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Non Overloading Impeller

09/23/2016 1:34 AM

I was reading going through a specification for HRSG (Heat recovery steam generator) feed pumps (often called as BFP - Boiler feed pump). The owner had specified that the Impeller of the pump should be non-overloading type.
Can anybody explain what exactly is this non-overloading type of impeller?

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#1

Re: Non overloading impeller.

09/23/2016 1:52 AM

It means that under low ΔP but high flow conditions, the impeller has been selected or trimmed so it won't overload the attached motor. This can be seen toward the right on pump curves by checking whether the BHP (brake horsepower) of the pump is less than the rated motor HP as furnished.

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#3
In reply to #1

Re: Non overloading impeller.

09/25/2016 4:21 AM

I'd just add that it's the combination of impeller and motor that is non-overloading (or not). The impeller clearly must meet the duty, but some impellers might have lower maximum power (usually but not always at high flow/low head) than others.

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#2

Re: Non Overloading Impeller

09/23/2016 10:29 AM

..."To understand non-overloading power when viewing a pump curve, first find the curve for your impeller size. Next, locate the dashed horsepower lines. If the point on your pump performance curve (a specific flow and pressure) is to the left of the dashed line, the motor size of the dashed line is not going to be overloaded. If your point is to the right of the dashed line, the motor will be overloaded.

In This Application, there is a 7 1/2 Horsepower Requirement at the Duty Point, but the Non-Overloading Horsepower is 15

Now, follow your pump curve all the way out to its maximum flow. The last motor size which every point along the pump curve falls to the left of the dashed horse power is the non-overloading motor size. This also corresponds to the motor size a program like Pumpflo will specify- because as you can imagine, overloading a motor is not good.

Selecting a non-overloading motor usually means selecting a motor 20-30% larger than the power required at the design point. This means that we will pay a slight efficiency penalty for selecting a non-overloading motor and also a higher upfront cost. The operational problems and costs we avoid, however, far offset these relatively minor upfront costs."...

https://hollandaptblog.com/2014/06/13/what-is-non-overloading-motor-power-how-does-it-affect-sizing-your-sanitary-pump/

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#5
In reply to #2

Re: Non Overloading Impeller

09/27/2016 4:04 AM

Have to keep in mind that pump curves are based on water at 70 degrees. High temps can affect performance.

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#4

Re: Non Overloading Impeller

09/26/2016 9:39 PM

You do have to admit it is curious wording to ensure you match a motor larger than necessary for the pump & design hydraulic conditions. Also, there is no sanity check on just what the hydraulic conditions are to size the motor.

In the paper industry, we often size the pump motor for full curve run-out. Usually this means that the pump power curve does not intersect the impeller curve, down to zero head. The hydraulic conditions are assessed, so that we don't put a 200kW motor on a pump that can never develop more than 100kW of load based on physical piping configuration, bypass valves or startup/shutdown conditions.

The reason for a specification like this is the writer's previous experience with a design engineer's poor anticipation of possible operating conditions the pump will encounter.

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#6
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Re: Non Overloading Impeller

09/27/2016 4:12 AM

Not clear what you're saying. If you're sure the system curve will keep the absorbed power within a certain range (eg there's no chance of it running out to low head/high flow condition) in principle you could base motor power on that. But he has client's spec to meet.

And in your example you use runout conditions. If 100kW pump power is maximum, even under these conditions, why would you consider a 200kW motor?

Also not all pumps have rising power with rising flow/falling head. Some axial and mixed-flow pumps have opposite characteristic, sometimes with a sharp increase in absorbed power close to shut-off.

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#7
In reply to #6

Re: Non Overloading Impeller

09/27/2016 9:44 AM

That's exactly what I'm saying, use some common sense.

As an example, if your feed pump has 60 meters of static head on the discharge before it gets to the drum, no need to size the motor for pump flow below 60 meters of head, plus friction loss. (curve run-out, in the U.S.)

Why the spec would call it some name that looks like a special impeller design, rather than an application parameter limitation also tells you something about the technical competence of your future client, and you should adjust your contingencies appropriately.

It doesn't sound like an axial or mixed flow pump would be a good selection for boiler feed pump, so not sure why you even mention, but a good point for general use. I usually see multi-stage double suction centrifugal pumps, with bypass piping to cover low flow high head operation.

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#8
In reply to #7

Re: Non Overloading Impeller

09/27/2016 10:16 AM

Agreed, if he's sure the delivery head won't fall below a certain figure, he can use that, provided he can get it past the spec, or re-educate the client! I wouldn't know, but he might have to be careful of start-up or test conditions, when the head might be lower.

I was making a general comment about axial or mixed flow pumps, wasn't suggesting they'd be suitable for boiler feed. Usually they're for high flow/low head applications, as I'm sure you know.

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