A once-through HRSG (called an OTSG) does not have a steam drum like a natural or forced circulation unit (Fig. 2.12). An OTSG is simply made up of serpentine coils like an economizer. Because water is converted to steam inside

The tubes, the water should have nearly zero solids. Otherwise deposition of solids can occur inside the tubes to the complete evaporation process. This in turn can lead to overheating of the tubes and consequent tube failure, particularly if the heat flux inside the tubes is high. Like natural or forced circulation units, these units generate single — or multiple-pressure saturated or superheated steam.

The concept of once-through steam generation is not new. Supercritical boilers in Europe have been using once-through designs for over half a century. A once-through unit does not have a defined economizer, evaporator, and super­heater section. The location at which boiling starts keeps moving depending upon the gas flow, inlet gas temperature, and duty. The single-point control for the OTSG is the feedwater control valve; valve actuation depends on predefined operating conditions that are set through the distributed control system (DCS). The DCS is connected to a feedforward and feedback control loop, which monitors the transients in the gas turbine load and steam conditions. If a transient in the gas turbine load is monitored, the feedforward control sets the feedwater flow to a predicted value based on the turbine exhaust temperature, producing steady-state superheated steam conditions.

Because there is no steam drum, the water holdup is much less than in drum-type units. Often Alloy 800 or 825 tubes are used to ensure dry running and also to limit the sensitivity to oxygen in the water, avoiding the need for active chemical treatment. A gas bypass diverter system is not required, because of the dry operability. The use of high grade alloy tubes minimizes exfoliation concerns, which are likely with carbon steel or low grade alloy superheater tubes. When boiler tubes are heated, they form an oxide layer inside the tubes, and when cooler steam flows through them the oxide particles are dislodged and carried off to be deposited inside the steam turbine. This process, called exfoliation, occurs when the tubes are cycled frequently between hot and cold conditions.

Once-through units can also be started up or shut down very fast compared to natural or forced circulation boilers, because the weight of steel and holdup of water are much smaller. On the flip side, the steam pressure decay when the gas turbine trips is likely to be faster than in designs that have much larger metal heat and a large water inventory. It must be kept in mind that a typical gas turbine HRSG can generally be started up in 80-100 min from cold, so the saving in start-up time may not be a significant issue unless the unit is designed for frequent cycling. There are also a few advantages of once-through units such as absence of downcomer and riser piping and drum and related material costs and fabrication concerns. From the heat transfer viewpoint there should not be much of a difference between once-through units and the natural or forced circulation units; hence the cross section and size of the HRSG or the areas of various heating surfaces should all be nearly the same. The flow configuration of the heating surfaces is generally counterflow except for the evaporator, which could be in parallel flow as in forced circulation units.

The two-phase steam-side pressure drop in the evaporator tubes is, however, quite large and could be in the range of a few hundred psi, which is an operating cost and must be considered in evaluating the design. In the natural and forced circulation unit, there is no additional pressure loss associated with the evaporator circuit, because the circulation system handles the losses and the static head available or the circulating pump balances this loss, considering other losses associated with the downcomer, evaporator tubes, and riser piping.

Another type of once-through unit is used in oil fields for secondary oil recovery operations (Fig. 2.13). These generate high pressure steam ranging from 1500 to 3000 psig at 80% quality for injection into used oil fields in order to recover additional oil. The steam pressure depends on the depth at which oil is available. The hot, wet steam dislodges the viscous layers of oil in the ground beneath, and thus more oil is recovered. This HRSG is also of once-through design, with water entering at one end of the coil and leaving as wet steam at the

Other. Because of concerns with departure from nucleate boiling (DNB), the final portions of the coil are in parallel flow and not in counterflow and are located behind tubes having lower steam quality. This feature helps to lower the heat flux inside the tubes where the quality of steam is high. The allowable heat flux to avoid DNB decreases as the steam quality increases, hence this measure. The feedwater in these generators is generally of poor quality and has high solids content, exceeding thousands of ppm of salts, because the water is taken from the fields nearby and basic, inexpensive softening methods are used in its treatment. Because sodium salts are soluble in water, the 80% quality steam, which still has 20% water, is often adequate to ensure that the salts are disolved and are not deposited inside the tubes during the evaporation process. Single-stream designs, in which a single tube handles the entire steam flow, are used for up to

100,0 lb/h capacity, whereas with higher steam flows, multiple streams are employed. Due to instability problems associated with two-phase boiling of fluids with multiple streams, a flow resistance at the inlet to each stream in the form of orifices or control valves, as explained in Q7.36, is used. Because water at ambient temperature is often used as feedwater, a heat exchanger is used to preheat the incoming water, using the hotter water at the exit of the economizer portion to minimize acid dew point concerns.