Standby Generator Sizing for a Fire Pump (long) 1

[wthall]

My client wants a back-up generator for his 125HP, 480V, 3phase fire pump motor. The fire pump controller is an 'across-the-line' starter type.

I have gone through some sizing software provided by generator manufacturers and I came up with a 200kW unit. The factory guy tells me I need a 500kW unit because of

NFPA 20 6-2
"Power Source(s). Power shall be supplied to the electric motor-driven fire pump by a reliable source or two or more approved independent sources, all of which shall make compliance with Section 6-4 possible."

NFPA 20 6-4
"Voltage Drop. The voltage at the controller line terminals shall not drop more than 15 percent below normal (controller-rated voltage) under motor-starting conditions. The voltage at the motor terminals shall not drop more than 5 percent below the voltage rating of the motor when the motor is operating at 115 percent of the full-load current rating of the motor."

Then I look at NFPA 20 6-6 'On-Site Power Generator Systems'

6-6.1
Where on-site generator systems are used to supply power to fire pump motors to meet the requirements of 6-2.3, they 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(s). A tap ahead of the on-site generator disconnecting means shall not be required.
6-6.2*
These power sources shall comply with Section 6-4 and shall meet the requirements of Level 1, Type 10, Class X systems of NFPA 110, Standard for Emergency and Standby Power Systems. The fuel supply capacity shall be sufficient to provide 8 hours of fire pump operation at 100 percent of the rated pump capacity in addition to the supply required for other demands."

NFPA 110
2-2.2 Type.
Type defines the maximum time, in seconds, that the EPSS will permit the load terminals of the transfer switch to be without acceptable electrical power.

Type 10: 10 seconds

I'm confused about the 15% vs. the ten seconds. Am I going to have to get this huge generator?

 

[DanDel]

FLA of a 125HP 460V motor is 302A per NEC. A 200kW generator at 80% PF can output 314A, which isn't really OK even for running. (80% loading) The LRA of that motor when starting is anywhere from 1825A to 2344A, based on the code letter, per NEC. Per your quoted spec, the generator has to be sized large enough to limit the voltage drop at start(1825A to 2344A) to less than 15%. Believe me, you need a significantly larger generator than 200kW. The actual size can be determined by running a Motor Starting Study, which requires knowing the generator, cable, and motor ratings and impedances.
The 15% is for the minimum voltage drop while starting, while the 10s appears to be for the amount of time that it takes the ATS to:
1. start the generator upon loss of normal power
2. allow the generator to come up to normal speed/voltage
3. transfer to generator power
The 15% and the 10s really aren't related to each other, but are simply different requirements. The transfer time is not related to the motor start, since the only time the generator is used for the fire pump motor is when normal power is lost and the ATS has already transfered to generator power.

 

[wthall]

I agree...the ten seconds is for the generator startup and the 15% is for the voltage drop. I was thinking that if it took 10 seconds for the generator to kick in, then there's no way that the voltage wouldn't drop more than 15%.

By the way, the FLA for the motor is 156 and the LRA is 908...appears that you were looking under the 230V column, DanDel. That's ok, your analysis of the situation was sound and I appreciate your input.

Now to break the bad news to the client...

 

[dpc]

Another option that is often used to get around this problem is to provide an engine directly coupled to the pump. The fire pump can have the motor and engine coupled along the same shaft. A clutch mechanism prevents the engine from turning if the motor is running. If no electric power is available, the engine starts automatically.

This eliminates the generator and probably reduces the size of the engine (a little). I'd talk with the fire pump manufacturer about it.

This does require a bit of extra space at the pump.

 

[DanDel]

Thank you. Actually, I was looking in the 460V column, but on the 250HP row. instead of 125HP. Sorry.

 

[MJCronin]

wthall,

dpc has a great idea....

If you have the space, the best soultion may be to install a diesel driven pump in addition to the electric pump. The cost of the diesel pump may be less than the new generator..... and the electric + diesel combination is found freqently in industry. Both types are described in detail in NFPA-25

There are many certified "used" diesel pumps available for sale by brokers on the internet....These are typically perfectly acceptable.

