Generator sizing for fire pump

[m sleem]

Assuming that the generator is dedicated for fire pump only, Is it true that using VFD for fire pump would reduce the generator sizing by 2~3 times due to voltage drop?

 

[GoldDigger]

Are there any VFDs that are suitably listed for use with fire pumps?
It is not something that I recall hearing about.

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[gadfly56]

Are there any VFDs that are suitably listed for use with fire pumps?
It is not something that I recall hearing about.

Sent from my XT1585 using Tapatalk

NFPA 20 recognizes the existence of VFD's when discussing soft start units in 10.4.5.5.

Here is a link to one controller providing this feature.

 

[Sahib]

NFPA 20 recognizes the existence of VFD's when discussing soft start units in 10.4.5.5.

Here is a link to one controller providing this feature.

But the VFD should not have any overload protection for the fire pump.

 

[publicgood]

NFPA 20 recognizes the existence of VFD's when discussing soft start units in 10.4.5.5.

Here is a link to one controller providing this feature.

The running load calc would be the same 125% FLA.

Soft starts typically limit to 300% in-rush. This is helpful for starting voltage drop (max 15%), which does reduce the genset size. Typical locked rotor is 5x. This means that there is a possible reduction in genset by about 1.6x.

NFPA 20 also discusses a requirement for emergency start function (across the line). We don’t have to meet the 15% during this condition, but we still need the controls to work, i.e. - we at least stay above 30% voltage drop to avoid electronics under voltage drop out. This needs to be considered as well.

 

[m sleem]

The running load calc would be the same 125% FLA.

Soft starts typically limit to 300% in-rush. This is helpful for starting voltage drop (max 15%), which does reduce the genset size. Typical locked rotor is 5x. This means that there is a possible reduction in genset by about 1.6x.

NFPA 20 also discusses a requirement for emergency start function (across the line). We don’t have to meet the 15% during this condition, but we still need the controls to work, i.e. - we at least stay above 30% voltage drop to avoid electronics under voltage drop out. This needs to be considered as well.

Would you please provide some more details for your calculations in equations assuming that we have 100hp fire pump?

 

[gadfly56]

But the VFD should not have any overload protection for the fire pump.

This feature is built into the fire pump controller. The controller manufacturer handles all those requirements.

 

[publicgood]

Would you please provide some more details for your calculations in equations assuming that we have 100hp fire pump?

Your 100HP across the line:



This results in a 450kW genset with 11% voltage dip. If you have a 300% limit via soft-start, you may find yourself in the 100kW range:



However, notice that is 14% voltage dip. This leaves you 1% of voltage drop on the feeder. #2/0 underground in non-metallic conduit for 100 FT: V = 725A (locked rotor) * 0.11Z * (150FT/1000FT) * root(3) = 20.72V. (20.72V/480V)= another 4.3% voltage drop. To get to that 11% range even with a 300% limit soft start you are at a 250kW range:



Given, there are opportunities to combat voltage drop by feeder sizing and distance. This is a pretty typical example. However, it is not just this aspect, it is the emergency start requirement by NFPA 20. The generator needs to be sized for this emergency across the line start; although, it can be greater than 15% - still, we don't want it to be greater than 30%. If it is over than 30% we have high likelihood of dropping out the fire pump controller. Up-sizing from the 300% limited (with VD of feeder in-mind), we land back near that 250kW range:

 

[publicgood]

 

[Sahib]

One problem with VFD is it introduces distortion so much so that any reduction in size of the generator due to lower starting kW and starting KVA is offset by it.

 

[Sahib]

One problem with VFD is it introduces distortion so much so that any reduction in size of the generator due to lower starting kW and starting KVA is offset by it.

So the answer to OP is no.

 

[publicgood]

So the answer to OP is no.

Some possible reductions, but agree - not 2-3 times smaller.

 

[m sleem]

Your 100HP across the line:
.........

Thank you, but really i don't understand how we get 1.6 or any possible reduction in generator sizing due to soft starting.

 

[publicgood]

Thank you, but really i don't understand how we get 1.6 or any possible reduction in generator sizing due to soft starting.

The worked example came up with 450kW in the across the line example and 250kW in the soft start. 450/250 = 1.8. Or you can look at it as ~40% reduction in generator size.

Where are we missing each other?

 

[Sahib]

The worked example came up with 450kW in the across the line example and 250kW in the soft start. 450/250 = 1.8. Or you can look at it as ~40% reduction in generator size.

Same issue as in VFD- Distortion, but may be overcome if bypassed after starting.

 

[gadfly56]

Same issue as in VFD- Distortion, but may be overcome if bypassed after starting.

If you had read the information at the link, you would have seen that's exactly what happens.

 

[publicgood]

If you had read the information at the link, you would have seen that's exactly what happens.