                                       
                                       

                                  MEMORANDUM



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 current use, feasibility, and technical issues regarding cooling water intake screens utilizing fine mesh (both traveling screens and wedgewire screens).  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

Facilities with fine mesh traveling screens

There are three primary sources of information on the use of fine mesh screens at facilities: EPA's industry questionnaire data, summary documentation developed by the Electric Power Research Institute (EPRI), and technology vendors.

The industry questionnaire (issued by EPA in 2000) requested information about use of fine mesh screens, but did not request detailed information about the mesh size. Instead, the survey asked respondents to check a box if they used fine mesh screens, defined as having a mesh size of 5 mm or less. As a result, the survey data could not be used as a source to identify the number of facilities with screens with fine mesh of 2 mm or less.  The survey data indicated that, of the 227 facilities that completed the detailed 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 type[1]
                          Design intake flow range[1]
                                 Fuel type[1]
                                     State
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, estimates 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 screens 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 type[1]
                          Design intake flow range[1]
                                 Fuel type[1]
                                     State
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 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.  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).  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 through-screen velocities (1.5 fps and 2.5 fps).  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 Through-screen Velocity (fps)
                Replacement Screen Velocity Increase Factor[2]
                       Compliance Screen Area Factor[3]
                                      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 Through-screen Velocity (fps)
                Replacement Screen Velocity Increase Factor[2]
                       Compliance Screen Area Factor[3]
                                      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 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.

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.

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.  The size and cost of this new screen technology are directly related to the required screen surface area.

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). 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.

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.  The availability of additional shore space at many existing intakes may be limited due to existing structures and other considerations.  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.



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.

