How far can I run into the SF?

[KernOily]

Dumb question here guys. I am sizing a new instalaltion for boiler feed. Pumps are split-case multistage. The bhp req'd is right on the ragged edge of 1500 and will be slightly over 1500 at times. Motor will be 1.15 SF. We are arguing over what the nominal motor HP should be: 1500 or 1750.

The question is, can I run a 1500 hp nominal motor into the SF on an extended basis, or is this a no-no?

Thanks!

Thanks!
Pete

 

[Zapster]

You may want to post this on the "Electric motors & motor controls engineering" forum.

 

[bulkhandling]

The purpose of SF is to protect the motor when the load ocassionally run out. It is hard to say what exactly is "occassionally" but for sure not for a extended time.

If you are using an existing motor, running into the SF but at the lower end may not be a bad idea, but at your case, It is strongly recommended to go to the next larger motor size especially your application is important.

 

[TD2K]

Hi Pete, long time no talk.

It's really up to the facility in question and how critical this pump is (BFW is fairly critical to me).

The SF (as its been explained to me) really goes back to the winding temperature. As you increase the load on the motor, winding temperatures increases which reduces insulation life. When the insulation fails, motor goes poof (my apologies to the electrical folks here for what I'm sure is a gross simplication). The SF and insulation class are both considerations, do you add a higher SF or a higher insulation rating? From my understanding as a process engineer, both effectively lead me the same way into a motor that can handle the higher loads.

A key question here are your ambient temperatures. I know on the North Slope in Alaska in winter, motors could be run quite a bit harder because with the colder air, windings stayed cooler. If you are in Death Valley in summer, the reverse would be true.

Back to your question and off my rambling thoughts. Most of the companies I work with for a new installation will NOT allow you to use the service factor they want to have. For a re-rate, they may allow you to use some of the service factor. How much usually generates many meetings.

Me? I'd go with the 1750 Hp. I can't say off the top of my head if that pushes you into the next starter size or bigger cables but I wouldn't want to run this service on the edge.

 

[katmar]

The problem is that "conventional wisdom" takes a long time to change. Thirty years ago you could probably have done this quite safely, but not any more. However, the stories about it being done still circulate and people are tempted to do it.

The motor manufacturers have improved their design techniques to the point that a 1500 HP motor is definitely a 1500 HP motor, and not a 1600 or 1750 HP motor. The manufacturers now know exactly how much copper to put into the motor to meet the specs.

Eight years ago I did a project that was a mirror image of one done 6 years before that. The client's spec for a particular large motor was that it should exactly as per the previously installed unit. We issued the same spec sheet and purchased the motor from the original supplier. Once it was running we noticed that it was running hotter than its older "twin". When we queried this with the manufacturer he was quick to point out that it was still within the code. The manufacturer was building the same specification motor with significantly less copper. As their knowledge increased they were able to decrease their internal SF to virtually nothing.

My vote goes with TD2K for buying the 1750 HP unit. The plant is always going to try to squeeze more out of the pump anyway, so rather be safe.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[KernOily]

Guys thanks for the replies. TD2K - good to hear from you. You still hangin' around up there in Washington?

'Many meetings' = well put. They have no internal spec for this issue, i.e. running into the SF, so it has generated a lot of hot air.

Most of the info I have found agrees with what y'all are saying here. I posted the same message over in the EE motors forum and got similar answers: the whole thing is a function of operating (windings and insulation) temperature and hence service life. The farther I run into the SF, the shorter motor life I can expect due to increased temperature.

The work at this point is centered around the load. The owner is trying to get his internal customer to come off the load a bit so we can run a 1500 safely. I should have mentioned this in the beginning, but this motor will be on VFD for times of reduced-load so we are already looking at elevated winding temperatures.


katmar - I was not aware of that (less copper for newer motors) but it sure makes sense.

Thanks!
Pete

 

[ERT]

A couple thoughts to add to these other good points:

1. Be careful about the VFD implication. Many motor mfg will say that a 1.15SF motor is only capable of 1.0SF if the wave form is generated by a VFD.

2. How confident are you on the BHP calculation?

3. How sensitive is the BHP to the input variables? Are you right on the BEP? Has there been several conservative assumptions that really mean you have a hidden safety margin? Or conversely, has there been a lot of wishful thinking regarding everything being exact to specification?

4. What are the implications of going with 1500 HP and getting it wrong? If you go with 1500 HP can you make the baseplate suitable for 1500 HP and 1750 HP?

5. Does the end user have this 1500 HP and/or 1750 HP motor at your speed in the plant already?

6. If the input parameters are off target, who pays for the fix?

Regards,

ERT

 

[Enkidu]

If you haven't purchased the driver yet, I would be inclined to look at motor efficiency. You may find the 1750 hp unit operating at 85% loading may be a long term cost saving. Just a thought.

 

[waross]

A call to the motor manufacturer may be productive.
respectfully

 

[djack77494]

Don't forget to consider efficiency losses. The bhp is what you need to turn the pump impeller. There will be a small loss in efficiency between it and the motor due to the coupling of the two. Don't cut it too close.
Doug

 

[stanier]

Have you also considered voltage variation. If voltage supply drops for any reason for extended periods the motor may run hot. You could up the voltage taps on the transformer to give you that extra little bit but that may have other implications for the system componenets. It depends on the grading of theprotection for the whole system.

Geoffrey D Stone FIMechE C.Eng;FIEust CP Eng

http://www.waterhammer.bigblog.com.au

 

[Artisi]

S.F. is exactly what it says- safety factor. So it is entirely up to you if you want to run into this area. Your motor selection should be based on a proper engineering study of how, why, when and for how long you will run beyond 1500hp in conjunction with your motor supplier, pump manufacturer and your electrical experts regarding constant voltage at the switch board.
Also - a motor of this size can probably be manufactured to suit your requitrements ie, rated at 1550 - 1600 hp to give you some margin.

Naresuan University
Phitsanulok
Thailand