Pipe Welding by the Numbers

About this book

A pipe weld is a small structure with a large responsibility. It must be as strong as the pipe, tight against the fluid inside, smooth enough inside not to disturb the flow or trap product, and sound all the way round, including the part the welder could barely see. It is usually made in an awkward position, often far from the workshop, and sometimes in a trench, on a rack or inside a plant that is still running. When it fails, it can release steam, gas, chemicals or product, so the codes that govern pressure piping ask more of pipe welds, and of the people who make them, than of almost any other welding.

This book is for pipe welders, piping supervisors, mechanical contractors and plant engineers who fabricate and install welded piping. It assumes a working knowledge of welding. It explains, with numbers, why pipe is welded rather than screwed, how wall thickness relates to pressure, how a joint is prepared and fitted, how position changes productivity, how roots are made and purged, how many passes fill a joint, what stainless and alloy piping add, how welds are tested, and how a piping job is estimated and recorded.

Early manuals as a starting point

Welded pipe was one of the first great markets for the oxy-acetylene torch. P. F. Willis’s practical manual of 1917 described gas mains laid out in long runs on the ground, turned on timber supports so the welder could always work from above, then rolled into the trench, with a pit dug where a joint had to be welded in place. The manual reported tests in which welded joints outlasted screwed connections, gave the number of joints a competent welder might make in an hour at each pipe size, and showed the cost of a welded joint falling further below the cost of a threaded coupling as the pipe grew larger. The Davis-Bournonville course of 1919 described machines that heated a whole joint at once with a ring of flame and pushed the ends together, and argued that a line with no couplings had nothing to leak.

Their arithmetic, joints per hour and cost per joint by size, is still how piping work is planned. Each chapter takes up one of their themes and works it through with modern processes and pipe.

The running case

This is an illustration. A mechanical contractor is building the piping for a dairy factory’s new processing hall: carbon steel steam and condensate lines, compressed air, cooling water, and stainless product and cleaning lines, together with about 4 km of DN 200 steel raw water main from a bore field to the site. Its piping supervisor wants to estimate the work honestly, plan fabrication and field welding, keep the repair rate down, and hand the client a weld record that traces every joint.

The contractor, its client, the pipes, prices and results are invented for teaching. They are not KEVOS® work. Piping that holds pressure must be designed, welded, examined and tested to the code that applies, by people qualified under it.

How to use the book

Chapter 1 compares welded and other pipe joints. Chapter 2 relates size, schedule and wall thickness to pressure. Chapter 3 covers joint preparation and fit-up, and Chapter 4 positions, rolling and fixed welds. Chapter 5 deals with the root pass and purging, and Chapter 6 with filling and capping. Chapter 7 covers stainless and hygienic piping, and Chapter 8 preheat, heat treatment and alloy pipe. Chapter 9 deals with inspection and pressure testing, and Chapter 10 with estimating and the weld log. Every chapter ends with an exercise and its worked answer.

Limits and professional responsibility

This book is educational. Pressure piping in Australia is designed, fabricated, examined and tested to codes such as AS 4041, with welding procedures and welders qualified to standards such as AS/NZS 3992, and pipelines and water mains have their own standards; the code named in the contract governs. This book quotes no values from those codes. Its figures are for teaching, and every number used in real work must come from the code, the procedure qualification record and the materials makers’ data. Welding in pits and trenches, and pressure testing, bring hazards that must be managed under the work health and safety laws of each jurisdiction.

An original treatment

READOS covers arc welding generally in Arc Welding by the Numbers, testing in Testing Welded Joints by the Numbers, and the metallurgy of welding steels in Weldability by the Numbers; this book links to them rather than repeating them. The explanations, the contractor case, the calculations, figures and exercises are original KEVOS® writing. The historical sources are public-domain works in the United States; their methods are summarised in new words, and none of their text, tables or illustrations is reproduced.