Blanketing of Boiler Feed Water Tank with Floating Balls? 5

[Htech01]

I am currently doing a study for an economical way to blanket a 5,000 M3 Boiler Feed Water tank. The tank doesn't have a blanketing system at the moment, and is atmospheric tank breathing to air. Therefore the plant experiences corrosion problems caused by the dissolved Oxygen in the water which was introduced by contacting with air at the tank.
The usual way to improve this, I think, is a Nitrogen Blankeing System. But someone says he heard about using floating balls in lieu of N2 Blankting system. To a little bit more explain this: to add floating balls on top of the water in the tank by a few layers, and this floating balls reduces the contact of water with air and thus serves as a simple blanketing system. If this works it will be much cheaper than the N2 blanketing system to operate.

Does anyone out there hear about this? or know anything about this?

It sounds like to work, but it does not completely prevent air contact and thus still have some Oxygen dissolve into the water. Then the question is how much will be the Oxygen content, or by using certain floating balls how much contact surface will be reduced in percentage?

Thanks,
HTech01

 

[Htech01]

The water is deoxygenated.
I found some vendors for these balls.
The material used for the balls are usually HDPE, PP or PVDF depending on the temperature and application.

 

[RayA1004]

Do you have the option to use a low-pressure steam blanket instead of Nitrogen? Maintain a slight positive pressure on the tank to exclude air.

 

[Htech01]

Yes, a steam blanket is one of the options.
But I think the nitrogen blanket would be more cost effective than the steam blanket.

The reason:
For steam blanket, I am thinking to purge steam from the bottom. Even so, to make the steam blanketing to be effective - to prevent air leak from outside - I have to maintain the positive steam pressure inside the tank which means I have to feed enough steam to maintain the temperature above 100deg-C. Considering the tank operation temperature of 80deg-C, I have to feed a lot of steam.

Any comments?

 

[Compositepro]

As I suggested before you can inject steam just below the water surface which is covered by balls. The surface will be 100C. The balls will minimize heat loss. The water at the bottom of the tank can be cooler. The balls, with steam injection, will act as the barrier you seek and not use a lot of energy.

 

[Millerd1]

Consider adding a floating roof to your tank. Insulating it will reduce heat loss, and the roof itself prevents air contacting the liquid surface.

A roof does the same thing as the floating balls, but eliminates the water surface exposed between the ball circumferences.

Most tank manufacturers can add a floating roof to an existing tank. It will cost 2-3 ft of tank volume to add one.

 

[bimr]

The use of steam is probably not a good idea. Large storage tanks are usually designed for atmospheric pressure. You put your boiler feedwater tank at risk of a collapse if your low pressure steam condenses. It happens all of the time when people are trying to clean a tank with steam.

http://www.trainorders.com/discussion/read.php?1,1894873

Floating balls has been widely used conserve heat in open tanks. However, floating balls will not be effective for this application, especially when one is trying to maintain ppb oxygen concentrations.

In the applications that use floating balls, the tank fluid level is a constant. The tank water level is not a constant in a boiler feedwater tank. When the boiler feedwater tank water level drops during operation, the exposed walls of the tank will be covered with water. Once the water has absorbed the oxygen and carbon dioxide from the air, you will have the tank corrosion that you are trying to prevent.

You also will have a problem when you lay up your unit. How do you propose to keep the oxygen out when the tank is drained?

Adding the balls to the tank would also provide a place that will allow microorganisms and crud to accumulate.

 

[gr2vessels]

Internal floating roof perhaps? One off investment, no continuous N2 supply. Or perhaps increase the Oxygen content to a controlled level (refer to Russian literature).
My two cents..
gr2vessels