Tetra Tech, Inc.

10306 Eaton Place, Suite 340

Fairfax, VA 22030

phone 703-385-6000

fax 703-385-6007

DATE: February 6, 2009

TO: Paul Shriner and Jan Matuszko, EPA

FROM: John Sunda, SAIC and Kelly Meadows, Tetra Tech

SUBJECT: Fine Mesh Screen Use and Feasibility

Tetra Tech and SAIC were tasked with summarizing the

technical issues regarding cooling water intake screens utilizing fine mesh

screens and wedgewire screens)

the effect of fine mesh screens on intake velocity and the size of the intake structure.

Fine Mesh Traveling Screens in

Facilities with fine mesh traveling 

There are three primary sources of information on the use of fine mesh screens at 

facilities: EPA’s industry questionnaire data

Electric Power Research Institute (EPRI)

The industry questionnaire (issued

fine mesh screens, but did not request 

the survey asked respondents

having a mesh size of 5 mm or

source to identify the number of facilities with screens with fine mesh of 2

The survey data indicated that, o

 

1

In this analysis, the term fine mesh 

less.

MEMORANDUM

10306 Eaton Place, Suite 340

February 6, 2009

Paul Shriner and Jan Matuszko, EPA

John Sunda, SAIC and Kelly Meadows, Tetra Tech

Fine Mesh Screen Use and Feasibility

Tetra Tech and SAIC were tasked with summarizing the current use, feasibility

technical issues regarding cooling water intake screens utilizing fine mesh (both traveling 

screens and wedgewire screens).

1

 Following the results of this research is a discussion of 

the effect of fine mesh screens on intake velocity and the size of the intake structure.

Fine Mesh Traveling Screens in Use in United States

traveling screens

primary sources of information on the use of fine mesh screens at 

EPA’s industry questionnaire data, summary documentation developed by the 

Electric Power Research Institute (EPRI), and technology vendors.

industry questionnaire (issued by EPA in 2000) requested information about use of 

but did not request detailed information about the mesh size

s to check a box if they used fine mesh screens, defined 

mm or less. As a result, the survey data could not be used as a 

to identify the number of facilities with screens with fine mesh of 2 mm or less.

that, of the 227 facilities that completed the detailed 

 

the term fine mesh is defined as screens with an aperture size or slot width of 2.0 mm or 

feasibility, and 

(both traveling 

the results of this research is a discussion of 

the effect of fine mesh screens on intake velocity and the size of the intake structure.

primary sources of information on the use of fine mesh screens at 

summary documentation developed by the 

requested information about use of 

mesh size. Instead, 

defined as 

the survey data could not be used as a 

mm or less. 

that completed the detailed 

aperture size or slot width of 2.0 mm or 
questionnaire, 7 facilities (3.1 percent) reported using fine mesh screens with a mesh size 

of ≤5 mm.

In 2007, EPRI published a report on fine mesh utilization (Fish Protection at Cooling 

Water Intake Structures: A Technical Reference Manual), which provided summary-level 

information on facilities where fine mesh screens have been installed (or in some cases, 

only tested). This report provides a list of facilities that employ fine mesh screens with a

mesh size less than 2 mm:

Facility name Mesh 

size

Waterbody type1 Design intake flow 

range1

Fuel type1State

Barney Davis 0.5 mm Tidal river/ estuary 500 MGD – 1 BGD Natural gas TX

Big Bend 0.5 mm Tidal river/ estuary > 1 BGD Coal FL

Prairie Island 0.5 mm Freshwater river 500 MGD – 1 BGD Nuclear MN

Brayton Point 1.0 mm Tidal river/ estuary > 1 BGD Coal MA

Brunswick 

Station

1.0 mm Tidal river/ estuary > 1 BGD Nuclear NC

Somerset 

Station

1.0 mm Great Lake > 500 MGD Coal NY

1 Waterbody type, design intake flow range, and fuel type identified by EPA.

The traveling screen vendor Passavant Geiger identified 3 facilities that employ fine 

mesh intake facilities: Barney Davis, Big Bend, and Somerset (Anderson 2008).

Fine mesh traveling screens are also used at numerous hydroelectric facilities throughout 

the US, but these facilities are not included in any analyses, as the operations of these 

screens may be fundamentally different than for screens at cooling water intakes.

