Foundation for a Natural Gas Generator 1

[oengineer]

I have a natural gas generator that requires a foundation to support it. I have obtained a vendor cut-sheet for the generator. The cutsheet does NOT provide the Operating Speed (RPM) or Power Rating (HP) for the natural gas generator.

I was looking through ACI 351.3R-04 Foundations for Dynamic Equipment and notice that they mentioned generators in the document. Would a natural gas generator be considered a "Centrifugal Equipment"? If so, it would mean that the foundation should be sized for 3 times the weight, correct?

Is the type of generator mentioned in ACI 351.3R-04 Foundations for Dynamic Equipment different than a natural gas generator? Would only "Turbine Generators" be consider for either a "Centrifugal" or 'Reciprocating" Equipment Foundations?

Comments/Suggestions are appreciated.

 

[oengineer]

Does a natural gas generator qualify as Dynamic Equipment?

 

[JoshPlumSE]

It depends on what your definition of dynamic equipment is. It probably is, but it's probably light enough that the foundation can just be designed per the "rule of thumb" method.... If the foundation weighs 4 times the weight of the equipment then you should be good. Three times may be good enough. Some of the 'rules of thumb' vary slightly.

 

[jayrod12]

Is it interior or exterior? If exterior and a slab on grade, we've always just provided an 8" concrete pad. It's not like a generator is so finicky that some slight vibration of the pad is going to affect the overall performance.

If interior, that's a whole different ball game.

 

[r13]

Except gas turbine generator in the powerplant, I consider the generator is a vibratory machine rather than dynamic equipment. It would be the best if you know its frequency range, otherwise, the 3-4 times "rule of thumb" is the way to go, but can be proven wasteful later.

 

[oengineer]

It depends on what your definition of dynamic equipment is. It probably is, but it's probably light enough that the foundation can just be designed per the "rule of thumb" method.... If the foundation weighs 4 times the weight of the equipment then you should be good. Three times may be good enough. Some of the 'rules of thumb' vary slightly.

The image shown below is from the ACI 351.3R-04 Foundations for Dynamic Equipment. If the natural gas generator does qualify as a 'rotating machine", then my goal is to adhere to the design criteria shown in the image below:

I would most likely use 3 times the weight of the generator, based on the information contain in the image.

Is it interior or exterior? If exterior and a slab on grade, we've always just provided an 8" concrete pad. It's not like a generator is so finicky that some slight vibration of the pad is going to affect the overall performance.

If interior, that's a whole different ball game.

I am not sure what you mean by "interior or exterior"? If by exterior you are asking if the natural gas generator is located outside, then yes the generator is exterior. Right now the plan is to use a Downturn slab/slab with haunch to support the natural gas generator.

Except gas turbine generator in the powerplant, I consider the generator is a vibratory machine rather than dynamic equipment. It would be the best if you know its frequency range, otherwise, the 3-4 times "rule of thumb" is the way to go, but can be proven wasteful later.

I can try to reach out to the vendor to obtain the Operating Speed (RPM) or Power Rating (HP) for the natural gas generator. I believe you are suggesting that this would influence which way I should go regarding the design of the foundation.

 

[r13]

I believe you are suggesting that this would influence which way I should go regarding the design of the foundation.

That's exactly what I meant.

 

[JoelTXCive]

I think that 'interior or exterior' does not matter. The real question is: "Are there going to be workers working around the generator full time where constant vibration could cause medical issues?" or, "is the vibrating generator going to cause issues for any nearby structures"

Also, how many horsepower is the generator. I do not have a code citation that I can give you, but many engineers do not worry too much about a true dynamic design for 200hp or less pieces of equipment. They just apply the 3-5:1 rule. (I have tried to find that in a design document, but never been able to.)

I too have struggled with the definition of reciprocating versus centrifugal. If you think about it, the engine's crank shaft is centrifugal, and the individual pistons are reciprocating. But....if you have a V-8 engine versus an in-line 6, then the reciprocating movements should be offset. I attended a 2-day seminar on dynamic design and never got a good answer to this issue.

 

[Brad805]

The sensitivity of these will depend on how many kW the unit is. Little ones like the one in the image are designed for the most part to be skid out to a remote site, attached to a light foundation, and fired up. Big models are different. If this is for your other project, I doubt it is a monster generator. I know of diesel fuel generators operating much larger sites. Those bear on some relatively small piles. If the unit is large and very expensive, you need to find a consultant that specializes in dynamic analysis. The general rules of thumb are inadequate for many.

