How to Recognise Stall Before You Replace the Trap

Why this matters

A stalled device and a failed-closed trap look identical from the front. The equipment floods, the output goes soft, the condensate at the trap reads cool, and every test a tech normally runs points at the trap. So the trap gets changed, the new one tests clean, the complaint comes back inside a month, and the second visit is on the shop.

The two conditions are separated by one comparison, and it is a comparison of pressures rather than a test of the trap. This procedure gets that comparison in hand before anything is unbolted, and it is worth doing first because a trap test on a stalled connection returns a confident wrong answer rather than a blank one.

Before any of it: what this system will do to you

Steam at any usable pressure burns severely and the first stretch of a leak is invisible, so nothing here is located by hand, by feel, or by reaching toward a sound. Condensate lines are the same hazard in a different form: a hot return line opened under pressure flashes at the opening and carries its water out with it.

Two steps below involve breaking a joint. Fitting a gauge or opening a test valve means isolating the steam supply and the condensate return, letting the assembly cool, draining it to a point piped away from any walkway, and confirming zero on a gauge you have just watched read something else. That is the isolation of stored pressure energy required by 29 CFR 1910.147 in general industry, and on a construction site the same work falls under the construction energy control requirements of 29 CFR Part 1926. Stored pressure does not care whether the boiler is firing.

Reading 1: the load condition at the moment of the complaint

Establish this before any instrument comes out, because it decides whether the rest of the procedure applies at all.

Ask what the equipment was doing when the complaint happened. Mild weather or design weather. Morning peak or evening trickle. Full production or one line running. Then ask the opposite question and get an answer to it: how does the same equipment behave at the other end of its range?

A device that performs at high load and fails at low load is the stall pattern. The differential across the trap is largest at high load, so a problem that appears only when the load drops is pointing at the differential. A device that fails at high load and recovers at low load is the opposite finding, and it points at supply capacity, valve size, or a fouled surface. A device that fails at every load has an obstruction, a shut valve, or a genuinely failed-closed trap, because stall by definition releases at full load.

Skipping this reading is what produces the classic wasted visit: a tech called on a mild-weather complaint arrives during the morning peak, finds the equipment holding setpoint, tests the trap while it is discharging normally, and reports no fault found.

Reading 2: the pressure inside the equipment, at that load

This is the number that does the work. Read the equipment's own steam gauge with the unit running at the condition that produces the complaint, not at whatever condition the site is in when you arrive. A reading taken at peak load on a stalled device is a true reading of a state that is not the problem.

If there is no gauge on the equipment, that absence is a finding in itself: nobody at that site has been able to check this device or its trap, ever, and no trap report written about it means anything. Fitting one is not part of a diagnostic walk. It is a separate scheduled job with the isolation, cooldown and drain described above.

Reading 3: the pressure in the return main, at that same moment

Read the gauge on the return main, and read it at the same time as reading 2 rather than from memory of a previous visit. Return main pressure moves with what the rest of the building is doing, and the number that matters is the one present while the equipment is complaining.

A return main that reads well above zero on a vented system is a second finding, and it is usually somebody else's failed-open trap blowing live steam into the common return. That raises the back pressure for every device on that return, which means the device you were called to is not necessarily the device that is broken.

Reading 4: the lift, with a tape

Measure the vertical rise from the trap outlet to the return main, physically, and convert at 2.31 feet of water per psi. Drawings are wrong about this more often than they are wrong about anything else in a mechanical room, because the pipe was routed around whatever was in the way on the day.

Add the return main pressure, the converted lift, and an allowance for friction in the return line. That sum is the total back pressure, and it is what the equipment must exceed for one drop to leave.

The temperature reading that will lie to you

A temperature check at the trap is the reading most techs reach for first, and on a stalled device it is the reading that produces the wrong answer with the most confidence.

Condensate standing inside a partly flooded device sits against a surface that is cooler than saturation, so it subcools. A trap in perfect condition on a stalled device discharges water well below the saturation temperature of the supply. Read the trap inlet on temperature alone and it presents exactly as failed closed.

So take the temperature if you like, but do not let it override the pressures. And take it without touching the line: use a contact probe with an insulated handle on metal that is already bare, a valve body, a flange face, the trap body itself, or an ultrasonic probe designed for the job, wearing gloves rated for the contact temperature. Do not open or brush lagging to make a spot: thermal system insulation on steam and condensate piping is presumed asbestos-containing under 29 CFR 1926.1101 and 29 CFR 1910.1001 until it has been sampled. A hand on a steam or condensate line to judge temperature is a burn, not a measurement.

Make the call

If the equipment pressure from reading 2 is at or below the total back pressure from readings 3 and 4, the device is stalled at that operating point. The trap has no motive pressure and no trap of any family, size or condition would drain it. Stop diagnosing the trap.

If the equipment pressure is comfortably above the total back pressure and the device is still flooded, the differential exists and is not being used, so the trap or the path is now a legitimate suspect and the usual trap tests apply.