PEERLESS pump has a lot of good info on fire pumps

Let us know what you decide.......


MJC

 

[wthall]

Sorry guys, I just don't have the space for the diesel motor to be coupled to the existing pump or the installation of a diesel pump.

Is there a way to be able to use this 500kW generator to provide power to more than the fire pump? My client wants to power the lights and exhaust fans also. He thinks he can use two transfer switches and use some fancy load sharing scheme to avoid upsizing the generator for the extra loads.

If you will recall, I had to go to the 500kW generator due to the 15% voltage drop requirement for fire pumps.

 

[esosa]

Another option may be to change the fire pump controller to a reduced voltage starter type. That would reduce your generator since your generator size is being driven up by the high starting current and corresponding high voltage drop.

 

[dpc]

I agree with esosa that a reduced-voltage starter is a viable option, assuming a centrifugal pump. These can be used with fire pumps if purchased as a listed fire pump controller. The NFPA requirements for voltage drop during starting are given for the **line** side of the controller. You should not need a 500 kW generator. Check with fire pump suppliers for information on reduced-voltage starting.

I think the generator could be used for other purposes provided the other loads are shunt-tripped if the fire pump needs to run and you have a connection to utility power for the fire pump.

 

[wthall]

You are correct, dpc. Since my last post, I have learned that the other loads can be shunt tripped if the fire pump needed to run...EITHER the lights & fans OR the Fire Pump.

I agree erosa, a reduced voltage starter is what my client needs but the fire pump controller is brand new...once I learned what they had ordered I tried to get them to change it but after the cost increase and the delay that would be involved, they decided to keep the full voltage one coming. Now they are paying the price...in peak KVA charges from the utility and this huge generator...that extra $8,000 isn't so much now...but I digress.

I'm stuck with the full voltage FPC.

 

[peebee]

A common transfer switch cannot be used to supply both legally required emergency loads, such as fire pumps, and other non-emergency loads per NEC 700.6(D). The generator itself can be shared, but separate transfer switches must be provided as stated by your client. Shunt tripping alone of those loads is not acceptable. See also 695.10, requiring a listed ATS for fire pumps. See also NFPA 20, 6-3.2.2.1: "Direct Connection. -- The supply conductors shall directly connect the power source to either a listed fire pump controller or listed combination fire pump controller and power transfer switch." See also NFPA 20, 7-8.2.3: "Each fire pump shall have its own dedicated transfer switch(es) where a transfer switch(es) is required."

You might want to consider an oversized alternator on your generator. Starting currents are reactive and have little impact on the kW rating of the engine; it's the upsized kVA rating of the alternator and the associated increased starting currents that you're after. An upsized alternater might just enable you to start across the line might be helpful if reduced voltage starting seems borderline.

Reduced voltage starters make sense too as previously mentioned.

Don't overlook the current limiting effect of your supply conductors. If your generator is any distance from the motor, and it sounds like it is, take a good look at the feeder sizing.

In general, any time the rating of a motor approaches the rated capacity of its source (be it a transformer, generator, whatever), you need to take a very close look at voltage drop issues.

 

[peebee]

By the way, since it sounds like you're probably not too familiar with NFPA 20, I'd strongly recommend getting a copy and reading it. There's a bunch of other requirements for fire pump electrical installations in there, including supervised monitoring (usually fire alarm system monitoring) of pump status, source availability, transfer switch status, etc.

 

[beghtesady]

Peebee

As others and you mentioned, reduced starting may be one way to reduce the starting current, however, as you may be aware, the fire pump, in the event of emergency, can also mechanically be operated. In that case, regardless of the method of starting, the fire pump will be starting at full voltage (across the line). Furthermore, reduced voltage started pumps may impact the required water pressure in the line since the reduce voltage means less torque.

 

[peebee]

I'm not quite following you.

Please clarify "can also mechanically be operated". Do you mean from a diesel engine? What's that got to do with electrical starting?

Also, regarding "reduced voltage started pumps may impact the required water pressure in the line since the reduce voltage means less torque", well, there's no motor out there that will start instantaneously, providing 100% pressure at time=0sec. What's your point?