Exchanger port side, flanged connections and gasketed joints on the hot-water skid

Exchanger, port side — Inzonex Modular Insulation, hot-water skid

Flanged joints / Gaskets and leaks

Flange gaskets and why joints leak

The metal does not seal. The gasket does, and it is the part with a life.

Two machined faces bolted together do not seal against each other. They seal against something soft squeezed between them. Everything that goes wrong at a flange goes wrong at that soft part. It relaxes, or it is the wrong material for the fluid. It was pinched during assembly, or the bolts around it were pulled up in the wrong order.

Materials

What goes between the faces

Compressed fibre

The general-purpose sheet, cut to shape on site. Cheap, forgiving, and the usual choice on hot water and LTHW. It is the one most likely to be found still in service long past the point where it has gone hard.

Graphite

Holds its properties far higher up the temperature range and does not relax the way fibre does. It is what steam work uses, and it appears on hot-water plant where the same fitter does both.

Spiral wound

Alternating metal and filler wound into a ring, usually with an outer centring ring. It tolerates thermal cycling better than any sheet gasket. It turns up on exchanger ports that heat and cool daily.

PTFE

Chemically inert and used where the fluid attacks the alternatives. It creeps under sustained load, so joints made with it need checking after they have been up to temperature.

Bolt-up

How the joint is actually made

Cross pattern, in stages

Bolts are pulled in a star sequence and in several passes rather than one bolt at a time to full torque. Pulling one side home first tips the faces and the gasket is squeezed at one edge and loose at the other.

Re-torque warm

A gasket relaxes on the first heat cycle. Check and re-torque the joints on a new or reworked circuit once they have been up to temperature. On a new installation, a joint nobody re-torqued is the first place to look for a leak.

Faces get looked at

A scored or pitted face will not seal whatever is put in it. The faces are inspected while the joint is open, because that is the only time anybody can see them.

Under insulation

The leak nobody sees

A weep is invisible under lagging

A joint that seeps a little declares itself by a stain and a smell. Wrapped in fixed lagging it declares nothing: the insulation absorbs it, holds it against the steel, and the first sign is a failure years later.

Wet insulation is worse than none

Water displaces the air the material works by and conducts instead of resisting, so a soaked section loses more heat than the bare pipe would. It also holds the steel wet, which is where corrosion under insulation starts.

Why the module opens

A cover that can be taken off in a minute is a cover that gets taken off. The joint is inspected, the gasket is changed, and the same cover goes back. Insulation that opens is still there in year five. Insulation that was cut away went in year one.

Reference

Gasket against service

Material	Where it is used	Why	What limits it
Compressed fibre	General hot water and LTHW	Cheap, cut on site	Hardens and relaxes with age
Graphite	Steam and higher temperature	Stable, low relaxation	Soft, damaged by careless handling
Spiral wound	Exchanger ports, cycling duty	Tolerates thermal cycling	Needs the right seating load
PTFE	Aggressive fluids	Chemically inert	Creeps under sustained load

Material · Where it is used · Why · What limits it

Asked about flange gaskets and why joints leak

Questions.

Answers distinguish modelled heat-loss estimates from equipment selection and service guidance.

Why is my flange leaking?

Nearly always the gasket. On a new joint it is relaxation after the first heat cycle, so joints are re-torqued warm. On an old one it is age, or a face damaged during a previous strip. It can also be bolts pulled up out of sequence, leaving the faces skewed.

What gasket do I use on a hot-water flange?

Compressed fibre sheet covers ordinary hot water and LTHW duty. Graphite where temperatures run higher or the same fitter is working on steam. Spiral wound on exchanger ports that heat and cool every day, because sheet gaskets do not like being cycled.

How do you tighten flange bolts?

In a star pattern and in several passes, not one bolt to full torque at a time. Then again once the system has been up to temperature. The sequence matters more than the final number. An evenly pulled joint at slightly low torque seals better than an unevenly pulled one at the right torque.

What happens if a flange leaks under insulation?

The insulation absorbs the leak and holds it against the steel. Wet insulation conducts instead of resisting, so that section loses more heat than bare pipe. The trapped water then starts corrosion under insulation on a surface nobody can see. It is the main argument for a cover that comes off in a minute.