Electrostatic Spraying NDTs For The Inspection And Protection Of Industrial Equipment

An electrostatic spray nozzle is a dynamic element in electrostatics, particularly in the area of electrostatic security testing. An electrostatic spray nozzle consists of an adjustable tip assembly, generally an internal valve type component, which is operative to adjust a lateral gap between an exterior surface of an electrostatic nozzle body and a century of an outer spray tip assembly. Electrostatic jets may be induced through either venturi or external venturi control. Spray jet nozzle bodies are designed specifically for electrostatic purposes and include venturi profiles with external tips and internal tips. Spray jet heads may include one-piece single cavity head and two piece head designs.

In electrostatic spraying, it is important to control the charge accumulation at any time. This is accomplished by selectively charging a finely divided droplet on a non-energized surface. This selective charge is achieved through charging the non-energized side of an existing charge stream while leaving an existing charged stream free to continue its journey across the charging source.

A simplified illustration of the electrostatic spray nozzle process is illustrated in figure 1. The shaker bottle is placed in a shaker cup filled with graphite or an inert gas. An electrical current is applied to the bottle through electrodes. As the charge progresses through the wire cable and sleeve, the wire cable is heated by an alternating current passing through the heating element. As the wire cable passes through the discharge electrode, a small amount of charge is deposited along the inner side of the wire cable near the shaker cup.

The concept of electrostatic spraying was actually discovered long before the first modern electrostatic spray nozzle ever existed. It was around 1900 that W.C Mullens developed a new way to charge the surfaces of plates without using an electrical current. His new technique allowed for the generation of fine divided droplets of fluid. These droplets were then directed by an electric current to a pre-determined location.

It was Mullens’ idea that these pre-determined locations could act as electrostatic holding points. This new concept of electrostatic spraying nozzle creation became the basis of many new generation electrostatic spray nozzles. They are generally used today in various applications. Some of these nozzles have become so effective that they have been patented and are commonly owned by individual manufacturers.

One type of electrostatic spray nozzle consists of a high velocity airstream or blower, usually housed within an externally accessible area such as a vehicle or an airplane. The high velocity airstream is designed in a manner to generate a fine stream of high velocity air droplets which are directed to a precise area. In most cases, this area is a very small radius, such as a few inches across.

A second type of electrostatic spray nozzle comprises an induction electrode. These electrodes are often introduced into an area through the use of high voltage cables. Once these electrodes come into contact with an electrically charged object, their electrostatic charges are suddenly released. This discharge then causes the droplets of gas to be directed towards a specific area. Many of these nozzles employ both the high voltage and the induction electrode in a single device.

In many cases, the power supply used to create the required high voltage is located within the device being sprayed. The majority of modern electrostatic spraying devices utilize the use of specially designed control panels. Some of these control panels are rather simple, consisting of just a couple of touch-sensitive buttons. However, more complex systems are also available. These are typically used by spraying equipment operators who require a relatively low voltage and a fairly tightly controlled industrial spray production.