Many process fluids present a risk of a static build-up, which can have devastating consequences
Common-place fluids used in manufacturing processes present a risk of a static build-up which when released could have devastating consequences, writes Dr. Nick Dixon of Innospec Ltd:
There has been a spate of fires recently where the exact source of ignition was difficult to prove with absolute certainty, but where the finger of blame was clearly pointing at electrostatic activity. It transpires that static is potentially the cause of a large proportion of unexplained industrial fire accidents and there is much research and development activity which aims to tackle this phenomenon.
The ATEX 137 and DSEAR (Dangerous Substances and Explosive Atmosphere Regulation) go a long way to addressing the problem with static plant and equipment, but there are a number of areas where the potential for static build-up and its subsequent impact and consequential damage is just as real and equally needs to be addressed.
The problem is that wherever there is flow of a liquid, there is the possibility of it generating a static charge within it. Factors such as the rate of flow, the conductivity of the liquid, and the diameter of the vessel/pipe all have a dramatic effect on the electrostatic build up. Restrictions such as filters or valves can also increase the flow rate and hence the potential for static build up.
Few people recognise that even though the system may be earthed, static charge can still accumulate and can discharge in the form of a spark. This can lead to a number of problems. Process manufacturing plant can suffer from pitting of the vessel. This is something which can be hard to detect, but which will cause expensive damage if ignored. Then there is the more obvious possibility of a fire where a flammable atmosphere is present.
Electrostatic hazards associated with dusts and powders have long been identified as a potential hazard in industrial processes, and a number of precautions can be used to control the build-up of static or to eliminate the risk of a fire or explosion.
But the hazards of liquids have been less well recognized and understood. An electrostatic charge can also build up wherever there is a flow of liquid, filtration or settling process occurring. A charge can build up within the liquid, especially those with low conductivity such as hydrocarbons. Even with the pipe, filter or vessel being earthed, a charge can remain within the liquid for a considerable period of time possibly, in some cases, even for minutes. This can ultimately lead to a discharge or spark that could result in a fire if a flammable atmosphere is present.
The flow of a liquid in a pipe results in the separation of positive and negative ions in the liquid. Hence, the electrical properties of the solvent play a major role in determining both charge generation and relaxation; the key parameters are dielectric properties and electrical resistance.
The conductivity of a liquid is expressed in terms of siemens per metre (S/m) or more commonly picosiemens per metre (pS/m).
Conductivity has a dramatic effect on a liquids charging ability. In fact it is possible to divide liquids into three classes depending on their conductivity. High conductivity are those above (>2000 pS/m), medium (50-2000 pS/m) and low (<50 pS/m).
With a high conductivity liquid any static generated within the liquid can be conducted to the pipe/vessel and dissipated safely to earth. For intermediate liquids, the rate of charge generation can be critical, i.e. when charge generation is rapid, there may not be time for the charge to be dissipated. Low conductivity liquids are unable to dissipate the static charge, and hence static buildup can occur even if the vessel is earthed.
It is these medium and low conductivity fluids that can hold the dark and dangerous surprises and can result is expensive and life-threatening damage.
Many hydrocarbons have a low base conductivity, and these liquids often require special attention in handling. The conductivity of a solvent is also related to its temperature and viscosity; hence, the conductivity of liquid will be lower when it is cold and when it is more viscous. It is therefore important in a manufacturing process to measure the conductivity of solvent at startup, or when the solvent is at its lowest temperature, viscosity, and conductivity.
Consequences of static buildup
Any liquid movement will cause friction, pipeline flow, mixing, stirring, settling, filtration, free-fall or splashing, jetting or spraying.
If a large-enough electrostatic charge is present in the solvent and there is an earthed object in close proximity, then a discharge or spark can occur. If there is a flammable atmosphere present and the spark has enough energy to ignite the solvent, then a fire can occur.
A nitrogen blanket can be used to prevent a flammable atmosphere, but this can still lead to problems with reactor pitting. An example of this is the damage caused to enamel-lined reaction vessels due to discharges of static electricity. Enamel itself cannot become dangerously charged, unlike some plastics, and is therefore used extensively in the chemical and pharmaceutical industries. However, experience in the use of enamel vessels has shown that under certain conditions high electrostatic charging can occur and the sparks generated can cause pitting of the reactor wall. If this remains unchecked, it can lead to corrosion, extensive damage, reactor downtime, and even reactor replacement. Similar experiences have been seen for glass-lined reactors.
Similar experience have occurred with enclosed mechanical plant, such as pumps and transmissions where static building has caused sparks to continuously ‘jump’ and cause pitting to surfaces, resulting in poor mechanical performance and failure.
The implications of static generated in flowing liquids or two-phase processes are still being widely overlooked. With flammable low-conductivity liquids there is always the possibility of a spark which can lead to a fire, but it can also result in reactor or mechanical equipment damage.
The conductivity of a liquid determines the rate at which generated static can be dissipated to earth. The higher the conductivity of the liquid, the more rapidly the dissipation can occur. Some of the options open to a process and production managers to control static in liquids are to reduce the flow rate in pipe work or to increase the conductivity of the solvent by using a conductivity improver in combination with suitable earthing of equipment.
As ATEX 137 and DSEAR regulations have increased the awareness of static electricity in manufacturing processes, and the issue of electrostatic build-up in liquids will need to be addressed as a important part of the whole electrostatic risk in current and future processes that involve solvents in motion.