Processing, August 2019

Cover Series Pump Systems C u r i n g t h e c a u s e s o f cavitation Cavitation suppression liner eliminates the vibration excessive noise and pump component damage that occurs as the result of cavitation By Geoff VanLeeuwen P E Blackmer avitation and net positive suction head NPSH issues are common detriments to effective pump operation that are often misunderstood Entire plant systems can C fail unexpectedly as a result of a mechanical seal or pump component failure Plant operators may say it sounds like gravel is being pumped through our pipe or the pipe is shaking or even the pump components have dissolved into nothing When such failures are assessed further operators often conclude that cavitation and NPSH are ultimately to blame for the seal and pump failure As a result all process engineers know that pump suction design is most critical to ensuring reliable operation of any piping system Although NPSH sometimes seems confusing it can be simplified when viewed as a part of a monetary budget In the same way that a company may default if withdrawals exceed deposits a pump may be destroyed if it withdraws more NPSH than the NPSH budget provided by the piping system If the pump has a surplus of NPSH i e NPSH required is less than NPSH available then vapor is not formed and there is no potential for cavitation Conversely if the system has an NPSH deficit where the pump withdraws more than what the system provides then vapor will form and destructive cavitation will occur What is cavitation Cavitation is the implosion of vapor bubbles within the pumps pressure stages Cavitation occurs only if vapor exists Systems without vapor do not cavitate So where does vapor form Vapor can form upstream of the pump within the pump or both Pump operators know cavitation is noisy and destructive When vapor implodes the liquid walls collapse and produce shock waves of incredible force that cause destruction upon impact Like a small jackhammer striking a surface at a rate of 10000 times per second cavitation shock waves damage all surfaces no matter the hardness or thickness These shock waves can cause excessive vibration and noise within the pump which can result in significant damage to internal components This cavitation damage will set off a chain reaction of negative effects that can include loss of operational efficiency elevated maintenance and part replacement costs pump downtime and in the worst case scenario total pump failure This article illustrates how a specific type of pump technology positive displacement sliding vane incorporates a technology known as the cavitation suppression liner that mitigates and even eliminates the harmful effects of cavitation The challenge Plant operators and process engineers often focus attention on the pumps NPSH required After all cavitating can be avoided if a pump requires less NPSH than the systems NPSH budget Unfortunately this perspective misses the fact that vapor is often formed upstream of the pump This is especially true when transferring fluids with high vapor pressures such as liquefied petroleum gas LPG anhydrous ammonia NH3 gasoline acetone various types of refrigerants and condensates and those that are highly viscous The vapor pressure of these liquids is greater than any NPSH provided In other words these applications operate at a deficit from the start The liquid demands 14 Processing AUGUST 2019 If the pump has a surplus of NPSH i e NPSH required is less than NPSH available then vapor is not formed and there is no potential for cavitation