Nitrogen vs steam blanket on top of hot water thermal storage tank

[sshakkour]

Hi...
I'm designing a 35 MW hot water stratified thermal storage tank (at 95°C) for an existing district heating system; and there is concern about system corrosion protection. The water will be treated with corrosion inhibitor and other chemicals, but I'm considering to have a blanket at the top of the tank to protect against oxygen dissolving into the water (oxidization?). Do you know of any issue with Nitrogen blanket given the type of application? How do I calculate the amount and pipe size needed? Any design consideration I need to be aware of?
Thanks in advance

 

[cheute79]

There shouldn't be any problem with using N2. You could use a conservation vent on this. You need to also make sure you have a pressure regulator to keep the minimumal nitrogen pressure (say, perhaps 75 mm Hg pressure, max, and maybe half that). the pipe size is based on N2 supply pressure (75 mmHg?), and most importantly on the size of the tank and the rate of draw-down. You only need to add N2 as fast as you remove water from the storage tank.

 

[sshakkour]

Thanks cheute. Now supposing that I need about 15 CFM of N2 and the pressure maintained is 7 mm Hg. How much cylinder volume do I need? How do I size the pipe?

 

[cheute79]

Cylinder volume depends on how often you want to change it out. You might want to consider using liquid N2 rather than compressed gas. Depending on how often you pump out at 15 cfm (~112 gpm), a standard cylinder could last a day or a month. If you continually pump in and out at 112 gpm, then the volume needed for blanketing (or inerting) doesn't change, so the nitrogen flow only compensates for leaks, and a cylinder could last a very long time.

For line size, assume a low flow velocity (minimum friction). Maybe 1-2 fps or less in the nitrogen line. If your pressure control valve (regulator) is quite close (less than 8-10 meters?), you can live with more friction loss. If it is very distant (50 or more meters?), you will want to have more diameter so that the pipe acts as a reservoir, and you have almost no friction loss.

 

[cub3bead]

These systems used to be made by Appalachian Controls. They have since been acquired by Fisher.

The systems were normally designed to maintain the tank at +0.5" WC.

Make sure you have a vacuum relief valve in the system if you are going to try and design you own system. If you don't include one and you get the outflow rate greater than the inflow rate you end up with a very nicely collapsed tank.

One of the biggest demands for the inerting gas will be caused by the diurnal temperature changes, assuming you are able to balance the in and out flow.