Water & Wastewater Treatment

www.wwtonline.co.uk | WWT | OCTOBER 2014 | 37 In the know Getting to grips with pumps Pump cavitation It can be an expensive problem for your business - but what exactly is pump cavitation and how do you avoid it? Pump cavitation occurs when the liquid in a pump turns to a vapour at low pressure. It occurs because there is insufficient pressure at the suction end of the pump, in other words, there is insufficient Net Positive Suction Head available (NPSHa). When cavitation occurs, air bubbles are created due to low pressure. As the liquid passes from the suction side of the impeller to the delivery side, the bubbles implode. This creates a BoB Went GROup COnsulTanT, XylEm WaTER sOluTIOns uK shockwave that hits the impeller creat- ing pump vibration and mechanical damage, possibly leading to complete failure of the pump at some stage. Why is it a problem? Cavitation can have a serious effect on pump operation and lifespan. It can affect many aspects of a pump, but it is o‚en the pump impeller which bears the brunt of its impact. A relatively new impeller which has suffered from cavitation will o‚en look like it has been in use for many years; the impeller material may be eroded and it can be damaged beyond repair. Pump problems caused by cavitation, sponsored by Smart liquid lime www.neutralac.co.uk such as vibration, can be severe and may lead to mechanical damage to the pump. Cavitation related problems also have the potential to reduce pump life from circa 10-15 years, down to just two years in extreme cases. What causes it? Cavitation occurs in a pump when the temperature and pressure of the liquid at the suction of the impeller equals the vapour pressure. Cavitation can occur at low pressures and normal operating temperatures. Locally it re- sults in the liquid turning to a vapour and creating very high temperatures and pressures, which can reach circa 10,000K and 1GN/m2. When cavitation occurs bubbles are formed. As the pressure in the pump increases, those bubbles collapse in the form of an implosion, which is equally as violent as an explosion. The implosion causes shockwaves to travel through the liquid and hit the impeller causing mechanical damage. How do you spot it/check for it? Cavitation causes pump performance deterioration, mechanical dam- age, noise and vibration which can ultimately lead to the complete failure of the pump. O‚en the first sign of a pump problem is a symptom such as vibration. It should be noted that vibration causes problems for many pump components including the sha‚, bearings and seals. How do you avoid it? Assuming no changes to the suction conditions or liquid properties during operation, cavitation can be avoided most easily during the design stage. The key is to understand Net Positive Suction Head or NPSH and take this into account during design. In order to understand this term more easily it is helpful to break it down into its con- stituent parts. Net refers to that which is remaining a‚er all deductions have been made, Positive is obvious and Suction Head refers to the pressure at the pump inlet flange. NPSH is defined as the difference between the pressure available at the pump inlet and the vapour pressure of the liquid. It is important to bear in mind that vapour pressure is different for different liquids and varies with pressure and temperature. It is also important to remember that the pressure available at the pump inlet is that which remains a‚er account has been taken of all the fric- tion losses, velocity head losses and Figure 1. Effects of wear including cavitation on an impeller

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