Clearing frazil ice in place - river intake 1

[aktill]

Hi folks,

I'm working on a de-icing system for a northern Alberta river intake. Basically it has to ensure that a series of fish screens in the intake structure remain clear during the danger periods for frazil ice accumulation (just after spring thaw and just prior to fall freeze-up from what I understand).

Since Alberta Environment is insisting that these screens be mounted flush with the outer wall of the intake chamber, I have nightmares of not being able to pull them up if they freeze!

As a result, I'm hoping to run a hot water flush system to the screens, which would ideally thaw them out (connected to a portable boiler of some description). I was thinking that a header running along the tops of the screens could feed a series of spray nozzles with enough potenency to have warm water reach the bottom of the screen.

The big problem I'm facing right now is nozzle selection - is this even a feasible application? Since the incoming water will be just above freezing, it will take a fair amount of hot water to make much of an impact on the screen temperature.

The screens are about 3m x 2m, and the worst case scenario for flow through intake pathways is 1400m3/hr per screen. We could ask the operators to reduce this for the couple of weeks in question, in hopes of being able to keep the screens clear.

Any help immensely appreciated - there's not much out there on combatting frazil ice.

 

[dicksewerrat]

Can you turn the gates into radiators? Make new ones out of one inch steel ppe. and pump your hot water through the new gates and return it to the heater. This would keep the ice down and save a lot of money in fuel costs.

 

[flamby]

Prevention is better than a cure. What prevents water from freezing has not only to be heat, keeping the water moving around is also as effective.

If you can install a piping, nozzle and pumping system which keeps the water moving in areas of concern, you can effectively prevent the water from freezing. Use cold water in normal case and hot water will only be necessary when freezing has taken place, hence an energy-efficient solution.

Ciao.

 

[BobPE]

flamby:

frazil ice is a bit different than normal ice and moving it around does not help much from stopping it sticking to things. Frazil ice is super cooled water in an unfrozen water stream and has strange properties. Like microwaving a cup of water and sticking something in it, the water immediately boils against what you put into it. I often imagine it as lumps of tar in the water, when the lumps impact things it sticks like glue. Frazil ice is similar and the problem is that the impaction to things like screens is unpredictable. There are many theories out there and specific solutions should only be reached on a case by case basis. If you braeak up the flow, you only have smaller lumps of frazil forming water that may actually build up quicker on surfaces.

Let us know more about your intake and the problems you are having and we might be able to help more...

BobPE

 

[aktill]

Hi folks,

Thanks for the input so far, very appreciated. I'll try to answer you questions as best I can.

Re: gates as radiators. The isolation gates are actually downstream of the fish screens by about a meter, so any heat they might give off would be lost to the incoming water flow without impacting the screens much.

Re: Circulation. The intake is almost constantly in use, so having water flowing through the screens all the time isn't an issue. If an intake port needs to be shut down, we'd shut the gate and likely pull the screen up to prevent screen icing...though that may just cause frazil accumulation on the face of the gate. But like Bob said, frazil is a different sort of creature.

To give some more info:

The intake is being designed at the moment, so nothing actually exists yet - I'm just trying to head off what I can see as an operational problem for the future. It consists of (will consist of?) six intake ports, with fish screens upstream of isolation slide gates.

My new thought is based on a recomendation from the US Army report on Frazil Ice Blockage of Intake Trash Racks...keep enough warm water flowing into the intake stream so that the frazil won't stick.

Since this is an isolated pump station, there's no source of hot process water, so I'm looking at using a portable boiler that only gets hooked up during frazil danger times. Whether it's a lot of warm water or a little hot water seems to be dependant on what sort of rental boilers are available.

Hopefully that clears things up a bit. Seems like a better idea to me to try to prevent buildup rather than thaw frozen screens, since it's the more energy efficient solution. I'm also led to believe that monitoring differntial pressure across the screens won't yield a quick enough response time to avoid blockage, since it's "too late" by the time a significant reading is noted.

 

[dicksewerrat]

Why can't the fish screens be radiators? Even if the hole size is small. 1/2 copper or stell pipe can be put together in almost any configurartion.

 

[aktill]

They can't be radiators because they need to be easily removable for debris removal, and because they sit about 11m below floor level. Hooking up the requisite piping to the screens each time would be difficult.

As a final complication, the regulations regarding fish screen bar spacing and through-slot velocities is very tight, making alterations very difficult.

I think having some form of standalone system is required.