Site specific written down procedure for hot tapping
Even though there are API codes and guidelines available for conducting welding/drilling on live systems, each facility is unique. It is recommended that each site develop an exhaustive procedure for carrying out hot tapping on live equipment at their premises.
The procedure should detail the technical requirements and the responsibilities of the various agencies/personnel involved in the execution of the job
The procedure should indicate fluids used at the facility for which hot tapping is prohibited. As an example, fluids that can cause stress corrosion cracking in metals should not be considered for hot tapping.
Hot tapping as the last option
Taking a branch connection or performing maintenance by welding on live system should be the last option for any operator considering the serious hazards posed by the activity.
All options available other than welding on live equipment/pipelines should be evaluated and documented.
- Considering the serious hazards, a well-documented approval matrix should be put in place for authorization of each of the hot tapping jobs.
Hot tap design
A request for welding on live system should be created. The request should include, as a minimum, parameters like:
a) Size and actual thickness of the hot tap location
b) Operating pressure and temperature
c) Fluid characteristics (flammability, toxicity, corrosivity)
d) Velocity / flow
Based on the request, the modification to the system should be designed per relevant national/international codes and standards. The site/facilities engineering standards should be followed while designing the modification.
The design engineer should be informed that the branch connection should be done by Hot Tapping.
Design engineer should design the nozzle reinforcement to meet the machine loads during the drilling operations.
Design engineer should do a field visit to check the location where the welding/drilling is to be performed.
Design engineer should discuss the hot tap procedure with service providers/contractors entrusted with the job of welding/drilling on the live system
Design should be validated by persons considered competent.
Design should include a testing requirement for the modifications.
Feasibility for safe hot tapping
- Feasibility of hot tapping should be done by a team from the process plant facility and the service provide/contractor.
- A preliminary risk assessment should be done to check whether the Hot Tapping can be done safely at the specified location in the process unit.
- The service provider, based on their experience, will provide inputs on safe execution of the hot tapping. All relevant design information (as listed above) should be shared with the contractor in order to make an accurate determination of feasibility.
Making sure that metallurgy of the existing pipe is suitable for hot tapping
- Hot tapping is normally only done on carbon steel piping and equipment.
- Hot Tapping is not carried out on ferritic alloy steels where special welding requirements like preheat, post heat and post weld heat treatment are required.
- Hot Tapping on metallurgies other than carbon steel should need special procedures and is outside the scope of this document.
- Positive Material Identification (PMI) on the pipeline / equipment is necessary where hot tapping is to be done, to ensure that the material is indeed carbon steel and there is no mix up during the original construction.
- If there are welding joints that intersect hot taps, then PMI should be done on the weld joints also
- Hot Tap welding should not intersect electric resistance seam weld in the pipe.
Inspect the existing weld joints that intersects hot tap weld joint for integrity
- Hot Tap should be avoided on existing weld joints. The location should be chosen in such a way that Hot Tap welding does not interfere with the existing weld joints.
- Longitudinal seam weld joints should be identified by suitable in-situ etching methods.
- If Hot Tap cannot be avoided without intersection, then the interfering weld joint should be volumetrically inspected (either UT or RT) to make sure that there are no defects in the weld joint that can lead to burn through during hot tap welding. The UT or RT should be done for the purpose of hot tapping and certified.
- The area over which hot tapping is to be done should also be checked by the Mechanical Integrity inspector to detect sub-surface defects if any.
Making sure that the metal thickness at the hot tap location is adequate
- The location where thickness is to be measured should be hard punched and this should not be changed without consent from the site inspectors.
- Thickness should be measured by devices that work based on Ultrasound. Any other method of measurement needs to be approved by owner inspector.
- The thickness should be exactly measured at the location where the welding is expected to be done. It is recommended to measure thickness over a 2” band. Recommended minimum thickness for hot tapping is about 5.0 mm.
A critical review of the measured thickness may be necessary if:
Hot tapping welding is planned on systems with thickness close to the minimum thickness of 5.0 mm
Hot tapping is being considered on corroded pipelines. (e.g. Nominal thickness is 10 mm and measured thickness is close to minimum required thickness of 5.0 mm)
Hot tapping is planned on a system which has undergone a fitness for service evaluation
High Temperature thickness measurement should be cross-verified adequately to ensure that the measured readings are dependable
Making sure that the metal where hot tapping is being done meets the hardness requirements
- It is recommended to measure and certify hardness to be less than 200 BHN (Brinell Hardness Number). Higher hardness material can crack during welding. Higher hardness can also cause problems during drilling of coupons.
