Simon Hogg Alstom Power, Rugby, UK
Admission conversion improves part load performance1 November 2005
Conversion to partial arc steam admission can increase the part load performance of steam turbine plant. The author compares the effect on performance of the two types of steam admission, giving examples from Spruce and Eggborough, and describes the added benefit of retrofitting the HP cylinders of throttle controlled machines that operate under non base-load conditions with partial arc admission.
Throttle controlled large steam generating sets featuring full arc steam admission (FAA) into the HP cylinder, irrespective of load profile, are common in UK power stations, but less so in some other countries – for example Germany, France and the USA – where partial arc admission (PAA) designs are often found, particularly in non-base load stations. Throttle control turbines exhibit low thermal and mechanical stressing of the turbine components, but suffer from a significant reduction in performance when operating at low loads owing to valve throttling losses. This situation can be improved by operating the boiler in sliding pressure mode, but many boiler designs are not capable of this type of operation.
Eggborough power station is currently undergoing an HP cylinder retrofit which aims to take advantage of a change to partial arc admission as part of the upgrade. This will also require modification of the HP valve governor system for sequential valve operation. This is the first example in the UK of a change in admission arrangement as part of a retrofit, although many other examples exist on retrofitted power plant outside the UK.
Partial arc steam admission offers an alternative approach to boosting part-load performance of the steam turbine set that is independent of the boiler design. In a PAA control stage, the inlet belt is divided into a number (usually 4) of discrete partitioned inlet segments. This can be achieved by using separate nozzle boxes, or, in some more recent designs, by partitioning the inlet belt. Control is achieved by restricting the steam admission to the segments sequentially, so when operating at part load with, for example, the valve controlling the steam flow to one of the segments fully closed, valve throttling losses are eliminated (all the other control valves remain fully open at this point). The efficiency of the first HP stage of a PAA machine is lower than that of the corresponding FAA cylinder.
The moving blade design for the control stage has to be extremely robust to withstand the cyclic steam loading experienced during part-load operation, and a mixing zone is required downstream of the first stage to ensure uniform flow into stage 2 at all loads. So PAA machines operating at full load are slightly less efficient than the equivalent FAA machines at the same conditions. However, the effect of valve throttling losses on performance is such that for loads of typically 90% or less, PAA machines become more efficient than their FAA counterparts. (A simple mathematical model for predicting PAA valve points may be found in ‘Conversion to partial arc admission when retrofitting steam power generation plant – unlocking additional performance at part load conditions’ S I Hogg, IMechE Seminar 20 September 2005)
Retrofitting steam turbine HP cylinders provides an opportunity to convert the load control arrangement to one that better meets the current operating requirements of the turbine. FAA control is best suited to units that spend the majority of time operating at or close to base load.
PAA is used extensively outside the UK, particularly on impulse turbines, on units that are designed to operate for appreciable periods at less than full load. Conversion from full to partial arc admission allows the turbine to operate more efficiently at part loads by limiting valve throttling to the partially open valves.
In addition to the turbine hardware, the scope of the conversion must also include modification of the turbine governor to allow sequential opening and closing of the HP control valves. Often this involves replacement of original mechanical governors with modern electronic systems capable of providing both full arc and partial arc admission but it is not essential that governor and turbine are upgraded at the same time. A turbine retrofit configured for PAA will operate satisfactorily under full arc until a suitable opportunity arises to make the governor upgrade. Conversion from full arc to partial arc admission as part of an HP cylinder retrofit gives some, or all, of the following benefits:
• For a cycle operating with constant boiler pressure, the efficiency of the unit at part load will be better for partial arc admission than it is by throttling to control the power on a full arc machine.
• For a cycle operating with sliding boiler pressure, it may be very useful to have an overload arc available in order to respond to fast changes in power demand.
• Turbines that feature partial arc can be readily adapted to the specific requirements of a utility by careful selection of appropriate admission arc areas.
(Comparison of) HP cylinder designs.
Figure 1 shows impulse HP cylinder designs configured for full and partial arc admission. For FAA (Figure 1a), the inlet pipes from the HP valves supply an annular inlet belt at the start of the HP expansion. The design of the first turbine stage of the steam path features a conventional disc and diaphragm construction that is of the same basic design as the rest of the stages in the expansion.
