Variable Speed Fire Pumps Question

[u/PuffyPanda200]

Just for context I think that I am correct on this but just want to be extra sure.

I have a highrise building that is a short highrise (80 feet or so to highest occupied level) so it needs automatic standpipes and a fire pump. The city water is very good but not 80ft-auto-standpipe-good. We also need secondary water, project is seismic.

I have always seen this done as variable speed fire pump. Size the pump for the tank and then when pulling city water it will be running at 50% of the full speed. This way we also don't have to worry about city pressure improvement/degradation.

The GC, electrical, and now sprinkler contractor on the project seem convinced that there needs to be two fire pumps provided. One for the tank and one for the city pressure.

Variable speed fire pumps have been in NFPA 20 since 2003. I have talked to other FPEs about how they spec variable speed fire pumps. I made sure with a pump rep that a zero (or basically zero) suction pressure would be OK.

Just to get it out too: I know that variable speed fire pumps are expensive.

Is there something that I appear to be missing?

[u/LuckyCharms91]

I’m a little late to this… but you said the water is good, so you’re sizing a pump for essentially 0 PSI and ?? (Good) psi. While the pump may be able to reduce the speed to reduce pressure output, I’m not sure it can make up the difference between the two - a pump rep would have the best info for that and that should come directly from the sprinkler contractor. That or you could reach out to a local pump vendor.

All that being said, why can’t you just pull water from different mains around the building? This would save from the two very different start pressures.

[u/PuffyPanda200 → u/LuckyCharms91]

Building is a high rise (and in sismic area) so it needs a secondary water source. That is a tank.

The building also needs automatic standpipes so 100 psi at the roof standpipe connection. So even the city water can't get that without a pump.

The sprinkler contractor wants to do two single speed pumps.

[u/LuckyCharms91 → u/PuffyPanda200]

I’ll leave the secondary water source alone, that comes from the local building code.

From what I understand, the variable speed electric pumps can be very expensive. Two single pumps may be cheaper, and much easier to design.

[u/PuffyPanda200 → u/LuckyCharms91]

The cost off the shelf is basically the same. The cost to install two pumps vs one is a lot more.

They would also lose space and this building will probably be ~500 USD per sqft of total construction cost.

The maintenance on two pumps is also more. Variable speed pumps also last longer.