Stall Phenomenon #1: Causes & Resulting Problems

What Causes Stall to Occur

Are you familiar with the term ‘stall phenomenon?’ Even though you may not have heard the term ‘stall’ before, there are probably many of you who have encountered the following problems:

Heaters often rupture
The sound of waterhammer inside the heater is often heard
Even heating is not possible

If you are experiencing these kinds of problems with your equipment, there is a good chance that stall is occurring. The phenomenon of stall can be described as follows:

Stall occurs when a lack of trap operating differential pressure (the difference between the inlet and outlet pressure of the trap) causes condensate to no longer be discharged from the trap and instead to pool inside the heat exchanger.

Steam traps themselves don’t have the ability to actually drawn out or transport condensate. So why does condensate get discharged from a steam trap? The discharge of condensate takes place due to a pressure difference between the trap inlet (primary) pressure and outlet (secondary) pressure. So it follows that in order for a trap to operate properly, the trap inlet pressure must be higher than the trap outlet pressure.

Steam systems are originally designed to operate in this way, but with the use of equipment such as temperature control valves, during operation this pressure differential may actually become reversed.

Normally, heat exchangers are designed to meet the maximum load that can be expected. The heat transfer surface area of a heat exchanger is fixed and cannot be changed, but the load will fluctuate depending on the operating conditions. For example, if the flow of air in a hot air heater is reduced, the only way to maintain the current temperature of the hot air is to reduce the temperature (pressure) of the steam being used as the heat source.

If the amount of valve opening on the temperature control valve is throttled down toward fully closed, the pressure inside the heat exchanger will drop and as a result the trap inlet (primary) pressure will drop and become lower than the outlet (secondary) pressure, and condensate will no longer be discharged from the trap.

The Mechanism Causing 'Stall' in Hot Air Heaters that Use Steam as the Heat Source

Control valves, temperature sensors and controllers are used to control 0.3 MPaG steam in order to achieve hot air at 80 °C. The stall phenomenon often occurs in this type of situation. In this animation, control and the conditions at stall can be confirmed by means of the needle on the pressure gauge, the display indicating degree of valve opening, and the indicated temperature reading.

Problems Resulting from Stall

The condensate that cannot be discharged by the trap accumulates in front of the trap, which means that it pools in the heat exchanger. This condition is known as the 'stall phenomenon'. The stall phenomenon can be eliminated by restoring the pressure at the trap inlet (primary pressure). However, condensate remains pooled inside the heat exchanger during the time it takes for the inlet pressure to recover, and as a result of this pooling, the kind of problems listed at the beginning of this article occur.

Waterhammer

Waterhammer may occur when steam comes into contact with the pooled condensate and the difference in temperatures causes some of the steam to instantly condense. In equipment that has narrow tubing such as in the shell and tube heat exchanger in the illustration below, the impact of the waterhammer on the tubing may cause the tubing to rupture.

Uneven Temperature

Stall that occurs in equipment such as jacketed kettles leads to the serious problem of uneven product temperatures.

As explained previously, stall is the condition that occurs when the differential pressure necessary for trap operation cannot be maintained. Therefore, merely replacing the trap does not prevent the occurrence of stall.

There are several methods that can be considered for the prevention of stall.

Reduce the trap outlet (secondary) pressure.
Install a PowerTrap.

Continue to ‘Stall Phenomenon Part 2 (Methods for Preventing Stall)’