When selecting a pump you will need to know if it is fit for purpose so it will perform efficiently throughout a healthy and long life. The main way of doing this is to check the pump’s curve by using a graph which displays a pumps duty under different conditions – this can normally be found on the pump’s datasheet and looks like the below:
At first, this graph may feel a little overwhelming but I promise you once you have read this article it will make a lot of sense.
This graph has a number of symbols and labels such as H (m), Q (l/s), Q (l/s), a continuous line, loads of numbers and ‘U5K’ inexplicably hovering in the top left corner. Let’s go through them one by one and give them some meaning.
This simply denotes the name of the pump the graph refers to. In this case the pump curve is describing the Jung Pumpen U5K pump which is a submersible pump mainly used for drainage.
The formatting of pump curves differ depending on the manufacturer of the pump and what the curve is intending to show. For example, if the graph was being used to describe multiple pumps there would be a line for each of them and each would have their own label.
H stands for ‘Head’ which is a term used to describe how much pressure the pump will have to overcome. Most of this pressure comes from the resistance caused by gravity when pumping the water vertically, however, some ‘head’ can be caused by other factors but this will be covered by a separate article.
The (m) stands for the unit of measurement (Meters), therefore, H (m) is Meters Head or Total Meters Head. To keep things simple, imagine Meters Head as the vertical distance between the position of the pump and discharge point – technically known as Static Head which is a major factor of Total Head.
As the ‘H (m)’ is labelling the horizontal axis of the graph we know the numbers that run up the side of it must describe the number of Meters Head, making the U5K have a maximum head of just over 8 meters.
The other axis of the graph describes the flow rate which is the amount of fluid that passes a given point per unit of time. This can be described in various ways but this graph primarily uses ‘Q (m^3/h)’, I will break this down below.
Q = Flow Rate
M^3 = Amount of fluid in meters squared
h = Time frame in hours
Therefore, when we look at the numbers that run along the bottom of the graph we can see the maximum flow rate (Q) for the U5K is about 11.5 cubic meters per hour or 11.5 (m^3/h).
In terms that are easier to visualise the U5K can pump a maximum of 11,500 litres per hour (one cubic meter is the same as 1000 litres).
This is just another way to express the same as the above but using different units. This time the flow rate (Q) is described using litres (l) per second (s) and has its own scale (0-3). On this scale we can see the U5K has a maximum flow rate of about 3.25 l/s.
One way of confirming this is to divide the previous figure of 11,500 l/h by the number of seconds in one hour (3,600), which gives us 3.19 l/s which is very close to the sum we came to a moment ago (3.25 l/s).
OK, so we now know how to read the maximum head and flow rate on the graph. Now it is time to look at how to see if a pump is suitable for a particular application. This is where the continuous line comes into play.
Say we have a need for a pump that can pump 10 cubic meters per hour (m^3/h) at 4 meters head [M (h)] and we wanted to know if the U5K would be suitable for the job. We could use the pump curve (continuous line) to find either what flow rate could be achieved for a given head or vice versa. Let us input our requirements one at a time.
If we start by finding the required head (4 meters) on the vertical axis and follow the horizontal line to where it meets the pump curve. At this point on the curve follow the line down to the bottom which brings you to a point just before the number 8 which tells us the following:
At 4 meters head this pump can achieve just under 8 cubic litres per hour [8 (m^3/h)] which is less than what we require meaning this pump is unsuitable.
N.B. It is good practice to find the flow rate for a given head rather than the other way around.
Sometimes, when sizing up a pump you find the requirements fall on the pump curve but it is close to one end or the other. This means the pump will be able to do the required but it is still not ideal. Generally speaking, a pump is happiest while working around the middle of the curve.
Imagine what is happening while a pump is working on the far left of the curve. Although it may be delivering the desired flow rate at the given head, the motor will be running at the upper edge of its comfort zone while pumping less fluid than optimal. This means there is less opportunity for heat to be displaced as fluid is the key component in cooling the pump. Due to this, it is likely the life of the pump will be significantly shortened and the pump will become very uneconomical.
On the flip side, if a pump was performing on the far right of the curve the motor will be almost freewheeling as there is far less head, therefore, less pressure to overcome. A pump running continuously at maximum speed will soon have its impellor degrade and experience cavitation which will drastically reduce its efficacy and it will be far less economical.
In conclusion, when sizing up a pump you will need to know what you want to achieve in terms of head and flow rate and look at the pump curves for any pumps you think may be suitable. It will be a case of trawling through all of the information available for each pump until you find something you are happy with.
Alternatively, you could give us a call on 01622 882 400 and one of our friendly staff will help advise on the most suitable pump for your requirements, totally free of charge.