What is Pump Suction Height and Why Does it Matters For Pump Selection?
Suction height is an important consideration for anyone searching for a liquid diaphragm pump solution. But what is suction height, and why is it so crucial?
Suction height is a relatively simple concept in theory. However, it is greatly impacted by outside factors like atmospheric pressure and media temperature. Failing to consider the physical limitations of suction height can result in insufficient performance and severe pump damage through cavitation. Because of this, having a deep understanding of pump suction height is essential when selecting a liquid pump solution.
What is Pump Suction Height and How is it Measured?
Diaphragm pumps create suction on the downstroke, creating a low-pressure vacuum which forces media into the inlet. Suction height, or suction lift, is the vertical distance between the pump’s inlet and media source in liquid applications. The lower the liquid is below the pump's inlet, the harder the pump must work to move it upward. Suction height is measured in meters of water, abbreviated mH2O, which represents hydrostatic pressure generated by liquid. While suction height seems like a simple concept, it is highly dependent on a variety of environmental factors.
Physical Limits of Suction Height: A Critical Consideration
When searching for a pump, it is important to consider the physical limitations of suction. Liquids will begin to evaporate when exposed to a vacuum. Additionally, the vapor pressure of a liquid is influenced by its temperature. Because of this, different liquids will start to evaporate at specific vacuum levels. If the media doesn’t evaporate, there are still limits to how high it can be drawn. Even a vacuum of 0 mbar (abs.) only generates a finite amount of suction.
Theoretically, the maximum suction height a pump can generate is about 10.33 mH2O, which is equivalent to atmospheric pressure at sea level. However, that limit assumes the pump is operating under ideal conditions, with perfect vacuum at sea level. Factors like atmospheric pressure, liquid temperature, friction within the piping, media viscosity, and pump performance all play a major factor in limiting suction height. Atmospheric pressure is lower at higher altitudes, which leads to lower suction height. Friction in the system and increased media temperature can also cause changes in pressure, further reducing suction capabilities. A suction height of 7 to 8 mH2O is the practical maximum achievable under most normal operating conditions.
Suction Height Impacts Cavitation and Flow Rate
Selecting a pump that can handle the appropriate suction height is essential for avoiding cavitation. Cavitation occurs when gas bubbles build up and quickly collapse in the pumped liquid. The collapsing of these bubbles generates powerful shockwaves, gradually eroding the internal surface of the pump’s components. It can also lead to excessive vibrations and noise, and in some applications, it can alter the media being transferred. Pumps operating near or outside the limits of their suction height capabilities are highly susceptible to cavitation.
Pump suction height also has a substantial impact on flow rate. Typically, pumping with higher suction height leads to lower flow rate, as the pump needs to overcome higher pressure differences to transport media. This is demonstrated on a pump flow curve, like this one for KNF’s NF 30.
As suction height is reduced, maximum flow increases. However, this also assumes ideal operating conditions. Changes in atmospheric pressure can also impact flow rate.
Meeting the Challenges of Suction Limitations
Strict limits and a heavy dependence on environmental factors make suction applications challenging. However, there are ways to mitigate these limitations. Proper installation is essential, as pump placement plays a major role in generating adequate liquid suction. Typically, a pump should be installed below the liquid level of the tank it is drawing from. This is not always possible, but the tank must be situated within the maximum suction height rating of the pump for the system to work effectively and avoid cavitation.
Selecting the right tubing is also essential for preserving flow and preventing pump damage. Tubing needs to be large enough in diameter to effectively move the media to the pump inlet. The tubing configuration should focus on having as many straight runs as possible and minimize joints and fittings. This can help reduce friction loss and prevent further performance losses.
Choosing the Right Pump is Crucial
Having a pump that can handle the appropriate suction height is essential for avoiding cavitation and generating appropriate flow. Diaphragm pumps are strong choices for generating suction height because of their characteristic design. KNF offers a wide range of pumps with strong suction, including the NF 30. The NF 30 offers a maximum suction height of 6 mH2O (minimum guaranteed, tested under dry conditions) along with a maximum flow rate of 0.3 l/min and maximum pressure of 1 bar (rel.). It is available with a variety of pump heads, valves, and diaphragm materials, as well as customizable brushless DC motors.
For applications requiring both a higher flow rate and good suction, a pump like the FK 1100 may be required. It has a maximum suction height of 4.5 mH2O (minimum guaranteed, tested under dry conditions), maximum flow rate of 12.4 l/min, and maximum pressure of 1 bar (rel.). It is also available in a high pressure, 1.1100 version, which offers pressure up to 6 bar (rel.).
