COLUMN | Common Pumping Mistakes
Double Trouble (Apologies to Fans of Stevie Ray Vaughan)
JIM ELSEY | Summit Pump Inc.
A regular reader of this column wrote to ask if they could double the speed on an existing centrifugal pump for the purpose of doubling the flow rate. The single-stage, volute-style pump is located at a remote dam. The pump serves as a water fill-station for firefighting tanker trucks. The existing pump performs fine, except the current flow rate is too slow for filling the tankers. The reader is familiar with the basics of the affinity laws and has an understanding that doubling the speed could, in theory, double the flow rate.
Abridged Response
Dear Reader: You don’t mention the size, model, type or manufacturer of the pump, but based on the stated flow rate, I can assist you on this hypothetical question without that information. The quick answer is that you will not be able to accomplish your goal of doubling flow by doubling the speed. You should contact the engineering department of the pump manufacturer to discuss your proposed plan, the pump boundaries and your system limitations.
There are several reasons why you rarely see a centrifugal pump operating above 3,600 rotations per minute (rpm). Exceptions are high-speed pumps (HSP) that are specifically designed for elevated speeds. HSP impellers will be small in diameter and often include a matching inducer located in front of the impeller eye. Two more exceptions are power plant boiler feed pumps (BFP) and rocket engines. Pumps that operate at high speeds will generally have impellers made with high-strength steel alloys.
Affinity Laws
For reference, I have illustrated the affinity laws below, where N is speed in rpm, Q is flow rate, H is head and BHP is brake horsepower. Subscripts are explained as 1 for existing conditions, and 2 is for the new condition.
Pump Boundaries First
The pump is only one part of the total equation; the system will dictate how the pump reacts and where it will operate on the performance curve, if at all. In no specific order, I will point out some of the pump boundaries and limitations that will prevent you from doubling flow by doubling the speed. I will focus mainly on the pump for this column and will address the system limitations in a future issue.
NPSHr
Doubling the pump speed will increase the net positive suction head required (NPSHr), which is approximately the square of the speed ratio. For example, if the initial NPSHr was 8 feet (2.44 meters), it would now be 32 feet (9.74 meters). This is an approximation but close enough for this discussion. See reference 1 for details.
NPSHa
Net positive suction head available (NPSHa) really falls under the future system discussion, but I wanted to address one aspect now because it reduces the net positive suction head (NPSH) margin/ratio. At the higher flow rate, the NPSHa will be less due to the increased friction in the suction pipe.
Suction Specific Speed: A Measure of Pump Cavitation Potential
In the suction specific speed (Nss) equation above, N is speed (rpm) and Q is the flow rate at the best efficiency point with the maximum diameter impeller. Nss is a function of impeller geometry. The main issues with Nss versus speed are covered in the conversations above on NPSH. In short, the eye of the impeller may not be able to effectively accommodate the increased flow.
Specific speed (Ns), the older brother to Nss, does not change as a pump's speed is changed, but Nss does change—we just don’t normally talk about it. In North America, we test most pumps at ≈ 3,550 rpm (big ones at 1,750), assign the calculated Nss to the pump for all other pertinent speeds and leave it at that. Without a full explanation or derivation of my above statement, I offer a simple explanation: The pump will have a higher Nss operating at a higher speed because the tested and published NPSHr for the pump is based on the 3% head drop. At higher speeds, the 3% head drop declaration point will be a different and higher value.
Impeller: Inertia & Balance
The pump manufacturer will be able to offer advice on the impeller’s value of inertia and the quality of balance. The inertia and balance do not change with speed, but the deleterious effects at higher speeds can be significant. The inertia will have a small effect on the torque and power requirements. Unless the level of balance is nearly zero, the increased speed will have a significant and negative effect on bearing and mechanical seal life.
Critical Speed
All single-stage volute pumps are designed so the first critical rotor speed is well above the operating speed. The manufacturer of your pump can advise you on the first critical speed of your model. Operating at or near the critical speed will create severe vibration issues. It is unlikely you will approach the first critical, but it is always possible.
Impeller Material
Unless the impeller material is a high-strength alloy steel, it is likely the tips of the vanes and edges of the shrouds will eventually fail. Impeller speed limits are based on the premise that the strength of any material can be exceeded if rotated too quickly. Impeller failure at high speeds is not easy to accurately predict, as it is a function of multiple factors including the strength of the base material, how the material was cast, erosion rate, surface finish, temperature and Brinell. Other than the prime objectives of performance and efficiency, impeller design considerations also include safety, reliability and longevity.
Impeller Tip Speed
Peripheral velocity of impeller in feet per second
(ft/s) = V = [π • D • (rpm)] ÷ 720
Equation 1
D is the diameter of the impeller in inches. Or alternately:
V= [(D) (rpm)] ÷ 229.2
Equation 2
Not knowing the pump size, I can’t calculate your impeller tip (peripheral) speed. Do not confuse tip speed with the rpm of the rotor.
Calculating impeller tip speed is easy to do with the previous equations.
