Understanding Pump Curves
Operating Range or Window &
Envelope
In the graph above, the area between the upper and lower black curve lines is the pump Envelope, and the yellow shaded area is the pump Range or Window. The 3 numbers located on the far left and just on top of each curve are the diameter in inches of the impeller represented by that curve. All three curve lines represent the same impeller but with a different diameter as indicated.
Envelope
The top curve line represents the performance of the pump equipped with the largest diameter impeller available from the manufacturer for this pump, in this case 7.97" (inches).
The middle curve line represents the performance of the pump when the impeller diameter is trimmed down to 7.2" . This middle curve line is assumed to be the performance of a pump under consideration and is called the design curve.
The lower black curve line represents pump performance with the smallest diameter impeller available or recommended by the manufacturer, 5.5". This means that the manufacturer does not recommend trimming the impeller below 5.5".
The top and bottom curves therefore represent the upper and lower limits or Envelope of possible or recommended impeller trims for this pump, and the middle curve line is the design curve for a pump under consideration or evaluation.
Operating Range or Operating Window
The yellow shaded area represents the range between the minimum and maximum flow rates a pump can continuously operate at without damage. This range or window is usually determined by the pump manufacturer.
The three main factors limiting the range of a pump are:
Recirculation Cavitation - Both Discharge Recirculation and Suction Recirculation can occur at low flow rates. If these phenomena become serious the pump may be damaged quickly by cavitation, which warrants limiting the amount of time a pump operates under low flow conditions. In general, the lower the specific speed, the more resistant to this type of damage. Pump designers can also manipulate the design and materials of pump construction to make a pump better suited to operate for longer periods at low flow rates without experiencing damage from recirculation cavitation.
Vibration - Any centrifugal pump operates with least vibration at the BEP. This is because the hydraulic environment surrounding the impeller is closest to being balanced at BEP, and above or below BEP vibration increases. As the flow rate moves further away from BEP vibration may become severe enough to cause damage quickly. The manufacturer or any competent person may then specify those specific flow rates above and below BEP, where vibration becomes excessive, as the upper and lower limits of the pump operating range.
The lower the Specific Speed number, the smoother the pump will operate at low flow rates. Some low specific speed pumps can operate all the way down to shut-off without severe vibration.
Centrifugal pumps operate with less vibration flow rates above and below BEP when equipped with a diffuser. Diffusers reduce vibration in centrifugal pumps by holding a more stable and balanced hydraulic environment immediately surrounding the impeller.
Pumps with low specific speed impellers and diffusers may be able to operate for extended periods at shut-off or low flow conditions without any damaging vibration.
Heat Accumulation - Although for performance testing the centrifugal pumping process is usually considered to be adiabatic (no heat gained or lost), a small amount of energy actually does get added to the pumpage in the form of heat. This heat accumulation becomes important in control systems for pumps operating at flow rates well below BEP.
Additional Information
In general, the lower the pump specific speed, the larger the safe operating window or range. The implementation of a diffuser also widens the safe operating window for any pump.
In general, the lower the Suction Specific Speed number, the larger the safe operating range of the pump. High Suction Specific Speed pumps (Nss>9500) will not tolerate flow rates far below BEP because they quickly begin to experience severe vibration and recirculation cavitation.
Many pumps can operate safely out of their range for various periods of time. However Mixed Flow and Axial Flow pumps are particularly vulnerable to problems when operated at low flow rates below the safe operating window. Pump specific speed and suction specific speed are rough indicators of how long a pump can safely operate out of its recommended flow range, or if the pump can ever operate much below its range. Low specific speed pumps (Ns < 8000) can usually operate for longer periods of time below their range without damage occurring quickly.
The problem that places immediate limitations on a pump operating outside its window is vibration. Severe vibration can damage a pump and its driver very quickly. Therefore, if a pump operating outside its safe range begins to vibrate severely the pump user should take steps to prevent this type of vibration from ever occurring.
All types of cavitation occur outside and even within the operating range of a pump. But cavitation cannot cause immediate and unacceptable damage unless that cavitation also causes severe vibration. Therefore, pumps can operate under Cavitation conditions for periods of time without encountering severe damage quickly. Those pumps that resist cavitation and cavitation damage are excellent candidates for jockey pump service where the pump may have to operate for periods of time at very low flow rates. Generally very low specific speed pumps (Ns < 1000) are excellent candidates for jockey pump service on systems where low flow rates are expected to occur frequently.
Follow the links below to pages focusing on other types of information provided in the Pump Characteristic and more, including graphs and explanations.