Where the two numbers are within a psi of each other, take a second set of readings at a different load rather than guessing. The load fraction where they cross is calculable from three temperatures, and that relationship belongs to the stall article; here the direct comparison at the complaint condition is enough.

Worked example: domestic hot water that goes cold in the evening

A shell and tube steam to water heat exchanger making domestic hot water in a mid-size building. The complaint is cold water at the far fixtures in the evening and at weekends. Mornings are fine. It had a new trap eleven months earlier, on a nearly identical complaint.

Reading 1. Fine at the morning peak, cold on light evening draw. Performs at high load, fails at low load. That is the stall pattern, and it is also why the previous visit found nothing: it was a morning call.

Reading 2. Shell gauge with light evening draw: 4 psig. The design supply is 15 psig, and at the morning peak the same gauge sits near 14 psig, which confirms the valve is doing its job rather than sticking.

Reading 3. Return main gauge, same evening, at the same moment: 5 psig. The return is nominally vented, so this is already a second finding to chase later.

Reading 4. Tape from the trap outlet to the return main: 8 feet. At 2.31 feet per psi that is 3.5 psi. Add 1 psi for friction in a long return run.

Total back pressure: 5 plus 3.5 plus 1, which is 9.5 psig. Saturation temperature at 9.5 psig is about 238 F.

The call. The shell holds 4 psig in the evening against 9.5 psig required. It is more than 5 psi short, so it is stalled, and it was stalled through last year's trap replacement as well. There is a three-temperature ratio that returns the load fraction where a device stops draining, and it is worth knowing why it does not belong here. It is derived for constant secondary flow with a varying outlet temperature. Domestic hot water is the opposite case: the outlet is held at setpoint and the flow varies with draw, so the required steam temperature at light load falls toward the setpoint rather than toward the 50 F inlet, and the ratio returns a number that means nothing. For a variable-draw exchanger the stall load comes from the manufacturer's own capacity curve at the constant-outlet condition. The direct comparison at the complaint condition is the finding and it stands without any fraction: 4 psig available against 9.5 psig required, on a device that reaches its drainage condition for perhaps an hour a day.

What that says about the previous repair. The trap replaced eleven months earlier was almost certainly fine when it was pulled. The visit before that had also replaced a trap. Two traps have been bought for a piping condition, and the exchanger has been sitting full of standing condensate for at least two heating seasons, which is a corrosion exposure on the shell side as well as a comfort complaint.

What actually gets fixed, and in what order. The 5 psig on a vented return main is the first thread, because it is costing every device on that return the same margin and it is usually one failed-open trap on a higher pressure branch. Suppose that is found and repaired and the return main comes back to near 0 psig. Recompute rather than assuming: total back pressure becomes 0 plus 3.5 plus 1, which is 4.5 psig, about 226 F saturated against a 250 F supply. The device now has 4.5 psig of back pressure to beat instead of 9.5, which is real improvement and is still not enough at light draw, so fixing the other trap improved the number and did not solve this device. That is the result worth carrying: the return main was a real fault and it was not the whole answer.

What remains is the 8 foot lift, and the honest options are to drain the exchanger by gravity into a vented receiver below it and pump from there, or to fit a device that supplies its own motive pressure. Both remove the lift from the trap's problem rather than asking a trap to overcome it.

The failure mode if this is skipped. A third trap goes in. It tests clean, because it is new and it is being tested at the only hour of the day when the device drains. The building gets the same call next spring, and by then the shell has spent another season holding oxygen-bearing water at temperature.

How to verify the diagnosis before you quote the fix

Two confirmations, and neither one needs the equipment opened.

First, take the same two pressures at high load. On a stalled device the equipment pressure rises above the total back pressure and the trap discharges normally, which is the observation that separates stall from a failed-closed trap: a failed-closed trap floods at every load, including the load where the differential is largest.

Second, watch the trap through a light load to high load transition if the site allows one. A stalled device passes nothing for a long quiet period and then discharges heavily once the valve opens up, which is a distinct rhythm from a trap cycling normally. Do that observation from a position beside the trap rather than in front of any test valve or vent, and do not crack a test valve to get a better look on a trap discharging into a pressurized common return.

Record both pressure sets with the load condition and the time next to them. A stall diagnosis that is written down as "stalled" with no numbers under it will be re-diagnosed from scratch by the next person, which is exactly what happened to the two traps already bought for this exchanger.

References

• Saturated steam tables from any engineering handbook, for the pressure and saturation temperature pairs used in the example
• Steam trap and pump-trap manufacturer literature, for the selection of a device that supplies its own motive pressure on a lifted return
• 29 CFR 1910.147, the OSHA general industry standard for the control of hazardous energy, and the construction energy control requirements of 29 CFR Part 1926 where the work is on a construction site, for isolating stored steam and condensate pressure before any joint is opened
• See related: What Stall Is and Why the Equipment Goes Cold at Part Load; How to Test a Steam Trap Without Taking It Apart; The Trap That Tested Good and Was Passing Steam