The recommended maximum impeller tip speed for centrifugal pumps is generally based on the liquid, the impeller material and the quantity of solids suspended or entrained in the liquid. The geometry and robustness of the design are typically assumed to be adequate or better with a design safety factor of at least 2 to 3.
For common metal impeller materials (bronze, stainless, cast iron and steel), the recommendations for the maximum impeller tip speed are listed below (conversions from U.S. Customary Units [USC] to Systems International [SI] are rounded). Operating below these speed ranges helps prevent cavitation, excessive erosion and premature mechanical failure. For perspective, 145 ft/s is 99 miles per hour.
- Clean water: 130-145 ft/s (40-44 meters per second [m/s]).
- Dirty water: 130 ft/s (39.6 m/s)
- Medium slurries up to 25% solids and 200-micron solids: 115 ft/s (35 m/s)
- Higher slurry concentrations and larger solids: 100 ft/s (30.5 m/s)
Next is an alternate list of suggested maximum impeller tip speeds for some common materials used for pump impellers. Unlike the list above, these are based more on speed than the liquid being pumped. Note these speeds are not adjusted for a safety factor or any suspended solids.
- Gray cast iron: 131 ft/s (40 m/s)
- Stainless steel (austenitic 316 series): 148 ft/s (45 m/s)
- Bronze and brass: 164 ft/s (50 m/s)
- Ductile cast iron: 164 ft/s (50 m/s)
- Stainless steel CA 15 (400 series martensitic): 311 ft/s (95 m/s)
- Stainless steel A743 Grade CA-6NM: 360 ft/s (110 m/s)
Wear & Erosion
As a rule of thumb, you can expect wear to increase by the cube (3rd power) of the speed ratio increase. This is expressed as: {wear & erosion} ∝ {liquid coefficient} X {rpm}3.
Power
According to the affinity laws, your power will increase by the cube of the speed ratio, or you can think of that as a factor of 8 in this case of doubling speed. Your remote site is probably not designed for supplying the required additional power. The foundation and base are likely not suited for the larger driver. If the driver is an electrical motor, the switchgear/starter, conductors and conduit will be too small.
Bearings
My assumption is this pump utilizes ball bearings in lieu of the fluid film type. Regardless of how they are lubricated (oil or grease), they will be limited by both speed and load factors. I doubt the bearings in your pump have the capability of handling the higher speed. The average ball bearing is rated for a maximum of 4,000 rpm. Axial and radial loads will be increasing in a nonlinear fashion with the speed increase.
Radial thrust is a direct function of the impeller diameter and width, which are fixed for this discussion; however, the radial thrust is also a direct function of developed head (pressure).
As pump speed increases, the axial force on the impeller will also increase proportionally to the square of the speed change because the head (pressure) is changing in the same proportion.
Seal Chamber/Stuffing Box
Mechanical seals and packing have speed limitations. Please check with your supplier. Mechanical seal applications are not limited solely by speed (rpm). Seals utilize a pressure (P) times velocity (V) value often referred to simply as PV. The PV value is the product of system pressure (in pounds per square inch [psi]) multiplied by shaft surface velocity (in feet per minute [ft/min]) at the seal interface. PV units are expressed in psi-ft/min and indicate the stress your seal experiences during operation. Think of it as a heat and wear indicator rolled into one simple calculation. On the other hand, mechanical packing limits are measured in feet per minute. Each packing type is rated for a specific feet per minute based on the yarn and lubricants used.
System Limitations
Rule number one of centrifugal pumps is that the pump will operate where the system tells it to operate (where the pump performance curve intersects the system resistance curve). The current system must be redesigned to accommodate for the increased flow rate.
Final Thoughts
The affinity laws are a useful mathematical tool for pump engineers and owners that allows them to make small (less than 25%) and mostly accurate changes to the pump speed or impeller diameter. The laws are typically used for adapting an existing pump design to a system without requiring actual tests or empirical data. In this case, the system and pump already coexist, and the proposed change is radical and forecasted to exceed the pump limits. The affinity laws do not account for the system resistance curve, and as Grandma always said, you can’t get something for nothing.
References
- NPSHr Doesn’t Play by the Rules, Pumps & Systems, Jim Elsey, January 2024
- Radial Thrust, Pumps & Systems, Jim Elsey, January 2021
- Is Variable Speed a Magical Fix for Pumps, Pumps & Systems, Jim Elsey, May 2021
- Centrifugal Pumps: Design & Application, Val S. Lobanoff and Robert R. Ross
- Centrifugal Pump User’s Guidebook, Sam Yedidiah
- Hydraulic Institute Standard 9.8, Pump Intake Design
- Limits for mechanical seals and packing, API 682 and ISO 21049:2004
Jim Elsey is a mechanical engineer with more than 50 years of experience in rotating equipment for industrial and marine applications around the world. He is an engineering adviser for Summit Pump, Inc., an active member of the American Society of Mechanical Engineers, the National Association of Corrosion Engineers and the Naval Submarine League. Elsey is also the principal of MaDDog Pump Consulting LLC. He may be reached at jim@summitpump.com.
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