316B RULE

TELEPHONE LOG

Name of Caller: John Sunda, SAIC

Date: October 14, 2008

Company Name: Eimco (Brackett Green)

Street Address: 1335 Regents Park Dr., Suite 260

City/State/ZIP: Houston, TX

Person Contacted: Trent Gathright

Title: Group Product Manager

Telephone #: 281-480-7955 Ext 27

 

Trent T. Gathright

Group Product Manager

P&I Screen Products

EIMCO Water Technologies

1335 Regents Park Dr., Suite 260

Houston, TX 77058

Tel: 281-480-7955 Ext. 27

Fax: 281-480-8225

Mob: 832-489-7956

Email: trent.gathright@glv.com

Web: www.glv.com

Called to ask about feasibility of fine mesh screens in high sediment and debris waterways, 

technical issues, and examples of fine mesh (<2mm) traveling screen installations.

He said that it is difficult to nearly impossible to retrofit existing screens with high screen 

velocities with fine mesh of 0.5 mm especially if there is a high debris or sand loading because of 

the rapid blinding of the screens. He said the Mississippi and especially the Missouri (big muddy)

have problems with sand and retrofitting high velocity screens with 0.5 mm mesh would be 

nearly impossible. I asked about larger mesh sizes and at what point (size) would the problem 

diminish. He said the problems would be significant when mesh size drops below 1.0 mm and 

that 2.0 mm and above would present no problem with sand. They have done numerous 1/8 in 

(3.175 mm) retrofit installations and screen velocity was not a problem in those cases. Basically, 

you can have fine mesh screens with low velocities like around 0.5 fps and coarse mesh screens 

with high velocities but you can’t have both fine mesh (0.5mm) and high screen velocities if 

there are any chances of debris/sand loading being a problem.
Regarding O&M cost issues, one of the biggest O&M cost items associated with fine mesh is the 

more frequent need for chain replacement. Factors that contribute to increased rates are more 

frequent or continuous operation, operating at faster rotation speeds, and higher depth 

differentials (head loss) across the screen. All of these tend to increase with reductions in mesh 

size. Labor is the biggest component for chain replacement. When asked about the relative 

increase in O&M for high debris/fine mesh screens, he said O&M costs should increase by about 

30%.

They recommend installing 2 speed screens so speed can be tailored to debris conditions, thus 

minimizing wear on screens. Typical coarse mesh screen speed is 10 fps and fine mesh will be 

around 15 fps and 30 fps. Their dual flow screens go up to 45 fps since screen panels travel both 

down and then up between washes.

Regarding screen velocities he said that 2.5 fps is probably a good cutoff point for acceptable 

screen velocities if the screen mesh isn’t too small. At higher velocities increased head loss 

across the screen become an issue. 

I talked about the fact that overseas they use fine mesh but probably with lower velocities and 

may use closer spaced bar racks. He said they also recommend use of closer spaced bar racks but 

only if the mesh size is smaller. For example it would make no sense to put 1 inch bar racks in 

front of 3/8 in mesh screens. 

Another problem with finer mesh is corrosion. They use stainless steel but also can use polyester 

screens but these are less strong and more easily damaged. He said that puncture of the screens is 

not that much of a problem.

Regarding types of debris, with regard to leaves mesh size doesn’t matter since the leaf will 

collect on the surface regardless. Algae can be a problem with fine mesh because it can form a 

matt on the surface essentially creating filtering surface that will catch finer particles. Lake 

Ontario has problems with algal blooms and the Mississippi and Missouri rivers can have 

problems with algae and leaf mats in addition to sand.

He will send me additional information regarding their experience later this week via email.