What stall means
Spirax Sarco defines stall as the reduction or cessation of condensate flow from a heat exchanger. It occurs when the pressure in the heat exchanger is equal to, or less than, the total backpressure imposed on the steam trap. The word describes condensate drainage, not a stopped motor, and it is a pressure condition: the steam space no longer has enough pressure to push condensate through the trap and away.
The manufacturer's discussion assumes a particular arrangement: saturated steam, a temperature control valve on the steam line to the exchanger, and a trapping device on the condensate line. The control valve varies the steam flowrate, and in doing so varies the steam space pressure to hold the outlet temperature of the heated fluid. That link between control action and pressure is the root of the problem, because lowering the steam pressure to match a smaller load also lowers the pressure available for drainage.
Compare the relevant pressures
The comparison has two sides. On one side is the pressure in the steam space, which provides the force that discharges condensate. On the other is the backpressure acting on the trap. Spirax Sarco describes the discharge as possibly subject to a lift and/or pressure in the condensate line, or falling to an open end where only atmospheric pressure acts.
A related Spirax Sarco page on lifting condensate shows how lift contributes. Backpressure there is made up of any pressure already in the condensate line plus the static lift of condensate in rising pipework, and the page gives an approximate conversion: one metre of lift is about one metre of static head, roughly 0.1 bar. A 5 m head, for example, gives about 0.5 bar of backpressure, which reduces the differential pressure available to push condensate through the trap. That page adds that under running conditions the loss of trap capacity is likely to matter mainly where upstream pressures are low, and that the upstream pressure is lowest at start-up. The lifting discussion distinguishes steam-main start-up conditions from temperature-controlled processes. These are different applications, their drainage arrangements should not be treated as interchangeable.
Spirax Sarco names three circumstances that can lower the steam-space pressure in its temperature-controlled arrangement: a rising secondary inlet temperature as heat load falls, a falling secondary flowrate as heat load falls, or a lowered outlet set point. These are relationships in the described arrangement, not instructions to change a plant setting.
Heat-transfer area can change the picture
The manufacturer points out that safety factors and standard sizes often leave a heat exchanger with more heating area than the duty needs. Compared with an exchanger perfectly sized for the same duty, the larger one operates at a lower steam pressure, so less pressure is available to push out the condensate. Because the oversized unit may still meet its thermal duty, waterlogging may not be noticed straight away.
Stall does not affect every exchanger. Spirax Sarco says that, as a general rule, the higher the secondary temperature above 100°C, and the more stable the load, especially near maximum output, the less likely stall is, but each application is unique and needs individual consideration. It names two ways to determine the dynamics: plotting the application temperatures on a chart or performing a calculation. The page also notes that some systems deliberately flood part of the steam space as a control method, which requires heat exchangers designed for it.
Use the principle, not a universal fix
Spirax Sarco lists a number of symptoms that may be evident with stall, including a cold or cool steam trap, a hunting control valve, fluctuating outlet temperature, waterhammer, reduced heat output and corroding heat exchangers, its wording is that one or more of them may be evident. The page does not discuss how to tell stall from a faulty trap, so none of these symptoms is established here as a diagnosis of any particular case.
These pages do not establish the cause of any individual heating problem, and they are not authorization to raise pressure, bypass a trap or alter site pipework. The selected trap, the exchanger pressure and the return arrangement have to be assessed together, and any corrective action belongs under the approved plant procedure and the equipment design.