Success - At Loos, the acronym SUCcess stands for "Start-Up-Control combined with Shutdown and Standby," a control and equipment variant facilitating automatic steam boiler operation in the operating modes normal operation, heat maintenance, cold standby and hot standby.

At the push of a button or using an external request signal, the steam boiler can be started up fully automatically and gently when cold, shut down and protected against overload during normal operation.

Cold Starts

Cold starts place a considerably higher mechanical load on shell boilers than normal operation. Characterized by the fact that the water in the boiler does not boil, cold starts occur after periods of down time or when multi-boiler systems are used with sequence control without pressure and temperature maintenance (cold standby).

A higher level of mechanical stress is involved because the temperature difference between the flame tube and the boiler shell is greater than in normal operation.

That is why the flame tube expands noticeably more than the boiler shell in normal operation. As a consequence, between the flame and boiler shell or the flame tube and the colder smoke tubes there is considerably more mechanical stress on the respective connecting and stay elements, such as the flame tube floor connection, stay tubes, flame tube reversing chamber connections or gusset stays. This stress is increased even more if there is either no or very limited vapor bubble formation during the start-up procedure, which is the case when the steam shut-off valve is closed, for example.

The natural circulation (fig.1) normally found in the steam boiler is not triggered. The result is temperature stratification in the boiler (cold at the bottom, hot at the top) with additional thermal stresses.

Overload and High Load Change Speeds

Every steam boiler is designed to continuously supply a specific nominal output. If the steam extraction quantity rises above this nominal output, the boiler's current working gauge pressure drops even though the burner is operating at maximum capacity.

Depending on the load peak, this more or less rapid decrease in working gauge pressure and the resulting reduction in the boiling point produce re-evaporation effects throughout the boiler's water content.

This means that additional vapor bubbles form in the entire water content of the boiler. Since steam has a greater volume than water and the vapor bubbles take a while to rise to the water surface in the steam chamber, the water foams.

On one hand, this can have the negative effect of producing uncontrolled high water or shutdowns due to insufficient water; on the other hand, it can also cause water entrainment in the steam outlet. The negative consequences are wet steam, water impact, corrosion, salt deposits and leaky valves in the steam condensate network.

Great changes in load, i.e. high load change speeds and the associated great fluctuations in pressure, can cause unfavorable flow conditions to develop even if the nominal output has not been exceeded.

The vapor bubble formation required to dissipate heat from the heating surfaces can stagnate, that is, cause many small bubbles to join together to form larger vapor bubbles which do not leave the heating surfaces immediately, thus making conditions favorable for local overheating.

Avoiding Stress And Premature Wear

Due to the reasons stated above, cold starts with steam boilers should be as gentle on the boiler as possible and follow the operating instructions exactly. Until a low working gauge pressure has been reached, the water content should be warmed up with the burner capacity as low as possible. The boiler's water level should be monitored with regard to the water's thermal expansion.

If the water level rises too much, it should be lowered using the blow-down valve. It is important that the boiler water is mixed well during the start-up process. This prevents unnecessary loads in the form of thermal stresses and can be achieved by opening the steam shut-off valve slightly.

A small steam quantity can flow into the connected network. The boiler's natural internal water circulation is triggered. Once the mean work gauge pressure has been reached, the steam extraction quantity should be slowly increased by opening the steam shut-off valve in steps. Water impacts in the lines due to the presence of condensate, steam boiler overload, as well as any unnecessary stress in the pipeline network connected can be avoided by slowly heating up the network.

Overload and High Load Change Speeds

Ideally, these interconnected topics should be taken into account when designing the systems and consumer structure. If great fluctuations in steam reduction and therefore also in the network pressure, e.g. from the steam accumulator, cannot be avoided, automatic units should intervene as needed to limit or prevent steam extraction and thus any resulting negative consequences.