Misapplication of pumps is one of the more frequent field issues I witness. The issues are normally the result of a lack of knowledge and understanding of the physical limits of the machine.
Centrifugal pumps, like all machinery, have specific physical limits and boundaries for their designed application and operation. The purpose of this column is not to define specific limits but to examine the process of proper selection and application.
What follows is a general checklist of eight potential misapplication issues. Always check with the manufacturer for specific restrictions.
During my career, I have installed scores of pumps with operating fluid temperatures from minus 200 to 1,000 degrees F and pressures up to 4,500 pounds per square inch (psi). In each case, the pump was properly and adequately designed for that application—usually because the users incorporated the proper environmental and ancillary support systems, often with centerline design supports.
Always check the instruction and operating manual (IOM) or contact the manufacturer for applications below 40 F and above 180 F. Typically, there will be a requirement for materials, isolators, seals and gaskets as the temperature departs from the midrange. Consult with the mechanical seal and bearing manufacturers for proper materials and support systems. There will also be a requirement for special lubricants and/or temperature control of the lubricants.
2. Pressure (Flange Ratings)
All pumps have a maximum working and design pressure rating. I recently witnessed a user attempting to place a pump with 300-pound flanges into a system that had more than 900 psi pressure at the suction of the pump. The user thought this was acceptable because the pump itself was only going to generate 200 psi.
How much pressure can a 150-pound flange be subjected to? There is no simple answer, but two main factors are the temperature and the casing metallurgy.
It is acceptable to subject a 150-pound (flange) rated 316 stainless steel pump to as much as 275 psi at temperatures below 275 F. For a 300-pound flange in 316 stainless steel, it would be 375 psi at 100 F.
The main point is that the maximum allowable pressure for the pump design must not be exceeded. The maximum pressure the pump will experience is the sum of the suction pressure (at its highest expected level) and the maximum developed pressure the pump could generate at maximum speed and impeller diameter.
Ambitious salespeople and unwitting customers will sometimes misapply a pump based on speed.
In a typical situation, these individuals will install a pump that has a speed limit (boundary) of 1,800 revolutions per minute (rpm) and apply the pump at 3,600 rpm.
While they may understand affinity laws, they probably do not understand that the subject impeller has (rotational) inertia limits and consequential deleterious effects because of high tip speeds (tangential velocity). The speed issues can result from a combination of fluid properties, impeller blade torque, erosion, vibration and strength of materials (think fatigue strength).
While this is not a complete or definitive list, normally accepted guidelines from American National Standards Institute (ANSI) 1.3 are as follows:
- Clean water: feet per second
- Dirty water: 130 feet per second
- Slurry (general): 100 feet per second
- Impellers (nonmetallic) or rubber coated impellers: 85 feet per second
Equation 1 can be used to calculate the velocity or tip speed. Note the relationship V= ὠ x r in the equation.
V = (N)(D) ÷ (229)
V = velocity in feet per second
N = speed in rpm
D = impeller diameter in inches
As an example, the velocity for a 10-inch impeller at 1,750 rpm is 76.3 feet per second.
Please note that there is also a minimum speed limit for pumps. This was not as much of an issue before the advent of commercially available variable speed drives, but it has become more of a problem.
Rotor dynamics can be complicated, even for small slow-speed pumps (below 1,750 rpm). For a general and non-technical discussion, think of all the close clearances in a pump and the radial and axial forces that the rotor is subjected to in normal operation. Some common rotodynamic forces are radial forces that result from operation away from best efficiency point (BEP), axial forces mostly on the impeller(s) resulting from different pressures on areas of varying sizes, shaft loading because of misalignment, oscillating suction pressures, changes in the system curve, and vibrations resulting from rotor imbalance and/or bent shafts.
As the pump slows down, the hydrodynamic stabilizing forces are diminished and the ability of the unit to counter the acting forces is consequently reduced. As a general guideline, I normally recommend that end users keep the operating speed above 650 rpm.
Always consult with the manufacturer, and keep the fluid properties in mind.
Note that pump bearings have speed limits, too. Ball bearing speed limits are determined by the type, size (usually diameter), cage design, load, lubrication and temperature. Hydrodynamic (fluid film) bearings are limited by load (which includes rotor weight), surface area, clearances, design (taper lands, for example) and lubrication properties.
All pump shafts have a speed, horsepower (HP) and torque limit. For single-stage pumps, many manufacturers express this limit as a horsepower per 100 rpm limit. Torque is inversely proportional to horsepower; the lower the speed, the more torque is applied to the shaft (see Equation 2).
HP = [(Torque) X (rpm)] ÷ (5,252)
HP = horsepower in foot-pounds per minute
Torque = torque in pounds-feet
While most shaft limits are based on speed, horsepower and continuous torque limits, the limits are further reduced if the pump is driven by an engine (internal combustion means intermittent, instead of continuous, torque). If the pump shaft is side-loaded as in the case of belt or chain drives, a striking reduction on the shaft limits occurs as a result of the cyclic bending fatigue factor.
With the advent of computerized programs for pump selection, correcting pump performance for viscosity is simple. Many users, however, sometimes overlook the details of what this does to the required brake horsepower.
Depending on pump size and impeller geometry, the viscosity limits for the average centrifugal pump will vary from 250 to 700 centipoise, and I have witnessed many pumps successfully pumping fluids in excess of 1,000 centipoise. The following two points are important to consider:
- An increase in fluid viscosity negatively impacts the torque and horsepower limit for the shaft. Check this viscosity correction factor to ensure a satisfactory and reliable installation.
- You may still be able to pump the highly viscous fluid with a centrifugal pump, but there will be a point of diminishing returns because of reduced efficiency. Perhaps you are using 25 HP to pump the viscous fluid with a centrifugal pump that would only require 5 HP with a positive displacement pump.
6. Suction Lift
As discussed in my column "10 Common Self-Priming Pump Issues" (Pumps & Systems, September 2015), there are physical limits to how far a pump can lift a fluid. I usually recommend 25 feet as the maximum lift for cold water. Other fluids with higher vapor pressures reduce the limit on the maximum lift. Always complete the net positive suction head available (NPSHA) calculations, which will also involve knowledge of the vapor pressure of the fluid. Most self-priming pump manufacturers limit the maximum fluid temperature to 165 F.
7. Air Entrainment
Depending on the specific speed of the pump impeller and other design factors, the air entrainment limit on most standard centrifugal pumps is 12 percent. The performance will be affected as the air entrainment percentage increases, and this phenomenon eventually will stall out the pump. Pumps that utilize vortex style impellers will be able to handle more air, typically as high as 24 percent.
8. Nozzle Loading
It has been said that the most expensive pipe hanger/support you can purchase is a pump. All pump manufacturers state a limit on how much force can be applied to the pump flanges when the equipment is installed and operated. There are design maximum allowable forces and moments in all three planes.
Dozens of additional limits/boundaries on pumps exist, and, perhaps if there is expressed interest, I will write more articles on the subject. Some added issues and limits are head generation per stage and per pump, specific gravity, metallurgy specific to fluid properties such as pH or suspended solids, NPSHA/NPSHR margins, elevation, orientation and vibration.
The most important point to remember is that the next pump you are putting into service is not indestructible. Always be aware of your equipment's limits.
ANSI 1.3 also ANSI B16.4 and 16.5.