International Use of Fine Mesh Traveling Screens

Facilities with fine mesh traveling screens

A review of sales and installation data from Passavant Geiger indicates that most 

traveling screens installed in Europe and elsewhere overseas by their firm are fine mesh

(Anderson 2008). Attachment B contains a list of hundreds of Geiger installations dating 

back to 1937; the mesh size for the majority of these installations is in the 1mm to 3mm 

range, with some as low as 0.5 mm and very few exceeding 4 mm.

Design considerations for European screens

The typical design for intakes in Europe is slightly different than for U.S. installations,

often employing an intermediate set of mechanically raked bar racks with more closely 

spaced bars in between the 3-4 inch spaced bars (typically called the trash rack) and the 

fine mesh screens. The second set of bar racks serves to protect the fine mesh screens and 

help reduce loading of larger debris on the traveling screens. The intent of this design is

to minimize condenser fouling and optimize condenser efficiency and downtime (i.e., 

minimizing injury to fish was not a primary design consideration). Use of a second set of

bar racks in addition to new traveling screens would increase traveling screen technology
capital costs by about 75%, while O&M costs would be much lower than for the 

downstream traveling screens as there are fewer moving parts (Anderson 2008). Capital 

costs could be even significantly lower if the cleaning rake system can serve multiple 

screen channels. Such intermediate bar rack systems could potentially be retrofitted on 

existing U.S. plants, but would be dependent on site design; EPA has not analyzed this 

technology for the proposed rule.

The installation data does not give through-screen intake velocities for these screens, 

which are generally a center-flow design. Some of the design data needed to calculate 

approach velocities is available, but other information is not provided. Making some 

basic assumptions,2estimates for some of the < 2.0 mm installations in the data result in 

approach velocities of approximately 0.5 to 1.0 fps. A corresponding calculation of 

through-screen velocities for conventional traveling screens3 with approach velocities of 

0.5 to 1.0 fps would be 1.2 to 2.4 fps.

Fine Mesh Wedgewire Screens

EPRI (2007) provides several examples of facilities that have installed fine mesh 

wedgewire screens.

Facility name Mesh 

size

Waterbody type1 Design intake flow 

range1

Fuel type1State

Logan 0.5 mm Tidal river/ estuary < 100 MGD Coal NJ

Westchester 

RESCO

0.5 mm Freshwater river < 100 MGD Solid waste NY

Cope 2.0 mm Freshwater river < 100 MGD Coal SC

1 Waterbody type, design intake flow range, and fuel type identified by EPA.

As with fine mesh traveling screens, fine mesh wedgewire screens are in use at a variety 

of facilities.

Attachment D presents a list of wedgewire screen installations provided by the Hendrick 

Screen Company. Unfortunately, slot width data was not provided for most installations. 

In general, Hendrick Screens has installed many 1.75 mm mesh installations in the 

western states of Washington, Oregon, and California due to Federal requirements for 

fisheries protection, with many of these installations being associated with hydroelectric 

dams, using either flat panel or T-screens (Isbill 2008). Hendrick Screens was not aware 

of any specific operational problems at any of their installations. Other examples 

provided by the vendor include a facility with 2 mm slot width on the Lehigh River 

(Conectiv Power), another with 3 mm on the Chattahoochee River (McDonough Plant 

cooling tower makeup), and one with 3.175 mm on the Ohio River (Buckeye Water 

 

2 Assumptions include a freeboard of 10 feet, available screen area of 83% of the channel cross-section, and 

that the mesh percent open area is 50%. These assumptions are based on best professional judgment for a 

typical screen configuration.

3

In center-flow screen designs, critical data to calculate intake velocity vary. Here, a comparison to 

standard traveling screens is provided as a reference point. Typically, the through-screen velocity for a 

center-flow screen would be lower, as there is a larger wetted screen surface.
District) (Isbill 2008). Unlike traveling screen retrofits, T-screens can be designed with a 

low through-screen velocity, thus limiting the debris accumulation rate. It is a general 

design requirement that T-screen installations have a maximum through-screen velocity 

of 0.5 fps. 

