The Rolled Groove Of The Steel Pipe

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Roll grooving, also known as press grooving or roller grooving, is a cold forming processing technique:

Using a dedicated roll grooving machine (press grooving machine), the lower part drives the cam wheel to rotate the steel pipe, while the upper convex pressure wheel is slowly pressed against the outer wall of the steel pipe under the action of the hydraulic cylinder, causing the metal at the end of the pipe to undergo plastic deformation and cold extrude a circular arc-shaped groove (groove). No cutting or material removal is performed.

Key distinction:

Roll Groove = Cold Extrusion Forming, no material removal;

in contrast, Cut Groove = Using turning method to cut off some materials to create the groove.

Roll Groove is currently the most common method in engineering.

The sole purpose of the rolling groove is to machine grooves at both ends of the steel pipe, so as to enable the use of groove-type connection (clamp connection / Grooved Coupling) for connecting the pipes:

1. Install the rubber sealing ring → Put on the clamp (flexible/rigid) → Tighten the bolt, and the two pipes are connected.
2. This connection system is widely used in: fire water systems, HVAC, water supply and drainage, petrochemicals, mining pipelines, etc.

There are various ways to connect steel pipes. Why not simply weld or use flanges? The significance of the rolled groove connection lies in:

In simple terms: The significance of grooving technology = It has advanced the connection of pipelines from the "welding era" to the "rapid mechanical assembly era" - safe (without using fire), efficient, flexible, and easy to maintain.

According to the "Technical Code for Grooved Connection Pipelines" T/CECS 151 and related operating procedures, the process is as follows:

▎Step 1 · Pipe Preparation

• Use a pipe cutting machine or a toothless saw to cut the steel pipe to the desired length.
• The incision must be smooth and the end face must be perpendicular to the pipe axis (skewness: for DN100 to 150, ≤ 1.0mm; for DN ≥ 200, ≤ 1.4mm)
• Use a grinding wheel or file to thoroughly remove any burrs, rust and oxide scales from both the inside and outside of the pipe end.

▎Step 2 · Positioning and leveling of the machine

• Place the slotting machine on a sturdy and level surface, and use a level gauge to calibrate the machine body.
• Select the corresponding upper and lower pressing wheel/embossing wheel molds of the appropriate size according to the pipe diameter (⚠️ Do not mix molds of different specifications)

▎Step 3 · Erecting steel pipes

• One end of the steel pipe is placed on the driving wheel of the rolling slot machine, while the other end is placed on the pipe support (the support is approximately at the position 2/3 of the total length from the pipe end).
• Adjust the height of the support to make the steel pipe horizontal (the end of the support can be slightly lower by 1 to 2 degrees to prevent "drifting of the pipe")
• The cross-section of the pipe end is tightly aligned with the positioning stop surface of the rolling groove machine, and the pipe axis is perpendicular to the stop surface.

▎Step 4 · Set the groove depth and roll

1. Tighten the pressure relief valve of the oil pump → Rotate the handle to make the upper pressing wheel just touch the outer wall of the steel pipe.
2. Rotate the limit nut to set the target slot depth (The limit device is mandatory! Do not use an unlimited position machine).
3. Start the motor → Drive the wheel to rotate the steel pipe → For each rotation, advance the pressing down by approximately 0.25mm, gradually deepening, never press all the way down at once!                                                                                                                                                                                                                                                        For example: A typical feed rate example (DN200): Approximately 10 to 11 rotations are required, resulting in a total depression of about 2.5mm. The feed is uniform for each rotation.
4. Once the stop nut is fully tightened and reaches the set depth, rotate the steel pipe for another 1 to 2 turns to achieve the shaping and polishing process → Stop the machine

▎Step 5 · Inspection and Removal
• Release the pressure relief valve → Remove the steel pipe→ Use a slot gauge/vernier caliper to measure whether the depth and width of the slot are within the standard tolerance → Only after it is qualified can it be put into use

