Hey
I am trying to find out how to calculate a combined pump curve. I have made the two pumps indivdual. But i can't figure out how to do the combined.
I have looked at this thread Liquid Flow Through Pumps in Parallel
ACCEPTED SOLUTION
Accepted Solutions
I just noticed that the flwn does not hit 4.5 bar. Could that somehow be done in another way. Maybe by programming?
I have just bought the book "Essential PTC Mathcad Prime" to improve my mathcad skills 
Well, it does, its just not zero as you'd like:
The function "polyfit" does a least-squares development of the coefficients of a polynomial fit, in the case of the graph above a second order polynomial. A third order fit got me 2.899 m^3/hr, Fourth order got 2.401, fifth got 1.819. You can do other fits--a spline fit will pass thru points exactly. Be careful, a seventh order polynomial seems closer to reality at 4.5 bar, butstrange things start to happen:
How close is "close enough"?
> Close enough is when i hit all the given marks, and I'm having a smooth curve
I guess you just have the choice between either an interpolation which hits all given 11 points, but is not perfectly smooth OR a regression of some kind which can be smooth, but will not exactly hit all points.
Here are a couple of regressions:
Polynomial fit of fourth order
You may try polynomials of higher order, too. You will get a better fit, but more "bumps" and probably will see it as less "smooth". If you choose order 10 or higher you get a function which will exactly hit all 11 points, but you won't like the outcome, I guess.
Here is a rational function which seems to fit pretty well
Localized polynomial regression using the built-in "loess"
Maybe something of use for you here. You may try to fit any type of function you think is worthwhile using genfit.
Here is another example using exponential association
WE
When the discharge pressure gets to 4.5 bar, the pump cannot push any more fluid. A curve that is built from the data points would predict a negative flow at a higher pressure; I chose to assume that for higher pressures the pump would stop producing flow but would resist attempts to push fluid back thru it, So flw becomes zero if the extrapolated curve is negative: 
You can simply replace flw with vsn if you so desire:
But you better be able to explain the negative flow.
I have attached an image which shows the difference between "flwn" and "vsn", and the combined curve .In the combined curve I would like to replace flwn with vsn, but I can't figure out how to do that. I hope I made myself understandable.
Fred already explained what he did and as you can see are "flwn" and "vsn" the very same function, the only difference is that "flwn" is set to 0 when "vsn" would yield negative values. So approx. from p=4,5 bar up "flwn" is zero (for good reason) while "vsn" would give you technically wrong negative results. Using the "max" function Fred avoids using an "if" condition.
But at first I am not sure if you would like to plot Qn over Pn, as Fred did, or if you rather would like plot Pn over Qn as the picture of your spline interpolation shows.
And then I still don't understand, why using "flwn" is a problem for you and using "vsn" is not. If the problem is just the name, you could equally well (re)define "vsn" after it is created by typing vsn(p):=max(vsn(p), 0 m^3/hr) (you don't even need to add the units for the zero value, but is sure is good habit).
You could redefine "vsn" in different ways, e.g. using the if-function
but this is not elegant and efficient as it would mean, that the function has to be evaluated twice in case the value is positive.
So we could use a programmed if-condition
but if you compare this to Freds solution, I guess you see why his way to make use of the max-function is the preferable one.
WE
I think I see what you mean.
In Freds sheet vsn hits zero at 4.623 bar.
If you chose a fourth order polynomial instead of Freds second order, the hit is at 4.548 bar. Would this 0.05 bar really be noticeable in a plot? After all, a regression is an approximation anyway (as is a hand drawing). And you don't gain more precision with what you do.
BTW, you dont have to deal with > and = in two different branches. You can use
or even shorter with the if-function (similar to Excel and similar unclear to read as well)
WE
Thanks a lot
This not a question for this task, but could I by programming leave out a part of a curve by programming for example all values below 4.5 bar and values above 5. bar
Can I type in anything that says not possible?
I have tried doing it like this, but I'm not getting the result that I would like to.
I've made to lines in paint, and i would like to leave out everything that is not between these to lines, and I thought I nailed it, but I didn't.
