Electrostatic Discharge

Electrostatic Discharge Within PTFE Hoses

Most applications of fluoropolymer hoses do not require the use of a conductive inner tube. Under certain applications, the potential for static discharge must he considered. Static electricity can he a hazard. Under those conditions where static discharge can occur, the use of conductive PTFE hose is recommended.

When two different materials contact each other, electrons from one material can move across its boundary and associate with the other. These electrons align themselves with the material contacted. If the two materials are good conductors of electricity, the positive and negative electrons flow back and forth between them, keeping them in balance. If one or both are insulators, the flow will not occur. A charge will then build up on the surface of one of the materials. When the charge exceeds the electric strength of the material, dielectric breakdown results.

In applying this to PTFE hose, we have to consider fluids and gases, which are poor conductors of electricity and the flow rates of those fluids and gases. In order for a liquid or gas to be a poor electrical conductor it will generally satisfy one or both of the following conditions:

  1. Be non-polar: that is an imbalance between protons and electrons, and/or
  2. Contain a non-mixable component or a suspended solid; such as water in kerosene

When a liquid contacts a PTFE tube that isn't a good conductor (white PTFE inner core), the result is phase separation, and the electric charge starts to build. The rate at which static electricity build up now become a function of the fluid flow rate. When the dielectric strength of the PTFE tube is exceeded, the electric charge will puncture the tube wall and ground itself on the stainless steel braid of the hose. In hydraulics, high pressures generally mean high velocities. Historically, fluids were filtered upstream of the hoses using metallic filter elements. The metallic element helped to ground the charge. But, today, most filtration is done with paper type and glass-fiber elements that have a tendency to inject an electrostatic charge into the fluid they are filtering.

Fuels and Steam are two Specific Areas of Concern

Fuels are, for the most part, "nonconductive" liquids and have a resistivity greater that 10 (to the power of 8) Ohms: i.e., gasoline and white spirits. hydrazine, benzene, diesel oils. etc. These fluids usually are transferred at fairly low velocities, but there still is a potential for an electrostatic discharge due to external factors, such as humidity and. to some extent, temperature. You should take all of these factors into account even at fluid velocities at or below 1 meter per second.

When using PTFE hose, you can offset the potential hazard of electrostatic discharge by using a conductive PTFE hose. Carbon is added to the PTFE inner wall during manufacture. The carbon layer directs the electrostatic charge down the inner diameter of the hose to the metal end fittings. This prevents the charge from building up on the inner tube wall. So, it's important to examine any application where nonconductive fluids are used and any of the above conditions exist. This section is not meant to cover all conditions or situations when they involve fuels, steam or other media, which may cause electrostatic buildup or potential discharge.

Following is a list of some of the chemical which meet at least one of the criteria necessary to create electrostatic discharge:

Cyclohexane - Lacquers Decalin - Lacquer Solvents - Diacetone - Naphtha - Dibutyl Ether - Naphthalene - Dibutyl Phthalate - n-Octane - Dibutyl Sebacate - Paint - Dimethyl Phthalate - Petroleum - Dioctyl Phthalate - Pinene - Dipentene Silicone Oils - Fuel Oil - Skvdrol 500 &700 - Gasoline - Steam - Hexane - Toluene - Hezene - Transformer Oil Hydrazine - Turpentine - Kerosene - Varnish - Versilube