This is a very crucial issue. The rolling groove process is essentially a cold plastic deformation operation, which alters the local state of the pipe end:

Physical phenomenon that are bound to occur

⚠️ The actual harm caused by improper operation:

✅ The conclusive result of correctly rolling the groove

As long as the operation is carried out in accordance with the specifications (with limited number of bits, stepwise feed, and controlled depth), the impact of the grooves on the strength of the steel pipe body is within the design tolerance. The groove dimensions specified in the national standards are designed based on ensuring sufficient remaining wall thickness (for example, for DN200 steel pipe, the minimum wall thickness requirement is 5.0mm, and the groove depth is only about 2.5mm).

According to the technical standards such as CJJ/T 154-2020 / T/CECS 151-2019 / CJ/T 156, a qualified trench must meet the following requirements:

Geometric dimension requirements：

Surface quality requirements

• The surface from the pipe end to the groove section is smooth without any bumps or obvious scratches or marks.
• The galvanized layer and the inner wall coating/liner must remain intact - it is not allowed for large areas to peel off or be damaged due to the rolling process.
• There shall be no cracks or peeling at the grooves.

The process mandates strictly

• The slotting machine must have a limit device and cannot use an unregulated homemade device.
• Different DN types must be used with corresponding molds. It is strictly prohibited to use them interchangeably.
• During the rolling process, the steel pipe must not have any longitudinal or angular displacement.
• The processing time should be within the limits specified by the procedures (to prevent rushing the process too quickly)

1. National Standard:

• GB/T 3091-2015 "Welded Steel Pipes for Low-Pressure Fluid Transmission": It stipulates that for steel pipes with a wall thickness of ≥ 2.5mm, the depth of the groove shall not exceed 15% of the wall thickness (for example, when the wall thickness is 3mm, the groove depth shall be ≤ 0.45mm).
• GB/T 5135.11-2006 "Automatic Sprinkler Systems - Part 11": It requires that the groove area of the socket-type connection pipe fittings shall not have cracks or deformations.

2. International Standards:

• ASTM A53 (American Standard): It is specified that after the grooves are machined, the compressive strength of the steel pipe should retain more than 90% of its original value.
• ISO 4144 (European Standard): It stipulates that the edges of the grooves must be smooth without burrs, and the roughness Ra should be ≤ 12.5 μm.

Rolling grooves and cutting grooves are both processes for pipe slotting.

The cutting groove process physically removes the pipe material from the outer diameter of the pipe, thereby cutting out a groove in the pipe.

Rolling and cutting are easier on the pipe and cause less damage. Rolling cold forming allows for the formation of grooves without removing any pipe material, eliminating the need for debris cleanup and offering high efficiency.

Pipes can be rolled with grooves within 30 to 40 seconds per square inch, while using the cutting groove process it takes 1 minute per inch. The rolling groove process also eliminates the need for cutting oil and the hassle of cleaning pipe metal debris.

Most of the products on the market adopt the rolling groove process. Won't the cutting groove process be replaced then?

In some cases, the cutting groove process may be more useful. For example, if the pipe wall is very thick, the cutting groove process will be more effective. For instance, if the pipe is used for transporting solid materials such as sand, and there need to be no protrusions inside the pipe to prevent the obstruction of sand flow, the cutting groove can effectively maintain the integrity of the inner wall of the pipe. Therefore, the appropriate slotting process should be selected according to the specific usage scenario.

The roll groove process is a cold extrusion technique used to "mold" a standard groove at the end of the steel pipe. The purpose is to enable the pipes to be quickly, reliably, and detachably connected using clamps - it replaces the fire risks of traditional welding and the bulkiness of flanges. The core of the operation involves positioning + staged slow pressure + size inspection. The deepening of the pressure weakens the strength, the shallowing reduces leakage, and eccentricity prevents installation. Therefore, each step is strictly controlled by a standard.

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