natural gas emergency generators

[mshields]

I have in the past sought approval from the AHJ in accordance with Article 700.12 and utilized natural gas generators without issue.

A colleague under similar circumstances has specified gas generators for Article 700 applications only to find that the gas pressure has proven to be problematic. i.e. at certain times of the day, it's fine. At other times, it's a problem. The problem is not that the unit won't run but that it won't start up in the requisite 10 seconds.

He claims he's seen this scenario multiple times and that specifying gas generators for Emergency applications is a bad idea - should never be done - etc.. This seems a bit extreme. I was wondering what the experience has been of this group of "know it alls "

 

[dkidd]

I always avoid it. Many localities won't accept it. A dual-fuel NG propane unit can be used.

 

[petersonra]

I have in the past sought approval from the AHJ in accordance with Article 700.12 and utilized natural gas generators without issue.

A colleague under similar circumstances has specified gas generators for Article 700 applications only to find that the gas pressure has proven to be problematic. i.e. at certain times of the day, it's fine. At other times, it's a problem. The problem is not that the unit won't run but that it won't start up in the requisite 10 seconds.

He claims he's seen this scenario multiple times and that specifying gas generators for Emergency applications is a bad idea - should never be done - etc.. This seems a bit extreme. I was wondering what the experience has been of this group of "know it alls "

I suppose what he is saying is possible. It suggests possibly that whoever sized the utility pipe coming into the installation did a poor job, or maybe a gas regulator was installed that is too small.

Personally, I think NG generators are more reliable than diesel given that the rules on how long it has to be able to run with the fuel found on site is so limited that in any kind of serious weather problem you could find yourself running out of fuel and be unable to get more.

Maybe a dual fuel system is best.

 

[iwire]

We install a lot of article 700 generators supplied with natural gas in both MA and RI.

I even recall removing a very new diesel set to replace it with a natural gas unit due to some wetlands issue or something.

 

[charlie b]

I also have designed Article 700 systems with NG generators. But I have a Plumbing Engineer on my design team, and he makes sure the NG supply is reliable and has sufficient pressure.

 

[dkidd]

In Florida they are afraid of trees uprooting gas lines during a hurricane.

 

[iwire]

In Florida they are afraid of trees uprooting gas lines during a hurricane.

Makes sense.

 

[mshields]

thanks for the quick response

thanks for the quick response

We too have our plumbing guys take a look at it. The concern there is that the inlet on the unit is a constant and represents a bottleneck.

 

[infinity]

On our current project we have a 725 kw and a 1000kw natural gas generators for life safety and optional standby. I'm guessing that if the system is properly designed the fuel supply shouldn't be a problem.

 

[Ingenieur]

We too have our plumbing guys take a look at it. The concern there is that the inlet on the unit is a constant and represents a bottleneck.

The sets are spec'ed for a given pressure at the machine connection
usually down stream of all regulators, etc
so the inlet is never the limiting factor

another way to look at it is equivilent lengths
assume 1000' pipe
900 of 2"
100 of 1"
it's not a 1" restriction or drop
it's a 1000' pipe of 900/1000 x 2" + 100/1000 x 1" = 1.9" pipe

perhaps another dedicated regulator can be tapped off the utility supply side and adjusted to satisfy demand

 

[petersonra]

In Florida they are afraid of trees uprooting gas lines during a hurricane.

What good will a few hours of diesel do you during a hurricane?

 

[infinity]

What good will a few hours of diesel do you during a hurricane?

I think that's something that NYC learned during Sandy. The 2 hours of diesel didn't do much good when the power was out for days. Now we're installing NG generators instead.

 

[mshields]

Thanks

Thanks

thanks

 

[Barbqranch]

Also, is the pressure regulator located near the generator, or is it many feet away where the gas enters the property? Or, is it as I have heard, a system where the pipes in the street are already at low usage pressure (always a bad way to do it, but I believe in some older areas they did that).

 

[shortcircuit2]

Article 700.12(B)(2) and 701.12(B)(2) call for an "on-premises fuel supply"

Article 700.12(B)(3) and 701.12(B)(3) specifically says "shall not be solely dependent on a public utility gas system"

So how do you connect to natural gas?

 

[texie]

Article 700.12(B)(2) and 701.12(B)(2) call for an "on-premises fuel supply"

Article 700.12(B)(3) and 701.12(B)(3) specifically says "shall not be solely dependent on a public utility gas system"

So how do you connect to natural gas?

The Exception allows for this where the AHJ agrees. Many do in cases where the conditions warrant.

One thing to keep in mind though is the fact that gensets used for an Article 700 application must also be certified as NFPA 110 compliant. Many natural gas machines are not, especially when you get into larger machines.

 

[ron]

I think that's something that NYC learned during Sandy. The 2 hours of diesel didn't do much good when the power was out for days. Now we're installing NG generators instead.

FYI, current NYC code based on the 2008 NEC:

Subsection 700.12(B)(2) Revise first sentence and add a FPN to read as follows:
Where internal combustion engines are used as the prime mover, an on-site fuel supply shall be provided sufficient for not less than 6 hours of operation at full demand load.
FPN: Some installations may require more than 6 hours of fuel supply. See Articles 517 and 708.

Only certain circumstances is NYC allowing NG for 700 or 701 generators. Mostly residential highrises.

 

[steve66]

In Florida they are afraid of trees uprooting gas lines during a hurricane.

I also would also recommend not specifying them in California or anywhere a large earthquake is a high probability.

But I've never had a problem with gas pressure. I find it hard to believe the utility won't guarantee some certain pressure. That seems almost as basic as providing 120 volts, and not 110 or 100.

Different generator brands and sizes have different pressure requirements, and of course different delivery pressures are available. So it seems like an engineer could make sure the delivery pressure is suitable for the generator, or even drop the pressure with an additional regulator at the generator.

 

[ron]

I find it hard to believe the utility won't guarantee some certain pressure. That seems almost as basic as providing 120 volts, and not 110 or 100.

Our plumbing eng says that you often can request a non-interruptible service for a higher cost, which is not necessarily uninterruptible, but it is better than a regular gas service.

 

[shortcircuit2]

The Exception allows for this where the AHJ agrees. Many do in cases where the conditions warrant.

The exceptions are to 700.12(B)(3) and 701.12(B)(3)...

AHJ's use this exception also for 700.12(B)(2) and 701.12(B)(2)?