Sizing of Motor for Cent Pump with ARC 2

[nirlt]

I am facing a Problem in selection of Motor rating for High pressure ~150 bar boiler feed water pump.

the pump requires Autostart facility i.e. its discharge valve will be in open condition at time of starting and also it has an Auto Recirculation Valve (ARC) in discharge line for minimum flow bypass(for pump safety.

Should the selected motor have rating sufficient to cater the 'End of the curve flow' of the pump or not.

Or speaking the other way round - will the pump operate at End of Curve point anytime?

 

[BigInch]

If you order enough driver power for the pump to get to the end of the curve (and the intersection with the system curve), or if the system curve decreases such that there is little resistance and hence little power required, the pump will get to the end, possibly overspeeding in the process. Whether the pump should do that or not depends on what flow (and heads) you will want or can have at both normal flow & head (BEP condition) and what the maximum flow and head conditions will be. Overspeed should be avoided.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[Scipio]

Really it comes down to a judgement call. At that size, for instance, API 610 recommends the driver have a nameplated power of at least 110% of the power requirements at the rated point. It should also likely be sized to handle plant startups, where the boilerfeedwater will be colder than normal operating temperatures (higher specific gravity, higher power requirement). End of curve sizing is good to have, but frequently that 110% rating will put you in the neighborhood anyway. I've had clients who simplified the decision process just by looking at the motor frame required for a rated power, then buying the motor with the most horsepower available within that frame size.

Something else to consider, if this pump is an auto-start, does that mean it's set up to come onstream automatically with another pump that's already running? If that's the case, regardless of an open discharge valve, the back pressure in the discharge system against the pump's discharge check valve (or even the ARC valve) will typically be enough to keep the pump from running out beyond the minimum flow rate until it's stabilized. The only time you have to really worry about runout in an auto-start scenario is if the pump is starting up against an un-pressured system, which isn't likely without operator assistance anyway.

Long story short, if your startup procedures work properly, the pump should never run out on it's curve. But that's a big 'if' and a fairly reasonable expenditure for that extra 10% horsepower availibility if it's required.

 

[nirlt]

Dear Scipio,
Thanks for he useful information. I would like to eloborate few more things

point 1.

There r 2 pumps 1 working and 1 standby (not in parallel operation anytime)

point 2.

Standby pump shall autostart as soon as the working pumps trips - due to some reason.
it has auto reciculation vale in pump discharge for pump safety.

point 3.

During plant startup operator assistance would be definitely provied but not in case if the pumps starts in above mentioned fashion (point no 2).

point 4.

rated BkW is 900kW and end of the curve(EOC) point power requirement is 1136 kW.MOtor size for EOC requirement
is 1150kW.

Question in my mind is that- when standby autostarts will its operating point anytime shift away from rated point towards EOC and hence motor will have to be sized for catering EOC power requirement or not??

 

[BigInch]

If flow resistance (boiler inlet pressure) drops below normal at any time, the pump will try to move farther out on its curve, if not... it won't. Determine the minimum boiler inlet head you will have then find the corresponding maximum runout point on the pump curve.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

 

[nirlt]

Dear Biginch,

I have found out the minimum pressure that the pump will see
it is 145 bar and the power corresponding to this point is 950 kW

Now at rated pt power is 917 kw, that means as per project standards the minimum motor rating required is 1.1 X 917 = 1008 kW

Can i select mator rating considering maximum power requirement as 1008 kw in place of earlier 1136 kW.

Please advice.

 

[BigInch]

If the end of the curve power selection value was 1136 kW and you don't need to go there, yes you could select 1008 kW. That is enough power allow you to sustain the flow at minimum head/pressure max flow (145 Barg), if you need to do so. Your selection requirement at BEP 917 kW + 10% = 1008 kW > 950 kW needed for at the lower head and faster flow, so that higher flow can be sustained until resistance of the downstream piping and equipment builds, flow drops and pushes the pump operating point back to the higher head at the BEP condition.

There is no need to consider any flowrate past the maximum flowrate that you can have, due to system conditions, flow or pressure control, or any other operating limitation.

http://virtualpipeline.spaces.msn.com