CNG Vessel Sizing

[Strategery]

I am interested in a Compressed Natural Gas (CNG) vessel to supply natural gas to heat a building because the piping from our existing infrastructure is far away and it's going to cost an insane amount in scaffolding, etc. to pipe it, there. All I know is that the building's HVAC units require a total of about 34m³/hr of gas (1200ft³/hr). I would like the vessel sized to last something near 4 weeks.
My pathetic Google searching keeps showing wet capacity of vessels and "gas capacities" that don't really mean much without the pressure. I suppose my 34m³/hr also does not mean much without a pressure; but, IIRC, gas is delivered to heaters at barely any pressure at all, isn't it? Something like 1psi? Arbitrarily close to atmospheric?

My crack at it would be something along the lines of (24hr/day)(30days/month)(34m³/hr) = 24480 m³ of atmospheric gas per month.
Then, convert that gas to a volume of liquid by looking up what pressure natural gas liquifies at. Find a vessel that is 1.25x the wet volume of natural gas (because you can't fill them >80%). Verify that it is rated for a suitable pressure and see what it costs.
Profit?

Please point out my errors and mock or flame accordingly.
Thanks!

 

[chicopee]

You may want to contact an area distributor that would be willing to do the engineering, installation and supply. For instance, in my region, a chemical plant owner was approached by a LNG distributor to install all piping, associated controls and bulk heads and to deliver LNG by trucking in the product 90 miles from the distributor source. LNG is stored in two tanker trailers connected to the LNG bulk head and delivery is made periodically each week. This offer by the LNG distributor was presented after the plant owner had invested in the installation of a 30,000 gallon propane tank. Try a similar approach.

 

[zdas04]

First off I'm going to assume that your units are SCM and SCF. Your first step is OK-ish, but then you veer from the track. The 30 days/month term doesn't add any value. Just multiply your volume/day times number of days. I get 806,870 SCF [22,848 SCM]. You will not liquify the natural gas in this process, just raise it to a very high gas pressure.

Density of methane at 14.73 psia [101.56 kPaa] and 60F [15.56 C] is 0.0451 lbm/SCF [0.723 kg/SCM] (assuming compressibility is negligible). CNG is stored at a nominal 3600 psia [24.8 MPaa] and 70F [21.1 C] so the density is 12.346 lbm/ft^3 [197.7 kg/m³] (calculating compressibility as 0.876). This is all you need. The volume of the vessel (with no safety factor) is:

Vol_receiver = Q*ρ_SCF/ρ_CNG=2950 ft³ [84 m³]

If you pick a 90-inch [2.3 m] diameter receiver then it would have to be 67 ft [20.3 m] long. Getting a vessel rated for CNG pressures much bigger than 90 inch is outrageously expensive, but then getting a 90-inch pressure vessel 67 ft long would be tens of millions of dollars.

The storage gets smaller with LNG, but sources of LNG and the gassification equipment for your load are hard to come by. There is a reason that people pay the cost to run pipe to their buildings.



David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[EdStainless]

The cost/difficulty of getting LNG very greatly by location.
In some locals LPG (propane, butane) sell for the same cost per BTU as nat gas.
I would check with suppliers and see what they can offer.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[georgeverghese]

Due to the volume and pressure of this flammable gas inventory, this unit will have to kept some distance away from this building - check with your safety engineer how far away. It will also need road access for removing spent vessels and installation of a fully pressurised charge. Operational safety problems during removal and reload also. All these taken into account ?

 

[LittleInch]

Dave has the numbers right, but not the cost. CNG systems are now made in their thousands to transport gas in India and Asia.A standard trailer holds about 20m3 volume at 200 to 250 bar. So for a month you need about 4 or 5 trailers.

How much? Don't know but they often make these things from fibre glass a multiple small tubes.

Search CNG containers.

Here's an example.

http://www.weldship.com/about-us/recent-news-and-events1.html

Don't confuse LNG, LPG with CNG. LNG is not currently available in that size of system, LPG is, but usually more expensive than gas. CNG is now very common in lots of places so the cost of equipment is low and safety issues have been worked out.

Let us know if you're going ahead.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[zdas04]

That configuration makes more sense than trying to do it in one vessel. Probably 6 of the 22" X 40' would do the job, I don't know what these tubes cost, but I would expect them to not be inexpensive.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[LittleInch]

What quite a few places do is have a set of multiple tanks and then get regular deliveries of cng. The first bit just flows across then they plug in a compressor to pull the gas volume down from the trailer into storage. Lots of tanker loads though for the OP sized load, pipelines are a much better idea.....

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Strategery]

zdas04: I should have said LNG from the start. I mentioned calculating the liquid volume in the post because I was pretty sure that a CNG vessel would be WAY too big; but, I put in a misleading thread title.
I think the idea is dead before it's off the ground. We are unfortunately stuck with Natural Gas which is infuriating me that those selections are set in stone when propane should work just as well and is super prevalent.
I pictured the LNG equivalent of a rental propane bullet that a company like Superior could supply and fill in required intervals. The filling is what kills it as I don't think there are any LNG trucks that can supply us in our region.