Everyone has heard of it, everyone has seen or experienced it. So why is there so much mystery surrounding equipment "stall"? (Stall is a condition in which heat transfer equipment is unable to drain condensate and becomes flooded because of insufficient system pressure.
Stall occurs primarily in heat transfer equipment where the steam pressure is modulated to obtain a desired output (i.e. product temperature). The pressure range of any such equipment (coils, shell and tube, etc.) can be segmented into two distinct operational modes: Operating and Stall.
Operating: In the upper section of the pressure range, the operating pressure (OP) of the equipment is greater than the back pressure (BP) present at the discharge of the steam trap. Therefore a positive pressure differential across the trap exists, allowing for condensate to flow from the equipment to the condensate return line.
Stall: In the lower section of the pressure range, the operating pressure (OP) of the equipment is less than or equal to the back pressure (BP) present at the discharge of the steam trap. Therefore, a negative or no pressure differential exists; this does not allow condensate to be discharged to the return line, and the condensate begins to collect and flood the equipment.
In Figure 3 we can see how this concept applies to a real world application. Assume the following operating conditions:
It is clear that any time the air heating coil in Figure 3 modulates below 15 psig (1.03 bar), the system will fall into a stall condition. This condition is most likely to occur when there is a light heating load, causing the coil to operate at a fraction of its capacity and a fraction of the design pressure.
In a stall condition, condensate accumulates within the equipment. When equipment becomes flooded by stalled condensate, a variety of problems will occur, ranging from minor to catastrophic failure.
Stall happens for a variety of reasons, but it always comes back to the fact that there is not always enough system pressure to return the condensate. The lack of sufficient pressure in the equipment may be caused by any one of the following:
The problems of equipment stall are well known and well documented. Over the years a variety of so-called "solutions" have been suggested to alleviate the stall scenario.
Installation of a vacuum breaker:
Objective: Relieve a vacuum within equipment, allowing for condensate drainage.
Installation of a safety drain:
Objective: Use a second steam trap above the primary trap to discharge condensate to drain when the system goes into a stall.
Shortcomings: A significant amount of condensate/flash steam and valuable Btu are lost down the drain when the system is in stall. Stall load may as high as 90% or more of the design load; therefore, 90% of the condensate coming from the equipment goes down the drain.