Welcome to the Clinic. Faculty G works the way a good service engineer works: start from the symptom, reason back to the disease, then fix the cause — not the part that happened to be within reach. The first patient is the steam trap, because the trap is where every system sin eventually presents. Lesson D5 taught you to select traps; this lesson is about why correctly selected, mechanically healthy traps still fail to do their job — and why the fault is usually written into the piping around them.
Two failure directions, four impostors
A trap can genuinely die two ways — failed open (a silent leak worth lakhs a year, lesson C9's arithmetic) or failed closed (a flooded, hammering, underperforming heater). Lesson G2 covers finding both. But the Clinic sees a third category constantly: the trap that tests healthy on the bench and still floods the plant. Four classic impostors are responsible for most of those calls — steam locking, air binding, double trapping and back pressure. Learn their faces and you will stop buying traps to cure piping.
Steam locking: the trap held hostage
Picture the practice plant's rotating dryer. Condensate forms inside the cylinder and leaves through a syphon pipe — a dip tube to the rotary joint, then a length of pipe to the trap. Now let the water level fall below the syphon mouth for a moment: steam enters the syphon, and the trap (a float type, doing its honest duty) shuts against it. The condensate forming in the cylinder now cannot reach the trap — a plug of steam sits between them, held prisoner by the very valve that is waiting for water. The cylinder floods while the trap sits warm and shut. The lock only breaks when the trapped steam slowly condenses; then the slug of backed-up water arrives at once, the trap discharges furiously, and the cycle repeats. From the operating floor it reads as a trap that "works in bursts" and a dryer that will not hold temperature.
The cure is never a different trap family — it is a path for the locked steam to escape: float traps ordered with a steam-lock release (an internal needle bypass bleeding the syphon into the trap body), fitted as standard on syphon-drained rotating equipment; short, generously sized syphon tails; and on multi-can dryers, the modern answer from the paper industry — a blow-through system that accepts a controlled steam flow and separates it downstream (the thermocompressor's cousin, lesson F5).
Air binding: the cold impostor
The same hostage drama with a different gas. At start-up, or downstream of a process that releases dissolved gases, a pocket of air reaches the trap ahead of the condensate. A thermostatic trap treats air as its oldest friend — cooler than steam, so the element opens and discharges it (that is why lesson C8 built air vents from trap elements). But a thermodynamic disc can seat on rushing air exactly as it seats on flashing steam — the disc cannot tell one fast gas from another — and an inverted bucket vents air only through a small bleed hole in the bucket, slowly by design. Either way the trap sits closed and cold — the giveaway that distinguishes air binding from every hot fault: a trap that is shut and cool while its equipment struggles is not seeing steam at all.
The cure: give the air its own door. A parallel air vent piped around the trap (lesson C8's placement rules), a disc trap with an anti-air-binding disc where the maker offers one, or on bucket traps a thermostatic vent in the parallel line. Symptomatically: equipment slow only after shutdowns points to venting (lesson C8); equipment that stalls mid-run on gassy condensate points here.
Double trapping: two guards, one gate
Two traps in series look like belt and braces — usually installed when a flooding problem made someone "add protection". They guarantee the problem instead. The upstream trap discharges two-phase condensate (lesson C11); the downstream trap sees that flash, shuts, and floods the pipe between them. The upstream trap now discharges into a water-logged pocket with no differential, and the pair settle into exactly the flooded misery the second trap was bought to prevent. One drain point, one trap — and if redundancy is genuinely wanted (a critical turbine drain), the second station goes in parallel with isolation, never in series. The same physics condemns its cousin: a trap discharging into another trap's body drain, or two equipment drains married into one trap — lesson D5's group-trapping sin, seen from the pipework's side.
Back pressure: the stolen differential
Every trap was sized (lesson D5) on a differential: space pressure minus return pressure. The space side gets the attention; the return side quietly changes. A 5-metre lift after the trap costs ~0.5 kg/cm² (0.1 per metre). An undersized common return (lesson C11) at full plant load adds more. A neighbour's failed-open trap blowing live steam into the same return (lesson C9) pressurises it further. Sum those against a temperature-controlled heater whose space runs at 0.7 kg/cm²g at part load, and the differential is gone — condensate backs up and the "trap failure" file opens on lesson D1's stall, caused three flanges downstream. Thermodynamic discs suffer earliest — most makers derate them when back pressure exceeds roughly half the inlet pressure, after which they cycle furiously and wear out their own seats.
The cure is bookkeeping before hardware: measure the actual return pressure at the trap outlet at full plant load (a ₹500 gauge point saves the file), re-run the differential, and then either lower the return's pressure (fix the leaks feeding it, resize the line) or accept it honestly and pump (lesson G3).
Installation sins, briefly
A closing gallery from the survey files, each worth one sentence at the design desk: float traps mounted off-level (the float arm needs its designed plane — follow the body arrow); disc traps discharging upward into a wet riser they must lift on every cycle; inverted buckets that lost their prime on superheat or vacuum swings and blow through silently until re-primed; thermostatic traps lagged by an enthusiastic insulation crew — the element then holds condensate back until it cools far below the line (fit them bare, lesson C7's one exception); and no strainer, no test point — the two fittings lesson D5's trap station demanded, whose absence turns every future diagnosis into guesswork.
- Trap works in violent bursts on rotating or syphon-drained equipment → suspect steam locking; ask whether the float trap has its lock-release fitted before changing anything.
- Trap shut and cold while equipment starves → air binding; give the air a parallel exit and watch the "fault" vanish.
- Ever find two traps in series, remove one — the system will thank you the same afternoon.
- Before condemning any flooding trap: gauge the return pressure at full load, count lift metres at 0.1 kg/cm² each, and recompute the differential it actually has. Below ~0.5 kg/cm², think stall and go to lesson G3.
- Most "failed" traps in a well-selected system are healthy traps in guilty piping.
- Steam locking = a steam plug between condensate and trap; the cure is a release path, not a new trap family.
- Air binding shows as a shut, cool trap — discs and buckets are the vulnerable families.
- Never two traps in series; never two duties into one trap.
- Back pressure is stolen differential: 0.1 kg/cm² per metre of lift plus whatever the return line and its leakers add. Measure it before you spend.
Steam locking was diagnosed on paper machines before anyone had written down why. Early twentieth-century dryer men knew that certain cans "sulked" — flooded for no reason, then cleared themselves with a rush — and their fix, passed hand to hand, was to crack a tiny cock fitted between syphon and trap until the sulking stopped. That cock was bleeding the steam lock. When trap makers later built the steam-lock release needle into the float trap body, they were casting a mill-floor folk remedy into iron — where it remains, an option box on every float-trap datasheet, ticked by everyone who has ever run a dryer and mysterious to everyone who has not.