Mesh Size and Sediment

The traveling screen vendor representative for Eimco stated that 0.5 mm fine mesh 

requires low screen velocities (i.e., approximately 0.5 fps) and that retrofitting a high 

velocity traveling screen with 0.5 mm mesh could be very difficult on large rivers such as 

the Mississippi and Missouri Rivers (Gathright 2008). The Missouri River is known for 

having high levels of suspended sediment, which can create problems in “blinding” of the 

intake screens. Blinding of the screens occurs when the sediment and debris accumulate 

on the screens at a rapid rate. If increased screen rotation and backwashing is not 

sufficient to remove the sediment, then the desired cooling pumping rate may not be 

sustained, which would force the facility to reduce the pumping rate or cease 

withdrawals, leading to a reduction (or cessation) of power generation. Typically, the

problem of screen blinding in rivers with high sediment loading diminishes as the screen 

mesh size approaches 1.0 mm and does not present a problem if 2.0 mm screens are used

(Gathright 2008). 

The primary reason for the difference in performance of screens with different mesh sizes

is due to the typical distribution of sand particle size in the river water. In a study of sand 

grain size distribution from the Fraser River Port in British Columbia, 90% of the sand 

particles were < 0.5 mm in size, with the percent content increasing rapidly below 0.5

mm (Smith 2007). The particle size distribution graph shows that 0.5 mm was somewhat 

of an inflection point where grain size content diminished more gradually as the size 

increased, approaching 0% at 2 mm. Thus, a screen with a mesh size of 0.5 mm would 

capture a significant portion of the suspended material, while a screen with a mesh size 

near 2.0 mm would capture very little of it.

Problems with larger, less-dense debris particles such as leaves will not be affected as 

much by mesh size, since such debris particles will be captured on the screen regardless 

of mesh size and, therefore, no changes in operation would be expected with finer mesh.

In summary, EPA recognizes that high sediment waterbodies pose a challenge for fine 

mesh screens. However, a mesh size of 2.0 mm has been shown to be effective in 

handling the high sediment loads. EPA also acknowledges that facilities located on high 

sediment rivers face constant challenges related to sediment, as existing intake screens 

may become clogged or suffer premature failure or condenser tubes may require more 

frequent cleaning.
Effects on Through-Screen Velocity

When retrofitting from a coarse mesh screen to a fine mesh screen, the open area of the 

screen is reduced, which leads to an increase in the through-screen velocity.4 To avoid 

increased impingement that would result from an increase in intake velocity, EPA 

assumed that a facility would enlarge its existing intake structure to regain the lost open 

area, thereby accomplishing the installation of fine mesh while retaining the same 

through-screen velocity.

As discussed in the TDD, EPA evaluated regulatory alternatives that contain compliance 

requirements for maximum mesh sizes and/or maximum through-screen velocities. At an 

existing facility, replacing a traveling screen with one that has a different mesh size or a 

different through-screen velocity affects the size of the screen surface area necessary for 

compliance. The screen area available for intake flow is the total area of the wetted 

surface of the screen material (i.e., effective screen area) and does not include screen 

framing, screen panel edges, or other structural elements. 

Replacing a conventional traveling screen with a fine-mesh traveling screen

As discussed above, replacing a coarse mesh screen with a fine mesh screen will reduce 

the effective screen area and may also increase the through-screen velocity. Tables 1 and 

2 illustrate the magnitude of the change in the required effective screen size associated 

with different screen technology mesh sizes and through-screen velocity requirements. 

To illustrate these effects, EPA analyzed two through-screen velocity scenarios: one 

where the maximum allowable through-screen velocity is 2.5 fps (Table 1) and another 

where the maximum allowable through-screen velocity is 0.5 fps (Table 2).5 EPA also 

analyzed three fine mesh slot width scenarios: 2.0 mm, 1.0 mm, and 0.5mm. 

The tables calculate the “compliance screen area factor” which represents the increase in 

effective screen area needed to comply with the applicable mesh size and through-screen 

velocity requirements. For example, a compliance screen area factor of 3.0 means that 

the area of the compliance screens (i.e., the new CWIS) will need to be three times larger, 

and thus a new intake system with a similar design would need to be roughly three times 

the size of the existing one. A factor of 1.0 means the existing intake is already large 

enough and that screen upgrades would simply involve the replacement of existing screen 

units.

