What is the difference between plain-end and threaded-end GBT 3091 steel pipe?

When you order welded steel pipe to the Chinese standard GB/T 3091, one of the first details your supplier will ask about is the pipe end. The same pipe can be supplied with a plain end, a threaded end, or a grooved end, and that choice changes how the pipe is joined on site, how fast the installation goes, and how much the connection costs. For buyers and engineers comparing plain-end and threaded-end GB/T 3091 steel pipe, understanding the difference before ordering avoids expensive rework and on-site machining.

This article explains what plain-end and threaded-end GB/T 3091 steel pipe are, how each end is made, where each type performs best, and how to specify the right end for your project.

What Is GB/T 3091 Steel Pipe?

GB/T 3091, titled "Welded steel pipes for low pressure fluid delivery," is one of the most widely used Chinese standards for welded steel pipe. It covers straight-seam high-frequency electric welded (HFW) pipe, straight-seam submerged-arc welded (SAWL) pipe, and spiral-seam submerged-arc welded (SAWH) pipe used to carry water, air, steam, and gas at low pressure, as well as hot-dip galvanized pipe made to the same standard. Steel grades commonly supplied under GB/T 3091 range from Q195 to Q345, in both black and galvanized surface finishes.

The standard also defines how pipe ends must be prepared. It requires the two ends of the pipe to be cut flat, with the end-face bevel no greater than 3 mm and all cut burrs removed. Beyond that basic requirement, the standard allows the pipe to be delivered with plain ends, threaded ends, or grooved ends, and each option suits a different method of connection.

What Is a Plain-End (PE) GB/T 3091 Pipe?

A plain-end pipe is cut square at a 90-degree angle to the pipe axis and deburred, with no thread, groove, or bevel applied. It is the simplest and most common end preparation, and it is what most GB/T 3091 pipe is supplied with by default.

Plain ends are joined by welding. The pipe can be butt-welded, socket-welded in small diameters, or connected with slip-on flanges. Because no material is removed from the wall, a plain-end pipe keeps its full wall thickness right up to the joint, which is why welding-based connections on plain-end pipe are the standard choice for pressure service and for larger diameters.

• Simple to produce and lower base cost than threaded or grooved ends.
• Full wall thickness is preserved at the joint.
• Flexible: the same pipe can be welded, flanged, or grooved on site.
• Easy to stock and manage across projects.

What Is a Threaded-End (TE) GB/T 3091 Pipe?

A threaded-end pipe has taper threads machined onto the outside of each end, so the pipe can be screwed directly into threaded fittings, couplings, or valves without any welding. Under GB/T 3091, pipe supplied with threaded ends must have threads that comply with GB/T 7306.1 or GB/T 7306.2, the Chinese standards for 55-degree sealing pipe threads that correspond to ISO 7-1. The taper on the thread lets the connection tighten and seal by interference as the joint is made up.

Threaded ends are a fast, low-skill connection method. A fitter simply applies sealant or thread tape and tightens with a wrench, which makes threaded-end pipe popular in regions such as the United States, the Middle East, and Southeast Asia where threaded connections are the established practice for small-diameter lines.

• No welding, so it is safe in confined or fire-risk areas.
• Fast installation with a wrench and sealant.
• Reliable sealing from the taper thread.
• Easy to dismantle, inspect, and reuse.

The main limitation is that cutting threads removes steel from the pipe wall, so the wall at the joint is thinner than the rest of the pipe. For this reason, threaded ends are normally limited to smaller diameters and low-to-medium pressure service. Under GB/T 3091, the dimensions for pipe supplied with threaded and grooved ends are listed in the standard's appendix for the size range where these connections are practical.

Plain End vs. Threaded End: Key Differences

When to Choose Plain End

Choose plain-end GB/T 3091 pipe when the line will be welded, when the diameter is too large for practical threading, or when the connection must handle higher pressure. Plain ends are also the right choice when the pipe will be grooved or flanged on site, because the plain end leaves the full wall thickness available for whatever connection method the contractor selects. For large-diameter water mains, structural lines, and any system where the joints will be welded, plain ends are the standard. When the project calls for big diameter steel pipe, plain ends are almost always the practical option, since threading is not economical or reliable at large sizes.

When to Choose Threaded End

Choose threaded-end GB/T 3091 pipe for small-diameter, low-to-medium pressure lines where welding is impractical or unsafe, such as fire sprinkler branches, plumbing, compressed air lines, and temporary or modular systems. Threaded ends also make sense when the crew on site has no welding capability, or when the line may need to be dismantled later for maintenance or relocation. If the pipe is supplied galvanized, confirm with the supplier how the threads are protected, because the zinc coating is interrupted where the thread is cut.

How to Specify the Right End When Ordering

When you order GB/T 3091 steel pipe, state the end type explicitly. A complete order should include the standard and grade, the outer diameter and wall thickness, the length, the surface finish (black or hot-dip galvanized), and the end preparation. If you need threaded ends, confirm the thread standard and whether couplings are required, and specify the thread length. If you are unsure, plain ends are the safest default, because a plain end can be threaded, grooved, or welded on site, whereas a threaded end cannot easily be converted to a welded connection without cutting the threads off.

EZ Steel Industrial Co., Ltd. is a manufacturer and integrated supplier of GB/T 3091 steel pipe and pipeline works, backed by more than 12 quality checkpoints, ISO 9001 quality management, and full mill test documentation. The company supplies GB/T 3091 welded pipe from Q195 to Q345 in black and galvanized finishes, with plain, threaded, or grooved ends as required, plus the fittings, flanges, and valves needed to complete a project package. Contact the team with your size, grade, and end requirements to get a quotation for your next project.