The “best” depends on the user’s assessment.
FIXED – A fixed pressure at an input point typically a tank OR if the required input pressure is being determined then a guess pressure (typically 1000 kPa) in which case only Pressure 1 is entered.
If the input pressure is not a constant, but a curve (e.g. if the input is a pump, the pressure will drop off as the flow increases), then the curve can be described in one of three ways:
LINEAR – signifies a pump or variable water supply pressure being entered as a series of (at least two) points on a curve of pressure vs flow. The program linearly interpolates between the entered points. For better accuracy, the points should be closest in the vicinity of the operating point.
CURVE – signifies a pump or variable water supply pressure but in this case, a series of coefficients are calculated from the entered values of flow and pressure for a polynomial fit to the points. All eight points must be entered. This may be a slightly more accurate method of describing a pump curve than the LINEAR method but remember that the curve will not necessarily pass through all points (least squares fit).
The polynomial takes the form:
p = a + bq + cq2 + dq3
where p is the pressure and q is the flow and
Q18 – signifies a variable water supply pressure where the pressure between each pair of entered points is proportional to the flow to the power 1.85. This is typically the data supplied by a water supply authority.