Stall: Why Temperature-Controlled Heaters Waterlog
A temperature-controlled steam heater cuts its steam pressure as load falls. At some point the pressure in the heater drops below the pressure its trap must discharge against, and condensate stops draining. The heater floods, hammers and corrodes. That is stall, and it is the reason behind many “bad traps” that are not bad at all.
You'll learn
- Explain why stall happens
- Predict when a heater will stall
- Recognize its symptoms
- Choose a cure
Before this: U-value, LMTD and fouling, Return lines and condensate pumps
When steam pressure is not enough to drain
A heater with a temperature control valve does not see full steam pressure. The valve throttles the steam to deliver only the heat needed. At full load the steam space might be at a few kg/cm²; at light load it might be close to atmospheric, or even below.
The trap needs the steam space to be at a higher pressure than the return line it discharges into, or condensate cannot flow out. When the steam space pressure falls below that, condensate backs up into the heater: it has stalled.
Predicting it
- At light load the control valve throttles until the steam in the heater condenses at 95 °C.
- The trap discharges into a return main 3 m higher, with about 0.2 kg/cm² of back pressure in it.
- Steam space pressure at 95 °C: -0.17 kg/cm²(g) — below atmospheric.
- Pressure needed to discharge: about 0.5 kg/cm²(g) (back pressure plus roughly 1 kg/cm² per 10 m of lift).
- Hammering and banging in the heater or its condensate line at low load.
- Temperature control hunting: the heater floods, output falls, the valve opens wide, the flood clears, and the cycle repeats.
- Corrosion and tube failures from standing condensate and carbonic acid.
- Traps replaced repeatedly with no improvement.
Cures
- Pumping trap or pressure-powered pump: drains the heater by gravity into a small vessel, then pushes the condensate out with motive steam whatever the heater pressure. The standard cure.
- Drain by gravity to a vented receiver: if the layout allows the condensate to fall freely, with no lift and little back pressure, stall disappears.
- Vacuum breaker: lets air in when the steam space goes below atmospheric so condensate can drain by gravity — useful only where the return is low enough.
- Design: oversizing a heater makes stall worse, because it reaches the low-pressure condition at a higher share of load. A correctly sized heater, or two smaller ones, helps.
Key takeaways
- Temperature-controlled heaters run at reduced steam pressure at low load.
- When steam space pressure falls below back pressure plus lift, condensate stops draining: stall.
- A heater condensing at 95 °C is at -0.17 kg/cm²(g), below atmospheric.
- Cure with a pumping trap or gravity drainage; a bigger trap does not help.