Hot tap is a pressure installation technique that allows connections to be made on existing live (operating) piping or equipment without shutdown or interruption to critical plant operation. As per API Publication 2201, "Hot tapping is the technique of attaching connections to equipment in service by welding and drilling."
A more comprehensive definition is provided in Shell standards which define hot tapping as "A method of making connections to piping (or other equipment) by attaching a fitting to the system, usually by welding, followed by cutting through the pipe wall at the point of attachment utilising an appropriate ("hot-tapping") machine. This operation is generally carried out on a live ("hot") system without decommissioning or interrupting flow."
Justification for Hot Tapping
Hot tapping involves safety hazards and should be undertaken only if the required modification cannot be delayed until the next available shutdown. It requires close coordination between Engineering Team, Operations Personnel, Integrity Assurance Team, Inspection Team, Loss Prevention Team and the Hot Tap Contractor.
Required Documentation
The following documentation is recommended to be in place before starting of hot tap operation:
- Detailed Engineering calculations confirming suitability of hot tap.
- Design drawings and sketches.
- Thickness measurements from ultrasonic examination reports and lamination checks.
- Hot Tap Procedure.
- Detailed welding procedures.
- Detailed Risk Assessment.
- Approved Hot Tap Check list
The Basic Steps in Hot Tapping
Hot-tap operation involves the following basic steps:
- Welding a suitable tapping fitting such as split tee or reinforced flanged fitting.
- Installing a hot-tapping valve.
- Installing the hot-tapping machine on the valve to execute the pressure cut which involves removal of section of run pipe.
Limitations
Most engineering specifications do not permit hot taps under the following situations:
- Piping with wall thickness below 6mm. (Thicknesses lower than 6mm increase the risk of burn-through during welding)
- Piping and equipment made of materials that require postweld heat treatment (PWHT).
- Piping and equipment made of materials that may undergo metallurgical deterioration due to heat of the welding operation.
- Piping and equipment that contains a flammable mixture of gas and air.
- Piping and equipment that has internal coating, cladding, glass, cement or refractory lining.
- Piping and equipment that are jacketed.
- Piping and equipment that are subject to vacuum.
- Piping where slugging can occur during the hot-tapping operation.
Section 216-2.4 of ASME PCC-2 (2018) provides a list of conditions for which hot tapping poses unacceptable risks. This includes oxygen or oxygen enriched atmospheres, monel piping handling suflur compounds, stainless steel piping containing potassium carbonate solutions and piping in hydrogen service.
Process Conditions - Maintaining pressure and flow
A minimum flow rate should be maintained in the line to be hot tapped to allow the heat from the welding operations to be dissipated. Too high a flow rate or too high flow velocities can cause rapid cooling of the weld and make it susceptible to cracking. On the other hand too low velocities can cause increased pipe wall temperatures at the weld location, weakening the material and increase the possibility of a blowout. Exxon Mobil specifications recommend a maximum velocity of 5m/s in liquid lines and 10m/s in gas lines. There are other Owner specifications which prescribe flow velocities in the range of 0.5m/s to 1.75m/s for liquid lines. The minimum recommended velocity in liquid lines is 0.5m/s and in gas lines is 0.4m/s. Section 216-3.10 of ASME PCC-2 recommends a minimum flow rate of 0.4m/s for liquid and gas lines. The maximum and minimum acceptable velocities need to be evaluated on a case to case basis.
Hot tapping on flare line poses a big challenge because there is normally no flow in this line. To maintain a minimum flow rate, the flare line is purged with gas to maintain the recommended minimum flow velocity.
During the hot-tapping operation the operating pressure should be kept as low as practically possible without disrupting the plant operations.
Checking minimum wall thickness of existing pipe
A detailed inspection of the hot tap location on the run pipe shall be carried out. This may require removal of existing insulation, paint coating in the area proposed for hot tap. It is recommended to examine additionally about 200-300mm on either side of hot tap location. The area shall be properly examined by ultrasonic examination for remnant wall thickness and presence of imperfections. The location shall be approved for hot tapping if it meets the minimum pipe wall thickness requirements and is free of laminations.
Welding of Hot Tap Fitting
The proposed location for all hot tap fittings shall be marked for inspection and approval. The location shall be checked for possible obstructions to the fitting installation, access to welding of the fitting, installation and operation of the hot tap machine. The fitting can be either a preformed full encirclement split tee, pipe to pipe set-on branch connection with reinforcement pad or flanged outlet (o-let) fitting. The size of set-on branch shall be at least one pipe diameter smaller than the diameter of the run pipe. Full size hot tap connections are not recommended as there is a possibility for the hot tap cutter to cut into the run pipe. For hot-tap connection of size 2-inches and smaller, flanged outlet fittings are recommended. Flanged o-let fittings should not be used on lines subject to severe vibrations. Consideration shall be given to providing adequate stiffening brackets to branch outlet fittings when their use cannot be avoided.
