DrumPlusTM + diverter damper: a faster way to switch from simple to combined cycle

To accommodate the fluctuating nature of renewables, combined cycle power plants are moving away from continuous base load operation to cycling operation involving a higher number of starts/stops, shorter start-up times and longer part-load operation. In addition to operational flexibility, combined cycle plants have also been evolving in terms of overall plant efficiency thanks to advancements in gas turbines, which have been scaling in size and operating at much higher temperatures than before. The elevated exhaust temperature and flow translates to higher pressure, flow, and temperature for the bottoming cycle. For the HRSG, this means thicker and larger pressure part components due to higher design temperature and pressure. But thicker pressure part components militate against flexibility as they experience larger thermal stress during a stress cycle. 

As indicated by the arrows in Figure 1, the pressure part components critical to lifetime under cycling conditions are the hottest headers in the HP and RH system as well as the HP drum, where the water–steam separation occurs. The HP drum is the component with the largest wall thickness and therefore most critical for lifetime. 

One of the strategies to resolve the challenge of a thick drum is to eliminate the drum and adopt a once-through HRSG (OTSG) design. In such a system, the outlet of the evaporator system always remains dry with suitable feedwater control. But due to the absence of a circulation system and blowdown to remove contaminants, a once-through system requires installation of a proper condensate polishing plant.

The other approach to reduce drum fatigue is to retain the drum while minimising the size and thereby reducing the thickness. The NEM patented DrumPlus^TM design follows the latter concept and works in a similar way to a convention natural circulation drum.

In the DrumPlus^TM design the HP drum is replaced by a ‘knockout’ drum for primary water–steam separation and ‘separator bottles’, which are a group of small vessels for secondary water–steam separation. In Figure 2 the separator bottles and knockout drum are indicated in the general arrangement of a horizontal-exhaust-gas-flow three-pressure and reheat system. The knockout drum diameter and thickness is minimised compared to a conventional drum, resulting in reduced thermal stress. For an operating pressure of 160 bar, the typical design values for a conventional and DrumPlus^TM HRSG are:

• Conventional drum: 2000 mm inner diameter with 140 mm wall thickness.
• DrumPlus^TM drum: 1300 mm inner diameter with 90 mm wall thickness.

Thanks to the reduced drum wall thickness, DrumPlus^TM can support an unrestricted start-up of the gas turbine. The El Segundo FlexPlant^TM 10, in California (Figure 3), is an example of a DrumPlus^TM installation, in commercial operation since 2013. The facility’s two SGT6-5000F gas turbines can ramp up without restrictions from the HRSG, enabling around 300 MW to be put on the grid within ten minutes of a cold start, the remaining 250 MW coming on line within less than an hour.

Combined cycle plants equipped with a diverter damper (DD) provide operators with an extra degree of flexibility allowing them to operate in simple cycle (SC) or combined cycle (CC) mode and respond quickly to fluctuating power market requirements. However, large gas turbines such as F-class machines are required to ramp down their power and soak the HRSG before switching over from SC to CC operation. A conventional switch over involving the ramping down of the GT load and HRSG soak could take approximately an hour (for a cold HRSG start). This is necessary to protect the HRSG from very high temperature transients, to minimise low cycle fatigue in thick-walled components such as the HP superheater headers and the HP drum and to limit expansion differences. Such high temperature transients also increase the risk of magnetite layer cracking in the HP drum inducing growth of cracks in the drum wall.

However, with the combination of NEM’s DrumPlus^TM technology and a diverter damper, new possibilities open up for combined cycle power plants, allowing them to switch-over from SC to CC with the gas turbine operating at high loads.

This new feature is being implemented by NEM at the Jizzakh combined cycle power plant in Uzbekistan. This employs an F-class gas turbine with a horizontal-gas-flow HRSG equipped with DrumPlus^TM, bypass stack and diverter damper.

SC to CC switch-over at baseload

For this project, the client wanted to switch from SC to CC at baseload without GT ramp down.

To reduce risks to the HRSG during an SC to CC switch-over, the net heat input into the HRSG can be reduced in two ways: 

(I) Reduce the GT exhaust temperature at the HRSG inlet, by ramping down the GT to a lower load and soaking the HRSG components until they are heated sufficiently.

(II) Reduce the GT exhaust flow, by controlling the position of the diverter damper.

Option (I) is the ‘conventional’ procedure for a switch-over while option (II) is the ‘fast’ approach. For the conventional switch-over, the net time required to reduce the GT load to the soaking load, soaking the HRSG and ramping the GT back to full load is a bottleneck for the plant operator if the plant is required to increase power output quickly. The option to switch-over without ramping down in load saves both time and money. Hence, we are focussing on the ‘fast’ switchover approach which is enabled by the DrumPlus^TM technology.

There are risks associated with admitting too little exhaust flow and admitting too high exhaust flow to the HRSG. With very low exhaust flow, the exhaust flow distribution will not be uniform leading to poor heat transfer, local hotspots and increased row and bundle expansion differences. Also, start of steam flow is delayed increasing the risk of rows exceeding design temperatures. On the other hand, admitting 100% GT exhaust flow results in higher temperature and pressure transients. Although DrumPlus^TM is designed to handle temperature transients of the order of 30K/min (and for a conventional drum, the pressure transient is usually restricted to about 5 bar/min), the SH header and main steam lines experience high thermal stress.

The following parameters were investigated to construct an optimised switch-over procedure: (i) SH row and bundle temperature difference and associated expansions; (ii) row temperatures vs design temperatures; (iii) low cycle fatigue for the drum, SH headers and the main steam line; (iv) bypass valve sizing to limit pressure transients; and (v) total peak stress to assess the risk of magnetite stress cracking.

Taking into consideration the above mentioned factors, an optimised diverter damper operation regime was constructed for the GT cycling window of 40%-100%. The diverter damper is a large door like device and therefore cannot be controlled to precise opening angles. Given the controllability limitations of the DD, the objective of the operation philosophy is to allow a DD opening that admits GT flow that is approximately equal to a certain target flow (about 50% of the maximum GT exhaust flow, ie, the exhaust flow at 100% GT load). The DD operation curve is presented in Figure 4. Figure 5 shows the GT load profile for a conventional and fast (DrumPlus^TM + DD) switch-over for an SGT5-4000F type gas turbine. The net savings for the utility operator correspond to the shaded region in Figure 5, which is about 150 MWh. In addition to the time and money savings, the operator also saves on NO_x emissions, which are higher at part load operation.

With this optimised diverter damper curve operation, the HRSG with DrumPlus^TM can enable an SC to CC switch-over throughout the entire cycling load range without compromising on startup times or the integrity of the HRSG components.