Diesel Generator Distance from Building

[acraig]

I'm designing a generator system for a WWTP. The generator size will be 750-kW with a 24-hour runtime (approximately 1700 gallons).

The generator will be located outside near the building with the electrical distribution equipment. I'm trying to determine how far off the building I need to be.

I'm mainly finding a minimum of 5 feet through various sources. I've been told to look at NFPA 30, specifically chapter 17. Table 17.4.3 would place me a minimum of 25 feet off the building. Does anyone know the applicability of NFPA specifically, chapter 17, to diesel generators?

Any information would be helpful!

Thanks.

 

[ron]

I believe NFPA 30 and 37 allow you to be 5' away from the building. Look at the standards for free on the NFPA.org website.

 

[JoeStillman]

ASHRAE has rules about how close to a building ventilation air intake you can get with your exhaust. I think it's ten feet. I recommend you check with your mechanical engineers.

 

[kwired]

Some are inside the building so I'd say pretty close depending on other factors.

Most self contained units likely have minimum clearances in their instructions that are more for necessary air circulation reasons than any impact on the building. Distance from exhaust outlet to possible building entry points is most likely main concern other than cooling air circulation.

 

[JoeStillman]

Ventilation = breathing air, not cooling air.

 

[PD1972]

Not sure if you want to classify the generator as part of the processing equipment, but if you do then table 17.4.3 would probably apply? I have never designed for a wastewater facility so not sure how it would be classified. Consider asking the PE or AHJ to determine what is applicable. If nothing else just go with the most conservative approach, which would be chapter 17.

For non-process facilities look at table 22.4.1.1 for ASTs. Sub-base diesel tanks are atmospheric tanks with emergency venting so gauge pressure would be <2.5 psi.

Based on the above, I am not exactly sure how you arrived to a conclusion of 25 feet based off table 17.4.3? I am getting either 15 feet from property line or 5 feet from nearest important building not part of the process (doubled if there isn't fire protection).

 

[herding_cats]

ASHRAE has rules about how close to a building ventilation air intake you can get with your exhaust. I think it's ten feet. I recommend you check with your mechanical engineers.

Absolutely. We moved one at a hospital from complaints from the doctors, nurses. It was terrible from fumes, exhaust and noise.

 

[kwired]

Ventilation = breathing air, not cooling air.

Why not both?

I'm assuming breathing air means both intake and exhaust air.

Put the unit in a closed space that is fairly sealed off from the outside and I bet it possibly overheats before it runs out of oxygen for combustion.

Have seen that happen before with units that had automatic ventilation damper that failed to open, most those however did have exhaust piped to outside. These were rather old school units though. Today they are mostly all self contained/enclosed units intended to be installed outdoors.

 

[JoeStillman]

Why not both?

Not-both only in the context of the OP. the concern there was polluting the ventilation intake with diesel exhaust.

I've done dozens of generators inside the buildings they serve, but it seems to be getting more rare. I've never met an Architect who could take all the relevant design factors into account without an engineer, such as not locating it in the same room as the electric service or how to rig it in and out of the building, how to keep snow and rain from getting sucked in with the huge cfm the generator requires. Skin-tight and walk-in enclosures settle a lot of disputes with the interior decorators.

Air flow is key a major factor for indoor installations. There should be an exhaust fan on a thermostat to cool down (with ventilation air!) after shutdown; all air dampers should open on power failure etc. On more than one occasion, the dampers were powered open and the operators didn't have enough torque to push against the generator radiator fan.

 

[kwired]

Not-both only in the context of the OP. the concern there was polluting the ventilation intake with diesel exhaust.

I've done dozens of generators inside the buildings they serve, but it seems to be getting more rare. I've never met an Architect who could take all the relevant design factors into account without an engineer, such as not locating it in the same room as the electric service or how to rig it in and out of the building, how to keep snow and rain from getting sucked in with the huge cfm the generator requires. Skin-tight and walk-in enclosures settle a lot of disputes with the interior decorators.

Air flow is key a major factor for indoor installations. There should be an exhaust fan on a thermostat to cool down (with ventilation air!) after shutdown; all air dampers should open on power failure etc. On more than one occasion, the dampers were powered open and the operators didn't have enough torque to push against the generator radiator fan.

Ran into a case one time small town municipal POCO that installed four large diesel generators inside a building. The primary purpose of these was to reduce peak demand charges from their power supplier though they could be run in the event of power loss from their supplier. No automatic startup / transfer, but they needed to detect the inlet and exhaust louvers were open before they could be started. Designers didn't account for opening those louvers in the case of total power failure. An after thought add on portable generator unit was sitting in there and one the operators told me all about this and it was there to open louvers during total power failure conditions.

 

[ron]

Ran into a case one time small town municipal POCO that installed four large diesel generators inside a building. The primary purpose of these was to reduce peak demand charges from their power supplier though they could be run in the event of power loss from their supplier. No automatic startup / transfer, but they needed to detect the inlet and exhaust louvers were open before they could be started. Designers didn't account for opening those louvers in the case of total power failure. An after thought add on portable generator unit was sitting in there and one the operators told me all about this and it was there to open louvers during total power failure conditions.

Normally open spring release dampers (or a gravity damper) is the way to go on generators, so you don't need power to open them.

 

[kwired]

Normally open spring release dampers (or a gravity damper) is the way to go on generators, so you don't need power to open them.

The more I think about it, not sure what they have for roof vents, they are rather large and directly over top of each diesel engine, but the inlet is an overhead door at each bay. I'm pretty certain the corresponding door must be open before the engine will start not sure about what the roof vents are designed like.

 

[JoeStillman]

The more I think about it, not sure what they have for roof vents, they are rather large and directly over top of each diesel engine, but the inlet is an overhead door at each bay. I'm pretty certain the corresponding door must be open before the engine will start not sure about what the roof vents are designed like.

I think a skin-tight housing will just have gravity dampers.

 

[topgone]

The more I think about it, not sure what they have for roof vents, they are rather large and directly over top of each diesel engine, but the inlet is an overhead door at each bay. I'm pretty certain the corresponding door must be open before the engine will start not sure about what the roof vents are designed like.

I see a parallel to this dilemma of providing sufficient airflow. In a project a long time ago, I was part of the commissioning team, a compressor house where the personnel were having difficulty opening the main doors to the compressor house because the room pressure was so low compared to the outside atmosphere, and doors just wouldn't open out!
Depending on the size of the generator, care should be taken when designing for airflow requirements to prevent problems, and that includes cooling/ heating.

 

[Birken Vogt]

Care to prevent injury also. Slamming doors in a generator room are no joke. Papers and trash also get sucked into the radiator fan. Even hands and shirts can get sucked into things if they get too close.