Torque vs Speed Pump Curve
Torque vs Speed Pump Curve
(OP)
Question:
I have noticed pump companies provide speed vs torque curves for their pumps. These curves are % Max Torque vs % Max Speed. This implies that this curve can be applied to any pump so long as motor rpm and torque (at synchronous operating speed) is known.
My question is 2 fold:
1. How can you make this assumption? How can you assume the torque is proportionally the same in two completely different types of pumps? It seems long multistage pumps (Reda or Woodgroup Style) would have far more friction than a simple stage OH2, for example. Maybe I am making an underlying assumption somewhere?
2. How is this curve developed, and why does it start higher (around 15% torque at 0 rpm),then decrease to 5% torque at 12% Speed? Since this is not a transient curve, and (I think) rpm is assumed to change slowly, it seems % torque should always increase, starting from zero, and inertia effects should not show up.
I hope this is worded clearly. Thoughts anyone?
Thanks,
Theron
I have noticed pump companies provide speed vs torque curves for their pumps. These curves are % Max Torque vs % Max Speed. This implies that this curve can be applied to any pump so long as motor rpm and torque (at synchronous operating speed) is known.
My question is 2 fold:
1. How can you make this assumption? How can you assume the torque is proportionally the same in two completely different types of pumps? It seems long multistage pumps (Reda or Woodgroup Style) would have far more friction than a simple stage OH2, for example. Maybe I am making an underlying assumption somewhere?
2. How is this curve developed, and why does it start higher (around 15% torque at 0 rpm),then decrease to 5% torque at 12% Speed? Since this is not a transient curve, and (I think) rpm is assumed to change slowly, it seems % torque should always increase, starting from zero, and inertia effects should not show up.
I hope this is worded clearly. Thoughts anyone?
Thanks,
Theron





RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
Patricia Lougheed
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RE: Torque vs Speed Pump Curve
I have seen this this same thing from multiple different manufacturers. Thus, my question.
RE: Torque vs Speed Pump Curve
There's also friction (static and kinematic) to account for.
You'll be lucky to find out what the actual proof looks like.
RE: Torque vs Speed Pump Curve
Most pump curves I have seen are geared to specific pump or small group of pumps.
However in absence of pump curve, sometimes it is assumed for purposes of motor starting calculations that torque is proportional to speed squared (in which case you just need one point to determine proportionality constant... not a whole curve). I believe that is related to pump laws under certain idealized conditions.
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(2B)+(2B)' ?
RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
T = 3300 * SG * dH_ft * Q_gpm / (3960 * eff * 2 * pi * rpm)
can be plotted for any speed, remembering that for a given point on a pump curve at one speed, the head and the flow will change as speed reduces, but efficiency is assumed to remain equal for all speeds, which is "nearly" true.
The flow varies directly with the ratios of speed, Q2 = Q1 * RPM2/RPM1
and head varies by the ratio of the speeds squared,
H2 = H1 * RPM22/RPM12
If you looked at the BEP full speed point and saw an efficiency of 0.76, it would be assumed to be equal at all speeds when sketching up a theoretical Torque vs Speed curve and the curve begins at 0,0 and increases with speed squared.
In a real curve the efficiency, being much lower than 0.76 at very low speeds, dropping to around 20% eff at 10% of maximum speed, due to break away effects of bearing and stuffing friction EPete mentions plus internal flows not being fully developed at low speeds, causing the Brake Power and Torque to rise above the theoretical curve for all speeds lower than around 10% of maximum speed. That is pretty much true for all centrifugal pumps, as are the affinity laws for flow and head, so the curves for one point on a pump curve modified for a speed change will all follow the same tracks as speed decreases and all will have the same shape, even though the actual shape of the pump curves may be vastly different between different types of pumps.
The Torque vs Speed curve you are seeing only represents one point on the pump curve as it is modified by % speed. Since one point on the pump curve does not make a full curve, what you see is not dependent on the shape of any particular pump curve and the result is the same for all pumps.
RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
Thank you for the thorough reply. I may assume the curve the manufacturers provide is basically a general curve, and by the definition above these are not exactly the same but only the same in characteristic.
Thanks!
-Theron
RE: Torque vs Speed Pump Curve
They also base their curves on water at a specific temperature so any variation you have from that has to be taken into consideration.
They also prevaricate sometimes so if you want to be absolutely certain about the curves, test the pumps yourself. I worked for a company once that did just that in a test lab and we knew which pump curves we could trust and those that had to be taken with a grain of salt.
On the other hand, pump physics is a pretty mature science so they can't be too far off or their competitors would nail them in the market place. I have no problem using pump curves as published when sizing and selecting pumps. Unless I'm dealing with NPSHr curves, and then I add my grain of salt.
rmw
RE: Torque vs Speed Pump Curve
GaTechTheron, I've never encountered a pump manufacturer that would not discuss their pump's actual characteristics forthrightly when the questions were posed in good faith, intelligently, and with adequate information being provided regarding the actual pump installation and its connected system.
Unless a very large or important (translation--very expensive) pump is involved (where pump-specific, witnessed test bed derived performance curves are included in the purchase price of the pump), it is important to recognize that the generally published data is only representative of the performance that can be expected within about 10%. Although there may be some optimism in the published data, the variations that a specific pump may display (when compared to the published data) are primarily the result of reasonable manufacturing variations and tolerances.
Just for the record, I have never worked for or had a financial interest in any pump manufacturer, distributor, ....
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Torque vs Speed Pump Curve
These conditions are rarely if ever found with site testing, hence the continual cry from end users "the curves are wrong, the pump doesn't perform as we expect" etc etc.
It's always easier to blame the pump / manufacturer for all site / installation short-comings than accept faultly installation, suspect measuring equipment or under conditions other than what the pump was selected to achieve.
Just for the record I've work for manufacturers, suppliers and for end users so have seen all side of the problems and can say from experience that only "sometimes" is it actually the pump at fault.
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.)
RE: Torque vs Speed Pump Curve
Great quotation! What excellent advice for all to observe!
Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.
RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
If you search the net there is a number of references to this subject, this is one of then.
http://centrifugalpump.org/speed_torque_curve.html
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.)
RE: Torque vs Speed Pump Curve
RE: Torque vs Speed Pump Curve
rmw
RE: Torque vs Speed Pump Curve