Modularised MSR upgrade

3 September 2002

Duane Arnold has undergone an extensive life extension and uprating programme to increase capacity by 16%. To minimise component installation time, the moisture separator components were modularised and installed as four completely pre-assembled sections. By Eric Sorenson, Chester Sullivan, Abraham Yarden and Clement Tam

When Duane Arnold BWR came on line in early 1974, it had an initial design rating of 565.7MWe, and was provided with two of the earliest Moisture Separator Reheater (MSR) vessels designed with two-stage reheaters. These consist of a two-pass HP reheater bundle taking throttle steam, and a separate two-pass LP reheater receiving extraction steam from an intermediate stage of the HP turbine. The HP reheaters presented problems from the very outset in that their steam/condensate outlet mixture carried far too much excess steam. Several efforts were made to control this excess loss through the use of orifices. However, these efforts were ultimately unsuccessful, and a continuing power loss resulted.

As a result, the HP reheaters in these MSRs were taken out of service in the late 1970s. While this, of course, reduced the MSR output steam superheat to the LP turbine from about 92°C to about 22°C, it did produce optimum power production and operational stability. During a mini-uprate in thermal output where the design rating of 1593MWt was increased to 1658MWt, electric power production was restored to the original design rating of 565.7MWe with the HP reheaters in the MSRs still shut down.

As a part of the current uprate programme, the rating will be increased over several years to 1912MWt and 660MWe. A new, larger HP turbine has been placed in operation. Of course, with the HP reheaters in the MSRs out of service, the full value of this larger HP turbine could not be realised. In fact, the throttle-flow ratio was limited to only 87%. In addition, a detailed MSR inspection identified, among other things, the fact that the existing excessive tube span between the tube adjacent support plates could exceed a vibration threshold in the course of the uprating of the steam cycle, and therefore constitute a reliability issue.

MSR upgrade options

In the light of all these factors, a variety of MSR upgrade options were developed and evaluated. Starting with the essential base of completely modernising the MS section of the MSR, these options included the addition of:

• Replacing the original LP and HP reheaters with advanced design reheaters in the same, efficient HP/LP configuration taking both throttle and extraction steam.

• Replacing the LP and HP reheaters with a single, modern four-pass reheater taking either throttle or extraction steam.

• Replacing the HP and LP reheaters with a modern single stage, two-pass design taking either throttle or extraction steam.

MSR redesign and reconstruction

A thorough evaluation of these three reheater options indicated the superiority of combining the base MS modernisation with the first of the above options. It combined the superior efficiency of two-stage MSR reheaters with the configuration of the new, larger HP turbine which provided for the steam extraction required.

Thus the interior structures of the MSR shells were minimally altered, and up-to-date internals - MS components, and LP and HP reheaters - were installed. This would take full advantage of modern MSR design technologies. Figure 1 is a cross-section of the restored MSRs, showing the MS perforated plates, double-pocket chevron vanes, and the two-pass, HP and LP reheaters and vertically oriented U-bends.

Moisture separators

Each MSR vessel received approximately 1295t/h of about 85 quality steam (1092Btu/lb) at 13barg and 190°C from the HP turbine exhaust. Four MS banks of modern, double-pocket chevron-vanes (Figure 2) are arranged on each side as a V-shaped formation at a 45°C slant from the vertical. They are fitted with perforated plates in front of the chevron vanes to improve steam-flow distribution along their length and height.

A recent MS innovation was employed at Duane Arnold to reduce erection time and minimise personnel radiation exposure during the MSR restoration process at this BWR plant. Four structures containing the chevron moisture separator banks, the integral perforated plates and the supporting frame were prefabricated in the manufacturing plant and shipped as individual modules to the site. In addition to greatly speeding up the installation process and minimising personnel radiation exposure, the added benefits of in-shop quality control over field assembly within the MSR shells are obvious.

HP and LP reheaters

The tubeside LP reheater bundles (Figure 3) receive HP extraction steam at about 32bara and 240°C; the tubeside HP reheater bundles receive main throttle steam at 67bara and 285°C. Each advanced design reheater bundle employs Type 439 stainless steel finned tubing (27 fins per inch) in an optimised geometrical pattern to minimise cycle steam pressure loss and to optimise heat transfer rate. Instead of the existing horizontal tube bend arrangement, a modern, vertical orientation was fitted into the existing high width-to-height ratio of the tube field. Experience has shown that this tube reorientation can reduce condensate subcooling by as much as 75% and cut excess steam consumption by up to 50%, as well as increase heat transfer area significantly.

The tube bundles are supported by a slip plate design (Figure 4), which drastically reduces the damaging effects of thermal stress by allowing the controlled, in-plane expansion of adjoining structural support plates. In addition, bypass steam losses were drastically reduced through the use of flow restricting bars welded to the slip plates themselves (Figure 5), and tube support plate holes were radiused.

Remote, manually operated throttling valves were installed at the discharge of the second pass condensate lines of all LP and HP reheaters. These valves, in conjunction with integral temperature sensors at the outlet ends of selected reheater tubes, permit occasional manual discharge flow trimming to minimise excess steam flow through the reheaters and maximise overall MSR thermal performance.

Overall consequences

Superheat produced by the MSRs has increased from about 22°C to 92°C, which significantly benefits LP turbine blade life by reducing the erosive effects of wet steam. As predicted, nearly 100% MS efficiency was achieved through the beneficial effects of more even steam distribution across the modern, double-pocket chevron vane banks in the modularised MS assemblies. The Terminal Temperature Difference (TTD) across the reheaters and the cycle steam pressure drop through the MSRs both met or exceeded guarantees. The installation of a new main turbine, oversized in anticipation of an increase in thermal power, was expected to result in a decrease of about 10MWe due to throttling losses across the turbine control valves. The refurbished MSR was restored, and second stage reheat was expected to just offset the throttling loss. Post installation testing showed a gain of 4MWe beyond the offset of the turbine control valve throttling losses. The net gain over previous operation is therefore 4MWe, even though the new MSR was responsible for a gain of about 14MWe.

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