When design an industrial plant in an existing context we have the necessity to connect new systems with the ones already present. Tie-in in general is the connection point between new and existing and can be the replacement of an equipment, adding of an instrument or an insertion of a branch in an working pipeline, insertion of a valve in an existing line, etc.

There are different types of tie-in depending on the working condition of the existing plant:

1. cold tap tie-in in conjunction with a total plant shutdown;
2. cold tap tie-in in conjunction with a unit or system shutdown;
3. hot tap tie-in, hazardous fluids;
4. hot pat tie-in, non hazardous fluid.

The execution of the tie-in, up to the first valve, has to be planned carefully because the time duration is defined by the shutdown. The sequence of activities to get the completion of the tie-in is:

1. define on p&i of the connection point;
2. buy materials up to the first block valve;
3. get existing documents (layout drawings, support drawing, etc.);
4. study an hypotetic location of tie-in for survey;
5. survey at site (tie-in location in respect of the work accessibility ;
6. issue isometric and layout drawing;
7. prepare the existing system (blinding and cleaning);
8. mechanical work execution;
9. NDE and hydrotest (where applicable);
10. system ready for start up certificate.

When the tie-in shall be carried out during a plant shutdown the activities above shall be planned to finish the last activity before the plant start-up.

Hot tap is a tie-in is to be performed with the existing system in operation, special precaution shall be taken during the welding and the drilling works to prevent fluid spillages. The sequence of activities to get the completion of tie-in consisting in the insertion by welding of a branch in an existing line is:

1. define on p&i of the connection point;.
2. buy materials up to the first block valve;
3. get existing documents (layout drawings, support drawing, etc.);
4. study a possible location of tie-in for survey;
5. survey at site (tie-in location in respect of the work accessibility and drilling machine dimension);
6. read with ultrasonic tests the actual thickness and wall defects (if any);
7. prepare special Welding Procedure Specification;

1. prepare work permit and safety plan;

1. issue isometric and layout drawing;
2. get approved the check list;
3. monitor pressure and temperature in existing line;
4. weld the insert (and reinforcing plate) or the split tee;
5. install valve;
6. perform hydrotest and NDE;
7. install drilling machine and drill;
8. close valve and dismount drilling machine;
9. install blind flange.

The design shall take into account that the valve (full bore gate, ball or sandwich type) shall be positioned as close as possible to the tie-in point. In case of tie-in in a pipeline take care of the pig passage for maintenance and adequate fittings shall be provided.

Then same sequence can be applied to the hot tap tie-in in tank when an extra nozzle shall be added. When the cleaning of tank containing flammable liquid is impossible or impractical the operation is permitted if the auto-ignition point is lower than the inner wall temperature during welding. In such case the only precaution is to leave at least 1 m of liquid above the tie-in point to avoid hot surfaces in contact with vapours.

In case you have to insert a valve in an existing line that can’t be stopped, the sequence is a little bit complex because a temporary by-pass has to be installed. Four hot taps shall be performed across the valve insertion point, in the farthest two a valved by-pass shall be installed, in the other two tie-ins stoppled fittings permits the line spool cleaning and work in safety for the insertion of the new valve.

The critical activity in hot tap design is the calculation of the maximum pressure in the existing line during the welding. During welding we shall keep into account that a part of the pipe wall thickness is melted and part of the remaining is hot (heat affected zone) so the allowable stress during welding of the pipe is lower and shall be calculated accordingly. For further details see

 When the welding is not practicable (the pipe thickness is too low or the pipe is lined) bolted split tees can be used, in that case an adequate gasket shall be inserted. The gasket to be very plastic to compensate pipe ovality and surface imperfection.