The Basics of Centrifugal Pump, System, and Variable Speed Curves
This blog aims to provide basic information on centrifugal pump curves for individuals who are experts in other subjects but who apply pumps to their applications.
Centrifugal Pump Curves
Occasionally, people ask me how much flow a particular pump provides. Unfortunately, the answer is a curve rather than a specific number. This is because the flow varies with the developed "Head" expressed in feet of the pumped fluid. You may be familiar with the practice of expressing pressure in heights of fluid such as "inches of mercury" or "feet of water." A centrifugal pump produces the same feet of head of a heavier fluid (e.g., salt brine or sulfuric acid) as it does a lighter fluid (e.g., gasoline). The resulting pressure depends on the specific gravity of the fluid:
For water, PSI = head / 2.31.
For other fluids, PSI = (specific gravity x head) / 2.31
It is also important to note that this curve reflects the developed head, or in other words, an increase in head across the pump. Typically, an application requires a certain flow at a particular discharge pressure. You need to take the suction pressure into account when determining the developed head required from the pump. For example, an application requires 200GPM of water at 52PSI. 52PSI is 120' of water. If the pump is drawing from a tank with a water level that causes a positive pressure of 10 feet of water at the pump (specifically the eye of the impeller), then the pump only needs to increase the head by 110 feet rather than 120'
Viscosity Corrections and BEP
You may have noticed the note below the curve regarding viscosity corrections. Most catalog curves for centrifugal pumps use water with a specific gravity of 1.0 and a viscosity of 1.0 cp in the models. But, the actual curve for a centrifugal pump will depend on the specific gravity and viscosity of the fluid at its operating temperature. The specific gravity only impacts the power requirement (actual operating power = water curve power x specific gravity) of actual fluid). The viscosity impacts both the power and the flow versus the TDH curve, and the correction is not as straightforward as it is for the specific gravity. Fortunately, many sizing programs, such as the one available from Goulds Pumps, can account for both specific gravity and viscosity.
It is advantageous to operate a pump at its Best Efficiency Point (BEP). It is better in terms of energy costs, but it is also better for the pump's longevity, especially for higher energy pumps. Sometimes pumps are not sized to operate at the BEP because of availability or cost. It is also the case that sometimes a pump operates at various parts of the curve, such as the level in the tank on the suction side or the system demand changing. There are limits beyond which a pump should not operate, both a minimum required flow rate and the maximum flow rate indicated by the end of the curve. Operation outside of these limits can cause severe damage to a pump. (For a detailed explanation, please see a detailed explanation from Kevin McKenzie in his blog.)
NPSHr and NPSHa
Two other items on our pump curve are the Net Positive Suction Head Required (NPSHr) curve and the Power curve. The Net Positive Suction Head Available (NPSHa) needs to exceed the NPSHr, or the pump will cavitate, damaging itself.
NPSHa = the absolute pressure above the liquid (barometric pressure for an open tank or sump) in feet of the liquid +/- static suction head – vapor pressure of the fluid - friction loss in the suction piping (in feet). See the end of the blog for more detail.
NPSH might be another potential limit to how far out on the curve a pump can operate. This is another reason why a pump might be sized to the left of its BEP since the NPSHr increases further out on the curve. You can see that the required power also increases with the flow rate. It is a common requirement that pump motors' sizes correlate are non-overloading (not requiring more than the nameplate horsepower) to the end of the curve.
As I mentioned initially, people frequently ask me how much flow a pump provides. The answer is that a pump provides the amount of flow where the pump's curve intersects its system curve. The system curve represents how the Total Dynamic Head varies with the flow rate. It consists of a static portion and a dynamic portion. For instance, if you transfer water from a tank with a water level 10' above the pump suction and discharge 80' above the pump suction to another tank, the static head would be 80' – 10' = 70'. This portion of the Total Dynamic Head does not change with the flow rate. The dynamic portion is due to friction through the piping and various components such as valves, filters, and heat exchangers. It is "0" when there is no flow but increases exponentially (at least in almost all industrial and commercial piping systems) with increasing flow.
Sometimes people consider changing to a bigger impeller to increase the flow rate in their system. If the existing motor can accommodate the bigger impeller, or if it is possible to install a higher horsepower motor, the bigger impeller will certainly increase the flow rate. However, the amount of flow increase depends on the system curve. The System Curve 2 captured below is all dynamic head, resulting in a steeper curve. Increasing the Impeller diameter with the steeper curve only increases the flow rate by 15GPM, while the same increase in impeller diameter with the flatter curve would increase the flow by 28GPM. As you can see, you would want to consider the system curve before deciding to spend money on changing the impeller and possibly the motor.
System Curves and Oversizing the Pump
Engineers frequently size pumps with a significant margin on the TDH. That makes sense if it is not possible to calculate the required TDH accurately or if there may be changes to the system in the future. When using a significant margin for the TDH, it is advisable to include a variable speed pump controller or a valve in the discharge piping to throttle back the flow if necessary. That is because being overly conservative in the TDH in selecting a pump can cause it to operate off the end of its curve in actual operation (see the following curve below).
Variable Speed Applications
Variable speed pump controllers are beneficial in many applications. Here are some pump curve considerations in variable speed applications:
In the example below, imagine that the application was for 325GPM of a salt brine at 220'TDH. There was also a second operating point at 160GPM at 160'TDH. The engineer wanted a 1800RPM operating speed because of the abrasive nature of the fluid, but no pump was suitable. He decided to use a 3600RPM motor but increased the impeller to the full-diameter impeller to run at a slower speed with a variable speed pump controller. He sees that for his maximum required operating speed of 2716RPM, he only needs 34.5HP. As such, he decides a 40HP motor appears more than adequate. Why is this wrong? Horsepower is proportional to torque times speed. If you decrease the speed by 25%, you need 33% more torque to get the same horsepower. Your 40HP motor would become overloaded!
Ends of the Curve
Notice that both the maximum and minimum flow rates shift to the left as the speed decreases. I recently had an application where the maximum and minimum flows could occur at the maximum and minimum TDH. Someone might think they are fine as long as they meet the maximum flow at the maximum TDH and the minimum flow rate at the minimum flow, but the curve below shows that the maximum flow at the minimum TDH is off of the end of the curve. Also, the chart under the curve shows that the minimum flow rate at the maximum flow rate is the worst case for meeting the minimum flow requirement of the pump.
Hopefully, you have found this blog helpful. If you have additional questions, please call our office (800-959-9161) and ask for one of our application engineers, or click on the link to the right for our CONTACT US page.