Hot tapping is the technique of attaching a mechanical or welded branch fitting to piping or equipment in service, and creating an opening in that piping or equipment by drilling or cutting a portion of the piping or equipment within the attached fitting.

HOT TAPPING MACHINE CONSIDERATIONS:

Before hot tapping is attempted, the machine, cutter, and pilot bit should be carefully inspected. During hot tapping, consideration must be given to the possibility of operational upsets which may alter the process temperature or pressure.

The two primary concerns when welding on in service piping and equipment are burn through and cracking. Burn through will occur if the un melted area beneath the weld pool can no longer contain the pressure within the pipe or equipment. Weld cracking results when fast weld cooling rates produce a hard, crack susceptible weld microstructure. Fast cooling rates can be caused by flowing contents inside the piping and equipment which remove heat quickly.

The metallurgy of the weld materials, the hot tap fitting, and the weld rod must be compatible with the metallurgy of the equipment to be welded or hot tapped. For example, low hydrogen electrodes are often advisable to minimize weld cracking problems. Special welding consideration may be needed for high tensile strength steels to avoid weld cracking and the need for post weld heat treatment.

BURN - THROUGH PREVENTION:

To minimize burn through the first weld pass to equipment or piping less than ¼ inch (6.4 mm) thick should be made with a 3/32 inch (3.4 mm) or smaller dia welding electrode to limit heat input. The use of low heat input levels can increase the risk of cracking in high carbon equivalent materials. Subsequent passes should be made with a 1/8 inch (3.2 mm) dia electrode or smaller if the metal thickness does not exceed ½ inch (12.8 mm).

For equipment or piping wall thickness greater than ½ inch (12.7 mm), where burn-though is not a primary concern, larger dia electrodes can be used. Excessive welding current to be avoided. Use of low hydrogen rods may be preferable to reduce the possibility of burn-through.

FLOW IN LINES:

For metal thickness less than ¼ inches (6.4 mm), some flow during hot tapping minimizes the potential of several undesirable conditions. Overheating liquids, burn through caused by elevated metal temperatures and fluid thermal expansion in closed systems are less likely when flow in pipeline is maintained.  Higher flow increases the cooling rate and the risk of cracking.

For metal thickness between ¼ inch (6.4 mm) and ½ inch (12.8 mm), flow also increases the weld cooling rate and risk of cracking. Minimizing the flow rate reduces the risk of cracking and keeps the risk of burn through low.  For metal thickness greater than ½ inch (12.8 mm), the effect of flow on both weld cooling rates and the risk of burn through is negligible. Under certain condition such as when tot tapping or welding on a flare line, there may be insufficient or interrupted flow that result in a flammable mixture during the welding operations. In these circumstances it may be necessary to purge or flood with steam, inert gas or hydrocarbon gas to prevent formation of flammable mixtures.

METAL THICKNESS:

Minimum base metal thickness of 3/16 inch (4.8 mm) is recommended. The actual minimum thickness is a function of the thickness required for strength plus a safety factor, usually 3/32 inch (2.4 mm), to prevent burn through.

METAL TEMPERATURE:

Welding should not be performed on lines or equipment when atmospheric temperature is less than -50deg F (-45deg C), unless special precautions are taken. Preheating may be required by the welding procedure to avoid cracking whenever the base metal has high carbon equivalency or high tensile strength.

HOT TAP CONNECTION AND WELDING DESIGN:

Generally hot tapping or welding closer than 18 inches (46 cm) to a flange or threaded connection, or approximately 3 inches (7 cm) to a welded seam (including longitudinal seam of welded piping) should be avoided.

PIPING AND EQUIPMENT CONDITION:

Welding or hot tapping should not be performed under vacuum, unless an engineering evaluation is conducted to determine safe operating procedures.

Welding and tot tapping should not be performed on piping or equipment containing the following materials:

• Vapor/air or vapor/oxygen mixtures near or within their flammable explosive range. The heat of welding may cause a vapor mixture to enter the flammable range.
• Carbon and ferrite alloy steel is susceptible to high-temperature hydrogen attack during process operations. Therefore a study must be conducted to ensure that the equipment has been operated within the Nelson curve for the particular steel involved.
• Peroxides, chlorine, or other chemicals likely to violently decompose or become hazardous from the heat of welding.
• Caustics, amines and acids (such as hydro fluoric acid).
• Certain unsaturated hydrocarbon as ethylene. These may experience exothermic decomposition due to high temperature caused by welding, creating localized hot spots on piping or equipment walls that could lead to failure.
• Oxygen or oxygen enriched atmosphere. The oxygen may cause a vapor mixture to enter the flammable range and may affect the base metal being welded.
• Compressed air, unless known to be free of flammables and combustibles, such as lubricating oil residues.

HOT TAPPING OR WELDING ON TANKS IN SERVICE:

The hazards include, but not limited to the following:

• Tank venting, with vapors reaching the exterior area where welding is taking place.
• Product within the tank rising and overflowing.
• Inadvertently allowing the liquid level within the tank to fall below the point of welding, thereby losing the heat sink provided by the liquid and exposing the vapor space within the tank to an ignition source.

Exterior welding on tanks shall not be conducted unless controls are established to prevent flammable vapors from reaching the area of welding. If flammable vapors are detected work must be stopped.

When hot tapping or welding on  a tank in service, maintain liquid in the tank at a level at least 3 feet (1 mt) above the area where the work is being performed. No attempt should be made to hot tap or weld above this liquid level in atmospheric pressure petroleum storage tanks because of the potential danger of an explosive atmosphere inside the tank vapor space.