In the partial arc cylinder design (Figure 1b), the inlet belt is divided into a number (usually 4) of discrete arcs. Each arc is supplied by one HP governor valve. Partitioning of the inlet belt is achieved by separate nozzle boxes, or, on more modern designs, by dividing the annulus internally into separate arcs and mounting the plates containing the stage 1 nozzles directly onto the inlet belt. Figure 1b shows the latter design. Shock loading on the first stage moving blades as they pass in and out of active arcs imposes high dynamic loads on the blades. Changes in temperature in the inlet region of the turbine under varying load are greatest with partial arc admission. These constraints imply robust designs for the inlet region of the rotor and casings, seen in the stage 1 moving blade design in Figure 1b. The blade is wider than its full arc counterpart in order to cope with the increased dynamic duty.
There is a mixing length immediately downstream of the control stage. This feature is designed to allow flow redistribution when operating at part loads (one or more inlet arcs fully or partially closed), so that stage 2 operates with essentially uniform inlet conditions around the annulus regardless of the output of the unit. This allows the HP steam path from stage 2 onwards to use the same turbine stage design technology as that used for full arc cylinders. One consequence of the axial requirements of the control stage when retrofitting HP cylinders is that it is sometimes possible to fit in one extra stage in a full arc admission design compared to its partial arc counterpart. This is advantageous at full load, but at partial loads the benefit for full arc admission of the more efficient first stage plus possibly one extra stage, will be quickly overcome by the increased valve throttling losses as load is reduced.
Figure 2 shows reaction technology equivalents of the full and partial arc impulse HP cylinder designs shown in Figure 1. The disc and diaphragm construction of the impulse technology has now been replaced by the ‘drum’ rotor construction of a reaction machine with fixed blades mounted directly into the inner casing wall. In many respects the comparison between the designs for different steam admission arrangements is similar toimpulse machines. The full arc cylinder (Figure 2a) features a full annular inlet belt and stage 1 uses the same blade design technology as the rest of the cylinder. The partial arc cylinder (Figure 2b), has a partitioned inlet belt and a more robust stage 1 design. The diameter of the first stage is significantly larger than the other HP reaction stages. On partial arc reaction technology HP cylinder designs, the control stage is an impulse stage. The increased diameter allows a larger heat drop across the first stage so that lower loads can be achieved at the PAA valve points. It also allows a greater mixing volume after the control stage and prevents direct impingement of the exit flow from the control stage onto the stage 2 fixed blades.
PAA control stage
Modern partial arc HP cylinder designs feature integral partitioned inlet belts with stage 1 nozzle plates mounted directly onto the inlet belt. Figure 3 shows two different designs for control stage moving blades. The requirements for very robust blade designs to withstand the high mechanical and thermal dynamic stresses during part load operation were also discussed in the previous section. The pin root control stage moving blade design shown in Figure 3 features an alternate torsion integral shroud arrangement. The blades are made in two blade sets with the shroud contact faces arranged such that when the blades are mounted onto the rotor disc, pre-twist causes the torsional moments on adjacent blades to alternate in direction. The industry has seen many examples of trouble in the past, but this very stiff arrangement with continuously contacting integral shrouds has been completely reliable since its introduction in the 1960s. This type of design is usually employed by Alstom on the company’s impulse steam turbine products.
An alternative control stage moving blade design is shown in figure 3b. In this design, the impulse blade profiles are electrically eroded from a solid ring. The resulting ring of blades is welded to the shaft disc using a large full penetration U-shaped weld. The impulse wheel is thereby made an integral part of the rotor. This type of design is usually employed on Alstom’s reaction steam turbine products.
Performance at different loads
Figure 4 shows a typical expansion for a HP cylinder in a sub-critical steam turbine cycle under full load operation. Steam conditions upstream of the inlet valves are 160 bar, 540°C and the HP cylinder exhaust pressure is 40 bar, say. If the pressure drop across the valves is taken as 5% of the inlet pressure, the pressure immediately downstream of the valves at the inlet to the first stage of the HP steam path is 152 bar. The isentropic internal efficiency of the HP steam path is ?h/ ?hs = 90 %, say. The overall efficiency of the HP expansion including the inlet valves is then ?h/ ?hsv.
