Pumps in parallel

That is excactly how it should turn out.

I was wondering how I could learn more about MathCad, there is so many things I would like to learn. Do your know any websites besides this page?

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"?

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 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

Sure you can. You may also combine two constraints with the AND operator rather than using "also if".

If you return a text like "not possible" for illegal arguments you may run into problems whith calculations using your functioon (you can't do math with text) but you may return 0 or NaN (Not-a-Number) instead.

What did you expect to see?

You function HM never ever returns a value which could be plotted.

You have to say which value should be returned if pp is in the range from 1.5 to 3 bar!

Probably using soemthing like "else HM(pp)" at the end.

Even better would be

if 1.5 bar <= pp <= 3 bar

     HM(pp)

else

     NaN