WHAT IS HOT TAPPING

Hot Tapping in Piping

A hot tap is a process which is used to cut or make a hole through hot tap cutter on a live line under the specific temperature and pressure. when someone wants to establish a new service to an existing pipe, it is not convenient or practical to take the system out of service, therefore a hot tap is required.

The coupon, is the cut –out section of pipe which is removed after the hot tap is made. There is no way to guarantee that is coupon will not be dropped, but we can do our level best to avoid this by using the pilot drill with u-wires which is known as coupon retention.

Below are the factors that affect the coupon retention.

1. High Flowrate

2. Viscous fluids

3. Broken u-wires

4. Dirty pilot drill, that the u-wires cannot freely rotate

5. Incomplete cut and pull back the pilot drill

Keep in mind, doesn’t matter how hard we try and put our efforts to mitigate the risk of coupon falling, drop the coupon is risk in hot tapping that we highlight in risk assement report before we execute the job.

HOW DO WE PERFORM A HOT TAP

For a hot tap, there are three major elements to make a tap into a pipe, the hot tap fitting (Split tee welded or extruded type/reducing fittings (weldolet nipploet with flange end connection or threaded type end connection), the valve, and the hot tap machine (which includes the cutter,pilot drill and adaptor).

First of all we weld the ho tap tee onto pipe and mount the valve and hot tap machine and do the pressure test to make sure there is no leak prior to start cutting coupon or making hot tap.

HOT TAP STACKUP

BENEFITS

1. No need to shut down the plant

2. New connection to be achieved on live line

3. No need to stop production

4. Minimize the risk of leaking

PIPELINE PRODUCT ON WHICH HOT TAPPING CAN NOT BE DONE

1. Acetylene

2. Ammonia

3. Chlorine

4. Elemental Sulphur

5. High pressure steam

6. Hydrofluoric acid

7. Sulfuric acid

8. Oxygen

9. Vacuum conditions

WHAT WE REQUIRE PRIOR EXECUTION

We need to know the parameters prior execution as listed below.

1. Design pressure

2. Design temperature

3. Corrosion allowance

4. Design Factor

5. Flow rate

6. Velocity

7. Stack up drawing to check the hot tap machine travel or reach to make sure the operation is doable

WHAT IS RECOMMENDED PRESSURE AND TEMPERATURE FOR HOT TAPPING

From safety point of view and safe operation the pressure inside the pipe should not be more than 70 bar, again it depends on companies capabilities who design hot tap machines which are operated at more than 70 bar pressure inside the pipe.

From safety point of view the operating temperature should not be more than 370 degree centigrade and less than 20 degree centigrade for safe hot tapping.

TYPE OF HOT TAP CONNECTION

Type of new tie-in or hot tap connection depends on size and service as well. Below given the general instructions to choose the type of connection.

1Reduced Branch Fittings

Reduced branch fittings mean weldolets and nipplotes etc. these kind of connections to be used in following conditions which company to company varies.

a. For on-plot piping the new tap connection shall be NPS 3 maximum, and the ratio of branch to header pipe shall be equal to or less than the 1/3, and keep in mind reducing kind of fittings like weldolet and nippolet cannot be used on the pipelines that have vibrations already, and minimum size for the reducing branch shall be NPS 1.

b. For off-plot pipeline the new tap connection shall be NPS 3 maximum, and the ratio of branch to header pipe shall be equal to or less than the 1/4, and keep in mind reducing kind of fittings like weldolet and nippolet cannot be used on the pipelines that have vibrations already, and minimum size for the reducing branch shall be NPS 1.

2 Pre-Fabricated Fittings

Welded kind of fitting which is known as split tee is designed and used usually with nozzle and flanged connection. It is designed to withstand with the design pressure and temperature. It is normally used for higher sizes, it is recommended to use the flanged end connection hot tap tee above NPS 3.

Pre-fabricated extruded fittings are also used for hot tapping and which is where the number of welds on split tee reduced (only extruded tee to flange butt weld)

ADDITIONAL REQUIREMENTS FOR SPLIT TEE

1. Split tee design shall be as per the design codes like ASME B31.3, ASMBE B31.4, ASME B31.8

2. Split tee branch shall be kept as short as possible and minimum shall be 150mm

3. Scraper guide bars are used if the line is pigable and we use the term flow thru plug, which does not let the pig getting into the line.

4. Coupon reinforcement is used for the larger dia pipes bigger than NPS 18.

5. Shop hydrotest is required for split tees to make sure it will withstand with design pressure, and normally the hydrotest pressure is considered the 1.5 times the design pressure and medium for hydrotest shall be potable water.

6. If the spilt tee’s run thickness is lies within the range where the PWHT becomes mandatory as per the design codes then we do NDT before and after the PWHT for the split tee fitting and again it depends on companies projection specifications.

MAXIMUM & MINIMUM RECOMMENDED VELOCITY DURING SPLIT TEE WELDING ON LIVE PIPELINE

1. Maximum Velocity

Maximum velocity during welding on live pipelines shall be as given below

For Liquids: 1.75 m/s

For Gases: 9 m/s

2. Minimum Velocity

Minimum velocity during welding on live pipelines shall be as given below

For Liquids: 0.5 m/s

For Gases: 0.4 m/s

HOT TAP LOCATION

Key instructions are given below regarding hot tap location.

1. In horizontal line, the vertical hot tapping at 12 o’clock is recommended.

2. In vertical pipeline, the horizontal hot tapping may be performed but the company approval is required.

3. Lateral tap at 45 degree is acceptable but the company approval is required

4. Hot tapping shall not be executed on upstream of rotation equipment’s and controls valves unless strainers are installed, because after completion of hot tap the swartf will be therer in l,ine that may stuck and damage the valves etc.

5. While split tee fillet welding keep in mind the fillet should be 150mm away from the welded seam

6. While split tee fillet welding keep in mind the fillet weld should be 450mm away from the flanged connection or threaded connection

7. Hot tap shall not be executed on the lines where already seam weld is available.

8. Keep in mind the chosen location for hot tapping following things need to be cheched

· Diameter and ovality

· Pipe wall thickness with UT

· Internal or external corrosion

· Defects in pipe wall

· Laminations

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