These days, armed with a variable frequency drive, we can change pump speed with the turn of a dial, a cursor, or a touch pad. If we go too fast, our power will increase, pressure will increase, and we could break something. But what happens if we slow down?
Electric variable frequency drives (VFDs) have become increasingly affordable and are being touted for energy savings in electrically driven centrifugal pumps and for process control. We are now often asked, “What is the minimum safe speed for our pump?”
Process Issues:
For centrifugal pumps, changing the speed changes the pump curve. The changes can be calculated using the pump’s “Affinity Laws,” as shown in Fig. 1.
From Fig. 1, we can see that an equivalent point on a pump curve will reduce directly with speed, the head (or pressure) will be reduced by the square of the speed change, and the power will be reduced by the cube of the speed change.
For example, a ten per cent reduction in speed will translate to a point on the pump curve: flow will be 90% of the previous, the head will be 0.9 x 0.9 = 81% of the last, and the power will be 0.9 x 0.9 x 0.9 = 72.9% of the previous. A relatively small reduction in the speed will produce a significant drop in head and power.
The actual flow rate in the pumping system will depend on the shape of the system resistance curve and how it interacts with the new pump curve. To understand this clearly, let’s revisit the concept of system resistance curves.
The Differential Head required by the pump (HD) is a combination of the static difference in levels (static head HS) and pipe friction. At zero flow, there is no pipe friction, so the curve starts at HS. As flow commences, the head curve rises with increasing pipe friction. For flow to begin, the pump must develop more than the static head HS. Otherwise, the liquid in the upstream reservoir will flow back through the pump, unless a non-return valve is present.
The system will operate at the point where the pump curve intersects the system resistance curve.
The pump–system combination in Fig. 3 shows a useful variation in system flow as the pump speed is varied between 100% and 70% of its speed.
What will happen if we drop the speed further?
In this example, at 60% speed, the pump does not generate enough head to overcome the static differences in levels. Without a non-return valve, the liquid will flow backwards through the pump. Not an acceptable scenario! If there is a non-return valve, it will be held shut by the back pressure, the pump will not be able to open it, and we will be operating at zero flow. An even worse scenario and a great way to destroy a pump!
The controlling factor for minimum speed will often be the system hydraulics. If there is a significant static differential head in the system, there is a real danger that the pump head will drop below the system’s static head, and the pump will operate at zero flow or experience reverse flow if there are no check valves in place. The pump speed must always be high enough to ensure that the pump head exceeds the system’s static head by a sufficient margin to achieve minimum flow.
System Stability:
A mostly frictional “steep” system, such as a closed loop system or a long pipeline, is ideal for VFD control. In this type of system, the ratio of flow to speed will be very linear, and flow control will be straightforward.
Conversely, with a “flat” system where the differential head is mostly static, VFD flow control may be challenging, as shown in Fig. 5. If the pump curve is also reasonably flat, even a slight speed change at lower speeds can result in a significant change in system flow. A steeper pump curve may help, but control of “flat” systems is often problematic, particularly at low system flows.
For “flat” systems, it is valuable to draw the pump curves at various speeds against the system curve and calculate the gain or percentage flow change versus the percentage speed change. Then, plot this against flow, as shown in Fig. 6. This will allow for the prediction of speeds where VFD control may become impractical. In these instances, control valve throttling may be a more effective solution than adjusting pump speed.
Mechanical Considerations
In most VFD retrofit applications, the goal is to utilise the VFD to reduce the speed to a previously fixed maximum speed. Usually, this will not present any mechanical problems for the equipment because operating stresses are dramatically reduced when the speed is decreased. If a major speed reduction is envisaged, aspects to consider include:
- Hydrodynamic Bearings require a minimum velocity to maintain an oil film. Generally, a speed above 500 rpm will be sufficient, but it’s worth checking at lower speeds. A general guideline for the minimum velocity of an oil film in a plain sleeve bearing is approximately 10 metres per minute. For a 25 mm diameter shaft, this is 126 rpm.
- Mechanical seals require a minimum speed to maintain a fluid film and ensure adequate lubricant and seal flush flow. This minimum mechanical speed is typically a few hundred rpm and is usually not an issue, but should be considered. High-performance seals for pressures above 40 bar should be discussed with the seal manufacturer when reduced speeds are envisaged. Gas seals typically require a velocity of 1.5 metres per second at the seal balance diameter to ensure face lift-off.
- Mechanical seals with pumping rings for cooling circulation loops will need to be checked to ensure sufficient flow is achieved, particularly if the speed reduction exceeds 50 per cent of the original design speed.
- If cyclone separators are present in the seal system, they require the pump to provide a differential pressure of typically at least 1.4 bar to be effective. This could dictate the minimum pump speed.
- When the pump itself has a shaft-driven cooling fan in hot service, the reduced cooling capacity of the shaft-driven fan may need to be addressed. At low speeds, heat soak from the pumped liquid will continue. Auxiliary cooling may be required, as would be the case when the pump set is on hot standby.
- The cooling capacity of electric motors at reduced speeds is often questioned due to the lower cooling fan speed. A centrifugal pump’s power requirement reduces with the cube of the speed change, and the required driver power is dramatically reduced. For this reason, reduced driver cooling fan effectiveness at lower speeds is rarely an issue for centrifugal machines. However, electric motors in other VFD applications, such as constant-torque service, often require auxiliary cooling at low speeds.
Conclusions
Lowering the pump speed will reduce flow, drop the generated head dramatically and reduce the power consumption even more. Internal loads and stresses within the pump will be reduced accordingly. The questions to be asked are:
- Most importantly, will it still pump? Yes – if the pumping system does not have any significant back pressure. Perhaps not if there is a static differential in the system. Operation at zero flow is a risk that should not be tolerated.
- Will the system remain controllable? Check the interaction between the pump curve and the system curve.
- Will the pump set be okay mechanically? Probably, but worth checking when the speed change is severe.
Most people ask about the mechanical aspects when considering the minimum pump set speed. It is more likely that the interaction between the pump and the system will be more critical.
As always, reliability starts and finishes with understanding your system.
