Significance of Pump end curve 2

[mechengineer80]

Please explain the significance of this statement "Capacity at end of curve shall not be less than 120% or 110% of rated flow" . if a pump does not fullfill it then what will happen if the pump operates at 120% or even above it . any mechanical effect on pump?

 

[Artisi]

Seems to be some conflict with the statement, 120 or 110, needs to be one or the other, can't be both.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[1gibson]

Mechanical effect on the pump, you are operating at conditions that will not have guaranteed vibration levels, will not have been considered for motor sizing or NPSH requirements, and will not be covered by manufacturer's warranty in the event of premature wear or other damage.

Poorly worded for a few reasons, but most important is runout flow should be based on the % of pump's best efficiency point, not on the pump's rated flow. The assumption that these flows are going to be the same or very similar isn't always valid. That language will require a less than ideal selection (hypothetically, say rated point is 115% of BEP) to operate in a potentially dangerous (from a reliability standpoint) region of 1.15 x 1.2 = 1.38, 138%) of BEP. That can be a pretty tall order depending on pump type, and to meet that requirement may lead to excessive motor size, excessive NPSH requirements, and difficulty passing vibration during performance test.

Strictly speaking, it can't be both 110 and 120, because the requirement is 110 or 120."Or" can't include "and." If you posted this in the language forum, I would argue that meeting both (end of curve flow more than 120%, which is more than 110 AND more than 120) would not comply with the specification. If I asked for my car to be painted red or blue, I would not be happy if it ended up striped, polka dotted, or purple.

So, that spec puts the end of curve flow firmly between 110% and 120% of "rated flow" which is oddly specific and has little to no basis in reality. Unless your reality includes relying on a lot of assumptions being correct, and that reality is not engineering.

 

[rotw]

1gibson,

Quoted from the OP
quoted
Capacity at end of curve shall not be less than 120% or 110% of rated flow
unquoted

This statement could imply that it is needed to have a wider curve to the right (i.e. the curve extend by xx% from Rated flow). Reference to BEP would be irrelevant. For instance you can imagine a situation where the Rated flow is 110% of BEP and then have the end of the curve extending only by 5% from Rated flow. This would not fulfill the quoted statement while I guess it still satisfy the requirement of having a sound pump selection with respect to the BEP. Nevertheless I've never heard about such requirement, more typical of cent. compressors I'd say.

Apart from the "or" issue, what then confused me is that the OP added :

quoted from OP
if a pump does not fullfill it then what will happen if the pump operates at 120% or even above it
Unquoted

Also just a note on the operation to the right of the curve: I have heard that when a pump operates on the right of the curve it results in a low DP over the pump and then a reduction of the Lomakin effect leading to poor damping capability of the seal and ultimately rotor-dynamic issues (severe or less severe depending of the type of the pump).

Regards

 

[LittleInch]

As 1gibson says, I suspect this is typo in either the OP or what he is quoting from and it should say " 120% of BEP or 110% of rated flow." That makes sense. What that means is that the pump and it's motor is capable of more flow beyond the rated limit without overload of the motor or excessive vibration etc of the pump.

Operating beyond these limits could damage the pump or overload the motor.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[mechengineer80]

Dear Floks
Let me correct my statement to remove any confusion. that is "Capacity at end of curve shall not be less than 120%of rated flow"( 110% was just written to mention that perhaps some companies can go for more stringent criteria. Any how rotaryworld pointed well the phenomina of reduction in "Lomakin effect "for operating pump above this range, which i needed to know. Thanks for all.

 

[PumpSmart]

Simple.
You could cut performance curve to 110 or 120%.