Whether you are purchasing a car, a cell phone or industrial equipment, being an uninformed consumer puts you at a distinct disadvantage. When it comes to industrial pumps, if you cannot figure out what you need, even the most knowledgeable applications engineer will have a hard time figuring it out for you. By following a few basic tips the next time you purchase a pump, you can simplify and speed up the process while buying exactly what you need.
Engineering a Well-Designed SystemWhen purchasing a new or replacement pump, it is important to know what types of pumps are available and which design most closely fits your application. Industrial pumps often are expensive, but the results of a pump failure are even more costly. The time and money spent researching, selecting and purchasing the optimal model are insignificant compared to what could be spent on repairing, replacing or cleaning up after the wrong pump has been installed.
As important as it is to know what pumps are available, it is even more important to know the design parameters of your system. A liquid property or system detail that might seem trivial often can be an important piece of data for the proper pump specification.
Determine the System Hydraulics. The three most important pieces of hydraulic data are flow, total differential head (TDH) and net positive suction head (NPSH) available. These three parameters ultimately will dictate the type of pump suitable for the application.
Flow is the volumetric flow rate through the pump. This flow varies with the TDH as indicated on a given pump curve.
TDH is a measure of the energy a pump supplies to the pumping fluid. It is proportional to the pressure differential between the pump inlet and outlet. Calculating TDH instead of differential pressure offers a more convenient method of specifying hydraulics because TDH accounts for specific gravity, thereby eliminating the need to specify the additional parameter. TDH can be calculated from the differential pressure with the following formula:
TDH is in feet and ΔP is in psi. TDH also can be calculated from the Bernoulli energy equation, although this approach can be more time consuming and complicated.
NPSH often is overlooked in pump specifications. However, having adequate NPSH available is vital to the life and performance of most pumps. Insufficient NPSH will cause cavitation bubbles to form inside the pump, which can cause irreversible damage, especially to the impeller. The pump’s performance also might be well below what is expected. The available NPSH is particularly of concern in closed systems where the vapor pressure of the fluid is high.
NPSH is commonly miscalculated and underestimated by considering only the available static suction head. When the suction source is vented, atmospheric pressure contributes 14.7 psia of suction pressure, which is equivalent to 34' (14.7 psia x 2.31 = 34' of water) at sea level. Dynamic losses due to the piping configuration should not be considered negligible and should be determined carefully, usually by the system owner.
Underestimating the NPSH available can greatly increase the price of the specified pump, as a much larger, slower-speed unit or one with an expensive inducer might be provided when it is not needed. Conversely, if the NPSH available is low, providing the pump supplier with inaccurate NPSH information could result in poor pump performance.
Software is readily available on the Internet to evaluate this important parameter. In addition to calculating available NPSH, the programs can greatly simplify the determination of system requirements. Pump users who are not comfortable with making these calculations on their own should familiarize themselves with one of these programs or hire an engineering consultant to avoid making costly errors.
Know What You Are Pumping. There are two main reasons you need to know what you are pumping:
- The physical properties of the fluid will determine the
size and power requirement of the pump and driver.
- The corrosive properties of the fluid will dictate the construction
materials and sometimes even the model of the pump required.
Specific gravity is important because even small differences in specific gravity can have a major impact on the system performance (as indicated in the TDH formula). The heavier the fluid, the more energy the pump must supply to the fluid to achieve the same discharge pressure.
Viscosity influences the size of the motor and the size or type of pump. The higher the viscosity of the pumping fluid, the more power the motor must supply to reach the desired operating point. If the pump’s motor is too small, it will overload, often resulting in motor failure.
Vapor pressure influences the available NPSH. A high vapor pressure will decrease the amount of NPSH available while a low vapor pressure will have a negligible effect. Vapor pressure is of particular concern when pumping refrigerants, high-temperature liquids and other volatile liquids that are near vapor point.
Set a Reasonable Safety Factor. It might seem prudent to add a huge safety factor to your design to ensure that the pump will work, but that is seldom the best solution. When it comes to pumps, more is not necessarily better. Often, too large a safety factor can increase the pump size enough to dramatically increase the initial price. Also, an oversized pump will operate at a lower efficiency, increasing the cost of ownership, and can cause the pump to operate below its minimum recommended flow point, resulting in unstable operation or reduced equipment service life.