Now consider the case where the cylinder shown in Figure 4 has an inlet arranged for full arc admission and is operating at 70% load. This expansion is shown in Figure 5. Throttling and hence losses across the inlet valves have now significantly increased. Load is roughly proportional to steam flow. The condenser pressure is fixed and so for 70% flow, all steam conditions back up through the turbine train to HP stage 1 inlet must scale in proportion. The effect of temperature is relatively small and so the HP exhaust pressure drops to approximately 40 x 0.7 = 28 bar and HP stage 1 inlet pressure becomes approximately 152 x 0.7 = 106 bar. The internal efficiency of the HP steam path is essentially insensitive to volumetric flow and so ?hf/ ?hfs remains at 90%. But, the HP efficiency including valves ?hf/ ?hfs has now become significantly lower than shown in Figure 4 owing to the large valve throttling losses.
Suppose now that the cylinder in Figure 6 is configured for partial arc admission. The 70% load point typically corresponds to operation with two arcs closed on a four arc admission machine with arcs of equal area. The valve losses associated with the two open arcs will now increase to more than 5% because now 50% of the valve system is carrying 70% of the full load flow. Assuming valve losses vary with flow squared, the pressure downstream of the open valves at inlet to the stage 1 nozzles of the open arcs will be about 145 bar. The steam conditions from HP stage 2 through the turbine train to the condenser must be the same for 70% load, regardless of type of HP admission stage and so the HP exhaust pressure in Figure 6 must have the same value as that in Figure 5. The internal efficiency of the HP steam path from stage 1 inlet to exhaust is reduced compared to that in Figure 5 because of the mechanically robust and therefore aerodynamically less efficient control stage blades and the losses associated with the mixing zone downstream of stage 1 (see section 2). In Figure 6 the internal efficiency of the HP steam path has been lowered to ?hp/ ?hps = 85% to reflect these increased losses. However, despite this lower internal efficiency, the overall efficiency of the HP cylinder including valves ?hp/ ?hps can be seen from the figure to be higher than that of the equivalent full arc cylinder shown in Figure 5. So, at part load conditions, despite the lower internal efficiency of the HP steam path, partial arc admission offers better performance than the equivalent cylinder with full arc admission.
This result is extended to a load range from 100% down to about half load in the typical example shown in Figure 7 . In this example, the partial arc admission machine is configured for 2+1+1 steam admission (i.e. the valves controlling the steam flow to two of the four inlet arcs can be closed in series). At full load, the turbine with partial arc has a slightly worse heat rate than the full arc turbine, because of the higher HP stage 1 losses. Initially, as the valve controlling the first arc is closed, the heat rate of the partial arc and full arc machines degrade at a similar rate. However, on the partial arc machine, the throttling losses reduce as the first valve progresses towards full closure, with the effect that for this example, by about 94% load the heat rates for both admission types are equal. Below 94% load, the partial arc admission machine has the better performance. At 90% load the first arc is fully closed and so HP valve throttling losses are eliminated and the partial arc performance is more than 0.5% better on heat rate than the full arc performance. The cycle then repeats as the second arc is closed as shown in Figure 7. Below 70% load the partial arc machine will throttle constantly on 2 valves and so the heat rate will degrade accordingly as illustrated in Figure 7. This example shows that for units with load profiles that require significant periods of operation at part loads, partial arc admission can offer a significant performance advantage over units featuring full arc admission. Figure 7 shows that at full load, full arc admission has a heat rate advantage of 0.1% over partial arc. However, as load reduces the position is quickly reversed and at 70% load , for example, partial arc has the heat rate advantage by 1.7%. The effect on fuel consumption can be illustrated by some examples, as follows:
• 24 hours at 100% load.
FAA better than PAA by: (24 x 100 x 0.1)/(24 x 100) = 0.1%
• 24 hours at 70% load.
PAA better than FAA by: (24 x 70 x 1.7)/(24 x 70) = 1.7%
• 6 hrs at 100% load + 6 hrs at 90% load + 12 hrs at 70% load.
PAA better than FAA by:
(6x100) + (6x90) + (12x70)
The author acknowledges the contribution of J.A. Hesketh of Alstom Power, Rugby, UK, who developed the method for calculating valve points.
Figure 1 Impulse technology full (a) and partial (b) arc steam admission HP turbine cylinders. Figure 2 Reaction technology full (a) and partial (b) arc steam admission HP turbine cylinders. Figure 3. Partial arc control stage moving blade designs – pin root design (a) and (b) welded design. Figure 7: Variation of heat rate with load – full arc admission v partial arc admission.