
                                  MEMORANDUM

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 Water Intake Structures

In cold climates (where prolonged periods of sub-zero air temperatures are common), cooling water intake structures (CWISs) must be designed and maintained to handle ice accumulation on and around critical intake components.  Ice can threaten CWIS operations in multiple ways; surface ice may develop in thick sheets and damage intake screens and equipment, blocks of ice may drift and collect in front of an intake screen, blocking water flow and damaging the screens or pumps, or small fragments of ice may become lodged in the open space of the intake screen.  Under the right conditions, ice can also form below the surface and collect on submerged portions of the intake structure, rapidly restricting water flow.  

This memo describes the various types of icing that are typically encountered at CWISs as well as the design features and operational measures that facilities use to minimize the effect of icing on maintaining normal operations.  Lastly, it describes the measures used to address icing at facilities EPA visited in preparation for the Phase II rulemaking.

Types of Ice

Depending on the meteorological and hydrological conditions at the CWIS, several different types of ice may form.  Each type behaves differently and has a different impact on CWIS operations. 

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 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 dynamic 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 crystals are not carried to a significant depth, and the result is the formation of a layer of surface ice (as opposed to frazil ice, described below) above the liquid water.

Dynamic 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 waterbody can occur.  If the initial water temperature is slightly above freezing and cooling is rapid, a small amount of supercooling occurs, and small, disk-shaped crystals form and are distributed throughout the turbulent mass.  These small crystals are the initial stage of frazil ice 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 hazard to exposed intakes.  On lakes, an accumulation of wind-driven ice floes near a shoreline intake can produce a deep, nearly solid layer of ice restricting or completely blocking intake ports.  Under such conditions, reliable intake operation becomes extremely 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.

Surface 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 crystals that form in supercooled water and, without sufficient buoyancy, can be carried throughout the water column by turbulence.   Frazil ice formed under dynamic conditions adversely affects hydraulic characteristics of intakes.

Two kinds of frazil ice have been identified: active and inactive.  Freshly formed frazil crystals dispersed in supercooled 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 only a short time after their formation.  With the reduction of supercooling and the return of water to 32°F, frazil crystals 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 aptly called the "invisible 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 encourage frazil ice formation are:

    # Clear night sky
    # Air temperature of 9.4°F or less
    # Daytime 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 generally accumulates in the late evening or early 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.

Design Features

In many cases, locating the intake 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 completely eliminate them.  Submerging lake intakes in deep water and sizing inlet ports for a velocity 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 crystals and supercooled water to considerable depths, making accumulation of ice on and around the intake likely. 

Alternatively, 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 channel into an earthen forebay with a retention time of approximately one hour.  Surface ice formed on the newly quiescent 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) ice clogging, the intake structure can be constructed with materials with a low heat transfer rate and smooth surfaces to prevent the accumulation of ice crystals.  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 crystallize or grow rapidly on wood or plastic--screens can be constructed using fiberglass-reinforced 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 adhere to objects whose temperature is slightly above the freezing point.
 
Operational Measures

Facility operators can employ a number of operational measures to address icing.  These measures generally fall into one of three categories: physical measures, thermal measures, and agitation.

Physical measures are most common with surface ice; barges or other structures are placed in the waterbody to guide floating debris and ice away from the CWIS.  A facility might also use a curtain wall or floating boom to keep surface ice away.  Once ice has formed on or collected near the CWIS, operators can also manually remove ice with hand tools or operate a mechanical rake or other cleaning mechanism.

Thermal measures are commonly used by power plants.  The typical 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 prevent ice buildup on the screens.  A facility may also backflush its cooling water flow, which would serve to route warmer water back to the CWIS.

Agitation is often important in controlling 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 climates where icing 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 typically 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 operations and rarely experience screen failures or unit shutdowns due to icing.  At least seven of these facilities recirculate heated effluent to their CWIS to prevent ice buildup.  At least three facilities also use an ice deflector or barge to divert floating ice away from the CWIS.

Some facilities also have desilting sprays 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 from the CWIS.

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 CWISs in the Great Lakes are located offshore, where surface ice would not be a concern.

Two of these facilities are able to recirculate heated effluent to the offshore CWIS 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 any problems with icing.

Conclusions

Many 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.