Increasing the size of the pump does not always decrease the likelihood of a pump failure. Many failures are a result of inappropriate pump selection and improper operation or maintenance. A large safety factor sometimes is necessary to account for unknowns such as varying system hydraulics or fluid properties; however, a better practice is to determine the system properties accurately and set a moderate safety factor. The appropriate size of the safety factor will vary for each application, but this parameter should account for the precision of the calculations and measurements; potential hazards and consequences of pump failure; and the costs incurred as a result of the safety factor.
Selecting the Appropriate PumpBecause so many different types of pumps exist on the market (figure 1), selecting a pump can be a difficult and time-consuming task. Finding a knowledgeable pump application engineer or consulting engineer often is a good first step. A thorough understanding of the system and pumping fluid also will greatly simplify the pump selection process.
Pump and System Curves. Once the system properties have been determined, the TDH can be compared to the flow in the form of a system curve. Developing a system curve instead of just determining a single operating point is necessary to ensure consistent system performance. The TDH required by the system will vary with the flow. It is important to determine the system requirements over the entire operating range of the pump, especially if the flow is unsteady or the TDH is crucial. Although a single, continuous operating point is desirable from the pump supplier’s point of view, it is not always feasible.
All models within a particular family of pumps will have comparably shaped operating curves with different ranges of TDH and flow, whereas different types of pumps often will have very different pump curves.
The pump curve shown in figure 2 is from a sub-ANSI centrifugal pump. The operating point is designated by the red arrow where the system curve (blue) crosses the pump curve (bold black). The bold black curve represents the operating curve with the maximum impeller diameter, and the thin black curve corresponds to the minimum impeller diameter. The pump can operate along any similarly shaped curve within that range by varying the impeller diameter. The green lines overlapping the pump curve represent pump efficiency; the best efficiency point (BEP) is the maximum efficiency value of this curve. Ideally, when sizing centrifugal pumps, you should always select an operating point to the left of the BEP as operation will be most stable in that range.
Design Considerations. The next step in selecting a pump is to determine generally what type of pump is best for the application: centrifugal or gear, horizontal or vertical, sealed or sealless, metallic or non-metallic, etc. A low-flow high-head application, for instance, might be a good fit for a regenerative turbine pump, while a low-flow medium-head application could be suitable for a sub-ANSI pump such as the one shown in figure 2. Many applications will accommodate a several different pumps, in which case, durability, efficiency and price should be considered.
Some design considerations that should be taken into account when selecting a pump are the pumping temperature, the presence of solids in the fluid, the amount of space available for the pump and the desired piping configurations. Irregular and undesirable system attributes such as low or unsteady flow, insufficient NPSH and highly corrosive fluids should be considered in pump selection as well. When ignored, these conditions can adversely affect the pump operation and life.
Low or unsteady flow can pose a problem for certain types of pumps. If the flow through the pump fluctuates drastically and the possibility exists that the flow will drop below the pump’s minimum requirement, it is a good idea to install a power monitor to avoid pump failure. The monitor will detect when the pump is running dry and shut it off. Note that running a sealed pump dry likely will result in the failure of the seal, which could be expensive. The same scenario in a sealless pump could cause bearing failure and could permanently damage the magnets in that type of pump, or the motor in a canned motor unit.
Insufficient NPSH is another potentially problematic issue. If the NPSH available is too low for a regular centrifugal pump at the desired TDH, consider a regenerative turbine pump. Regenerative turbine pumps are suitable for low-flow high-head applications because they operate well with a low NPSH.
Another solution for a low NPSH application is to run a larger pump at a low speed; however, this can be a costly alternative. The easiest and most economical option typically is to redesign the system to provide more NPSH, often by simply increasing the suction source height or pipe size.
Highly corrosive fluids can present material selection issues. Corrosion-resistant construction materials should be used for highly corrosive pumping fluids. Settling for a cheaper pump with less desirable construction materials can pose a safety hazard and also will increase the long-term cost of pump ownership.
Sealless pumps are good choices for corrosive and hazardous fluids. Because they do not have mechanical seals, they usually require less maintenance than sealed systems. Even small, seemingly insignificant repairs to systems pumping corrosive fluids can be expensive because of the safety risk involved in handling the chemicals.
In conclusion, the pump buying process can be simple and efficient as long as a few vital steps are taken to ensure success:
- Calculate the flow, TDH and NPSH of the system.
- Determine the specific gravity, viscosity and vapor pressure of the
- Construct a system curve with which to compare pump curves.
- Keep in mind important design constraints associated with the
particular system conditions.