Fire Pump??

Status
Not open for further replies.
P

Polarcat

Member
have an owner that has asked me to price a natural gas gen-set for a fire pump. he wants to compare it to a price he got for a diesel fire pump.

my questions are this,
when sizing the gen set do we have to go with locked rotor rating?
would we still have to provide normal power from the utility with a transfer switch? both if they decide to go with diesel or backup genset?

I think I know the answers but would like anybodys input for clarification/reassurance. if that makes sense
 
Using a generator with a fire pump brings a different set of rules into play. If the generator is dedicated to the fire pump, the overcurrent device on the generator needs to be sized to carry locked rotor just as the utility supply device does. The transfer switch must be rated for fire pump service and it is arranged and connected to work with the fire pump controller.

You should consult with the engine supplier to size the generator and be sure to tell them it for a fire pump so that they quote the correct transfer switch. It's A LOT more expensive than a standard one, so hold on to your wallet when you get the quote.

Also, I'd advise you get a copy of NFPA 20 and study it well. It's available from NFPA in PDF format for a nominal fee.
 
It is for a commercial building (98 room hotel to be exact)

if we use a genset sized according to locked rotor of course, do we still need to provide a service from the normal power (utility co)?

if they go with a diesel fire pump do we again need to provide a normal power as back up?
 
I think only your AHJ can answer those questions. But my opinion would be that a genset sized for the locked rotor current qualifies as an "on-site power production facility" and a utility source would not be required. (See 695.3(2)).

My opinion is also that a diesel generator would not need a backup. (How is electric going to back up a diesel motor anyway? You would have to put an electric motor and a diesel motor on the same shaft, or provide 2 pumps. )

One other question though: what are they going to do during generator or diesel pump maintenance? Will the AHJ be OK with occasional outages, or will they have to empty the hotel. I know it's unlikely a fire would occur during maintenance, but its something to think about.

Steve
 
You would have to put an electric motor and a diesel motor on the same shaft, or provide 2 pumps.
Click to expand...
I have seen that for a well pump. There was a "tee" drive gear box device. A vertical electric motor on top, the back up diesel off to the sides and a vertical shaft down the well casing to the pump.
Don
 
I can't imagine that a natural gas genset sized to drive an electric fire pump and the fire pump & motor itself would be more economical than a diesel fire pump. With the diesel unit, you are just buying a pump, motor and controller. With the electric set up, you are buying a pump, motor, generator and controller. Not only that, but with the electric you are going to have to have something to tell the generator when to start and shut down.

Maybe I'm wrong, but I don't see the economics. :roll:
 
benaround said:
Unless the genset is going to be big enough for the fire pump plus?????
Click to expand...

Agreed. But then you will need a utility supply as well, since the generator won't be dedicated to fire pump service as required by NFPA 20.
 
OK heres my understanding. we would size a NG genset to the fire pump and provide a second transfer switch with a emergency panel to utilize the genset as backup power for the building. the genset will have to breakers in it one that feed the fire pump directly and one that feed the auto transfer switch(and back up power panel).

the one breaker that feeds the auto transfer switch will be a shunt trip type and hooked up to the fire pump controller. in the event that the fire pump comes on line it will trip the secondary breaker and then the genset becomes dedicated to the fire pump.

this way you can utilize the genset for backup power to offset the cost.,
 
I have spoken with a couple of colleagues and I'm not sure I agree with the definition of an "on site power production facility" as presented above. IMO, a standby generator is not an on site power production facility. It is an acceptable alternate power supply, but not a primary source as required by NFPA 20 9.2.

You might want to check with your AHJ before you move forward with this, but as I said, this is my opinion only.
 
Good catch Brad. You are absolutely correct. From the definitions in NFPA 20:

3.3.27 On-Site Power Production Facility. A power-production facility that is on site, that is the normal supply of electric power for the site, and that is expected to be constantly producing power.

