An HRSG does not necessarily operate throughout its entire service life under the nominal conditions for which its design point was defined. When the gas turbine load changes, the thermal profile of the gases and the ratio between the available energy and the water flow rate through the economiser also change.
It is under these conditions, far removed from the nominal operating point, that a phenomenon may occur which should be checked as early as the design stage: economiser steaming.
The problem is not merely that some of the water begins to evaporate before reaching the evaporator. This vapourisation can alter the hydraulic distribution within the tubes and create operating conditions that affect stability and, in severe cases, the integrity of the tube bundle.
For this reason, analysing an HRSG solely at its nominal point may not be sufficient. Behaviour at partial load is also an integral part of the design.

Under normal conditions, the economiser recovers heat from the flue gases to raise the temperature of the feed water before it enters the evaporation section.

The water must be maintained in a supercooled liquid state throughout the economiser.

The phenomenon whereby this water begins to vaporise partially within the economiser tubes themselves, in an area where the design does not yet allow for steam formation, is known as ‘steaming’.

The formation of this vapour phase alters the hydraulic behaviour of the circuit. It is therefore not simply a matter of recovering more heat than anticipated, but of entering a condition that differs from that for which that section of the HRSG was designed.

The approach point represents the difference between the water temperature at the economiser outlet and the corresponding saturation temperature.

Whilst this margin exists, the water leaves the economiser whilst still in a subcooled liquid state.

When the margin decreases until it disappears, the water reaches a state in which it can begin to vapourise inside the economiser.

For this reason, the approach point should not be understood solely as a thermal parameter of the set point. Its behaviour as operating conditions change is crucial to determining whether the economiser will continue to operate within the intended range.

Partial load alters the thermal equilibrium under which the HRSG operates.

When the gas turbine operates outside its rated point, the temperature profile of the gases changes. Under certain conditions, the economiser may receive more heat relative to the flow rate of water circulating through its tubes.

The result is a reduction in the margin relative to the saturation temperature.

If that margin disappears, partial evaporation of the water inside the economiser begins.

This behaviour explains why an HRSG that operates correctly under nominal conditions may experience problems when operating at other load points.

The crucial issue, therefore, is not merely to check that the economiser operates correctly at 100 per cent load, but to verify that it continues to keep the water in a liquid state under the various operating conditions envisaged for the plant.

The formation of steam within the economiser causes a change in the flow behaviour.

When steam bubbles appear, the distribution is no longer uniform across the different pipes. Some may receive a higher flow rate, whilst others are left with a reduced flow rate.

This inequality is the root of the problem.

Pipes with a lower flow rate lose their ability to dissipate the heat they are absorbing and may experience localised overheating.

Consequently, a phenomenon that begins as a thermal deviation can develop into a hydraulic and mechanical problem.

The instabilities associated with steaming can manifest themselves in various ways within the jet.

• Uneven flow distribution between pipes.
• Localised overheating in the pipes with the lowest flow rates.
• Vibrations of the tubular beam.
• Water hammer associated with the collapse of steam bubbles.
• Internal erosion.
• Tube fatigue.
• In serious cases, a rupture of the tube.

The significance of this phenomenon lies precisely in this chain of effects.

The economiser may have been correctly sized to handle the anticipated heat load and yet still perform inadequately if the actual operating conditions cause the water to evaporate within the tube bundle itself.

Prevention begins before the HRSG is commissioned.

During the design phase, the performance of the economiser must be verified not only under nominal conditions, but also under the partial loads relevant to the intended operation.

The aim is to verify that the water remains sufficiently far from saturation and that it does not begin to evaporate within the economiser at any of the operating points considered.

This check is particularly important when the plant’s operating profile includes load variations.

Designing solely around the nominal point may mean that the conditions under which the balance between available heat and water flow is most unfavourable are not analysed in detail.

Steaming highlights a wider issue in the design of heat recovery equipment: achieving the rated performance correctly does not, in itself, guarantee stable behaviour across the entire operating range.

In an HRSG, the relationship between flue gas temperature, water flow rate and saturation conditions changes as the load changes.

For this reason, the analysis of the economiser must take into account the actual conditions under which the plant will have to operate.

The approach point thus ceases to be merely a design parameter and becomes a margin that must be verified against various operational scenarios.

When designing an HRSG or reviewing the performance of an existing plant, the analysis should focus on the relationship between the thermal conditions and the water flow rate through the economiser.

In particular, the following should be reviewed:

• The nominal design conditions for the economiser.
• The planned partial loading points.
• The temperature profile of the gases under those conditions. The temperature profile of the gases under those conditions.
• The flow rate of water passing through the economiser.
• The water outlet temperature.
• The margin relative to the saturation temperature.
• The possible occurrence of vapourisation inside the pipes.
• The stability of the flow distribution between the different circuits.

The aim is to check that the economiser continues to function as an economiser throughout the intended operating range.

Streaming should not be viewed as an isolated phenomenon.

Its appearance indicates that, under a particular operating condition, the heat balance of the economiser is no longer maintaining the water within the intended subcooled liquid range.

From that point onwards, hydraulic instabilities, localised overheating and mechanical stresses may occur, affecting the reliability of the tube bundle.

Therefore, the best strategy is not to react when vibrations, water hammer or damage to the pipes occur, but to ensure during the design phase that such conditions do not arise in the first place.

A properly designed HRSG must be capable of operating not only at the design point, but also under the partial-load conditions it will encounter during its service life.

At AITESA, the design of an HRSG is based on the actual gas conditions, the required steam output and the anticipated operating profile for the plant.

In the case of the economiser, this involves checking that the thermal behaviour remains within the expected parameters even when the load changes, ensuring that the necessary margin relative to saturation is maintained and that no vapourisation occurs within the bundle.

When a plant exhibits instability under partial load, or when the design, modification or refurbishment of an HRSG is being assessed, the analysis must be carried out based on the specific operating conditions of the equipment.

Please provide AITESA’s technical team with details of the observed behaviour, the operating conditions and any available information regarding the HRSG.

Based on this initial data, the case can be analysed and an assessment made as to which aspects of the economiser and the operating regime should be reviewed.