The location of hot tap fitting bore shall not intersect the run pipe longitudinal or circumferential welds. A minimum gap shall be maintained between the fillet weld of the hot tap fitting and the existing girth welds such that the heat affected zones from both the welds do not overlap. ASME PCC-2 recommends a weld toe-toe distance which is lesser of 150mm or 8 times the wall thickness of run pipe. Longitudinal welds that will be covered by full encirclement split tees or reinforcing pads should be ground flush with the surface of the pipe for a distance equal to the length of the split tee or width of the reinforcement pad plus 75 mm beyond each end.
The heat input during welding of the fitting on the pipe results in a weakening effect on the run pipe as there is reduction in strength of pipe wall at higher temperature. There is a possibility of blowout occuring during the welding operation if the heat input is not controlled. A detailed calculation shall be performed to estimate the maximum temperature of the pipe wall that will be attained during the welding operation. The remnant pipe wall thickness should be able to withstand the operating pressure of the line at the elevated temperature.
Appendix B of API STD 1104 covers recommended welding practices for making repairs to or installing appurtenances on pipelines and piping systems that are in service. Welding procedures should be qualified in accordance with these procedures unless other engineering specifications exist. Appendix B of API STD 1104 provides suggested welding sequence for reinforcing pad, reinforcing saddle, full encirclement sleeve, encirclement tee, encirclement sleeve plus saddle and encirclement saddle.
Hot Tap Valve
The hot tap valve can either be a full bore gate valve or ball valve. The bore of the valve shall be clearly specified in the method statement or procedure. This is to ensure that the selected drill or cutter is at least about 3mm smaller in diameter than of the valve bore. The hot tap valve pressure and temperature rating should be at least equal to that of the existing piping system and the materials should be compatible with the intended service. The valve is usually subjected to shell test and seat leak test in the field even if these tests have been carried out by the vendor to ensure safety during hot-tapping operation.
Temporary Supports during Hot-tapping
The process of hot-tapping introduces additional sustained loads on the existing pipe due to branch fitting, hot tap valve and hot tap machine. The additional sustained loads due to branch fitting and valve will be permanent while that of the hot tap machine will be temporary. The stresses induced due to the additional loads shall be calculated and if the existing supports are deemed insufficient, additional temporary or permanent supports should be provided. A dynamic load factor of 2 may be considered in the calculations due to the nature of hot tapping operation which will induce dynamic loads.
Testing of Welds
The root pass and final pass of all welds shall be subject to dye penetrant, magnetic particle or ultrasonic examination to ensure that the welds comply with the requirements of the applicable codes and procedures. Additionally, hot tap fittings shall be pressure tested after completion of welding and prior to starting the hot tap operation to demonstrate the strength of the hot tap branch weld. During this strength test, the run pipe will be subject to external pressure. Hence calculations shall be performed to ensure that the run pipe will not collapse (buckle) due to external pressure. If the calculations indicate occurence of local buckling, the test pressure may be reduced such that the stresses do not exceed the yield strength at test temperature. The test pressure shall be at least equal to the maximum operating pressure of the run pipe which is subject to hot tap.
Hot Tap Calculations
Detailed hot tap calculations shall be made to ensure that the entire hot tapping process is carried out in a safe manner. The following calculations are recommended:
Calculations to determine the adequacy of remnant pipe wall thickness in existing pipe considering the temperature derating due to heat input from welding operation. This calculation will determine the extent of reinforcement needed on the run pipe.
Calculations to check the adequacy of existing pipe supports to take the additional loads of hot tap fitting, hot tap full bore valve and hot tap machine. A dynamic load factor shall be applied for the loads from hot tap machine.
Calculations to check that the existing piping is safe for buckling during hydrotest considering that the pipe will be subjected to external pressure while carrying out hydrotest of the fitting.
Applicable Codes and Standards
EEMUA Publication 185 - Guide for Hot Tapping on Piping and other Equipment.
API RP 2201 - Procedure for Welding or Hot Tapping on Equipment in Service.
ASME PCC-2 - Article 216 Welded Hot Taps in Pressure Equipment or Pipelines.
API STD 1104 - Welding of Pipelines and Related Facilities
API RP 2200 - Repairing Hazardous Liquid Pipelines