Pipe fittings, and the ones that fail when fitted the wrong way up.

A cone joining a large bore to a small one, both on the same centreline.

Changing pipe size where the centreline must stay straight.

• Vertical lines, where there is no top or bottom for anything to collect in
• Pump discharge, where the line steps up from the pump nozzle to the pipe size
• Anywhere the centreline has to be maintained — through a wall penetration, a pipe rack, a header
• Gas and vapour service generally, where liquid pooling is not the concern

• Horizontal pump SUCTION lines. It puts a high point at the top of the cone where air collects, and that air goes into the pump
• Horizontal lines carrying settling solids, where the low pocket silts up

The same cone, offset so that one side stays flat and in line. Which side you put flat is the whole point of the fitting.

Changing pipe size on a horizontal run without leaving a pocket.

• FLAT SIDE UP on a pump suction — this is the one everybody quotes, and it is right. A flat top leaves no high point for air to collect against the pump, and air in a suction line is how you lose prime and cavitate
• FLAT SIDE DOWN on a line carrying settling solids, so there is no step for sediment to build against and the invert runs continuously
• FLAT SIDE DOWN where the line must drain completely to a low point
• Any horizontal size change on a liquid line, in preference to a concentric reducer

• Fitting it without deciding which way up. An eccentric reducer installed the wrong way round is worse than a concentric one, because it makes a deliberate pocket
• Vertical lines, where the offset does nothing and just shifts the centreline

A quarter turn. Long radius has a centreline radius of 1.5 pipe diameters, short radius 1.0, and the difference in head loss between them is not small.

Turning a line through a right angle.

• Long radius as the default for everything. It costs less head, erodes more slowly and is what most specifications assume unless told otherwise
• Short radius only where the space genuinely will not take a long radius bend
• A fabricated bend of 3D or 5D radius where erosion or head loss really matters — slurry lines and pump suctions

• Short radius on abrasive slurry: the outside of the bend is where it wears through, and the tighter the radius the faster
• Any elbow immediately upstream of a flow meter or a pump suction — both want straight run, and the swirl a bend leaves takes several diameters to die out

Half a right angle. Two of them make a shallow offset at roughly half the head loss of two 90° bends.

Turning a line through a shallow angle, or stepping a line sideways.

• Offsetting around an obstruction, in pairs
• Slurry and sludge lines, where every degree of turn costs wear — two 45s beat one 90 comfortably
• Bringing a branch into a header at a shallow angle so the streams merge instead of colliding
• Where a long-radius 90 will not fit but a sharp turn is not acceptable either

• Using a pair of 45s where a single long-radius 90 would fit — more joints, more gaskets, more places to leak, for no gain

A branch at right angles to the run. Equal if the branch is the same size as the run, reducing if it is smaller.

Taking a branch off a line, or combining two into one.

• Any branch connection at full or reduced size
• Reducing tee wherever the branch is two sizes or more below the run — cheaper and stronger than a full tee with a reducer bolted to it
• As a combining tee where two streams merge, accepting that the merge is turbulent
• With the branch pointing UP on a liquid line, if the branch must not collect sediment

• Taking a branch off the bottom of a horizontal line unless you want the sediment. Take clean branches off the top or the side
• Merging two streams of very different velocity in a plain tee — the faster one drives back into the slower line. A 45° lateral handles that far better

A screen in a leg angled off the run, so the debris it catches falls away from the flow and can be blown down without opening the line.

Catching debris before it reaches something that will not survive it.

• Upstream of anything with a close clearance or a soft seat — control valves, meters, pump mechanical seals, plate heat exchangers
• On commissioning, to catch the weld slag, rag and gravel that every new pipe contains
• Where a compact in-line strainer is wanted and the dirt load is light
• With a blowdown valve on the leg, so it can be cleared without shutting the line down

• Heavy or continuous dirt loads — the pocket is small and it will block. Use a basket strainer, or a duplex pair
• Fitting it without a way to tell it is blocking. A strainer with no differential pressure indication is discovered when the pump cavitates
• Leaving the commissioning screen in permanently. Fine mesh fitted for start-up is meant to come out; it blinds and collapses if it does not

A vertical pot with a removable basket in it. Much more screen area than a Y, and the basket lifts out through a cover on top.

Catching debris where there is a lot of it, or where the screen must be cleaned often.

