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.

Characteristic Curve Combined curves

This is the combined curve. That means, it has a combination of curves that we have studied so far. Note:- in this curve, I have not added NPSH curve as it was not practical to put it from visibility and clarity of the graph.

Most of the time, manufacturer will be providing this graph. Here we can see different parameters such as power, flow, head and efficiency in one single graph.

It is very important to learn this graph as most of the problems on field related to pump and pumping system can be solved by studying this graph along with the data taken on site.

This graph also helps to explore the possibility of energy conservation. This is an excellent tool to understand and implement energy saving measures. In this version, we will not be focusing on this topic, but just for reader to understand I am giving following example.

Suppose we have an old system which was designed during project stage.The system has a single pump pumping water to an overhead tank at the rate of 200 cu. m/hr. And as a safety, the o/h tank was allowed to overflow at the rate of 20 cu.m/hr back to the supplying tank. Initially, this pump must have been selected to operate near BEP (Best Efficiency Point). Along the years, due to system improvement, head was reduced (one heat exchanger is removed which reduced the frictional head, this will shift operating point further right of BEP).Due to this, 10cu.m/hr more water is pumped to o/h tank. This means now o/h tank is overflowing 30cu.m/hr of water. Now if the system runs as it is, operating point has shifted to right side of BEP. This means efficiency will be reduced. If we throttle discharge valve, we can bring back the operating point back to BEP. At BEP, eff was 75% and pump was operating to right of BEP at 66% eff, so by throttling, we can save 9% of energy (only from efficiency). Actually, power savings will be more as duty point has shifted towards left, where power consumption has also reduced.

Characteristic Curve NPSH curves

NPSH Curves(Fig-1) shows both, NPSHa and NPSHr. NPSH stands for Net Positive Suction Head. Now what is this?. Pl refer NPSH chapter for details.

Now, Please refer this graphs just for illustration. Here Y axis is not to scale. Means, you can consider two graph overlapped on one just to show the diff. As the flow from the pump increases, NPSH required by the pump keeps on increasing. At the same time, NPSH available in the system keeps on decreasing. When the flow is q1, NPSH available is na1 at point A which is more then na2 at point B where flow q2>q1. Similarly, NPSH required at point B is more then NPSH required at point A.

Curve-2  This graph shows NPSH available and NPSH required in same graph in with Y axis showing head in meters. As flow increases, the margin between NPAHa and NPSHr reduces. The point A, at which flow is q, the margin is known as Safe pressure margin at impeller eye over liquid VP. The safe margin goes on reducing from point A to point B(where it is zero). For proper functioning of the pump, NPSHa must be always more then NPSHr by at least 0.5m.Note:- For every pump, a particular force (Head) in terms of mlc is required for every speed and flow rate to prevent the liquid from vaporization.