Starting currents (locked rotor amps) of 2x to 3x times only lasts for a fraction of a second when compared to full load amps (FLA). For some units, this LRA can be 10x to 15x running currents (FLA). This is the case for across-the-line start. If you have a soft start (auto transformer, or solid state type), these can be shaved significantly. A variable frequency drive completely takes the LRA out. This is NOT the reason for starting pumps against a closed valve.
Starting low specific speed pumps against closed valve is almost always recommended. This is typically done to reduce the sudden shocks to the system - think transients. Power at zero head is not a big deal usually as it is much lower than running horsepower at rated conditions. If mechanical seals are specified, then the shaft deflection at the stuffing box should be carefully calculated not to exceed 2 mils (typical design criteria for mechanical seals).
For high specified speed pumps - typically and higher - the power at shutoff head (zero flow) starts to rise higher than rated power. At even higher speeds, pressure rise and power rise at shutoff is dangerously high - particularly so for axial flow pumps. For this reason, you will not see a closed valve on a very low head, high flow pumps - think storm water pumps here - New Orleans, Southern Florida, Houston, etc.
In water distribution systems, specific speeds are usually between and and starting these pumps against a closed valve which takes anywhere from two to eight minutes is VERY common.
But I also see a reduction in peak starting amps.
The highest instantaneous current will occur within the first half cycle (8.3 millisconds in 60-hz land). Consider how little difference in speed there is going to be in the first 8.3 milliseconds, remembering that acceleration torque must accommodate both inertia and pump torque.... BUT the pump fluid torque is proportional to speed squared (neglecting the friction component which does not depend on lineup). If the pump is accelerating from 0 to 1% full speed during that first 8.3 milliseconds, then the fluid torque is changing from 0 to
one ten thousandth of full speed torque. Any difference in speed resulting from a difference in that minuscule component of fluid torque during acceleration over that very short time period is neglibible. Speed is effectively the same in both cases over the first half cycle. There is no reduction is highest instantaneous current. Your instrumentation is not measuring the highest instantaneous current, but performing some kind of averaging. I will say the terminology for describing this type of measurement is tricky. The true peak instantaneous can be up to 2*sqrt(2) time the (rms) locked rotor current. The true peak is obvious if you have an waveform recording. Many instruments will try to give an RMS which is not an obvious conversion for a non-sinusoidal waveform.
Accordingly, the instantaneous relay settings can be determined for a given motor without considering the fluid lineup. The overload relay settings do consider the fluid lineup.
That link is confusing. "Opening up on a system throttle", and "adjusting flow in the direction towards BEP". Sounds like double negatives to me. For a straight centrifugal pump I would have said "closing down on the throttle valve will decrease BHP and current draw", which is not the case with axial and mixed flow impellers.
The link is my FAQ, so I'm always interested in comments that might improve it. But I think you're missing some context in your quotes.
"Opening up on a system throttle valve". The word throttle is used as an adjective indicating the normal function of the valve (we could almost drop the word throttle, but at least it clarifies we're not talking about a recirc valve). Opening is a tense of the relevant verb. It does not seem ambiguous to me.
In a completely different sentence, I said
"for mixed flow centrifugal pump, the curve is non-monotonic. As a very rough thumbrule peak BHP is near BEP so current increases when adjusting flow in a direction toward BEP.". What I was saying is that it's not as simple to estimate which direction to change flow to increase BHP on a mixed flow pump as it was on a radial or axial flow pump, but (as a very rough thumbrule) the highest BHP tends to occur near BEP.
That particular FAQ has also been controversial for the statement that axial flow pumps and mixed flow pumps are "commonly referred to as
"centrifugal pumps"". I am not saying the principle is centrifugal, only that many people (including some textbook and standards authors) lump radial, mixed and axial flow together under the term "centrifugal". Likewise I observe plant personnel often do not differentiate these types of pumps. But even though some people may refer to them all by the same name, there are differences that become important when you are trying to talk about expected bhp vs flow characteristic. The very fact that the common terminology does not match what an engineer might conclude considering the pump principles is a good reason to mention the terminology imo.
If anything, maybe I should enhance the standard caveats - consult manufacturer's curve for your specific pump rather than relying on these generalities. I also agree my FAQ is now confusing since the 3rd party link that I provided is now broken. Graphs would be nice, along with a label "typical"
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(2B)+(2B)' ?