Gary Loeb, Commonwealth Edison, Illinois, USA, David Taylor, Peerless Manufacturing Co., Texas, USA, Abraham L. Yarden, Thermal Engineering International, California, USA
The many changing forces acting upon today’s power industry are forcing operators to re-examine many of the ‘given’ factors that influenced past practices. Increased competitiveness due to deregulation has put pressure on power producers, particularly within nuclear generation, where MWe output rather than heat rate has been the dominant performance criterion.
As a result of this changing environment, Commonwealth Edison of the USA has revisited some of its older nuclear power plants with a view toward their life extension and re-licensing.
Commonwealth Edison’s four older, essentially duplicate 820 MWe boiling water reactors (BWRs) at Dresden and Quad Cities, each incorporate four OEM-supplied moisture separators (MSs) that reflect the design technology of their times. An economical redesign and reconstruction of the MSs, using today’s advanced technology, offered significant MWe output gains.
Commonwealth Edison therefore decided to implement a MS redesign and reconstruction programme which is now in progress at Dresden and Quad Cities and which will be completed in 2001.
Many 20 to 30 year old nuclear power plants elsewhere in the USA, Japan and the UK are also approaching the expiration of their initial licensing life, but they still represent a significant resource that can in many cases be uprated and upgraded to meet today’s industry goals.
Design and operation
Figure 1. Double-pocket moisture separation panels
The original four MSs serving each 820 MWe unit at the Dresden and Quad Cities nuclear power stations are essentially 7.3 m high, 4 m diameter vessels containing four vertical moisture separator panels, slightly offset vertically, which function in parallel and divide the vessel internally into two equal volumes. At full load, 306 kg/s of high pressure (HP) turbine outlet saturated steam at 15.5 bar enters each MS at approximately 88 per cent quality through the centrally located, 1.2 m inlet pipe.
In an effort to distribute this steam flow evenly across the four moisture separation panels in each vessel and slow down the incoming high velocity steam, a V-shaped deflector directs the steam to the two upper and two lower panels. However, the deflector is apparently insufficient to reduce the steam’s aggressiveness as it approaches the moisture separation panels.
These panels, consisting of original technology single-pocket chevrons, have not been fully efficient in withstanding the onslaught of wet steam flow, reducing to a significant degree their moisture separation effectiveness. After passing through these panels, the steam exits the MS through a 0.9 m pipe to the low pressure (LP) turbine inlet.
The actual performance of these MSs for steam demisting between the HP and LP turbines was difficult to measure. But now, 30 years later, there is strong evidence to indicate that they are not approaching the dry steam presumed. During recent years, they have deteriorated to the point where they probably now produce only minimal 98 to 98.5 per cent steam quality (80 to 85 per cent separation efficiency).
After the separated moisture is collected in the drain channels of each moisture separation panel, it is drained through vertical ducts to a sealed horizontal compartment in the bottom, semi-elliptical vessel end. Here it is collected together with condensate drained from the upper surface of this sealed area. Due to a slight pressure differential that can exist between the cycle steam and the pressure in this condensate drain compartment, steam blowback up the condensate drain ducts can impede the drainage of separated liquids.
These factors – moisture carried through the moisture separation panels, moisture bypassing the panels themselves, and poor condensate drainage from the drain channels – were the main factors to be addressed in this MS redesign and reconstruction programme at Dresden and Quad Cities. The more significant of the adverse effects of these factors are lost power production and increased LP turbine maintenance.
It was also determined that over the years some flow-assisted corrosion (FAC) damage had also occurred to the vessel walls together with other damage in the moisture separator support structure and other internal elements.
Figure 2. Commonwealth Edison’s Dresden nuclear generating station
A complete system analysis which included computational fluid dynamics (CFD) studies was conducted to determine the specific and most effective steps necessary to increase moisture separation to 100 per cent within the four existing MS vessels in each of the four 820 MWe units at Dresden and Quad Cities. The resulting redesign consisted of new and improved internals which were manufactured to fit manway access and handling assembly in a confined space.
