gas-fired emergency genset - issues?

[lielec11]

A mechanical engineer I'm working with seems to believe that the fuel supply for an EMERGENCY generator cannot be natural gas, but that it's fine for a STANDBY generator. I'm looking through NFPA 110 and can't seem to find anything that states this. It mentions gas-fired prime movers, but are they talking about natural gas or liquid gasoline? If there are any other codes that might disallow natural gas fired emergency generators let me know so I can look into it but I don't believe it to be the case. Thanks.

 

[steve66]

Gas fired means natural gas.

I believe the NEC states that gas fired emergency generators are permitted where allowed and approved by the AHJ.

For an area where earthquakes are possible, a natural gas fired generator would not be a good idea, since an earthquake could easily damage electrical lines and natural gas lines at the same time.

A standby generator is completely optional, so it could be whatever.

 

[GoldDigger]

If there is only a single source of natural gas for the generator, it may still be allowed if the utility can give suitable assurance of the reliability of that supply.
Otherwise a dual fuel generator with local fuel supply may be required.

 

[lielec11]

Thank you both for you replies. I figured it will come down to the AHJ. Looking into the NEC, I believe 700.12(B) is what GoldDigger is referring to.

 

[paulengr]

A mechanical engineer I'm working with seems to believe that the fuel supply for an EMERGENCY generator cannot be natural gas, but that it's fine for a STANDBY generator. I'm looking through NFPA 110 and can't seem to find anything that states this. It mentions gas-fired prime movers, but are they talking about natural gas or liquid gasoline? If there are any other codes that might disallow natural gas fired emergency generators let me know so I can look into it but I don't believe it to be the case. Thanks.

The basic issue as pointed out is a question of what specific events are you trying to plan for. In many areas the possibility of losing both electrical power AND natural gas simultaneously may be low while for others it may be quite high. If that is not an issue then long term maintenance on a natural gas generator is a lot less painful.

The second issue is that of standby vs. emergency generators. A standby generator can be as simple as one that you go down to the local hardware store to purchase and plug into it. An emergency generator has a number of requirements such as an automatic transfer switch, automatic or manual testing/exercising, and so forth. A standby generator really has very minor requirements and you can implement as many or all of the features that you get with an emergency generator except one. You cannot use short-circuit only protection with a standby generator. Overload protection is required. One could argue the idea that being able to run an emergency generator for just a few more minutes while the generator coils, feeder cables, etc., are burned up and destroyed while the load itself melts from pulling too much current under a low voltage condition, but on an extremely theoretical basis the whole idea is to provide backup power in an emergency at any cost.

So from a practical point of view you can simply implement the generator as a standby generator and do absolutely everything that the emergency generator rating gives you save the ability to skip out on some electrical protective relaying, or you can go the emergency generator route and deal with all of the additional requirements.

This is akin to putting in emergency stop buttons that have a lot of Code requirements under OSHA as opposed to putting in "Stop" buttons that do exactly the same thing and have no special regulatory status and are completely optional except for one special case (presses).

 

[iwire]

So from a practical point of view you can simply implement the generator as a standby generator and do absolutely everything that the emergency generator rating gives you save the ability to skip out on some electrical protective relaying, or you can go the emergency generator route and deal with all of the additional requirements.

This is akin to putting in emergency stop buttons that have a lot of Code requirements under OSHA as opposed to putting in "Stop" buttons that do exactly the same thing and have no special regulatory status and are completely optional except for one special case (presses).

I do not understand this, a standby generator cannot do the same thing as an emergency generator when emergency circuits are required.

 

[GoldDigger]

I do not understand this, a standby generator cannot do the same thing as an emergency generator when emergency circuits are required.

I think what paulengr is saying is that if all you legally need is an optional standby generator you will be wasting money if you specify an emergency generator instead.

 

[iwire]

I think what paulengr is saying is that if all you legally need is an optional standby generator you will be wasting money if you specify an emergency generator instead.

Seems a bit obvious but maybe.

 

[John120/240]

If there is only a single source of natural gas for the generator, it may still be allowed if the utility can give suitable assurance of the reliability of that supply.
Otherwise a dual fuel generator with local fuel supply may be required.

