A main that stands full of water for twenty years and has to work on the worst day

Fire protection is the only piping system on a site bought for a duty it may never perform. A process line is judged every shift; a fire main is judged once, possibly never, on the assumption that nothing has changed inside the pipe since it was tested. A fire main is a dead leg by design: mains pressure, ambient temperature, no flow to sweep anything along, and usually the last system anyone walks.

The pressure side is almost trivial. The wall is the hard part, settled at quotation by three choices: the finish, the seam, and what the pipe is threaded into.

The main is sized by flow; the pressure rating is not the constraint

A factory fire main runs around 8 to 12 bar and a sprinkler installation less. Against that, the pipe is enormous. The A53 page prints a maximum allowable pressure table running from 425 psi at the 36 inch end to 17,150 psi at the small-bore heavy-wall end, and every entry between is several times what a fire pump can produce.

So the wall is not there to contain pressure. It is there to be eaten slowly, still be thick in year twenty, and survive the thread cut at the riser. Schedule 80 where Schedule 40 was specified buys time without changing the mechanism.

One specification covers two finishes, and the finish is the corrosion decision

A53 describes welded black and hot-dipped galvanized pipe in one document, which makes the finish look like a coating option. Black pipe is bare steel and begins to oxide as soon as the test water is drained; galvanized carries zinc that sacrifices itself in front of the steel. The seamless and ERW comparison names galvanized pipe as common in fire protection work, alongside fence posts and light structural frames.

The zinc budget is finite. The Z275 coil page gives 275 g/m² across both faces, about 137.5 g per square metre per face, and at 7.14 g/cm³ roughly 19 microns a side. That is the depth of a forklift scratch and of a thread cut, which is why a main threaded after coating rusts at the couplings first. The coil range runs 0.12 to 3 mm in Z60, Z80 and Z275.

Cut, drilled or welded after coating, the zinc has to be restored, or the part coated after fabrication. The galvanized processing chain runs cutting, bending, welding and polishing, then zinc, then assembly.

Wet, dry and preaction are three different corrosion environments

The system type decides the environment inside the pipe, and it is rarely stated on the enquiry. A wet system holds water permanently: the first reaction consumes the dissolved oxygen in a closed volume and the rate falls, until a drain or top-up brings fresh oxygen in. A dry system holds air and drains after each trip, so most of the bore stays dry except at the low point, where test water never quite leaves. A preaction system is dry until a detector confirms fire: a dry system with a water hammer problem attached.

The seam and the thread are where it fails

Two features fail before the wall. The first is the thread, which removes wall where the pipe is thinnest in service and water sits in the coupling gap, and takes the zinc with it. Threaded-and-coupled ends are normal on small-bore drops, which is why a galvanized main should be inspected at the joint rather than at mid-span.

The second is the seam. Welded pipe is adequate, but tolerance differs: the comparison page notes wall tolerance is tighter on seamless, which matters when a design sits close to a code minimum. A106 puts a number on it at plus or minus 12.5 per cent cold drawn, and describes the grade as suitable for bending, flanging and welding, so a riser can be rolled and flanged from one heat. Above the pump discharge, where the class climbs, seamless is the conventional answer; the A53 grade covers the general case in the same family.

The pipe is carried, so the frame is part of the order

Very little of an industrial fire main is buried. It runs on a rack, in a trapeze hanger, down a wall on brackets, across a pump house on a skid, and that steel is bought on the same enquiry more often than not. Square tube from 10 × 10 to 100 × 100 mm in 1 to 12 m lengths covers guards and brackets; H-shaped steel from 5 to 34 mm thick, 6 to 12 m at plus or minus 1 per cent, covers the rack; the A36 universal beam and 50 × 150 section cover lighter bays. Q235 plate and A36 plate from 0.3 to 200 mm cover base plates and receiver saddles.

The steel structure page is blunt that the material needs derusting then galvanizing or painting, and that is a line item. Anything drilled, welded or cut on site should be galvanized after fabrication; pre-coated steel arrives breached at every hole, which is why a hot-dip galvanized H-beam settles the argument at the mill.

Where the water is seawater, the ladder starts higher

Coastal plants and ships take fire water from the sea, and a stagnant seawater main is nastier than a flowing one, because the chlorine that would have kept the surface clean is not being generated. The 254 SMO page lists firefighting systems beside seawater cooling and ballast water: a super austenitic carrying molybdenum in the 6.0 to 6.5 band with nearly double the strength of the 300 series. S31803 and S32205 duplex pipe are supplied in 12 m lengths, hydrostatically tested in the shop, with third-party inspection (TUV, SGS, BV, ABS, LR) available, from one tonne. 2205 pipe belongs to that family and lists a 550 MPa yield, and S32760 ships at a PREN of 40 for seawater at elevated temperature.

Where the main cannot be flushed, the answer sometimes leaves stainless. The titanium rod page quotes below 0.005 mm a year in seawater, one twentieth of 316L, on a passive film that repairs itself after a scratch. That decides a sea water fire main on a ship, and it is also why titanium does not cover the whole system: Grade 2 tensile at 345 MPa is not a pump casing.

What the enquiry should carry

Seven items settle most of it. The system type, because it decides the finish. Design pressure and pump churn pressure, because the second is higher and is what the riser sees. The bore from the hydraulic calculation. The minimum schedule the insurer requires, stated as a schedule and not a wall. The finish, and whether threads will be cut after coating. Whether the pipe comes cut to a spool list. And the witness test, which is priced separately from the metal.

Bring the fabrication into the same enquiry and it stops being a pipe list. Cutting covers shear, plasma and waterjet work on brackets and base plates, bending the saddles, punching the bolt holes, and plate welding the skid. A last pass through surface treatment and the package arrives complete.