The largest steel item on the site is the one nobody inspects

A penstock arrives as pipe, gets welded in a trench, is backfilled, and then disappears for the life of the scheme. On a small scheme it is usually the heaviest single steel purchase, bought from a pipe mill against a hydraulic drawing written by a civil engineer. The mill quotes grade and wall; the engineer designed for a head and a flow; the pressure vessel calculation between them often has no owner.

Head sets the wall, diameter only sets the tonnage

Static head converts at 9.81 kPa per metre: 100 m is about 0.98 MPa at the bottom of the run. Hoop stress is the pressure multiplied by the diameter, divided by double the wall thickness: a 3,000 mm pipe at 12 mm carries roughly 122 MPa, which against a 345 MPa yield in Q345 plate leaves a factor near 2.8. Double the head and the same wall reaches about 245 MPa, no longer a sensible place to sit; 20 mm brings it back to 147 MPa.

The number that gets forgotten is surge. A turbine rejecting full load closes a valve in seconds, and the wave running back up the pipe can add a third again to static head before relief catches it. That allowance has to be agreed before the wall is calculated. Longitudinal stress is half the hoop value, which is why the longitudinal seam, not the girth, is the weld the mill has to prove.

A 12 metre plate decides how much welding is in the trench

A 3 m outside diameter pipe has a circumference of 9.4 m. Roll a 12 m plate into a can and the plate length becomes the longitudinal seam, one per can. Four kilometres of penstock in 12 m cans is 333 of them and 332 girth welds at 9.4 m each, so the line carries about four kilometres of longitudinal seam and three of girth - seven kilometres of weld, most of it made in a trench.

Longer cans move that work into the shop where the rotator, the submerged arc set and the radiography bay already are. They stop at the transport envelope: a standard flatbed carries 12 m, and beyond that needs a permit and a route survey. The cheapest penstock is usually designed around the longest can the road will take. Shop work means plate bending to a checked circumference, cutting that leaves a weld preparation rather than a burned edge, and plate welding under a qualified procedure.

A bend is a thrust problem wearing a pipe

At a 90 degree bend the water changes direction twice over, and the force on the fitting is twice the pressure times the area times the sine of half the angle. For the 3 m pipe at 0.98 MPa that is 2 x 0.98 x 7.07 x 0.707, close to 9.8 MN - a thousand tonnes pushing the bend out of the ground.

No wall thickness resists that, so it goes into an anchor block. The steel side of the order is the collar welded to the pipe, the gussets behind it and the embedment plates cast into the concrete: A36 plate, 0.3 to 200 mm thick, and A36 universal beam from 5 to 34 mm thick and 6 to 12 m long.

Thin walls fail inwards

A penstock is designed for internal pressure and then spends its life resisting everything else. Emergency closure can drop the crown below atmospheric, and a buried pipe also carries backfill and traffic where the alignment crosses a road. Both punish stiffness, which is a function of diameter over thickness, rather than strength.

At a diameter-to-thickness ratio around 250 the wall stops being governed by pressure at all. Below that the minimum is set by handling, weld quality and ovalisation during backfill; a 3 m pipe is rarely specified under 8 to 10 mm. Above it, stiffener rings are cheaper than plate, rolled from the same Q235 plate as the cans.

What the water does to the steel

Most high-head schemes run clean, soft water and specify carbon steel with a cement mortar or epoxy lining. The lining is replaceable; the shell behind it is not. Low pH, high chloride or acid mine drainage in the catchment will find every holiday in a coating, and then the pipe becomes stainless.

304/304L pipe covers outside diameters from 3 mm to 2,200 mm, with 0.08 to 170 mm wall, and large diameter thick wall stainless pipe goes to 2,500 mm, covering most small-scheme sizes in one can. Chloride above a few hundred milligrams per litre moves the choice to 2205 duplex pipe - 550 MPa yield, 800 MPa tensile, 1/2 to 12 inch NB in SCH20 to SCH80 - and brackish or tidal water to 254 SMO, with molybdenum at 6.0-6.5 per cent and nitrogen at 0.18-0.20 per cent, at close to double the strength of a 300-series grade. Outside, carbon steel still needs surface treatment: blast to a clean profile, then a coating that keeps air and moisture off the steel.

The intake, the rack and the things that are not pipe

Upstream sits a trash rack, usually stainless because it is thin, wet and impossible to repaint. Bars in square stainless tube from 10x10 up to 100x100 mm, 1 to 12 m long, welded into a frame sized for a debris load. The hoist runs on stainless wire rope from 0.8 to 21.4 mm diameter, 6x19 or 6x37 lay, to GB/T 20118 or API Spec 9A.

Access steel is galvanized and forgotten until it rusts through. Z275 coil, 0.12 to 3 mm with a coating from Z60 to Z275 g/m2, covers walkway decking and handrail; hot dip galvanized H-beam carries the rack guide frames where paint would fail at the waterline.

Test it before the backfill goes back

The hydrostatic test is the last chance to see the line under pressure, and it is worth holding at 1.5 times design for the full duration. Girth welds take radiographic or ultrasonic coverage at a percentage agreed with the owner; the longitudinal seam is the mill's responsibility and should arrive with its record. Ends are bevelled for the site joint and ovality is checked at the can ends, where fit-up is won or lost.

Ductility is not a formality: the A106 specification requires a length of pipe to be bent cold through 90 degrees around a mandrel, and the same expectation sits behind every plate rolled into a can.

What goes on the enquiry

Four numbers and one drawing: design head with the surge allowance, maximum flow, diameter, and water chemistry including chloride and pH. Then the alignment, because bends set the anchor blocks, and the transport route, because it sets the can length. After that the finishing scope - lining, coating, weld standard and test pressure - and whether the site welds are yours or the contractor's.

A penstock is bought as pipe and behaves as a pressure vessel. The order that survives forty years in a hillside is the one written by somebody who treated it that way.