LP generators for Emergency applications

Status
Not open for further replies.
mshields

mshields

Senior Member
Location
Boston, MA
Is there any reason why an LP gas generator couldn't be used to serve Article 700 loads.

I know that with natural gas, you need to get a green light from the AHJ in accordance with 700.12. With LP, the fuel is on site so this isn't an issue.

Will LP generators start in 10 seconds?

Thanks,

Mike
 
My first question is why are you considering LPG instead of diesel? While LPG does have an almost unlimited shelf life and starts well in cold areas, it's operating expense is higher in terms of maintenance and cost per volume. That aside, as long as the LPG isn't tied to the utility/grid, you should be fine. I am assuming you have a dedicated LPG tank for the generator.

As far as startup time, that's a good question. Many of the LPG generators run at a higher rpm to account for the lower energy density that LPG has, (this is why they typically have higher maintenance costs and a shorter TBO). This higher rpm may take longer to get to before it can engage the transfer switch. You would certainly want to check with the manufacturer on that.
 
rlundsrud said:
As far as startup time, that's a good question. Many of the LPG generators run at a higher rpm to account for the lower energy density that LPG has, (this is why they typically have higher maintenance costs and a shorter TBO). This higher rpm may take longer to get to before it can engage the transfer switch. You would certainly want to check with the manufacturer on that.
Click to expand...

So what speed do backup diesel gensets typically run at versus LP gensets? How about NG gensets?

note: this is somewhat of a trick question.
 
rlundsrud said:
My first question is why are you considering LPG instead of diesel? While LPG does have an almost unlimited shelf life and starts well in cold areas, it's operating expense is higher in terms of maintenance and cost per volume. That aside, as long as the LPG isn't tied to the utility/grid, you should be fine. I am assuming you have a dedicated LPG tank for the generator.

As far as startup time, that's a good question. Many of the LPG generators run at a higher rpm to account for the lower energy density that LPG has, (this is why they typically have higher maintenance costs and a shorter TBO). This higher rpm may take longer to get to before it can engage the transfer switch. You would certainly want to check with the manufacturer on that.
Click to expand...
There are a very limited set of possible engine speeds that will produce 60Hz output unless you include gearing between engine and generator head.
What you normally see is that for the same RPM, 3600 or 1800, the LPG fed engine will simply produce less horsepower than the same displacement engine fed with petrol. And the generator rated output will be correspondingly lower.
You may be thinking of inverter type generators where output frequency is not tied to engine RPM.
 
mshields said:
Is there any reason why an LP gas generator couldn't be used to serve Article 700 loads.

I know that with natural gas, you need to get a green light from the AHJ in accordance with 700.12. With LP, the fuel is on site so this isn't an issue.

Will LP generators start in 10 seconds?

Thanks,

Mike
Click to expand...

I believe you need full load in ten seconds. The genset manuf can answer that question.
 
petersonra said:
So what speed do backup diesel gensets typically run at versus LP gensets? How about NG gensets?

note: this is somewhat of a trick question.
Click to expand...

They all run at 1800 rpm for 60 hertz. 1600 for 50 hertz.
Of course that is assuming you aren't talking about the one's that people pull out of their garage and plug their freezers and refrigerators into when power goes out.
 
qcroanoke said:
Of course that is assuming you aren't talking about the one's that people pull out of their garage and plug their freezers and refrigerators into when power goes out.
Click to expand...
I once had a generator that was just a large permanent magnet as the flywheel and fixed output coils. The motor governor (air vane) simultaneously provided frequency and voltage control.
And my "remote stop" feature was to put a shirt on the output.
--carefree youth-- :)
PS: Since it was two pole, the engine ran at 3600rpm.
 
mshields said:
Is there any reason why an LP gas generator couldn't be used to serve Article 700 loads.
Click to expand...

We use natual gas gensets quite often for article 700 loads. Pretty standard practice in MA and RI. Once in a while we have done LP.

Generally we don't go diesel until the KW starts to get higher.
 
iwire said:
We use natual gas gensets quite often for article 700 loads. Pretty standard practice in MA and RI. Once in a while we have done LP.

Generally we don't go diesel until the KW starts to get higher.
Click to expand...


I'm curious as to how that complies with the required onsite, 2 hour fuel supply?
 
GoldDigger said:
I believe that there is an exception if the natural gas supply is "reliable". But I would personally not want to rely on it. :)
Click to expand...

Here's the exception but I'm not sure if anyone would actually accept this. :)

700.12(B)(2) Internal Combustion Engines as Prime Movers.
Where internal combustion engines are used as the prime
mover, an on-site fuel supply shall be provided with an
on-premises fuel supply sufficient for not less than 2 hours?
full-demand operation of the system. Where power is
needed for the operation of the fuel transfer pumps to de-
liver fuel to a generator set day tank, this pump shall be
connected to the emergency power system.
(3) Dual Supplies. Prime movers shall not be solely de-
pendent on a public utility gas system for their fuel supply
or municipal water supply for their cooling systems. Means
shall be provided for automatically transferring from one
fuel supply to another where dual fuel supplies are used.
Exception: Where acceptable to the authority having ju-
risdiction, the use of other than on-site fuels shall be per-
mitted where there is a low probability of a simultaneous
failure of both the off-site fuel delivery system and power
from the outside electrical utility company.
Click to expand...
 
But they do accept it here in literally 1,000s of installations. :)

I believe Don has mentioned he believed natural gas supplies to be reliable.
 
follow up

follow up

I only mentioned the RPM difference as some RV LPG generators operate at 3600 RPM rather than 1800. This way the manufacturer can use the same engine for both LPG and diesel. I just wanted to cover my bases. And yes, a reduction gear is used to spin the generator at 1800 RPM.
 
LP is fine as long as you had 2 hours of it on site.

Just be sure the Article 700 loads can be picked up in 10 seconds. You have 60 seconds for 701 loads and whenever you want for 702 loads. LP and NG gens are not good with large block loads.
 
Status
Not open for further replies.
Top