What Is Pipeline Stopple?

Quick Answer: A stopple is a temporary pipeline isolation method that inserts a mechanical plug into a pressurized line through a hot-tapped fitting, allowing downstream work (modifications, repairs, or tie-ins) without depressurizing or shutting down the pipeline. Hot tapping is the process of drilling into a live, pressurized pipe to create the access point for the stopple or a new branch connection.

Hot Tapping Process

Hot tapping (also called pressure tapping) uses a specialized machine to cut a hole in a pipe while the line remains in service. The basic sequence is:

Step	Activity	Key Equipment
1	Weld a split tee or weldolet fitting onto the live pipe	Welding per qualified WPS; typically requires engineering assessment per API 2201
2	Bolt a full-bore gate valve onto the fitting	Valve rated for line pressure and temperature
3	Attach the hot tap machine above the valve	Cutter housing, boring bar, pilot drill, hole saw
4	Open the valve and advance the cutter through the pipe wall	Coupon is captured on the pilot drill
5	Retract the cutter and close the valve	Coupon retrieved for inspection
6	Remove the hot tap machine	Valve remains for future access or permanent branch

Pro Tip

Always retrieve the coupon. A loose coupon inside a pipeline can obstruct flow, damage equipment, or jam a valve. Verify coupon retrieval before retracting the machine.

Stopple (Line Stop) Procedure

Once the hot tap fitting and valve are installed, a stopple head is inserted through the valve to plug the pipeline bore:

Step	Activity
1	Install the stopple machine on the hot tap valve
2	Open the valve and advance the stopple head into the pipe
3	The folding head or inflatable element expands to seal against the pipe ID
4	Verify isolation by monitoring downstream pressure (should drop to zero or bleed off)
5	Perform downstream work (tie-in, repair, valve replacement)
6	Retract the stopple head and close the valve
7	Remove the stopple machine; install a blind flange or leave the valve for future use

Double-block stopple systems use two plugs with a bleed point between them, providing positive isolation equivalent to a double-block-and-bleed valve arrangement.

Hot Tapping vs Stopple: Comparison

Feature	Hot Tapping	Stopple (Line Stop)
Purpose	Create a branch connection on a live line	Temporarily isolate a section of a live line
Requires shutdown	No	No
Permanent or temporary	Permanent (fitting remains)	Temporary (plug is removed after work)
Typical application	New branch, instrument connection, bypass line	Valve replacement, pipe cut-out, tie-in
Size range	1/2 in. to 48 in. and larger	2 in. to 48 in. and larger
Governing standards	API 2201, ASME B31.3/B31.4/B31.8	API 2201, project specifications
Weld qualification	In-service welding per API 1104 or ASME IX	Same as hot tap (stopple requires a hot tap first)

Engineering Considerations

Parameter	Requirement
Minimum wall thickness	Must support welding loads without burn-through; ultrasonic thickness survey required
Material compatibility	Fitting material must match or exceed pipe grade; carbon equivalent (CE) evaluated for weldability
Pressure and temperature	Hot tap machine rated above MAOP; typical limit ~100 bar for standard equipment
Flow medium	Hydrocarbons, water, gas; restrictions apply for oxygen, chlorine, and highly toxic services
Welding on live line	Requires engineering assessment, purging considerations, and thermal analysis for burn-through risk

Common Mistake

Performing a hot tap on a pipeline without verifying minimum wall thickness. Corrosion or erosion can reduce the wall below safe welding limits, risking burn-through under pressure. Always conduct a UT thickness survey of the tap location before welding.

Read the full guide to pipe class specifications