Well Pumps - Variable Speed?

[avillahdw]

TheGoose With your experience then I have a question or series of them. Due to code I have to put a fire suppression system in the new house we are going to build. Option A I can put a large pump set deeper and get the flow I need. or Option B I can put a smaller pump say 5-10 gpm set more shallow (dynamic level at 10 gpm is a little bit higher than at 50 gpm) and put in a 2000-3000 gal tank with another pump which is high gpm to supply the system. Option A the well pump is more expensive with more copper in the hole (set deeper) but less complex as only one pump and no storage tanks. Option B has a cheaper well pump but the extra tanks and extra high flow pump are not cheap. Either way would have a pressure tank for the system. Money aside which do you think is better in the case as I describe it? Tradeoffs for certain but what is your opinion as a long time pump installer. I have had installers give me absolutely positive answers both ways. They each think there is only one way, their way, each one being the opposite of the other. By the way the well can support either option. Required flow is 50 gpm.

In response to this, take in consideration that normally one of the first things done in fighting a fire is to disconnect the power. Thus a separate meter would truly be the only way to provide for the fire suppression system.

 

[techman]

The hz limitation of a motor is usually limited by the voltage. To run a motor faster requires a higher voltage than nominal, due to the inductance of the motor. So as the hz goes up, so does the voltage required to push the current (called V/HZ in VFDs). At some level you will reach the insulation limit of the windings and the breakdown will occur. Other factors such as iron losses also increase with frequency, but usually just lower the efficiency. Industrial motors rated for VFD operation usually have 150% to 200% insulation ratings above the nominal nameplate voltage.

paul

 

[TheGoose]

Normally when using a true VFD pump/motor matched system you use a bigger motor and a smaller pump. For example, you would use a 1.5 HP motor with a .75 HP pump head. The motor will be a 3-phase motor even if you are using single phase power (the controller will convert it).

 

[TheGoose]

If you go with option A you still need a tank big enough to keep your pump from cycling. For a 50 GPM pump you need a tank with a drawdown of 50 gallons to keep the cycle time at one minute. That gets expensive. This is if you are not using a VFD of CSV to control the cycling.

I guess the main question is "How well do you want it to work when the SHTF?" Is there a back-up power source?

TheGoose

With your experience then I have a question or series of them.

Due to code I have to put a fire suppression system in the new house we are going to build.

Option A I can put a large pump set deeper and get the flow I need.
or
Option B I can put a smaller pump say 5-10 gpm set more shallow (dynamic level at 10 gpm is a little bit higher than at 50 gpm) and put in a 2000-3000 gal tank with another pump which is high gpm to supply the system.

Option A the well pump is more expensive with more copper in the hole (set deeper) but less complex as only one pump and no storage tanks.
Option B has a cheaper well pump but the extra tanks and extra high flow pump are not cheap.

Either way would have a pressure tank for the system.

Money aside which do you think is better in the case as I describe it?

Tradeoffs for certain but what is your opinion as a long time pump installer. I have had installers give me absolutely positive answers both ways.
They each think there is only one way, their way, each one being the opposite of the other.

By the way the well can support either option. Required flow is 50 gpm.

 

[buickanddeere]

Then it would be rated as such.

Sorry but I'm not certain what you mean .

 

[Silvic]

If you go with option A you still need a tank big enough to keep your pump from cycling. For a 50 GPM pump you need a tank with a drawdown of 50 gallons to keep the cycle time under one minute. That gets expensive. This is if you are not using a VFD of CSV to control the cycling.

I guess the main question is "How well do you want it to work when the SHTF?" Is there a back-up power source?

The code is quite specific in just about all areas except power which is laughable. When asked about backup power requirements the answer is none required. What a joke that is.
My plan/design has backup power and yes if I go option A it is a constant pressure design which bumps the cost of the well set pump but either way there is a constant pressure system involved so not cheap.

As to the response from the other poster on the FD first killing power, as you may or may not know the fire suppression systems are required so as to provide initial occupant escape time and to provide some fire suppression prior to the FD arrival. Once they are there, obviously they are there.

 

[TheGoose]

I would go with the option that has the least amount of equipment to maintain (one pump, one tank, etc). My advice is to skip the constant pressure stuff and go with the biggest tank you can afford. What horsepower pump are they proposing?

 

[Silvic]

That is kind of my thinking go with least equipment ie one pump no storage just pressure tank. The motor in the well now is a test pump and is 5 hp (3/phase) and the numbers say that if I go option A I will have a 5 hp motor. I can't remember the stages on the test pump in the hole. The constant pressure system we have been looking at is 11 stages, yes no typo.

Why would you go large pressure tank vs constant pressure?