Steve
 
Also something that I have been told by a couple of manufacturers is that the larger Natural Gas units have trouble starting within the required 10 seconds. If this is legally required you might check into that issue.
-Ed
 
Natural Gas fired generators are not that good for emergency service. Besides the slow starting time, how do you guarantee the fuel source? Diesel units have a day tank, the gas generator has just the incoming gas utility service. Would a fireman want to shut off the gas supply to a burning hotel? Propane tanks for storage might be an option, but the cost of tanks and the vaporizer system would be expensive.

Gas fired engines have to be a bit larger than diesel for the same power output, so the cost is higher. It used to be a 20 - 30% premium.

The quantity of gas required brings up area classification issues. Diesel by itself does not lead to classifying an area as Class I Div 2.

In our power plant designs we have the diesel fire pump as the primary protection with an electric fire pump as a backup. Sometimes the electric pump is fed from a bus that can be supplied utility or diesel generator power. This arrangement does not exactly meet NFPA, but the AHJ's approve our approach. We have never been able to get by with just an elelctric or just a diesel.
 
2002 NEC 695.3 (B) Multiple Sources. Where reliable power cannot be obtained from a source described in 695.3(A), power shall be supplied from an approved combination of two or more of either of such sources, or from an approved combination of feeders constituting two or more power sources as covered in 695.3(B)(2), or from an approved combination of one or more of such power sources in combination with an on-site standby generator complying with 695.3(B)(1) and (B)(3).
(1) Generator Capacity. An on-site generator(s) used to comply with this section shall be of sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load. Automatic shedding of one or more optional standby loads in order to comply with this capacity requirement shall be permitted. A tap ahead of the on-site generator disconnecting means shall not be required. The requirements of 430.113 shall not apply.

From above, sizing of on-site genset is not required to be locked-rotor based.
 
PT said:
From above, sizing of on-site genset is not required to be locked-rotor based.
Click to expand...

The 2002 Handbook agrees with you :

Where the alternative source of power is an on-site generator, the alternative source disconnecting means and the alternative source overcurrent protective device(s) for the electric-drive fire pump are not required to be sized for locked-rotor current of the fire pump motor(s). Rather, the circuit components of the alternative source are permitted to be sized according to Article 430, provided they are ?selected or set to allow instantaneous pickup and running of the fire pump load.? See 445.12(A).
Click to expand...
 
Well there are a lot of diff. of opinion here. I was just told the owner is seriously considering the backup genset for this project. I went back thru the codes and could not see any reason this genset could not be the sole source of power for the fire pump.

I guess it all boils down to "opinion" of reliable source. and if the local building dept and fire marshall are ok with the gas line being a reliable source, then the genset power ONLY would meet all applicable codes. Correct.

All right let me have it! I know it's coming! the verbal barrage of interpretations and opinions. JUST kidding, I find all responses very helpful. Gives me a broader range of knowledge and opens my mind up for different approaches of Interpretation/opinion.

"Whatever doesn't kill us make us stronger"!!!
 
I just don't see how a genset only can comply. If you start at 695.3(A), it is titled "Individual Sources" and says "one or more of the following". That is followed by (1) Electric Utility Connection and (2) On site power production facility.

So if you have a single source, it has to be one of those two. And we have decided that a standby generator is not an "on site power production facility".

(It seems to me you could probably just use the utility connection, and not have to worry about generators and transfer switches and natural gas supplies.)

Back to 695.3. Part (B) lists multiple sources, and a generator is a subcategory to "multiple sources". So again, the code tells us that a single generator isn't enough to supply a fire pump.

Steve
 
went back thru the code and I do agree with you, that a standby gen is not a power producing genset classification. so I would need 2 sources. (already sold them on the genset).

However the problem that I am facing with the utility service connection is, the incoming service is a 2400amp 120/208v feed from a pad mount t-former to a 2400amp BPS gear via under ground conduit with (6) of 500cu. the CT's will be in the transformer. so the tap before the disconnect is not feasible(would have to provide a JB prior to the gear and tap there, not very easy, very large, and definitely not economical.)

would I be allowed to provide a tap in the utility co Pad mount, (being that the CT's are there i would see no reason why they would not allow) and enter the building via undergroung conduit directly to the transfer switch and firepump controller (still sized for locked rotor).
would this arrangement require a disconnect/overcurrent protection?
 
Status
Not open for further replies.
Top