Tetra Tech, Inc.

10306 Eaton Place, Suite 340 Fairfax, VA 22030 phone 703 - 385 - 6000 fax 703 - 385 - 6007 DATE: April 10, 2009 TO : Paul Shriner and Jan Matuszko, EPA FROM : Tim Havey

and Kelly

Meadows, Tetra Tech SUBJECT: Icing at Cooling In cold climates ( where prolonged periods of sub cooling water intake structures accumulation on and around critical intake components. operations in multiple ways; s screens and equipment , b lock blocking water flow and damaging the screens or pumps become lodged in the open space of the intake screen also form below the surface and collect on submerged portions of the intake structure, rapidly restricting water flow. This memo describes the various ty pes of icing that are typicall y encountered at CWI Ss as well as the design features and operational measures that facilities use to minimize the effect of icing on maintaining normal operations. to addr ess icing at facilities EPA visited in preparation for the Types of Ice Depending on the meteorological and hydrological conditions at the CWI S, several different t ypes of ice may form. Each type behave on CWI S operations. 

### MEMORANDUM 10306 Eaton Place, Suite 340

April 10, 2009 Paul Shriner and Jan Matuszko, EPA Tim Havey

and Kelly

Meadows, Tetra Tech Icing at Cooling Water Intake Structures where prolonged periods of sub

- zero air temperatures are common) cooling water intake structures (CWI Ss) must be designed and maintained to handle ice accumulation on and around critical intake components. Ice can threaten CWI S operations in multiple ways; s urface ice may develop in thick sheets and damage intake lock s of ice may

drift and collect in front of an intake screen, blocking water flow and damaging the screens or pumps , or smal l fragments of ice may become lodged in the open space of the intake screen

. Under the right conditions, ice can form below the surface and collect on submerged portions of the intake structure, rapidly restricting water flow.

the various ty

pes of icing that are typicall y encountered at CWI Ss as well as the design features and operational measures that facilities use to minimize the effect of icing on maintaining normal operations. Lastl y, it describes the measures used ess icing at facilities EPA visited in preparation for the Phase II rulemaking.
Depending on the meteorological and hydrological conditions at the CWI S, several different t ypes of ice may form. Each type behave s differently and has a differ

1

common) , must be designed and maintained to handle ice

can threaten CWI S urface ice may

develop in thick sheets and damage intake ice may drift and collect in front of an intake screen,

l fragments of ice may Under the right conditions, ice can form below the surface and collect on submerged portions of the intake structure,

the various ty

pes of icing that are typicall y encountered at CWI Ss as well as the design features and operational measures that facilities use to minimize the Lastl y, it describes the measures used

Phase II rulemaking.
Depending on the meteorological and hydrological conditions at the CWI S, several a differ ent impact2

Ice Formation Ice begins to form when water temperature is reduced to 32°F and the water continues to lose heat to the atmosphere. For pure water, supercooling 1 to temperatures below freezing is necessary

to start ice formation, but with natural water, the required supercooling is much less. There are two general types of ice formation: static ice and d yn amic ice. Static ice forms where natural water loses heat to the atmosphere and there is little or no turbulence, supercooled water and existing surface ice cry stals are not carried to a significant depth, and the result is the formation of a lay er of surface ice ( as opposed to frazil ice , described below ) a bove the liquid water

. Dy namic ice formation occurs in turbulent water such as areas of greatest flow in rivers and lakes mixed by

wind and wave action. Under turbulent conditions , uniform cooling of a large fraction of the waterbod y can occur. If the i nitial water temperature is slightly above freezing and cooling is rapid, a small amount of supercooling occurs, and small, disk

- shaped cry stals form and are distributed throughout the turbulent mass. These small cr ys tals are the initial stage of frazil i ce production. Other ice forms can develop from this initial ice production in sizable quantities. Surface Ice Surface ice is simply

blocks of ice floating in the source waterbody

. In some locations , surface ice and ice floes can create a structural h azard to exposed intakes. On lakes, an accumulation of wind

