Part 1 - Choosing the correct system type
Pump systems generally fall under one of four types of configuration. Each configuration has its own set of factors to consider.
- Simple system. A simple system consists of a single input source and a single destination. Many pump systems fall under this category, such as a chemical unloading system, where the pump transfers the chemical from a chemical truck to a storage tank. Pumps that circulate the chemical back to the same tank, such as an electroplating system, are also a simple pumping system.
- Branched discharge. This is also a fairly common pump system configuration. A pump is used to move chemicals from a storage tank to various workstations. Each workstation may be located at a different distance from the storage tank, so the discharge piping is being branched to supply to the workstations. In such a system, we have to consider running the pump at variable speeds to optimize the delivery of the chemicals as needed by the workstations.
- Pumps in parallel. This is also a common pump configuration where two or more pumps are used at the same time to transfer chemicals from the same suction tank to be processed or to be discharged. Using this configuration allows the transfer volume to be scaled up as needed. Advanced systems use monitoring systems to balance the loading between the pumps to ensure the pumps run closest to the best efficiency point of the pumps.
- Pumps in series. This configuration is not as common as others. The main purpose of running pumps in series is to provide a boost to reach a higher pressure or a greater height. Such a system is used when using a single pump to achieve the pressure/height is impractical
Part 2 - Preliminary pump selection
Let’s review the information we have so far.
1) We have determined the duty point (flow rate and total head)
2) We have looked at the properties of the chemical
3) We have looked at the operating condition or environment of the application
4) We have looked at the different types of pumps to decide which pump types we may need
5) We have looked at different hazardous chemicals and their handling
6) We have touched on temperature effects on S.G. and Viscosity on energy requirements
7) We have also introduced some of the most common system configurations
Using the above information, we are ready to make the pump selection. If you have access to the manufacturer’s pump selection software, you can start with the duty point. That would provide the preliminary pump model(s) available for your application. Next, look at the pump types to narrow down your selection, e.g. if you need a seal-less pump, you may eliminate mechanical seal models. The next step will be to verify the sizing of your pump. You may require professional assistance from the manufacturer or their authorized service agents.
Part 3 - Adjusting for Viscosity, S.G., temperature, and motor loading
As discussed in Section 2 Part 3, the viscosity and specific gravity for some chemicals can vary greatly with temperature changes. If you have this information on hand, please use the maximum specific gravity and viscosity of your chemical in the pump selection criteria. This value is usually associated with the minimum temperature limit of your application. For example, if your pump application is outdoors and with insulation, the expected ambient operating temperature may vary between 10~30 ℃, then you use the physical characteristics at 10 ℃. If you do not have the physical properties available, contact the pump manufacturer and they may have the information in their database.
Since a higher S.G. and/or viscosity will result in a heavier load, there are generally two approaches to adjusting the pump for the application.
First, if there is room between the performance (H-Q curve) of the pump and your desired duty point, then you can lower the performance curve by either reducing the speed of the motor via a Variable Frequency Drive (VFD) or if a VFD is not planned, you may discuss the option of trimming the pump’s impeller with the manufacturer.
Second, if the pump selection software is still showing the required power of the application is still above the rated power of your motor, then the only option is to move up to the next available motor size. In some instances, the pump selection may also change accordingly.
It is highly recommended that you have the pump manufacturer check your pump selection after adjustments since there may be some other limitations that are not immediately apparent.
Part 4 - Checking the NPSHr and Best Efficiency Point (BEP)
Centrifugal magnetic pumps work by having a propeller or an impeller rotate to generate centrifugal force. The result of the centrifugal force is a low-pressure area at the center of the rotation and a high-pressure area at the edge of the rotation. The low-pressure area at the center of the rotation is what causes the liquid to flow into the pump (fluids tend to move from high pressure to low pressure). When we work with various chemicals with different physical properties, we need to understand if the chemical could vaporize at the low-pressure area of the pump. To make that determination, we calculate the Net Positive Suction Head available value (NPSHa) of the pump system and compare it with the Net Positive Suction Head required (NPSHr) of the pump. Please contact your magnetic pump manufacturer if you need assistance with determining the NPSHa value of your system.
Part 5 - Understanding the Best Efficiency Point (BEP)
By now, you may have a basic familiarity with the performance curve (H-Q) of the centrifugal pump. While it looks like the pump can be used for the entirety of the curve, each individual pump model has a design duty. This design point is generally where the pump has the highest hydraulic efficiency, also known as the best efficiency point (BEP). While it is rare for the desired duty to fall exactly on the BEP of the pump, we can make sure the pump is most suited for the application by making sure we choose a pump whose BEP is close to the desired pump duty. The recommended range is between 70% to 120% of the flow capacity of the BEP. Running a pump too far outside the recommended range would have some negative effects, such as low energy efficiency, reduced service life from parts wear, overheating of the pump, or a higher risk of cavitation to name a few.