How to Pick a Centrifugal Pump Part 9: Over Speeding HVAC Pumps & Motors
/By Chad Edmondson
There are times when slightly more capacity or a little more safety factor is warranted in a pump selection. Often we are asked about increasing the speed of the motor above the nameplate rated RPM. There are several things to examine when operating the pump above the rated RPM speed using a variable frequency drive.
Operating an HVAC centrifugal pump at a higher speed than the pump curve indicates provides several advantages. The first is the ability to gain additional capacity from the pump. In recent years on a constant speed pump application, the contractor would request a price to provide a larger impeller to gain that added capacity. Today, we might accomplish the same result by increasing the speed of the drive. The analysis used is similar to the reduction in speed to simulate the trimming of an impeller. It all starts with the Pump Affinity Laws.
Increasing Pump Capacities Using Pump Affinity Laws
The pump affinity laws show us the relationship between flow, head, speed, and horsepower required for closed hydronic systems. One mantra frequently heard in our industry is, “twice the flow needs four times the head.” The same is true for motor speed:
Twice the motor speed gives twice the flow.
Twice the speed gives four times the head.
Twice the speed requires eight times the horsepower.
How much you can over speed a pump might depend on the pump manufacturer. In general, Bell & Gossett recommends a maximum speed increase of 5%. There may be more increase available, but we recommend contacting the manufacturer to make sure there are no issues.
As an example, let’s assume we have a closed chilled water system with a capacity of 600 GPM of water at 80 feet head. Let’s assume we selected the below pump at 1770 RPM. How do the pump affinity laws work if we want to increase the speed by 5% using a VFD set at 63 Hz?
The design point is 600 GPM at 1770 RPM, 80 feet of head at 1770 RPM, and 14.5 brake horsepower at 1770 RPM. What do these numbers look like at 1858 RPM or 5% more speed?
The design point is 600 GPM at 1770 RPM, 80 feet of head at 1770 RPM, and 14.5 brake horsepower at 1770 RPM. What do these numbers look like at 1858 RPM or 5% more speed?
Based on this we would expect the potential capacity to be 630 GPM at 88 feet, but requiring 16.8 horsepower. Since the original selection would have a 20 HP motor to be non-overloading, the motor looks good. Let’s see what the curve looks like:
The selected pump is still non-overloading at 20 HP when running at a higher speed of 1858 RPM.
Limitations and Concerns
When considering running a pump at a higher speed it should be noted that the drive and pump motor must be rated for the higher amp draw caused by higher horsepower. If the amp draw is larger than the rating of the drive, the breaker or fuse disconnect will trip.
Another issue is the pump and motor bearings. Often when a pump is available at a nominal 1800 RPM speed and also at 3600 RPM speed, the components inside the pump change. The change occurs because the work done by the pump could increase. The changes include larger bearings and shaft, and a different seal and coupler. When is this the case? There really is no fixed answer. It is imperative that you check with the pump manufacturer when considering over speeding a pump.
One last concern is the net positive suction head (NPSH) required by the pump. NPSH increases at a greater rate than the speed. If we over speed the pump by more than 5%, we should check the NPSH available (NPSHA). Once we have the NPSHA, a safety factor is applied since the exact NPSH varies slightly with manufacturing tolerances and also due to entrained air in the hydronic system. The example above is a low energy pump; the suggested safety factor is 1.3. As we change the speed of any pump, the energy of the pump may change and the suggested safety factor will also change.
