LP gas vaporization rates 1

[mbsmith]

I have three 1,000 gallon LP tanks that cannot supply my 1.2 million BTU/HR load demand under 0 to -10 degree F conditions. It is my intent to add as many 1,000 gallon tanks as needed to ensure adequate gas supply. The tanks will be manifolded with two-stage regulation. My only reference states that one half-full 1,000 gallon tank will supply an intermittent withdrawal rate of 525,000 BTU/HR if there is no tank frosting, and that this value should be reduced to 1/4 the intermittent rate for continuous loading. My questions are, 1) how do I evaluate tank frosting, 2) what constitutes continuous loading, and 3) how do I evaluate the tank level (fullness)?

 

we have the same problems with LP in Minnesota. You may want to look into a blanket or a doghouse approach rather than adding more tanks. Also, a tank that is kept above 50 or 60% will have more vapor pressure at any temperature than a tank allowed to deplete down to say 20%.

 

[TD2K]

Tank frosting would be a visual appearance I would think on the outside of the tank. It's an indication that the pressure in the tank is getting low resulting in colder liquid. As the liquid gets colder, it can cause frosting on the outside of the tank.

Continuous would, in my opinion, be LP requirement for more than a short period of time. For a few minutes, you could pull quite a higher rate from the tank but for longer periods, you'd want to use the lower rating (if the LP load was lasted for 1/2 an hour, I'd say that was a 'continuous' load).

The problem is that you are not gaining enough ambient heat to vaporize the LP gas so you can draw it off. 1.2 MMBTU/hr works about to about 60 lbs/hr. To vaporize that, you'd need about 3.2 kW of heat entering your tank(s). You could try putting heat tape on the bottom part of the tank with an inch of insulation and using that heat to vaporize the LP you need. It's a fair chunk of heat tape I realize since this stuff is usually rated for a few watts per foot but you can get higher outputs. One thing is you want to make sure you have a relief valve on the tanks (I'm pretty sure you do and this isn't enough heat to overload them).

For telling the level in the tank, I've heard of a tape you put on your barbeque tank. When you pour hot water on it, the portion on the metal above the liquid warms up more than the metal below the liquid level and you got a colour change. You could check with your LP supplier for more information.

 

I would not suggest heating the storage tank at all if I did understand correctly. Instead, I would take the liquid with a pipe from underside of the storage tank into an independant heater and produce the vapor required. This heater can be investigated from the local suppliers. Even they can design and provide the required heater for you. For the required liquid supply, you need a forwarding pump between storage tank and heater. Thus, you do do not have to consider the tank level.
I suggest to talk to a local LPG loading station designer for the details. They can design and provide all the required materials.
I hope that this helps you.

Ibrahim

 

[butelja]

The typical approach for high volume users is to pull liquid off the tank and vaporize it externally in a heat exchanger. The usual sources of heat are either ambient (air heated) or steam.

 

[TD2K]

I agree that external vaporization is the way to go for large users.

But this user is talking about roughly 60 lb/hr of LP. Electric trace heating of the tank is going to be less expensive and much less complicated to maintain than an pump and vaporizer system. At the end of the day, it's just so many BTUs/hr to vaporize the LP, the question is how to get that heat into the liquid.