Quality Waterjet Newsletter 02/06/2007

Electrostatic Charge Generation in Waterjet Systems


Miller*, in his 2001 paper, provided some important and practical information to prevent damage caused by electrostatic discharges. Here are the highlights.


Static electricity is the result of imbalanced electric charge buildup – one part with excessive electrons and the pairing other with a deficiency of electrons. Electrostatic discharge is the transfer of electrons to balance out the pair. When two different materials are in contact, a “contact potential” between 0.001 and 0.1 volt is built. If the pairing materials are both conductive, electrons can move quickly in either direction to eliminate any imbalance. If one or both materials are non-conductive, the time to dissipate the electrical charges (“relaxation time”) will be longer.


Plain water for cleaning is not usually considered as “poor conductor”, but it can be. Disasters have occurred in the past because of electrostatic discharge in waterjet cleaning (e.g. three very large crude oil carriers were destroyed in 1969 during washdown by waterjets). It was found that water spray can create a charged cloud of water droplets which can store enough energy to cause electrostatic discharges. High pressure waterjet cutting often uses purified water. Lack of conductivity is used to judge the quality of purified water (e.g. by reverse osmosis (RO) or deionization (DI)). If a pink glowing cloud is observed around the nozzle, one should realize that this is an electrostatic discharge phenomenon known as corona discharge (or ionization of the nitrogen in the atmosphere). The electrostatic charging mechanism and the mathematical representations were explained in details in the paper.


The author suggested the following to control static electricity: (a) Bond all components together with a grounding cable which is connected to ground; (b) Use conductive piping and hoses as well as conductive joints (e.g. when gasket is used); (c) Minimize fluid velocity by using large piping or hoses (e.g. < 1m/s); (d) Use proper humidity or ionization to minimize static charge buildup; (e) Use additives to enhance conductivity of the liquid.


* Miller, Paul L. (2001) Electrostatic charge generation in waterjet systems, in Mohamed Hashish (Ed.), Proceedings of the 2001 WJTA American Waterjet Conference, Minneapolis, Minnesota, USA, August 18-21

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