The Dangers of Running Centrifugal Pumps Outside of the Preferred Operating Range
Operating centrifugal pumps outside the preferred range can cause significant damage. In this blog, we discuss how operating pumps at the left of the BEP can cause severe, cascading effects.
A hot topic with every customer is the need to improve reliability and reduce downtime. One way companies can achieve this with their centrifugal pumps is to avoid operating pumps at the left of the Best Efficiency Point (BEP), outside the preferred operating range. Estabrook is a leading regional sales and service organization located in both Ohio and upstate New York, focused on industrial pumps for industrial fluid handling applications.
As a subject matter pump expert for lines such as Goulds Pumps, Goulds Water Technologies, Gorman Rupp, Warren Rupp, we first look to apply the appropriate pumping technology to your application. We then look to optimize reliability and efficiency by sizing and selecting a pump that is going to operate nearest to the BEP. In this blog, we will discuss how the preferred operating range is developed and then its effects when running outside of its range.
Pumps with suction specific speeds above 7000 Nss are prone to experience damage to their components if they operate outside of a preferred range relative to a percentage of BEP flowrate (see Curve 1). Suction specific speed, also referenced as S or Nss, is a dimensionless number that describes the suction conditions that occur due to the relationship between rotational speed, flow, and NPSHr.
American Petroleum Standard 610 (API 610) states that a pump should operate in the preferred operating region of 70% to 120% of BEP flow rate, with a rated flow between 80% to 110% of BEP flowrate. It can be illustrated as a simple angled line on the right and left side of BEP on the performance curve or expressed as values. Curve 2 illustrates a single volute centrifugal pump curve with the preferred operating range shown with vertical lines. The intent is to ensure the proper application of that particular pump. Operating outside the recommended minimum and maximum flows run the risk of severely damaging the pump. Below we will address the possible causes of damage to a centrifugal pump operating outside of the preferred operating range and graphically illustrated in Figure 2.
Possible Mechanical Damage
If the pump is a single volute design, the impeller will endure higher radial thrust loads that act perpendicularly to the shaft when operating at lower flows. As the pump flow rate reduces below the pump BEP, the unbalanced, internal casing pressure force lowers the radial bearing life and increases the pump shaft stress.
Larger, higher head impellers will see a greater radial force than smaller, low head designs. Figure 1 illustrates the relationship between the radial thrust and a pump's reliability. Radial thrust is lowest at BEP and increases to its maximum at shut off, while pump reliability is greatest at BEP and reaches its minimum at shut off. The curve illustrates the sudden decrease of reliability to the left of the intersection of the two curves, at roughly 50% of BEP.
Impact of Temperature Increases
When a pump runs at a very low flow rate, the majority of the power input is converted to thermal energy, causing a rapid temperature rise. If the temperature rise continues, the liquid within the pump can vaporize and cause thermal expansion of the internal parts. This could result in seizure of the rotating parts, cavitation damage, complete pump failure, or destruction of the pump. Also, some fluids become more corrosive at elevated temperatures.
Problems From Internal Recirculation
Internal recirculation causes the formation of intense vortices with high velocities at their core and, consequently, a significant lowering of the static pressure at that specific area. This causes cavitation accompanied by severe pressure pulsations and noise that are potentially damaging to the life of the pump and the integrity of the impeller material. A small amount of recirculation from the impeller vane exiting back to the suction via the wear ring is typical and usually does not cause problems. As flow lessens, this secondary flow will increase, and cavitation can occur. If excessive shaft deflection is also present, it will cause increased wear ring wear and even greater recirculation flow.
The location of the damage to an impeller can help identify if the pump experienced classic cavitation or internal suction recirculation. If the damage is on the visible side of the inlet impeller vanes, the cause is classic cavitation. If the damage is to the hidden pressure side of the vanes, the cause is suction recirculation.
Another type of low flow recirculation, known as discharge recirculation, occurs when water changes direction at the vane exit and reenters the vane. This can lead to cavitation that erodes the low-pressure side of the vane.
Additionally, internal recirculation that occurs near the shut-off head will cause abrasive wear when solids or abrasives are present in the fluid. The impeller, volute, wear rings, and mechanical seal will all experience increased erosive wear.
Below is an image of a double suction pump that was operating at approximately 50% of BEP flowrate and experienced severe damage to the impeller inlet and wear rings due to suction recirculation.
There are many methods of resolving the problem of operating outside the preferred operating range. The solutions range from installing a bypass line to maintain minimum flow, opening a discharge valve, trimming an impeller, adding a variable speed drive, hydraulic re-rating, or resizing a pump. Contact your Estabrook Account Manager to assist you in identifying the right solution for your application.
Credits: Igor Karassik, Joe Evans, Ryan Utara
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