- driven ice floes near a shore line intake can produce a deep, nearl y solid lay er of ice restricting or completel y blocking intake ports. Under such conditions, reliable intake operation becomes e xtremel y difficult

. Ice jams can cause partial or complete blockage of river intakes. Jams below a river intake can also cause extremely high river stages, and an upstream jam can produce low water levels at the intake location, reducing its capacity

. S urface ice formation reduces heat loss from the water and usually prevents frazil ice formation. Frazil Ice Frazil ice is composed of small, free

- flowing ice cry stals that form in supercooled water and, without sufficient buoy ancy , can be carried through out the water column by turbulence. Frazil ice formed under d yn amic conditions adversely affects hy draulic characteristics of intakes. 2

1 Supercooled w ater has a temperature below the freezing point but remains in a liquid state. 2 Note that frazil is not the same as anchor ice, a generic term used to describe ice crystals that form underwater and adhere to each other and a submerged object, suc h as rocks or intake structures

.3

Two kinds of frazil ice have been identified: active and inactive. Freshly formed frazil cr ys tals dispersed in super cooled water and growing in size are in an active state. When in this condition, they

will readily

adhere to underwater objects such as intake screens or rocks. Frazil ice production and adhesiveness are associated with the degree of supercooling, which is related to the cooling rate of the water mass. Frazil ice particles remain in an active, adhesive state for onl y a short time after their formation. With the reduction of supercooling and the return of water to 32 ° F, frazil cry stals stop growing and change to an inactive, or passive, state. Passive frazil has no adhesive properties and is therefore less troublesome. Frazil ice has been aptl y called the “invi sible strangler.” When conditions favor its formation, the rate of buildup on underwater objects can be rapid; frazil ice formation can reduce an intake’s capacity substantially

or clog it completely

in a few hours. The climatological conditions that enc ourage frazil ice formation are:

 Clear night sk y  Air temperature of 9.4 ° F or less
 Day time water temperature of 32.4 ° F or less
 Cooling rate greater than 0.01 ° F per hour
 Wind speed greater than 10 mph at water surface Frazil ice generall y accumulates in the late evening or earl y morning hours and seldom lasts past noon. Conditions favorable to frazil ice formation vary considerably

from site to site, making it difficult to use weather data alone as a forecasting tool. Design Features and Operational Measures to Minimize the Effects of Icing Facilities can use a specific intake design to combat ice formation or they may use operational measures to prevent ice buildup. Desi gn Features In man y cases, locating the intak e at a significant depth is a simple method to reduce problems with icing. Research and experience on the Great Lakes and elsewhere indicate that the location and design features of submerged intakes can reduce intake ice problems, but probably

not comple tely

eliminate them. Submerging lake intakes in deep water and sizing inlet ports for a velocit y of 0.3 ft/s or less minimizes the amount of frazil ice transported downward into the structure. However, during winter storms, strong wind and wave action carry

ice cry stals and supercooled water to considerable depths, making accumulation of ice on and around the intake likely. Alternativel y, a facility can promote the formation of surface ice to prevent frazil ice. At the Billings, Montana waterworks, frazil ice was a severe winter problem in the turbulent Yellowstone River. In this case, the solution was to enlarge an off

- river intake channel4

into an earthen forebay

with a retention time of approximately one hour. Surface ice formed on the newly quiesce nt forebay

