Tetra Tech, Inc.

10306 Eaton Place, Suite 340 Fairfax, VA

22030 phone 703 - 385 - 6000 fax 703 - 385 - 6007 TO : Paul Shriner and Jan Matuszko FROM : Tim Havey , Tetra Tech, Inc. DATE: July

30, 2008 SUBJECT: Maximum Wedgewire Screen Slot Size Summary For reasons discussed below, establishing a numeric maximum slot size for pre wedgewire screen applic ations is unnecessary and would not eliminate certain site de sign considerations . Unrelated slot size for any

screening application conditions m ight require a facility to install a system that is over performance standard , or unintentionally restrict a permitting authority’s desire to adjust design requirements that account for site vary significantly

from one site to another overly restrictive for one facility

and insufficiently

restrictive for another. Discussion Wedgewire screen performance is a function of two exclusion principles hy draulic. Leaving aside hydraulic cons main factors influencing screen slot size accomplished. (I

t is noted that slot size also indirectly influences h ydraulic exclusion. Ultimately, the optimal slot siz smallest/fewest screen assemblies while minimizing Historically, intake screen slot size condenser tube is the narrowest point in the cooling water pathway and as such, is most susceptible to clogging from debris. As a general rule of thumb, the maximum screen slot size is never larger than 1/2 the condenser tube diameter, which is why a large number of facilities have intake screens with 3/8 that are 3/4 or 7/8 inches in diameter). 

### MEMORAN DUM

Paul Shriner and Jan Matuszko Tim Havey , Tetra Tech, Inc.
Maximum Wedgewire Screen Slot Size

For reasons discussed below, establishing a numeric maximum slot size for pre

ations is unnecessary

and would not eliminate certain site . Unrelated factors at existing facilities naturally limit the maximum slot size for any

screening application , including wedgewire. E xcessively prescriptive a facility to install a system that is over

- engineered to meet the , or unintentionally

restrict a permitting authority’s desire to adjust design requirements that account for site

- specific factors. Because biological factors can vary significantly

from one site to another , a sin gle numeric limit could simultaneously be for one facility

and insufficiently

restrictive for another.

Wedgewire screen performance is a function of two exclusion principles

— physical and hy draulic. Leaving aside hydraulic cons iderations for now, this discussion focuses on the main factors influencing screen slot size

— the means by which physical exclusion is accomplished. (I

t is noted that slot size also indirectly influences h ydraulic exclusion. Ultimately, the optimal slot siz e will balance these competing factors and allow for the smallest/fewest screen assemblies while minimizing impingement and/or entrainment

screen slot size s were derived from the condenser tube diameter. Th narrowest point in the cooling water pathway and as such, is most susceptible to clogging from debris. As a general rule of thumb, the maximum screen slot size is never larger than 1/2 the condenser tube diameter, which is why a large number of have intake screens with 3/8

- inch mesh panels (corresponding to condenser tubes that are 3/4 or 7/8 inches in diameter).

- 1

- - approved ations is unnecessary

and would not eliminate certain site

- specific at existing facilities naturally limit the maximum prescriptive to meet the , or unintentionally

restrict a permitting authority’s desire to adjust

Because biological factors can gle numeric limit could simultaneously be

physical and iderations for now, this discussion focuses on the the means by which physical exclusion is accomplished. (I

t is noted that slot size also indirectly influences h ydraulic exclusion. e will balance these competing factors and allow for the

impingement and/or entrainment .) the condenser tube diameter. Th e narrowest point in the cooling water pathway and as such, is most susceptible to clogging from debris. As a general rule of thumb, the maximum screen slot size is never larger than 1/2 the condenser tube diameter, which is why a large number of inch mesh panels (corresponding to condenser tubes- 2

-

Slot sizes for conventional traveling screens typically refer to a square opening (3/8” x 3/8”) that is punched or woven into the scre en face. Wedgewire screens are constructed differently, however, with the slot size referring to the maximum distance between longitudinally

adjacent wires. Because wedgewire screen slots are oblong, the slot size approximation method noted above should be conservatively modified to select slot sizes that are smaller than 1⁄2 the condenser tube diameter. As with any

screening technology

wedgewire screens function best when kept free of debris and intake flows are uninterrupted. As a passive technology, howeve r, wedgewire screens are designed to be somewhat “self

- cleaning” b y minimizing debris accumulation on the screen face in the first place instead of incorporating a robust cleaning mechanism (air

- burst cleanings are essential, but are meant to be used infre quently and for brief durations). Minimizing debris accumulation is best accomplished by maximizing the screens’ through

slot area (larger slot size) , which corresponds to a lower velocity ratio and smaller flow field. The tradeoff is a diminished ability to exclude smaller organisms. In general, decreasing the slot size increases the probability that debris (including fish) will be impinged. Adding biological criteria to design parameters changes the slot size considerations. The goal of protecting smaller organisms

— namely eggs and larvae

— may require using smaller slot sizes than would be required otherwise but the selection basis is still comparable: choose the slot size that will exclude the target species and life stages while maximizing the screen open area. This allows for the smallest flow field and reduces the number and size of the screen assemblies while still being protective enough

. Slot Size Tradeoffs

Smaller slot size Larger slot size Greater exclusion Lower exclusion Larger flow field Smaller flow field Higher velocity ratio Lower velocity ratio Higher impingement probability Lower impingement probability Larger/more screen assemblies Smaller/fewer screen assemblies