The existing system design parameters, especially the design through-screen velocity, 

have a direct effect on the compliance screen area factor. Table 1 shows the increase in 

screen area required for intakes when the maximum allowable through-screen velocity is 

2.5 fps for two different fine mesh slot sizes (2.0 mm and 0.5 mm) and for two existing 

 

4 Assuming that the intake flow volume remains constant.

5

If, after installing fine mesh screens, the new through-screen velocity exceeds the given threshold, then 

the CWIS must be expanded to a point where the velocity requirement is met. Table 2 shows a scenario 

where a facility would both install fine mesh screens and be required to meet a 0.5 fps threshold.
through-screen velocities (1.5 fps and 2.5 fps).6

The analysis assumes that the existing 

screens are conventional 3/8 inch (9.5 mm) coarse mesh traveling screens.

Table 1. Screen Area Increase Factors with a Maximum Through-Screen Velocity of 2.5 fps

Mesh Size (mm) Percent 

Open 

Area (%)1

Existing Throughscreen Velocity 

(fps)

Replacement 

Screen Velocity 

Increase Factor2

Compliance 

Screen Area 

Factor3

2.0 50 1.5 1.4 1.0

2.0 50 2.5 1.4 3.4

0.5 33 1.5 2.1 3.1

0.5 33 2.5 2.1 5.2

1 Open area of the selected mesh size. For reference, a 9.5 mm screen (coarse mesh) has an 

open area of 68%.

2 Change in through-screen velocity resulting from replacing a coarse mesh screen with a fine 

mesh screen.

3

Assumes new larger intake screens have a reduced design through-screen velocity of 1.0 fps. 

Larger intake screens (and larger intake) are assumed to be needed if design through-screen 

velocity for replacement screens exceeds 2.5 fps.

As can be seen in Table 1, facilities with existing through-screen velocities close to or 

below the median value of 1.5 fps would require no increase in screen area for 2mm 

mesh screens and a size increase factor of 3.1 for 0.5mm mesh screens. Facilities with 

existing through-screen velocities close to the value of 2.5 fps would require screen size 

increase by a factor of 3.4 for 2mm mesh screens and an increase in size by a factor of 

5.2 for 0.5mm mesh screens.

Table 2 shows that the screen area requirements increase considerably when the 

maximum allowable through-screen velocity is reduced from 2.5 fps to 0.5 fps.

Table 2. Screen Area Increase Factors with a Maximum Through-Screen Velocity of 0.5 fps

Mesh Size (mm) Percent 

Open Area 

(%)1

Existing Throughscreen Velocity 

(fps)

Replacement 

Screen Velocity 

Increase Factor2

Compliance 

Screen Area 

Factor3

2.0 50 1.5 1.4 4.1

2.0 50 2.5 1.4 6.8

0.5 33 1.5 2.1 6.2

0.5 33 2.5 2.1 10.3

1 Open area of the selected mesh size. For reference, a 9.5 mm screen (coarse mesh) has an 

open area of 68%.

2 Change in through-screen velocity if replacing existing screens with similar screen design.

3

Assumes all compliance screens have a design through-screen velocities equal to 0.5 fps.

For facilities with existing through-screen velocities close to or below the median value 

of 1.5 fps, 2mm mesh screens increase the required screen area 4.1 and 6.2 times the 

existing intake screen area, respectively. For facilities with an existing through-screen 

 

6 The selected velocities represent the median value reported for existing Phase II intakes in the 316(b) 

industry survey (1.5 fps) and a typical design value for older intakes (2.5 fps). Note that these values are the 

assumed existing through-screen intake velocities.
velocity of 2.5 fps, 0.5mm mesh screens increase the required screen area were 6.8 and 

10.3 times the existing intake screen area, respectively.7

Implications for facility costs

In general, the capital costs for replacement of existing traveling screens with fine mesh 

traveling screens are about the same as for replacement with coarser mesh screens, but 

the operations and maintenance (O&M) costs may be about 30% greater (Gathright 

2008). This difference is primarily due to energy requirements and equipment wear due 

to continuous use and increased screen rotation speed.8

If the analysis estimated a total screen area required that is greater than what is available 

at the existing intake (i.e., the compliance screen area factor is greater than 1.0), a new 

intake with a larger screen area will be needed. EPA assumed the new larger intake 

would have a through-screen velocity of 1.0 fps when estimating the screen area factor 

and technology costs for a new larger intake.9 The size and cost of this new screen 

technology are directly related to the required screen surface area.10

Technical feasibility of fine mesh screens and expanded CWISs

In an August 2008 presentation to EPA, EPRI stated that field deployment of fine mesh 

traveling screens with favorable screen operating performance (i.e., can properly handle 

debris loading) included eight power plant sites in the US (Dixon 2008).11 These plants 

represent various waterbody types, flows, fuel types, configurations, and locations 

throughout the country.