. This insulating ice cover prevented the formation of additional ice and provided the opportunity for river frazil carried into the forebay to combine with the surface ice and revert to a passive condition. To prevent ( or at least minimize ) ic e clogging, the intake structure can be constructed with materials with a low heat transfer rate and smooth surfaces to prevent the accumulation of ice cry stals. Metals such as steel are more susceptible to frazil ice formation because they

have a high heat conductivity and act as a sink for the latent heat released with the ice begins to form , which encourages ice buildup. In contrast, ice does not readily

cry stallize or grow rapidl y on wood or plastic

-- s creens can be constructed using fiberglass

- reinf

orced plastic with low thermal conductivity and a smooth surface. Any exposed metal surfaces can be coated with an inert material such as black epoxy paint to effect better thermal properties and to increase radiation heat gain. Heated screen elements have also been used successfully at power plant installations (see examples below)

. In these cases, screen elements (such as the bars in a trash rack) are wrapped in conductive material, where an electric current is converted to heat. Frazil ice does not a dhere to objects whose temperature is slightly above the freezing point. Operational Measures Facilit y operators can employ a number of operational measures to address icing. These measures generally

fall into one of three categories: phy sical measures , thermal measures, and agitation. Phy sical measures are most common with surface ice; barges or other structures are placed in the waterbod y t o guide floating debris and ice away from the CWIS. A facility might also use a curtain wall or floating boom t o keep surface ice away

. Once ice has formed on or collected near the CWI S, operators can also manuall y remove ice with hand tools or operate a mechanical rake or other cleaning mechanism. Thermal measures are commonly used by power plants. The ty

pical method is to construct a pipe from the discharge of the condensers to carry a portion of the heated effluent back to the CWIS. The hot water, in combination with the agitation from the discharge, acts to preve nt ice buildup on the screens. A facility may also backflush its cooling water flow, which would serve to route warmer water back to the CWI S. Agitation is often important in control ling frazil ice at an offshore intake structure . A facility can injecting steam or compressed air at the intake opening and backflush the intake with settled water. Site Visit Data As part of the process in revising the 316(b) regulations, EPA has conducted dozens of site visits to power plants across the country , including facilities in northern climates5

where ic ing is a significant concern. The rulemaking record contains the full set of cite visit reports, but a summary of the methods used by

sites that EPA visited is presented below. Midwest EPA visited several facilities in Nebraska, Kansas, and Missouri and gathered data on several more

. Most facilities stated that they experience significant icing; these facilities ty picall y have more problems with floes of surface ice, which can collect near the intake and block water withdrawals. Intake screens at these facilities can also form ice (especially

in the open space of the screen panels), making them heavier, more difficult to operate, and less able to provide cooling water. Despite the threat of ice, facilities in the Midwest have adapted their operatio ns and rarel y experience screen failures or unit shutdowns due to icing

. 3 At least seven of these facilities recirculate heated effluent to their CWI S to prevent ice buildup. At least three facilities also use an ice deflector or barge to divert floating ice away from the CWI S. Some facilities also have desilting spray s or other devices designed to expel sediment that accumulates in the area of the intake screens; these technologies could also be used to agitate the source water or push ice floes away fr om the CWI S. Great Lakes EPA visited three facilities located on the Great Lakes. Frazil ice is the more common problem here, as winds on the lakes create a more turbulent environment. Additionally , many CWI Ss in the Great L akes are located offshore, where surface ice would not be a concern. Two of these facilities are able to recirculate heated effluent to the offshore CWI S to inhibit ice buildup; the third uses a heated bar rack. Northeast EPA also visited a number of sites in the northeastern U.S. None mentioned any significant problems with icing, but one did note that it is forced to operate the air sparge on its wedgewire screens more often in the winter to prevent ice buildup on the screen. Other Sites EPA also visited sites in the Mid

- Atlantic and southeast, but these facilities did not indicate an y problems with icing.

3 One facility did note, how ever, that w ater levels were abnormally low due to an ice dam upstream of the facility, forcing them to shut down one generating unit.6

Conclusions M any facilities face challenging operating conditions and icing is a significant concern to many power plants. However, as evidenced by EPA’s site visits, power plant operators have installed design features and operational measures to reduce the impact that icing can have on their ability

to generate power. These technologies are in common use across the affected regions and are known to be effective.