Or, more commonly, “How far up will a pump suck?”
A common question asked on our hire desk is “how far up will a pump suck?”. The simple answer is, at least in theoretical terms, 10.34 m. However, as simple answers go, it’s a little bit wrong (or at least not quite right). The reason is that the 10.34 m limit assumes perfect conditions: that you’re pumping at sea level, that the pump produces a perfect vacuum, that the water is cold and that there are no friction losses in the suction hose. In reality, the actual limit is 7 or 8 m.
Why a theoretical 10.34 m limit on suction lift?
Part of realising of how this limit arises from comes from understanding the fundamentals of how an end-suction pump operates. Firstly, pumps don’t suck – they produce a vacuum and rely on atmospheric pressure (or more simply, the weight of the atmosphere) to “push” the water up the suction hose. The water flows from an area of high pressure to an area of low pressure – a low pressure region (i.e. a vacuum) is created inside the pump and the pressure differential raises or lifts the water up the suction hose.
The 10.34 m limit is the point where the weight of the atmosphere “pushing” the water up the hose equals the weight of water in the hose.
Why a 7 or 8 m real-world limit on suction lift?
Since real-world pumps aren’t capable of producing a perfect vacuum, a more realistic limit of 7 or 8 m can be calculated. This is the point where the weight of water in the hose equals the force from the difference between atmospheric pressure and the less-than-perfect vacuum the pump is producing.
Maximum Suction Lift Explained in Pictures
Confused yet? Take a look at the diagram below – Column A is a hollow tube with a cross-sectional area of 1 cm² (why 1 cm²? It makes the following calculations a bit easier) representing the suction hose. If one end of the tube is open to the atmosphere (analogous to the pump being switched off), and the other end is submerged under water, then there will be no pressure differential and so no force applies. Therefore, the water isn’t “pushed” up the tube.
Column B is also a hollow tube of cross-sectional area of 1 cm². If the top end of the tube is sealed and subjected to a perfect vacuum (analogous to a theoretical super-pump) and the other end is submerged underwater, then a pressure differential will exist and the weight of the atmosphere will “push” the water up the tube. The height the water rises to, and therefore the maximum possible suction lift, can be calculated as follows:
Atmospheric pressure at sea level = 14.7 psi = 1.034 kg/cm² (effectively the weight of the atmosphere acting on every square centimetre of the water’s surface).
Vacuum Pressure inside sealed tube = 0 psi = 0 kg/cm² (a perfect vacuum)
Weight of 1 cm³ of water = 0.001 kg
(Atmospheric Pressure – Vacuum Pressure) / Weight of Water
(1.034 kg/cm² – 0 kg/cm²) / 0.001 kg = 1034 cm = 10.34 m
So this gives a value of 10.34 m for the maximum possible suction lift under perfect conditions. In reality this will never be achieved – even the most efficient pumps aren’t capable of producing a perfect vacuum and then you still need to factor in friction losses in the suction hose, water temperature (the warmer the water, the less the suction lift) and even the altitude (as you move up from sea level the atmospheric pressure decreases, effectively reducing the amount of “push” available).
So, what’s a more realistic figure? Let’s ignore water temperature, friction losses and altitude and concentrate only the how much vacuum a real-word pump can produce. Taking a look at the diagram, Column C is also a hollow tube of cross-sectional area of 1 cm² with a sealed top end subjected to a partial vacuum with the other end submerged underwater. Now since the tube is only subjected to a partial vacuum (just like a real world pump produces), the pressure differential is less, meaning that the water is raised or lifted up the tube to a reduced height.
Performing the same calculation again:
Atmospheric pressure at sea level = 14.7 psi = 1.034 kg/cm²
Partial Vacuum pressure inside tube = 4 psi = 0.2812 kg/cm² (a realistic amount how much of a vacuum a pump can produce)
(1.034 kg/cm² – 0.2812 kg/cm²) / 0.001 kg = 752 cm = 7.52 m
So, in this instance our real-world pump would have a maximum suction lift of 7.52 m, however you would still need to factor in other effects such as friction losses in the suction hose – making the actual value even lower.