13.4.1 Separators - part 4
Chevron or corrugated-plate separators
Although mesh separators are most effective in removing moisture, their major limitation is the massive size imposed by the low limiting velocity. A mesh separator which is almost 100% efficient at 1.5 m/s may only be 60% efficient at 1.7 m/s, since the steam velocity is then just sufficient to prevent the water falling, and re-entrainment occurs.
Chevron separating elements do not have a clear upper limiting velocity and are therefore more tolerant of possible excessive local velocities. Chevron separator performance also tends to be as good as wire mesh performance at twice the steam velocity. Modern designs of chevron separator regularly extract 98% of the water from 12% wet HP exhaust steam. Because chevron separators are able to give satisfactory performance at higher steam velocities, they require a smaller frontal area for the same steam flow, giving a more compact arrangement.
Chevron separating elements are produced in various forms. Basically, the elements are parallel corrugated plates which induce separation of the water droplets by inertia and impaction. Earlier types incorporated hooks or stops in the corrugations to induce even more sudden changes in the steam flow. These elements were complex to make and the slight performance improvement did not warrant the additional complexity and expense. Current designs employ simple corrugated plates, sometimes with curved sides or, more recently, with flat plates in a zigzag pattern.
The droplets captured on the corrugated plates coalesce to form a film of water which drains down the corrugations. If this film is allowed to accumulate over a large depth of plate it could grow to a thickness which would result in re-entrainment of water by the steam flow. Hence, the chevron plates must be either of a limited depth or the corrugations must be fitted with water-collecting channels shielded from the steam flow. The design of the final drain-collecting device, at the bottom of the vane, also requires detailed attention to ensure that it does not induce re-entrainment.
A typical MSR design using chevron separators is shown in Fig 2.104. After general steam distribution within the vessel, the flow through the separators is controlled by perforated plates located in front of each packet of chevrons.
In order to avoid any problems from wet steam erosion and off-load corrosion, components used in the construction are normally stainless steel. Separator plates, for example, may be in austenitic stainless steel and the distribution plates in 13% chrome ferritic steel. Internal supports for these elements are designed to avoid resonant vibration and to allow for differential thermal expansions.