• Pump suction on river, sea or recycled water, where the load is real and continuous
• Where the screen needs cleaning on a routine, and doing it must not mean cutting into pipework
• Duplex, with two pots and a changeover valve, where the line cannot be stopped to clean it
• Ahead of expensive equipment on a dirty supply — membranes, heat exchangers, dosing skids

• Tight spaces: you need clear headroom above it equal to the basket length, or the basket cannot come out. Check that before locating it, not after
• Duties with no isolation either side, unless it is a duplex — otherwise cleaning means draining the line

A bellows or a rubber sleeve between two flanges, there to absorb what the pipe cannot: thermal growth, vibration, and the small misalignments of real installation.

Taking movement out of a line so it does not end up in a nozzle.

• Both sides of a pump, to keep vibration out of the pipework and pipe strain out of the pump casing
• Where a line crosses a structural joint or enters a building, and the two sides move independently
• On hot lines, to take thermal expansion that has nowhere else to go
• Where a pump or vessel nozzle has an allowable load that the pipe would otherwise exceed

• Fitting it without tie rods on an unanchored line. Internal pressure tries to push a bellows apart with a force equal to pressure times its full cross-section, and it will extend until it fails or pulls the pipe apart
• Using one to correct a misalignment that should have been fixed. It is for movement, not for bad fabrication
• Anywhere it cannot be inspected — it is an elastomer or a thin bellows and it has a finite life

A float in a chamber on top of the line. Air collects, the float drops, the air goes out; water arrives, the float lifts and it seals.

Getting air out of a pipeline, and letting it back in when the line drains.

• At every high point on a pipeline, without exception. Air collects there and nowhere else
• On the discharge side of a pump, where air comes out of solution as the pressure changes
• A double-orifice type where the line can drain — the large orifice lets air IN fast so the pipe does not collapse, the small one bleeds accumulated air out under pressure
• Upstream of a flow meter, because trapped air reads as flow that is not there

• Leaving them off "because the line is short". A single trapped air pocket can take a large fraction of the bore and the head loss goes with it
• A small-orifice-only valve on a line that can be drained. It will not admit air fast enough, and a large pipe under vacuum buckles

Three pieces — two ends and a nut — that let a threaded line be broken without unscrewing everything back to the last free end.

A deliberate break point in small-bore threaded pipework.

• Either side of anything that will be removed for service — a small pump, a meter, a dosing valve
• At the end of a threaded run, because otherwise the only way to dismantle it is to unscrew the lot
• Small bore, up to about DN50, where flanges would be clumsy and expensive

• Buried or inaccessible locations — a union is a joint, and joints are what leak
• Large bore and higher pressure, where a flanged pair is the right answer
• Using one as a flexible connection. It takes no misalignment at all

A telescoping flanged spool with tie bars, adjustable in length, so a valve or meter can be lifted out of a rigid flanged line without cutting anything.

Getting equipment out of a line that has no slack in it.

• Beside every large flanged valve, meter or pump that will one day need removing
• In buried chambers and pump stations, where the pipework is anchored and there is no give at all
• Where flange faces have to be pulled apart for a gasket change without springing the pipe
• To take up fabrication tolerance on a long flanged run at final fit-up

• Treating it as an expansion joint. It absorbs a set-up length, not repeated thermal movement
• Leaving the tie bars loose after adjustment — they carry the pressure thrust, exactly as on a bellows, and a slack set will let the joint pull open

A solid disc bolted where a flange would go. A spectacle blind is two of them joined — one solid, one open — so the line can be swung between blanked and flowing.

Positive isolation, and closing a line for good or for now.

• Blanking a future connection, so the tee can go in now and be used later
• Positive isolation for entry into a vessel — a closed valve is not isolation, a blind is
• End of a header that will be extended, so the line can be pressure tested as built
• A spectacle blind where the same line is regularly blanked and unblanked, and there must be no doubt about which state it is in

• Relying on a shut valve where a blind is required. Valves pass, and a permit to work that accepts a closed valve as isolation is a bad permit
• Forgetting the blind is there. This is what a spectacle blind solves — you can see which way it is turned from across the room

Not a fitting so much as the joint everything else is made with — and the place most pipework actually leaks.

A demountable pressure-tight joint between two pieces of pipe.

• Anywhere the line must come apart: at equipment, at valves, at the ends of a spool
• Raised face with a soft gasket for most water duty; full face where the mating flange is cast iron or plastic, which will crack if loaded only on a raised ring
• Flat face and full-face gasket on GRP, PVC and cast iron, always

• Mating a raised-face steel flange to a flat-face cast iron or plastic one. The raised face acts as a fulcrum and cracks the weaker flange as the bolts are pulled up
• Mixing standards. ASME 150, PN16, JIS 10K and AWWA C207 Class D are all "the same size" and none of them share a bolt circle with the others
• Pulling bolts up in sequence around the circle. Tighten in a star pattern, in stages