Characteristic curve - Iso Efficiency curve

What is Iso efficiency curve? Performance curve with different impeller diameters are plotted against the co-ordinates (x and y). Q-H curves are plotted for each impeller dia. after taking actual running test. (similarly Q-n curve is plotted). The next step is to draw parallel lines to x-axis intersecting the Q-n curve. It is necessary to select points on each Q-n curve for the same values and project upwards from these points to intersect Q-H curves. Marking and joining these lines makes what is known as Iso Efficiency Curves.

This curve serves very well for indicating efficiency zones at a glance. When new pump is to be selected, this graph comes very handy as we get to know the various impeller diameter performance. we can select proper impeller dia considering future demand/ reduction in demand. If we have iso eff curve of diff model pumps, we can easily figure out which model to select depending on our present and future demand.

Characteristic Curve Best Efficiency Point/Zone

What is Best Efficiency Point/Zone.

It is always advisable to operate pump at BEP (Best Efficiency Point). But most of the time it may not be possible. Hence as a thumb rule, -10% to +10% of BEP (B) is known as BEZ (Best Efficiency Zone) (zone between A-C) .

It is not only economical to run the pump at BEZ(Best Efficiency Zone) (A-C), but it is also called as sweet point of the pump. That means, pump operates smoothly at this zone.

What if pump does not operate at this zone? Well there are many things that take place if pump is not working at this zone. The most important is Vibration. Why? There are many explanations which I will not discuss in depth in this version. Just to satisfy curiosity, cavitation is one of the factor. Design factor is another, where the pump is designed to operate at BEZ(Best Efficiency Zone) where hydraulic radial forces are taken care of. In case pump is operation to right or left of BEZ(Best Efficiency Zone), the hydraulic radial load is not balanced and it is passed to the shaft.

It is advisable to select a pump with duty point on the +10% side (C) i.e. right side of BEP(Best Efficiency Point) in new installation. We can always throttle the pump and bring the SH curve to intersect QH curve at BEP(Best Efficiency Point) (B). This is done in case in future, more demand of flow is required, pump will be still operate within BEZ(Best Efficiency Zone).

Characteristic curve - Duty Point

What is duty point? Please refer last chapter S.H.Curve.  When SH curve is superimposed over QH Curve, the SH curve line cuts QH curve at a point. This point is known as duty point.

That means the pump operates at this point. If we ref combined curve, vertical line dropped from this point, will give other parameters of pump such as power it is consuming, the flow it is generating etc. And horizontal line from this point will give the head developed by the pump.

System Head Curve

This is called System Head Curve. Pump curve supplied by the manufacturer indicates the increase in total head that takes place between the suction and delivery nozzles of a pump expressed in m.

However a pump operates in some systems where it has to overcome pipeline and pipe fittings resistance measured in meters. It is also known as pipeline characteristic.

Total head consists of there components at a defined discharge.

1) Static Head or static pressure dereference in m.

2) Friction losses in m (pipe and fittings losses).

3) Discharge (exit) velocity head in m (Vd²/2g).

The summation of these components constitute total head, H in m.

Pipeline Losses

Here we will consider point 2 in depth as it is a varying component related to the flow rate. The component is also called System Head (S.H). This plays an important role in determining total head H of the site under consideration.

When friction head is plotted against the discharge, a parabolic curve is obtained. Its because friction head varies as the square of the discharge. This curve is called System Resistance Curve (S.R). But popularly known as System Head Curve (S.H). This curve represents the relation between the pipe friction and flow rate. Reference plan for SH is taken as static head. Please refer fig. With increase in flow, resistance in the pipeline also increases. Hence a parabolic curve starts from the reference line i.e. static head.