API 2207: Preparing Tank Bottoms for Hot Work Flashcards
Prior to beginning hot work on a tank bottom, safety inspections should be conducted to determine the oxygen content of the tank. What is the acceptable range of oxygen content? (API 2207-17, Section 4.1.5)
19.5-22.5%
What must be done in order to weld on steel that has been in contact with leaded gasoline without using supplied-air respiratory equipment? (API 2207-17, Section 4.2.7)
On each side of a line that might be heated excessively by welding or other operations, an area at least 12 in. (30 cm) wide should be scraped down to bare metal.
Where should grounding leads from a welding machine be attached? (API 2207-17, Section 4.2.12)
Directly to a bared surface on the shell of the tank on which hot work is to be performed
In what position should compressed gas cylinders used for cutting operations be kept? Where should these cylinders be located? (API 2207-17, Section 4.2.13)
Compressed-gas cylinders used for cutting operations shall be securely fastened in an upright position and kept outside the tank and at a safe distance from open manholes.
Small openings are sometimes made in the tank bottom in areas where hot work is to be performed. What is the purpose of these openings? (API 2207-17, Section 5.2.1)
The hole can be used with a detector to “sniff” through the hole for the presence of flammable materials or hydrogen sulfide. These openings facilitate visual inspection and freeing the space below the floor from gas, if necessary.
Small openings are sometimes made in the tank bottom in areas where hot work is to be performed. What equipment should be used to make these openings? What type of drilling machine is NOT recommended? (API 2207-17, Section 5.2.1)
The use of open-type electric drilling machines (including battery-operated drills) is not recommended.
If minor repairs are to be performed on a tank bottom where flammable vapors are suspected to be present under the tank bottom, what procedure may be used to prevent ignition of the vapors? (API 2207-17, Section 6.2)
a) Drill and tap a hole for a 1 /2 in. (13 mm) pipe adjacent to the repair area, but no closer than 12 in. (30 cm) from any floor plate seam, floor attachment weld, existing floor patch, or the critical zone.
b) Connect a supply of nitrogen, carbon dioxide, or other inert gas to the 1 /2 in. (13 mm) tap, using metal tubing. A pressure control valve with a flow indicator should be used to prevent overpressuring the tank bottom. The gas pressure should be limited and not exceed the weight of the floor.
c) Prior to beginning the hot work, establish a flow of inert gas under the tank bottom in the vicinity of any proposed welding to ensure that any flammable vapors have been swept away or diluted so that they cannot support combustion.
d) When using inert gas, continuously monitor the work area to ensure that the tank ventilation keeps oxygen content equal to that in the atmosphere. Monitoring for potentially toxic chemical or hydrocarbon concentrations should continue.
e) When welding has been completed in a localized area, stop the flow of inert gas, remove the tubing, and promptly repair the hole in accordance with API 653 before moving to another area. A 6 in. (152 mm) diameter patch plate installed and inspected per API 653 would be typical
If major repairs are to be performed on a tank bottom where flammable vapors are suspected to be present under the tank bottom, what procedure may be used to prevent ignition of the vapors? (API 2207-17, Section 6.3)
a) Construct an earthen dike higher than the highest floor plate in the tank around the tank’s outer shell.
b) At the highest points in the tank bottom, drill and tap holes for 1 /2 in. (13 mm) pipe. These holes should be located to ensure that all flammable liquids or vapors under the tank will be displaced with water.
c) Install pipe nipples to be used as vents and checkpoints. The height of the nipples should exceed that of the temporary earthen dike.
d) Fill the space under the tank with water until seepage occurs at each nipple. Continue the flow of water to maintain a constant level within the dike to ensure that no voids develop under the tank
e) Watch for leaks inside the tank and plug them as necessary to keep the tank bottom dry.
f) When repairs are completed, empty the water from the dike to just below the nipples, plug the holes with a tapered pin or other device [see 6.2 e)], and backweld.
When using water flooding to displace flammable liquids and vapors from beneath a tank bottom, how high should the earthen dike be that surrounds the tank? How far away should the dike be from the tank shell? (API 2207-17, Section 6.3.2)
Higher than the highest floor plate in the tank
What methods may be used when performing sectional work on tank bottoms? (API 2207-17, Section 6.6.2)
a) Using a hand- or air-operated tool, cold cut the tank bottom section to be replaced and remove it. A coolant shall be applied continuously to the cutting edge of the tool to remove heat generated by friction. High-pressure water jet cutting may provide another cold-cutting option.
b) Remove the earth from under the section to be replaced. Fill the space with tamped sand. Seal the perimeter of the patch area with mud or some other noncorrosive, noncombustible material. Plastic sheeting may be used under the fill material to provide an additional barrier.
c) When large sections of the bottom must be replaced and it is questionable whether the seal will be effective, it may be possible to maintain an inert area under the patch area while hot work is being performed. The area should be continuously monitored with an oxygen analyzer while work is being performed in and around any area with an inert atmosphere. See Section 4 regarding the potential need for use of respiratory protection, and see Annex A on inerting vessels.
