After all available non-recoverable product, sludge and res- idue has been removed from the tank through closed connec- tions, the remaining material may be removed from the outside of the tank through an open manhole, doorway or door sheet opening. The door sheet may be opened using a water cutting technique that minimizes heat, thus reducing the potential for ignition. The tank shall be vapor freed (degassed) and ventilated during these operations.
6.5.1.1 Entry supervisors shall review the potential haz- ards, determine the safe procedures and requirements, have qualified persons conduct atmospheric testing and issue entry permits to begin tank cleaning from the outside of the tank.
6.5.1.2 Tank cleaning workers shall wear approved respira- tory and appropriate personal protective equipment, as required, throughout the cleaning operation.
6.5.2 Vapor and Gas Freeing
Before beginning sludge breakup and removal from out- side the tank, the tank atmosphere shall be vapor and gas freed (and the vapors degassed, if required) until the vapor-in- air (gas–in- air) mixture leaving the tank is at or below 10%
LEL. Because agitation of sludge and residue will release vapors and increase the vapor-in-air concentration inside the tank, cleaning from the outside of the tank shall stop if the vapor-in-air (gas–in-air) mixture leaving the tank rises above 10% LEL. Vapor and gas freeing (degassing) and ventilation shall be continued until the tank’s atmosphere is again below 10% LEL, when sludge breakup and removal from outside the tank can resume.
Figure 6-1—Example of Eductors Used for Tank Agitation
Circulation of contents
Jet eductors
Pump
From pump Elevation
Plan View
Eductor Detail A–A Jet
eductor
Jet eductor
Eductor A
A
6.5.2.1 Vapor and gas freeing, degassing (where required) and ventilation shall continue throughout the removal activity (except in inert tanks).
6.5.2.2 A qualified person shall conduct atmospheric mon- itoring for flammable vapor and gas and toxic exposures, con- tinuously or periodically as required by the entry supervisor, in the ventilation exhaust areas, around degassing systems and at open manhole, doorway or door sheet areas during the cleaning operation.
6.5.2.3 Cleaning shall stop if the flammable vapor or gas levels or toxic exposure levels in the work area exceed the permit limits.
6.5.3 Removal of Product, Sludge, and Residue After Opening the Tank
Dispersant surface additives and petroleum solvents may be added to heavy non-recoverable product, sludge and resi- due to increase fluidity.
6.5.3.1 Dilute concentrations of hyperchlorites or perman- ganate solutions may be used to assist in oxidizing pyro- phoric compounds or to assist in tank cleaning. The entry supervisor shall be aware that this usage has a potential to create a runaway exothermic reaction that can result in suffi- cient heat to create an ignition source and institute appropri- ate preventive measures.
6.5.3.2 Water streams, steam jets and petroleum solvent or chemicals may be injected through the open manhole to break up and dislodge heavy non-recoverable product, sludge and residue and float it to a pump out connection. Hose nozzles and suction hoses shall be bonded to the tank to reduce the potential for electrostatic buildup.
6.5.3.3 Approved mechanical powered equipment may be permitted to enter the tank through the doorway or door sheet opening to remove large amounts of dry, caked sludge, depos- its and residue from inside the tank. The entry supervisor shall be aware that the use of such equipment has the poten- tial for a source of ignition and institute appropriate protective measures. Mechanical powered equipment shall have explo- sion proof motors, ground fault interrupters and shall be bonded to the tank.
6.5.3.4 Mechanical robotic cleaning devices may be used inside tanks to carry cutter oil hoses and nozzles to specific locations where the solvent is applied directly into the heavy non-recoverable product, sludge or residue. This direct appli- cation of a pressure jet is more effective and uses less solvent than a hose stream operated from an open manhole. Mechani- cal robotic cleaning devices are particularly useful in remov- ing sludge from inert tanks.
1. Robotic devices may be provided with low light video, lighting systems and lens washers to assist the remote operator in directing operations.
Figure 6-2—Example of Water Balancing
Clean water supply Product cut to temporary storage or treatment
facilities Hot tap Above ground,
internal, floating roof
Storage tank being degassed
Water Balancing
2. Robotic devices shall have explosion proof motors, ground fault interrupters and shall be bonded to the tank.
3. Robotic devices may be equipped with oxygen, hydro- gen sulfide (H2S), toxic gas and flammable gas and vapor detectors.
4. Operators shall recognize that robotic devices have problems negotiating obstacles within the tank such as piping, columns, supports, roof legs, sumps, etc. and shall take appropriate action to avoid obstructions.