Maintaining fluid flow during hot tap operations
- Hot tapping operations are considered safe based on the fact that molten metal during welding solidifies and cools to less than about 400 deg C very quickly. This is necessary to achieve the mechanical properties able to withstand the system pressure. A positive flow in the pipeline helps in conducting the welding heat away and cooling the molten metal quickly.
- A positive flow should be ensured during the full cycle of hot tapping operations.
- The greater the thickness reduces the dependency on flow to cool the metal. However, flow has to be maintained.
- Operating crew will identify a direct quantitative measurement of fluid flow in the line where hot tapping is being done. The calibration of flow meter should be ensured.
- If the flow measurement is not available for direct determination in the line where hot tapping is being done, an indirect method of ensuring flow should be established. This should be validated by competent personnel at the facility.
Ensure welding procedures are in place
- Hot tap welding should be done as per an approved WPS.
- Welders should be qualified for the material and thickness.
- Based on the thickness of the metal, heat input during welding has to be controlled to make sure that metal does not get over heated.
- Welding heat input is dependent on the welding speed. Based on the maximum heat input allowed, welding speed should be calculated.
- The welder should be trained on a mock up piece to comply with the welding speed requirements.
Ensure that the new materials are used in fabrication of hot tap connection are as per specification
- All the materials like pipe, reinforcement pads and welding consumables should be checked by Positive Material Identification tools to make sure that there is no mix up.
Test the integrity of the hot tap nozzle before fixing
- The hot tap nozzle should be welded to the flange and this spool should be hydro-tested at the required values per the drawings and appropriate codes. This should be done prior to welding the nozzle on the existing equipment / pipeline by hot tapping.
Switch off Cathodic protection
- Cathodic protection can sometimes affect welding operations in pipelines. Ensure the same is switched off.
Have a risk assessment done for the complete hot tap job
Maintenance, Inspection and Operations crew on site should meet with the contractor and prepare a risk assessment for conducting the hot tap job safely.
The evaluations/inspections that were completed to certify the pipe / equipment as fit for hot tapping shall be shared with the contractor performing the work.
Inform the crew of the hazards of the fluid flowing through the pipe
The risk assessment should include a contingency plan in case of a mishap during hot tapping
Ensure availability of communication between the hot tap crew, supervisors and control room for emergency actions that may be required.
Fire department should be kept informed for responding in case of emergencies
Ensure that any scaffolding or platform, if erected for the hot tap job, is large enough for personnel to back off from the work area. Plan for alternate exits from the scaffold / platform.
Make sure that escape routes are clear.
Ensure that minimum personnel are present close to the hot tap location while welding is in progress.
Ensure enough ventilation and lighting in the work area.
Make arrangement for gas monitoring of the hot tap location continuously until welding is complete, approximately 30 minutes following completion of the work.
Prepare a briefing/toolbox talk to all the crew involved on the hot tap jobs regarding the safety and emergency procedures
Stiffening of parent pipe
- For large size hot tapping typically greater than 20”, there is always a possibility of the coupon springing back when the coupon cutting is complete. If spring back happens, coupon removal can be difficult. Consider installing welding stiffeners on the coupon to be cut prior to drilling and cutting.
Test the integrity of the hot tap nozzle after welding
- The hot tap nozzle should be pressure tested after welding is complete. During this pressure test, the parent pipe or part of the equipment on which the nozzle is welded is subjected to an imposed pressure. Hence, this test pressure has to be calculated by the design engineer and provided to the hot tapping contractor. If pressure testing is done above this calculated value, the parent pipe can buckle inwardly. The larger the branch of the hot tap, the higher the risk of buckling.
- The testing medium should be selected based on the temperature of the fluid flowing through the pipeline / equipment.
Check and certify the isolation valve
- The isolation valve to be installed on the hot tap flange should be hydro-tested in the shop prior to installation to ensure seat sealing. The gland packing should be changed prior to test to make sure that there are no leaks during the pressure test and subsequent hot tapping operations.
Test of hot tap drilling machine
- Check the drilling machine maintenance records / date.
- Check the hydrotest certification of all pressure parts of the drilling machine.
- Check the hydraulic hoses used in the drilling machine visually and ensure that they have been pressure tested.
- The pressure gages installed on the drilling machine must be calibrated and certificate should be valid.
- The packing used in the drilling machine should be new and compatible to the operating fluid at the operating temperature of the fluid.
Check and certify the gasket joints in the hot tap connection
- Do a pneumatic leak test and make sure that gasket joints are not leaking / passing. Both the gaskets between valve and nozzle and the gasket between valve and the drill machine should be tested.
- Check for gland leaks during the leak test.
Make sure that the drilling cavity is functional
- When the pilot drilling is done, the hot tap nozzle and the valve above will become pressurized. The vent valve should be in operation to safely vent at various stage of drilling operation.