 

[oengineer]

Also, how many horsepower is the generator. I do not have a code citation that I can give you, but many engineers do not worry too much about a true dynamic design for 200hp or less pieces of equipment. They just apply the 3-5:1 rule. (I have tried to find that in a design document, but never been able to.)

The only place I have been able to locate the mentioning of the amount of horsepower of the machine to warrant a dynamic analysis is in PIP STC01015 - Structural Design Criteria. Please see image below:

The sensitivity of these will depend on how many kW the unit is. Little ones like the one in the image are designed for the most part to be skid out to a remote site, attached to a light foundation, and fired up. Big models are different. If this is for your other project, I doubt it is a monster generator. I know of diesel fuel generators operating much larger sites. Those bear on some relatively small piles. If the unit is large and very expensive, you need to find a consultant that specializes in dynamic analysis. The general rules of thumb are inadequate for many.

The natural gas generator for this project is 450 kW. This a different project from the one in the other post.

 

[oengineer]

Just realized that 450 kW equals 603.46 hp.

I guess I just determined that a dynamic analysis is required.

 

[r13]

If this generator is to be set on the ground floor, I would consider design its own foundation with isolation joint around, by observing the rules of reduced allowable bearing stress, then verified with avoidance of operating frequency range.

 

[oengineer]

If this generator is to be set on the ground floor, I would consider design its own foundation with isolation joint around, by observing the rules of reduced allowable bearing stress, then verified with avoidance of operating frequency range.

I still need to determine the operating frequency range. The slab the generator is resting on is independent from any other structure or foundation. The foundation is just for the generator.

 

[oengineer]

It appears that the generator in questions has a power rating of 60 Hz. When converted this comes out to 3600 RPM.

For a "Downturn Slab/Slab with haunch" foundation, would it be acceptable to include the weight of the haunch in the mass ratio check ( I believe it is,but I would like to confirm because technically the haunch is not an actual "block" in the slab).

 

[r13]

What you gain by using a hunched foundation instead of thick mat? Personally I prefer the latter for less trouble comes with it.

 

[oengineer]

What you gain by using a hunched foundation instead of thick mat? Personally I prefer the latter for less trouble comes with it.

I understand what you are saying in this case, but a "Slab with haunch" foundation is what is preferred for this application by the PM of this particular project.

So...what are your thoughts on including the weight of the haunch in the mass ratio check?

 

[r13]

If the hunch is directly under the generator, or very close to its foot print (no criteria though), I would include it.

 

[Rabbit12]

A dynamic foundation analysis is a specialized analysis and should be done by someone with experience.

That being said your 600 hp generator is probably on the edge of "needing" an analysis. That being said, does your client have specifications? If so, they may have guidance on whether a dynamic analysis is needed. Many or our clients don't want dynamic analysis even for large pieces HP pieces of equipment. They ask we use the prescriptive weight requirements of 3-5 times the weight of the piece of equipment.

 

[oengineer]

A dynamic foundation analysis is a specialized analysis and should be done by someone with experience.

That being said your 600 hp generator is probably on the edge of "needing" an analysis. That being said, does your client have specifications? If so, they may have guidance on whether a dynamic analysis is needed. Many or our clients don't want dynamic analysis even for large pieces HP pieces of equipment. They ask we use the prescriptive weight requirements of 3-5 times the weight of the piece of equipment.

I have do not have access to client specifications.

Based on the area of the slab & if I include the weight of the haunch around the perimeter, I am able to satisfy the mass ratio of the equipment (the mass ratio of the foundation to the equipment is 3.97, including the haunches).

 

[Rabbit12]

What kind of generator? Reciprocating or centrifugal? If it's reciprocating many clients I've worked with have required a 5x multiplier.


If you don't have access to your clients specs you need to request access. If they don't have specs on the limit for dynamic analysis you need to get buy-in from the client to your approach. Don't roll the dice and design to the mass ratio without buy-in. If your foundation vibrates and damages the generator it likely won't end well for your company. Saving a few thousand on a foundation won't get you far if a quarter million dollar piece of equipment is damaged.