Hot Tapping

Hot Tapping

INTRODUCTION

 Definition

Method of making a connection into a live piping under operating conditions

Purpose

• Branch connections for supply of process fluids
• Connections for measuring instruments

 

Advantages

• Can be done without plant shutdown
• No loss of production or supply to customers 

Applications

Hydrocarbon, steam, chemicals, water etc.

 

Disadvantages

• Always urgently required
• HIGH RISK JOB!

LIMITATIONS

Materials

•Hot tapping is limited to carbon steels and austenitic stainless steels

•Shall not be done on materials which required PWHT

•Hot tapping shall not be done on lined / clad / internally coated pipes

•Limited to materials with maximum design temperature of 370°C and minimum design temperature of -20°C

Service

•HP steam, H2S, lean / rich amine, sulfuric acid, caustic soda, hydrogen (>700 kPa), acetylene, compressed air (>3000 kPa), oxygen etc.

MAINTENANCE ASSISTANCE

•Identify location based on approved isometric drawing

•Provide proper access and approach

•Freeze location

•Remove insulation, paint or coating material

•Mark all weld locations of stub, RF pad or full sleeve on main pipe by punching

•Clean the main pipe for about 150 mm on either side of the proposed location of the fitting by wire brushing or power buffing

•Visually examine the location for cracks, laps, pitting or any other abnormality

•Locate the stub so as to avoid welding on such minor flaws

 PRE-EXECUTION INSPECTION 

•Carryout Ultrasonic and Magnetic particle inspection over a band of 150 mm minimum width centred over the proposed weld locations to ensure freedom from laminations and other detrimental discontinuities

•Available thickness should be comparable with nominal thickness or atleast 4.8 mm

Maximum acceptable hardness is 200 BHN 

 MAIN PIPE CHARACTERISTICS

Spiral welded pipe

Avoid stub interference with weld.  If unavoidable, locate stub such that the spiral weld cuts coupon into two equal halves.  Weld stiffener to prevent spring back of coupon

ERW or EFW pipe

Avoid stub interference with weld.  If unavoidable, check weld by RT for acceptance

EFW seam is difficult to find.  Use buffing, radiography or etching using 5% Nitric acid

If seam weld is crossing across the fillet weld of branch or reinforcement, seam weld shall be ground flush with the pipe surface over a length extending 50 mm beyond each end of the proposed fitting

RF pad and Full sleeve welding

Do not weld on ERW seams.  A distance of atleast 7 times the main pipe wall thickness or 75 mm (whichever is greater) shall be maintained

Fillet weld of branch shall be atleast 1D or 150 mm away from any circumferential weld of main pipe (D=main pipe diameter)

CONTROL OF CRITICAL PARAMETERS

Velocity of fluid

•Minimum velocity for gases – 0.4 m/sec

•Maximum velocity for liquids – 1.75 m/sec

Pressure

•Shall not exceed 7000 kPa or design pressure of hot-tap machine, whichever is lower

Heat input during welding

•Weakening effect of material during welding may result into blow out of pressurized pipe

•Microstructure change due to excessive heat input or rapid cooling

•Heat input = Net factor x   -----------------

Velocity of fluid

•Minimum velocity for gases – 0.4 m/sec

•Maximum velocity for liquids – 1.75 m/sec

Pressure

•Shall not exceed 7000 kPa or design pressure of hot-tap machine, whichever is lower

Heat input during welding

•Weakening effect of material during welding may result into blow out of pressurized pipe

•Microstructure change due to excessive heat input or rapid cooling

•Heat input = Net factor x   -----------------

•Current and voltage is given in the WPS

•Welding speed and electrode diameter shall be decided based on allowable heat input

•Current and voltage is given in the WPS

•Welding speed and electrode diameter shall be decided based on allowable heat input

Net factor = 0.85 for butt welds

                                                                                        0.57 for fillet welds 

MINIMUM WELDING TRAVEL SPEED

PRESSURE TESTING

 •Branch pipe with flange and valve in open condition shall be pressure tested before cutting

•RF pad / full sleeve shall be pneumatic tested at 1.5~2 kg/cm²g

•Test is done to determine leak tightness of welds

•Strength test pressure is the highest practical pressure depending on the internal pressure and ability to withstand external pressure against buckling

•Water shall be used as the testing medium for low temperatures

•For high temperatures, glycol or therminol can be used