What are nits anyways
joefitz said:
...Is it suggesting that our pumps should be run at a rate less than "full"?
Yes, although the curve isn't making that suggestion. The curve simply tells you the optimal point at which to run a pump. Engineering suggests that you run a pump around its best efficiency point.
Bear in mind that each pump has its own performance curve, and performance curves are like finger prints or snowflakes; no two are alike. Sometimes, the best efficiency point is closer to maximum flow. Other times it is closer to maximum head. However, because the pump wasn't designed by monkeys, the designers probably put the best efficiency point (BEP) at a practical location on the curve.
joefitz said:
...Or that our pumps are more efficient when there is head pressure?
Yes, although again, that one curve shown is just representative. Due to the Affinity laws, and the design intent behind the pump, pumps are always most efficient when some head is applied.
joefitz said:
...From what I can make out, it appears the curve that the blue dot is on represents the "ideal" (most efficient) operation...
Yes, although it is a little more complicated than that. The curve that is concave up represents the system curve; the curve corresponding to head loss through the plumbing versus flow rate. This curve will intersect the pump curve, and as you can see, the pump was chosen correctly in that its unique pump curve intersects the system curve near the best efficiency. In practice, we are rarely that lucky.
joefitz said:
...The 10/8/6 in dia. curves (concentric circles) represent the diameter of the pipe?
No, you probably should ignore these. These concentric curves show how the pump curve changes when you change the diameter of the impeller. Each of those curves is like for an impeller that is an identical, yet scaled down version of the other. The 10", 8", 6" are impeller diameters.
However, you will notice a dotted curve that is concave upwards.
This curve shows what would happen if you changed the plumbing in your system so that the head loss is greater. Say you have a throttling valve (ball valve, of course
) on the pump discharge, if you throttle the valve more closed, the system curve changes, and it intersects the performance curve of the pump at a different point.
joefitz said:
And the 50/55/60 curvs (concentric circles) represent the efficiencies (with the rightmost intersection with the "ideal" curve representing the highest efficiency). Is that right or am I off base?
Yes, you are right. There is a 45%, 40%, etc... curve outside the 50% one, but these aren't shown.
joefitz said:
And how can we apply that graph to our own pumps?
Well, again it is hard to use
that specific graph for anything other than a general understanding of how centrifugal pumps operate, and the many variables involved in their operation. Suffice it to say, if the manufacturer has a performance curve available, you will estimate the head of your system curve, and make sure that the system curve head is not too close to the maximum discharge or maximum flow condition. Without best efficiency curves, it is hard to make any further decision. No pump manufacturer that I have come across gives out efficiency curves.
joefitz said:
It appears to be suggesting that some amount of head pressure is necessary to make the pumps operate most efficiently...
Yes. Again, pump efficiency is the ratio of power actually gained by the fluid to the shaft power supplied.
joefitz said:
I would think the amount of head required would greatly depend on the pump itself and its operating characteristics. Some pump seem to be designed for high head situations while others are not so I would think the amount of head pressure required to operate efficiently would be quite different for these pumps. Could it generally be stated that most pumps we use in our systems probably have enough head introduced by our return feeds and plumbing that there is no need to "artificially" increase the head pressure (by potentially throttling the pump)?
Yes, again, remember that each pump is unique. The manufacturers have designed the impeller for a specific set of operating conditions. Also, must of us have enough head in our systems that unless you were using a high head/low flow pump for a system with very little discharge head, you will be okay, and even then, these motors are resilient.
joefitz said:
And then there is the "practical" matter of the application of the "ideal" to the real world. Do you think it is a bad practice for us to be running our pumps at full throttle?
Again, it depends on the pump, but I would say probably not. The manufacturers have designed these pumps for reef use. Still, look at the pump curve, the closer you are to the middle of that curve, the better off you are.
Matt