When I was in Maintenance at the nursing home we had a letter on file from the natural gas supplier. It stated their reliability record, no outages in so many years, blah, blah etc.

 

[petersonra]

I think that the codes should be changed to require more fuel for emergency generators, or at least dual fuel capability so they can run off NG instead of relying on a finite fuel source. My understanding is at least one of the hospitals in town only has a few hours of diesel for its gensets.

 

[iwire]

I think that the codes should be changed to require more fuel for emergency generators, or at least dual fuel capability so they can run off NG instead of relying on a finite fuel source. My understanding is at least one of the hospitals in town only has a few hours of diesel for its gensets.

I cant say for hospitals but in general emergency gensets 'shall be provided with an on-premises fuel supply sufficient for not less than 2 hours full-demand operation of the system' is all that is required.

My understanding is that emergency generators are for evacuation not maintaining operation.

 

[kwired]

I think that the codes should be changed to require more fuel for emergency generators, or at least dual fuel capability so they can run off NG instead of relying on a finite fuel source. My understanding is at least one of the hospitals in town only has a few hours of diesel for its gensets.

That needs consideration. Most of the country NG is probably pretty reliable and fairly infinite availability, but propane, gasoline, diesel, are typically trucked to the site and not distributed via permanent pipelines, and have a finite supply without additional intervention.

It is primarily earthquake prone areas where you have greatest risk of losing both electric utility power and utility provided natural gas at same time.

 

[iwire]

That needs consideration. Most of the country NG is probably pretty reliable and fairly infinite availability, but propane, gasoline, diesel, are typically trucked to the site and not distributed via permanent pipelines, and have a finite supply without additional intervention.

It is primarily earthquake prone areas where you have greatest risk of losing both electric utility power and utility provided natural gas at same time.

Again, I don't belive it is the NFPAs postion that emergency generators need to run for the duration of a disaster. The person paying the bills can certainly choose to do so if they wish.

 

[petersonra]

I cant say for hospitals but in general emergency gensets 'shall be provided with an on-premises fuel supply sufficient for not less than 2 hours full-demand operation of the system' is all that is required.

My understanding is that emergency generators are for evacuation not maintaining operation.

That makes sense for a shopping mall. You can't evacuate a hospital in 2 hours. I wonder if there are longer times required for facilities like hospitals.

 

[JFletcher]

My understanding is that emergency generators are for evacuation not maintaining operation.

Depends. One of the waste water plants I used to operate had 2 8,000 gallon diesel tanks to give ~80 hours of operation (195g/hr consumption). They were also used during hot summer days to go off grid per the POCO's request, usually from 2pm-9pm.

 

[iwire]

Depends. One of the waste water plants I used to operate had 2 8,000 gallon diesel tanks to give ~80 hours of operation (195g/hr consumption). They were also used during hot summer days to go off grid per the POCO's request, usually from 2pm-9pm.

The generator supplying the pumps would not be an article 700 emergency load to the NEC.

 

[iwire]

I wonder if there are longer times required for facilities like hospitals.

As do I.

 

[kwired]

The generator supplying the pumps would not be an article 700 emergency load to the NEC.

but depending on what happens when that plant is down, it may be still be an emergency for someone.

Small village lift station manager told me a while back the most effective alarm they had on their lift station is when one of the closest users to the station calls to complain about sewer backup in their basement

 

[iwire]

but depending on what happens when that plant is down, it may be still be an emergency for someone.

Why is it you always make me feel like I am banging my head against a brick wall?

Let's look at the OPs post.

A mechanical engineer I'm working with seems to believe that the fuel supply for an EMERGENCY generator cannot be natural gas, but that it's fine for a STANDBY generator.

If we are talking code an emergency generator is a certain thing determined by the NEC / AHJ not the user.

What you are talking about, barring other codes is an optional standby or maybe legally required
system and as such has no fuel requirements at all.

The fuel supply and type is up to the user / bill payer.

 

[don_resqcapt19]

but depending on what happens when that plant is down, it may be still be an emergency for someone.

Small village lift station manager told me a while back the most effective alarm they had on their lift station is when one of the closest users to the station calls to complain about sewer backup in their basement

But just not an Article 700 "emergency" and maybe not even an Article 701 "emergency".