HOT TAPPING OR WELDING O THE DECKS OF FLOATING ROOF TANKS:

Welding should not be permitted on the decks of floating roof tanks in service. These tanks are subject to unique flammability hazards in the following specific locations:

• Inside the pontoons.
• Between the deck and liquid surface near the tank roof gauge float compartment.
• Near the roof seal vent.
• Near the floating roof lift leg vent.
• Between the primary and secondary seal.
• Near the roof drains.

HOT TAPPING OR WELDING ABOVE OR BELOW GRADE:

Provisions shall be made for an easily accessible means of egress. To assure that the atmosphere in excavations and confined spaces is safe for entry and hot work, tests shall be conducted for oxygen, flammable vapors and toxic air contaminants, and permits issued listing the requirements and approving the entry into the confined space and hot work therein. Confined space procedures may be followed.

HOT TAPPING OR WELDING ON LINED PIPING OR LINED EQUIPMENT OR CASED LINES:

Hot tapping or welding should not be permitted on in-service lines or equipment with cladding or with glass, lead, refractory, plastic or strip linings, unless specifically authorized by specialized procedures or following an engineering evaluation. When hot tapping or welding on underground lines which run through casings, care must be taken to assure that the annular space is gas free and that the work is performed on the pipeline and not on the casing.

HOT TAPPING UPSTREAM OF EQUIPMENT AND VALVES:

Avoid hot tapping upstream of rotating equipment or automatic control valves, unless such equipment is protected from the cutting by filters or traps.

BEFORE STARTING THE HOT TAP:

Each of the following consideration should be satisfied before starting the hot tap:

1. Review/follow manufacturer’s instructions.
2. Have the contents of the line or vessel to be hot tapped or welded been assessed, and MSDS’s reviewed for health hazards, to assure procedure is appropriate?
3. Is the material in the line or vessel stable under heated conditions?
4. Has the connection been designed?
5. Do the flanges, bolts, gaskets, pipe and valve to be installed meet the code for the line or vessel to be hot tapped?
6. Has the welding procedure specification been developed as covered with the standards?
7. Have approved work permits been obtained?
8. Review manufacturer’s instructions to ensure that the hot tapping machine has suitable pressure, temperature ratings, and adequate cutter travel for this job.
9. Has the valve been pressure tested and the cover fitted to assure that it will work and fit properly?
10. Has the exact location of the hot tap been identified and marked on the line or equipment?
11. Has the area to be welded been inspected for thickness and freedom from existing welds, laminations, hydrogen attack, or other metallurgical imperfections?
12. If laminations or defects have been found, has a thorough engineering evaluation been made to determine if and how to proceed with the work?
13. Has the metallurgy of the line or vessel been established, and is it compatible with the connecting fitting?
14. Can the area support the weight of the hot tapping machine, and is there adequate housing and support for the hot tapping machine and subsequent piping?
15. If PWHT of the welded area is required, was an appropriate review conducted?
16. Is there sufficient external clearance to install the hot tapping machine and extract the cutter through the valve?
17. Is there sufficient internal clearance to retract the cutter and coupon through the valve?
18. Is the hot tap fitting of the proper length to accommodate operation of the hot tapping machine?
19. Have oxygen, combustible gas, and toxic atmosphere tests been conducted in the hot tap area?
20. Has a fire watch been assigned and appropriate fire fighting equipment provided?
21. Have all personnel in the area been provided with appropriate personal protective equipment?
22. Is the area to be hot tapped located below the liquid level of the tank, or on  line or vessel in which flow has been established?
23. Is there adequate storage area and room for operational and emergency access?
24. Has a procedure been prepared and in place to isolate the work area in the event of a failure? Are personnel trained to implement the procedure?
25. Have the requirements been defined for weld inspection and for pressure testing, and is all of the testing equipment on hand and in good working condition?

BEFORE WELDING:

Each of the following conditions should be satisfied before welding:

1. Are the welders qualified for the approved welding procedure to be used?
2. Is a preheat of the weld area required?
3. Is the fitting properly positioned so that misalignment of the hot tapping machine will not occur?
4. Have the pressure and temperature of the contained materials been reduced as much as the process operation will allow?
5. Have the flow, pressure and level considerations are considered?

BEFORE CUTTING:

Each of the following conditions should be satisfied before cutting:

1. Has the weld been inspected and tested?
2. Has the hot tap fitting been pressure tested?
3. Have the hot tap valve, packing, gasket and bolts been checked for leakage?
4. Has the packing or seals on the hot tapping machine been checked?
5. Has the bleed off valve been checked to assure it will hold, is operable, and is not obstructed?
6. Are all bolts on the pilot and cutter bit tight?
7. Is the coupon catcher on the pilot bit?
8. Is the valve centered on the flange?
9. Has cutting depth end calculated to avoid cutting the opposite side of the pipe?
10. Has the boring bar been run through the valve to assure free passage?
11. Have the hot tapping machine and valve been purged, if recommended?

BEFORE REMOVING THE HOT TAPPING MACHINE:

Each of the following conditions should be satisfied before removing the hot tapping machine:

1. Have the manufacturer’s instructions been followed to be sure that the boring bar is fully retracted before closing the hot tap valve?
2. Has the hot tap valve been closed?
3. Has the bleeder valve been opened?
4. Has all the pressure been bled off of the hot tapping machine before removing the bolts from the flange?
5. Have provisions been made to contain or control any liquid or gas in the hot tapping machine?

AFTER REMOVING THE HOT TAPPING MACHINE:

1. The hot tap machine should be cleaned, removing the hydrocarbons/chemicals from the line or equipment.
2. All rags, absorbent pads, and other cleaning materials must be disposed of properly.