Understanding and Solving Equipment Stall


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.

What causes stall?

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.

 

Figure 1

Figure 2


In Figure 3 we can see how this concept applies to a real world application. Assume the following operating conditions:

Table 1
Design Pressure (DP) = 100 psig (6.9 bar)
Back Pressure (BP) = 15 psig (1.03 bar)
Incoming Air Temperature (T1) = -10°F (-23°C)
Exit Air Temperature (T2) = 70°F (21°C)
Design Condensate Load (DL) = 3000 lb/hr (1361 kg/hr)

Figure 3

 

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.

Effects of stall

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.

  • Problems associated with stall:
  • Inadequate condensate drainage
  • Water hammer (thermal shock)
  • Frozen coils
  • Corrosion due to cool condensate and the formation of carbonic acid
  • Poor temperature control
  • Short equipment life
  • Control valve hunting (system cycling)
  • Reduction in heat transfer capacity

Factors contributing to stall

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:

  • Oversized equipment (excessive surface area)
  • Overly conservative fouling factors
  • Back pressure at equipment discharge, due to elevation or pressure in the line
  • Modulating control
  • Equipment operating at lower pressures because of light load demands
  • Vacuum
Many types of heat transfer equipment are susceptible to stall because excessive safety factors have been built into the design. In attempting to provide an extremely robust heat exchanger, equipment manufacturers and engineers often "overdesign" equipment, which often leads to a stall scenario.

Solutions?

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.

Shortcomings:

  1. This practice will help only if the condensate is gravity drained to atmosphere; any pressure present at the discharge of the trap will not allow condensate drainage.
  2. Undesirable air is allowed into the system.
  3. Vacuum breakers often fail because a poorly chosen location downstream of the equipment causes a build-up of scale/sediment, impeding the operation. Such a location may also allow the hydrostatic pressure of a vertical water column to keep the vacuum breaker closed in a small vacuum.
  4. Valuable flash steam is lost.

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.

Continue on to : The Solution

 

 





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