Sometimes in the life of pumping equipment in a plant, the pump needs to be rerated for a different operating point and process condition. This may be caused by process changes, poor reliability or energy savings projects. The evaluation and execution of these ideas can be critical to the long-term success of such a rerate. These rerates typically involve an impeller change, motor change, speed change or combination of all three to achieve some desired new process operating point. Since most pumps are electromechanical devices, many areas must be investigated. Some of these key areas that must be carefully evaluated when rerating a centrifugal pump for a different operating point are outlined in this article.
If the rerate is to meet a new process condition, then where the pump is operating on its curve is primary concern. Bad things happen as the pump operates away from its best efficiency point (BEP). Several minimum flows must be satisfied on the pump curve. The pump manufacturer will usually have a hydraulic minimum flow for the pump. The minimum continuous stable flow is present to eliminate recirculation, followed by the minimum thermal flow. For best overall performance, it is best to keep the operating point above the hydraulic minimum flow.
Horsepower is probably the first variable with a design point change that will be evaluated. An increase in flow, head or specific gravity of the fluid will increase the brake HP (BHP) as shown in the equation below. To get this increase in design point, an increase in speed or impeller diameter will be necessary. Horsepower increases with a cube of the speed, so just a little speed can mean a large jump in BHP. Process fluid changes can be somewhat hidden, so be careful if the rerate involves a process change that involves fluid density or viscosity.
Where Q = flow, gpm
H = head, ft
SG = specific gravity
η = pump efficiency
With a different operating point comes different Net Positive Suction Head (NPSH) characteristics. Higher pump flows increase the NPSH Required (NPSHR) from the pump curve. Higher flows in the same piping increase friction losses in the piping. The increased friction losses on the suction side will further reduce NPSH Available (NPSHA). If a speed increase is involved in the hydraulic rerate, the NPSH margin will need to increase because the suction energy increases with the square of the speed and NPSHR will increase.
The impact of a speed change can be seen by comparing the NPSH on the two pump curves in Figure 1. A pump that operates at 900 rpm with a design point of 6,000 gallons per minute (gpm) at 70 feet has an acceptable suction energy and resulting NPSH margin that still leaves the NPSHA higher than the NPSHR. That pump will likely not have any issues with cavitation. The same pump that needs a rerate to get 7,000 gpm at 110 feet will need to run 1,200 rpm; However, at 1,200 rpm, the pump’s suction energy and NPSH margin increases so that the NPSHR is higher than NPSHA. Cavitation issues are likely to cause significant reliability issues with the new pump operating condition. A different pump should be the correct path forward rather than trying to force an existing pump to operate in a poor hydraulic condition.
For decreased flow operation (left of BEP), recirculation cavitation is a concern. The minimum flow to avoid suction recirculation is a function of suction specific speed. As shown in Figure 2, most pumps should likely be 40 to 50 percent of BEP flow for minimum flow to prevent recirculation cavitation issues.
Check submergence if a substantial increase flow occurs. An increase in flow will increase the amount of submergence for the same pump to prevent a vortex from pulling air into the pump suction. Minimum tank levels may need to be adjusted to accommodate the new design point.
If the pump hydraulic evaluation produces acceptable results, then the mechanical analysis is necessary to see what physical changes must happen to complete the hydraulic rerate. A summary of the mechanical areas to check can be seen in Figure 3.
A best practice is to install a new pump with a motor baseplate that is sized for the next-size motor for future expansion. This will make an HP increase easier, but all the details for the upgrade must still be compared. These would include the motor feet spacing in both directions, the shaft stick out, shaft height and motor bolt hole size. Some motors may have four-bolt holes in the feet, so sometimes the other holes can be used for mounting. Note that some motor dimensions are not dictated by standards such as the National Electrical Manufacturers Association, so it is a good idea to inspect the installation for any other potential interferences.
For a larger motor, the coupling sizing must be verified. This includes shaving enough torque/HP capacity. If that is satisfactory, verifying the maximum bore of the coupling must be checked because the larger motor will have a larger shaft size. This is where the coupling could get in trouble. Motor shafts are typically larger than pump shafts, so this is typically the critical shaft size. The distance between shaft ends should be maintained for the coupling to be properly engaged. Many pump motor upgrades require guarding modifications. Some guards may be adjustable, but many will need modifications to fit the new installation.
If the pump has a belt drive, evaluate belt drive sizing. If belts have enough HP capacity, the belt tension may need to be revised. The resultant load on the shafts as well as bearing capacity and life need to be considered, especially if a larger motor is installed. Some pump manufacturers may publish a maximum shaft load for a pump size.
Verify starter size if increasing motor HP. Standard starter sizes are one through five with the higher number corresponding to higher HP, typically up to 200 HP. Many times, the cable size may change for some sizes or remain constant. The heater element size for the motor feed and fuse size will also change. Correctly size these for long-term performance and reliability. If staying with an existing motor when increasing HP, care must be made to make sure that the pump motor does not normally run into the service factor. Motors are not meant to normally operate into the service factor rating for continued operation.
Sometimes with a motor upgrade, the wiring junction box is positioned in a different location just slightly. If the existing cable size is adequate, cable length or route location may be an issue. A larger motor may also stick out further on the fan end so that access or aisle passageways are affected.
If the pump has a variable frequency drive, consider the minimum speed and HP required as the HP decreases as speed decreases below rated speed (torque remains constant). Proper motor turndown ratio may be something to evaluate if the pump may run at a very low speed and overheating is a concern.
The reliability portion is a lagging indicator of the success of the hydraulic rerate. While the rerate may not have been carried out for reliability reasons, the reliability aspect must not be ignored or it will negate other aspects of an otherwise successful upgrade. Many pump reliability elements can be summarized into one main area: how far from the BEP the pump operates. If the rerate pushes the pump further away from BEP, it may have many reliability issues, which will be driven by the hydraulics of the pump operating point. It is generally accepted that the pump should operate between -10 to +5 percent of BEP for best reliability with an accepted operating range of -30 to +15 percent. The types of failure modes multiply the further away from BEP the pump is operated (see Figure 4), which will lead to high maintenance costs.
When a pump upgrade is complete, professionals should update equipment information in the maintenance database. This may include the latest spare parts information for the storeroom and bill of material changes. Add information such as new bearings or rotor information for a larger motor. Some pumps may have a couple of different impellers that will run with different numbers of vanes. All of these data are important to properly manage the new pump arrangement for best reliability.
Before moving forward with a hydraulic rerate, many areas of the new pump design and operating parameters must be evaluated for a reliable pump system. It is not the goal of any rerate to contribute to a less reliable system. Many issues may not show up initially, but they will emerge later if something is missed. Upfront evaluation of these key areas is a worthy investment for a successful hydraulic rerate.
Randy Riddell, CMRP, CLS, is the reliability manager for Essity at the Barton Mill in Alabama. He has more than 27 years of industrial experience with a career focus on equipment reliability. Riddell has a Bachelor of Science in mechanical engineering from Mississippi State University. He can be reached at [email protected] or 256-370-8105.