The wide variety of operating conditions at facilities with fine mesh traveling screens 

suggests that with proper design and operation, these screens are technically feasible at 

most facilities.12

 

7

Similarly, even if a coarse mesh screen with a 1.5 fps existing through-screen velocity were subject to a 

maximum allowable through-screen velocity requirement of 0.5 fps, there would be a required increase in 

screen area by a factor of 3.0.

8 EPA notes that its compliance cost estimates consider the increased operating energy requirements and 

equipment replacement costs for fine mesh traveling screens.

9 The design through-screen velocity of 1.0 fps for new expanded intakes is not a regulatory requirement; it 

simply reflects a best professional judgment (BPJ) design standard for a new intake structure. In part, EPA 

assumed that a new facility would be designed using a more conservative through-screen velocity to avoid

operational problems involving debris accumulation

10 See Chapter 8 of the TDD, which describes the costing model used for the proposed rule. Module 3 

contains the costs for expanding an existing intake structure.

11 The facilities listed were Hanford Generating Project, Barney Davis, Indian Point, Big Bend, Brunswick, 

Somerset, Dunkirk, and Prairie Island.

12 Further, the technology vendors stated that the distribution of fine mesh traveling screens has been 

limited due to the fact that few facilities have been required to install fine mesh screens. EPRI also 

concluded that the potential for future use of fine mesh screens is favorable, as handling procedures and 

screen designs have continued to improve (Dixon 2008).
However, as the size requirement of the compliance screens increases (i.e., as the screen 

compliance factor increases), the potential for problems associated with the availability of 

space to construct a larger intake also increases. This is especially true for shore-based 

intake technologies, since water depth is generally relatively shallow, thereby requiring 

any screen expansion to cover a proportionally longer length of shoreline.13 The 

availability of additional shore space at many existing intakes may be limited due to 

existing structures and other considerations.14 In some cases, the expanded intake 

structure would be built in front of the existing intake, which may lead to interference 

with vessel traffic or other uses.

In analyzing various compliance options involving fine mesh screens, EPA analyzed an 

option where all facilities would install 2.0 mm mesh screens and found that 

approximately 33% of facilities would be forced to expand their intake structure by more 

than 5 times its current width. Another option where all facilities would install 0.5 mm 

mesh screens would have required as many as 68% of facilities to expand their intake 

more than 5 times its current width.

 

13 New construction would be lateral along the shoreline; costs to deepen the existing intake wells as a way 

to expand the screen area would be expensive.

14 Examples might include limited ownership of shoreline property or conflicting uses of the shoreline.
References

Anderson, Dave. Passavant Geiger. Telephone Contact Record. Contacted by John 

Sunda, SAIC on October 13, 2008.

Smith, David, Thurber Engineering Ltd. Technical Aspects of River Sand. Summary of 

Dredged Sand Tests. March 27, 2007. Accessed on November 20, 2008 at website:

http://www.fraserportauthority.com/pdf/2.%20Thurber%20Engineering%20Ltd.%20-

%20Technical%20Specifications%20of%20River%20Sand.pdf

EPRI. Fish Protection at Cooling Water Intake Structures: A Technical Reference 

Manual. Palo Alto, CA: 2007. 1014934.

Dixon, Doug. EPRI. Black, Jon. Alden. Fine-mesh Traveling Screens. Presentation for 

EPRI-EPA Meeting. August 26, 2008.

Gathright, Trent. Eimco. Telephone Contact Record. Contacted by John Sunda, SAIC on 

October 14, 2008.

Isbill, Mike. Hendrick Screen Company. Telephone Contact Record. Contacted by John 

Sunda, SAIC on October 8, 2008.