spare OBA
aeration, and washdown systems. If recoating is
or air-line mask (preferably OBA) must be
necessary, it should be done with the four-coat
tank-coating system, MIL-P-24441. The cement used
to fill voids and pockets should be replaced with
12. After the tank is opened, ventilate it
latex cement, MIL-D-21631.
continuously by taking suction from the tank and
exhausting directly to the atmosphere. Ventilation
should provide a change of air in the tank every 3
minutes. The number and types of blowers needed,
ducting path and arrangement, and position of
suction in the tank are to be determined by the
production engineering and industrial hygiene
The tank must be assumed to contain
toxic/explosive gases until otherwise
determined. Observe all NO-SMOKING
13. Once forced ventilation of the tank has
regulations. Do not allow open flame,
begun (step 12), you can continue work around the
ordinary electric lights, flashlights, or
tank without using OBAs or air-line masks.
sparking electrical apparatus in or near
the open tank until safety is certified by
14. Before entering the tank, force ventilate it
for 30 minutes and then clean it thoroughly with a
firehose or manually controlled high-pressure water
cleaning nozzle. Continue ventilation during this
20. Do not perform any welding or hot work on
the tank until the gas-free engineer first determines
washdown. Be careful not to get water into the
that the tank is safe for hot work. After the welding
blower inlet line or to damage internal tank
equipment (level sensors). Pump out the tank as
is completed, inspect the coating for heat damage
necessary during cleaning.
and repair as necessary.
15. Have the gas-free engineer repeat gas level
measurements. When the tank is designated
Toxic gases are generated at various rates
gas-free and safe, you may enter using an OBA or
depending upon the sewage temperature, pH,
air-line mask. Personnel entering the tank must
oxygen, and the amount of sewage in the tank.
wear overalls, boots, gloves, and head covering. If
Toxic gases are likely to form in tanks where sewage
the tank is found to be unsafe, continue ventilation
until it can be certified gas-free and safe. If you
is allowed to remain for long periods of time.