Maximum suction

When explaining how end-suction pumps work, it is usually in response to the common question “how far up will a pump suck?”

The simple answer is, at least in theoretical terms, 10.34m. However, as simple answers go, it is not quite right.

The reason is that the 10.34m 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 7m or 8m. Understanding the difference between the theoretical and actual limit 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 to “push” the water up the suction hose.

The water flows from an area of high pressure to an area of low pressure and a vacuum is created inside the pump.

The pressure differential “raises” or “lifts” the water up the suction hose. The 10.34m 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 7m or 8m real-world limit on suction lift?

Since real-world pumps are incapable of producing a perfect vacuum, a more realistic limit of 7m 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.

This is shown in the diagram where Column A is a hollow tube 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. In Column B 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, creating a pressure differential where 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.

In reality the 10.34m level of maximum possible suction lift under perfect conditions 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 shows a hollow tube with a sealed top end subjected to a partial vacuum with the other end submerged underwater.

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 7.52m, although you would still need to factor in other effects such as friction losses in the suction hose – making the actual value even lower. This gives more of an insight into a common question that can often be misleading.