Pumps and friction

Pumps and friction losses are an important concept to grasp.

There is only one pump that doesn't care about friction. Gravity is a great, free pump that we can all and should use wherever possible.

OK if you have checked out the Pump Curves page you will understand that the higher your pump has to pump water the less water it will be able to deliver. This is particularly true when opting for low power systems. What this means is that you need to be careful when designing your pond and filters.

It is easy to build a Koi pond filter system that will run on less than a total head of 200mm. In such cases you can use extremely efficient pumps that will deliver in excess of 100l/hr/w. In other words, 100W of power in such systems will move 10 000l/hr through your filters. But we cover this in more detail on the website. 

In conventional terms every pipe, every elbow and every peice of equipment that comes before and after your pump presents an obstacle that the pump has to overcome.The work done in forcing water through a pipe, around a few elbows and past filters of various descriptions is work that the pump has to do as opposed to directing its energy in moving water.

It's called friction losses. It is exactly the same concept as 'drag' in motor cars. The faster you go in your car the more work the engine has to do to overcome the friction drag of the air around your car. If there was no air or friction losses, the smallest car would be able to travel almost infinitely quickly.

Exactly the same thing happens in pipes and with our pumps. The faster the water flows, the more the increase in drag.

We have seen people go to great expense and trouble to replace short bend 90 degree elbows with long sweep bends. With the velocity the same in the pipes, it will make just about zero difference.

Flowrates are improved in one way only. Slow the water velocity down, and you minimise the drag. Minimise the drag and you minimise the friction losses.

So, although it is inconvenient use 63mm pipe for your Koi pond pipework. Present your pump with two of these at every stage. There is not a single practial reason that we can think of for running your water at a rapid velocity through your pipework.

Some simple mathematics.

Consider a  50mm pipe. Assuming the internal diameter to be 44mm (allowing for a 3mm wall thickness) will give you a surface area across the pipe diameter of 1519mm^2 using good od pi*r^2.

A 63mm pipe of 3mm wall thickness will give you 2550 mm^2, some 67% more. In other words assuming a constant 10kl/hr passing through the pipe, the 50mm pipe will give you a water velocity of 1.828 m/sec. In a 63mm pipe this velocity will drop to 1.08 m/sec, some 67% less.

As with air friction the friction losses increase with the square of the value of the speed. In other words, the friciton in air at 80km/hr is more than double that at 60km/hr.

For water, because it it more dense and viscous than air the friction losses are amplified. Hence the faster you go in water the more quickly the friction losses build up. It's why submarines can't travel at mach 2.

Hence, a good rule of thumb is to keep friction losses at a minimum is to run at around 1m/sec or less through your pipes. If you keep this velocity, you can have as many elbows and bends as you like, and it will play a far lower role in restricting your water delivery.

It should be clear you cannot do that with 50mm piping, unless you are running at around 6kl/hr. Hence, if you have a 20kl/hr pump you should have 4 independent 50mm lines. Or two 63mm lines.

Note that the same applies to your suction line to your pump! In most cases this is less of a problem than the discharge line, but bear it in mind, especially if you have multiple bends and the like.

When it comes to water, speed is the enemy!