Article 17: What Happens When a Pump Operates Off-Peak?


            Last week I got a call from a friend asking for help with a pump at a Paper Mill. The problem: piping vibrates violently, pump is noisy, and failures are constant. Besides, pulsations started to affect the quality of the paper, obviously an unacceptable thing to the mills’ customers. The question was - what can be done?


As we looked closely at the details of operation, it became clear that this double suction pump, initially specified and sized to operate at the 3000 gpm (which is where its BEP is), actually runs at about 500 gpm. Several options were presented to the mill in the past, such as by-passing the excess flow, or buying a smaller pump – although would solve the problem with vibrations – are an expensive way to solve problems. Getting a new pump is “easy”, - as long as you do not pay for it from your own pocket. Piping modifications alone could exceed the cost of a new pump, and sometimes even that is not possible, and prohibitively disruptive.


The question was – could something be done to pump internals, while keeping the same casing, and not having to change the piping? Fortunately, the answer was yes. A new impeller, with modified hydraulics was designed to fit the existing casing, and shift the pump best efficiency point to 500 gpm.


When a centrifugal pump operates way to the left of its BEP, many nasty things happen. First, a radial thrust grows exponentially, resulting in significant shaft deflections – thus quick seal failures, reduced bearings life, worn out bushings and rings. Also, a hydraulic phenomenon called “rotating stall” sets-in, which is essentially a back-flow, leaving the impeller eye, and progressing backwards, resulting in violent piping vibrations, pressure pulsations, and wear-out of the components. The problem becomes worth especially when a hydraulic parameter called “suction specific speed” (NSS) is high.  Suction specific speed is an indirect indication of the impeller eye being too large, but also depends on several other factors, related to design, installation and application. There are certain engineering rules and principles related to minimum allowable flow - as function of pump energy, specific speed (NS), suction specific speed (NSS), and other factors, which – when violated – can cause trouble.


A properly redesigned impeller can thus be an effective way to solve such hydraulic problems, and is only a fraction of a cost of a complete pump replacement. And, these days, when funds are tight, and maintenance and purchasing departments are struggling to find better ways to extend the equipment life, - equipment upgrades through hydraulics optimization come to the rescue. Paper mills, chemical plants, refineries, municipalities, - all have the same goal: maintain production, while reducing expenditures by extending equipment life. Working with the end users, such improvements in equipment reliability are possible, and require sound engineering approach, where attention to details, and appreciation of hydraulics and mechanical interactions, can be realized effectively, and economically.



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