Effect of Low Starting Voltage on the Required Water Pressure

[tobee]

Can any mechanical engineer tell me what would be the effect (if any) of low starting voltage of an electric fire pump on the required water pressure? As you may know, the motor torque will drop in a squared amount as the voltage drops. Do you compensate for the drop in the initial water pressure drop? Any input in this matter is appreciated.

 

[peebee]

Not that you asked, but. . . .

Make sure you meet the voltage drop requirements of NEC 695.7.

 

[dpc]

Generally, if it is a centrifugal pump, the starting torque requirements are quite low. To be sure, you would have to do a transient analysis to verify that the motor would always be able to supply sufficient torque as it accelerates and reaches operating speed before it burns up.

Low voltage will increase acceleration time at the very least.

As peebee said, fire pumps require special consideration and the applicable Codes give little flexibility for creative engineering.

 

[peebee]

Maybe I misunderstood your original question. Are you talking about using a reduced-voltage starter such as an auto-transformer starter? So long as it's listed for fire pump use, and you meet the requirements of NEC 695.7 in the power wiring, you're set.

Yes, it will take a little longer this way for the water to get up to pressure. But even with full-voltage starting, there's a finite period of time required for the water to ramp up to pressure. They're both OK, and no, you don't need to compensate for that ramp-up time by any other means with either starter.

 

[tobee]

After reading section 695-7, It states that the maximum starting voltage drop is limited to 15% (regardless of the method of starting). However, my question has to do with the water pressure drop (due to drop in the motor torque). For example if I start a fire pump by a soft start method, my typical available starting torque would be about 16% of a motor that is started across-the-line. So, does the mechanical design engineer care about this short duration of water pressure drop or does he/she compensate it somehow in their pipe design?

Also, after reviewing the UL standard used for listing of controllers (UL 218), you will find out that the standard (and its listing)does not evaluate the effect of a voltage drop on the controller and it's impact on the fire pump motor. I imagine the reason is that the listing only looks at the controller as a stand-alone equipment rather than its role in a mechanical system.

 

[peebee]

Um, I think I already addressed your points in my last post. But just in case: realize that with FV, your motor will take something like 6 seconds to start. With the reduced-voltage starter, it will be longer -- I don't know how much longer, but I bet less than 15 seconds. A Google search or the Red Book or PowerTools would probably shed more light on the starting time if you're that concerned. But NFPA is cool with the starting time. No mechanical mods required, use a listed controller and limit the voltage drop per 695.7 and you're good to go.

You mentioned soft-start -- if you're talking about a solid-state soft motor controller, I don't believe such controllers are listed for use with fire pumps, but I could be wrong. If anyone knows otherwise, please clue me in. I believe most reduced-voltage fire pump starters are autotransformer type or some other "old-fashioned" induction type. Anyone who's ever used a computer realizes that the last thing we need is a digital control system in charge of life safety equipment (how many people will I piss off with that one?!?). KISS (Keep It Simple Stupid) is the name of the game in life safety.

 

[tobee]

For your info, there are numerous manufacturers that do offer Low voltage listed soft start controllers. To name few of these manufacturers, they are as follows:

1. Firetrol, INC
2. Hubbell Industrial Controls, INC
3. Joslyn Clark Controls, INC
4. Master Control Systems, INC

 

[dpc]

I guess I'm missing the point regarding water pressure. If you are starting from a condition of the pump not even running, I don't see how a longer acceleration time is going to cause a dip in pressure.

 

[tobee]

dpc

As far as I understand (I am not a mechanical engineer) the water line is constantly under water pressure. The reason for using the fire pump is to make sure that the required discharge pressure at the last sprinkler head is not less than a specified value (I guess 65 PSI). You may want to use the analogy of comparing the fire pump system to that of an electrical distribution system. Under all conditions of operation there is a set magnitude of voltage present at an electrical service (like the water pressure at the pump), upon turning on an equipment (activation of a sprinkler head) there would be voltage drop in the line ( drop in the water pressure due to discharge and flow resistance), if the voltage magnitude is below the rating of the equipment, it may not operate (lack of water pressure would not provide the required water coverage).

So, if we don't have enough water pressure (i.e., low voltage to begin with), would there be a problem or would they blame it on the electrical designer that did not give them the required torque as they thought it should have been there to begin with?