Using Spec Sheets to Enter Curve Fitting Values

[Samantha]

Hi ya'll,

Can someone tell me what i'm doing wrong please. Let me start from the beginning.Ok, first i get the spec sheet for the part(1" 2-way Electric Control PB1). On this spec sheet, there is usually a graph/chart that shows the relationship between PSI vs flow (attached ). I choose three values from the graph(3.1psi to 22gpm, 4.4psi to 30gpm, 6.5psi to 44gpm) and i enter them into the Curve Fitting Table-> Value Pressure Loss From Flow (Formula H=KQ^n).
I press Fit Curve and i look at the calculated values and they are wrong. How do i know they're wrong? Well, since there is a Netafim database (as a test run), i chose a valve that i know we use, and found the graph for it, entered a few values from the graph, and it did not match what the Netafim database came up with for the K,n values. Am i doing something wrong? Please help.

Thanks,
Sam

[Matt Binder]

Hi Samantha,

I don't believe you are doing anything wrong it is just the Netafim database used a different method of calculating the headloss (Kv) and therefore, gives differeing results in the K & n values but will produce the same outcome.

A good way to check your created valve is to use it in a design and use a higher flow through it to compare with your friction charts. I would recommend plotting at least 4 points along the friction chart in the database editor.

[jovivier]

Samantha,

I notice the curves do not go through zero on the charts so use the Valve Pressure Loss from Flow, with Intercept option. The Intercept is the C value and means a start-up head (pressure) is required before the valve can open and therefore the friction loss curves cut the Y axis above zero.

It is also best to enter as many points as you can - 3 is the minimum required but ideally more will give a more accurate K, n and C as Matt suggests.

[Samantha]

Thank you both for your help,