The four MSs at Dresden Unit 2 were reconstructed during the fall of 1999 and the four MSs serving Quad Cities Unit 2 were reconstructed early this year, all achieving a better-than-expected MWe gain. Activities are underway now to reconstruct the MSs at Dresden Unit 3 this fall and Quad Cities Unit 1 in early 2001.
Several redesign and reconstruction options existed, alone or in combination. However, studies indicated that replacement of the original, single-pocket chevrons with new technology, double-pocket chevrons (see Figure 1) within the existing internal structure would contribute the most toward complete moisture separation.
In addition, extensive structural and FAC-damage repair was required in order to assure the success of the life extension anticipated and to reduce to a minimum any cycle steam bypass around the moisture separation panels.
One particularly unique obstacle had to be overcome to fully utilize the repaired, existing internal panel-support structure. The standard single-pocket moisture separation chevron panels produced 20 or 30 years ago measured 254 mm deep by 2.31 m long. Today’s advanced technology, double-pocket chevron panels are punch-press produced to a standard size of 203 mm by 2.13 m.
Since it obviously was impractical to retool the modern chevron panel production facilities to duplicate the size standards of the past, structural spacers in both dimensions had to be installed in the existing internal support structure, so that modern, standard-size panels could be installed. In future similar MS upgrades, 25 mm spacers can be pre-welded on both sides of each 203 mm double-pocket chevron panel (to replicate the old standard 254 mm width). This will greatly reduce the internal structural modification required. Retooling is now being considered to further reduce these mismatches.
In these cases, the entering, two-phase, inlet steam velocity of approximately 46 m/s is reduced by 15-fold as it approaches the chevron panels. This relatively low approach, yet non-uniform, velocity field was considered to be acceptable. Had this not been the case, other options could have been applied to further reduce the aggressiveness and maldistribution of the steam flow and increase the moisture separation element’s defence against it.
These included such tried and true design modifications as the installation perforated plates directly upstream of the new double-pocket chevrons which would improve their separation capability by smoothing the velocity profile of the aggressive inlet steam. This option is an integral component of modern moisture separator reheater (MSR) design.
To prevent cycle steam blowback through the downcomers from the condensate drain channels, new larger loop seal pots were installed at the exit of all condensate drains. These pots are designed to be tall enough to create a head of water well in excess of any steam pressure differential that could exist between moisture separation area within the MS and the condensation drain header at its bottom.
All of this work had to be accomplished through a 610 mm manway in the shortest possible time. As a result the amount of preplanning required was extensive and considerable site preparation had to be accomplished. This work produced a modification time at Dresden Unit 2 of only 18 days, and based on learning-curve experience gained, the modification time was further reduced by two days when the MS at Quad Cities Unit 2 were reconstructed. The 16-day MS work window was critical to keeping the Quad Cities Unit 2 refuel outage within its planned duration of 19 days, breaker open to breaker closed.
It is now anticipated that with additional pre-outage, component subassembly and other learning curve efficiency increases, a 12 to 14 day outage is an attainable goal for upgrading these older MSs in the USA and elsewhere.
It was demonstrated by the redesign and reconstruction programme of the MS at Dresden Unit 2 that essentially complete separation of drainable moisture was achieved. The initial design target calculated to achieve a gain of 7 MWe. After reconstruction, discounting other outage improvements, it was confirmed that the actual MWe gain was 11 MWe – a total of 4 MWe over the calculated target. This increased the priority for reconstruction of the MS at Quad Cities Unit 2, which was scheduled several months later. Again, an 11 MWe gain was achieved. It is anticipated that the construction of the MS at Dresden Unit 3 this fall and Quad Cities Unit 1 next year will produce similarly good results.
The experience and success gained in redesigning and reconstructing the four MS vessels of each of these four older Commonwealth Edison 820 MWe BWRs point the way for the MS redesign and reconstruction in many other plants of this generation to help them meet today’s new industry challenges.