Static charge is a small variable which affects the quality of the product indirectly. One needs to understand what it is and how it can be eliminated or avoided. An insight by Ramesh Sahu, Product Manager, Skytech Systems (I)
Pharmaceutical industries use high quality- oriented process to ensure that a full proof quality product comes out with very minute degree of tolerance which is allowable. There are many external factors which affects the final quality of the product. Nowadays, the concept of QbD and PAT is being implemented in pharma manufacturing process which help to eliminate these variables at its generation point itself.
Static charge is also a small variable which affects the quality of the product indirectly, so we should understand what it is and how it can be eliminated or avoided. Static current can also have a major impact on human safety and is a health hazard. It can even cause an explosion as well.
Static electricity is a major cause of fire and explosions in many industries handling flammable liquid. The hazard of electrostatic spark ignition of flammable vapour can be minimised by taking actions to limit the accumulation of electrostatic charges to safe values. Primary importance is the proper bonding and grounding of equipment and containers. In addition, charge accumulation in liquids must be limited, in many instances, by controlling the rate of charge generation and/ or the rate of charge dissipation.
What is it and how it gets generated?
Static electricity is an imbalance of electric charges within or on the surface of a material. The charge remains until it is able to move away by means of an electric current or electrical discharge. Static electricity is named in contrast with current electricity, which flows through wires or other conductors and transmits energy.
A static electric charge is created whenever two surfaces contact and separate, and at least one of the surfaces has a high resistance to electrical current (an electrical insulator). The effects of static electricity can be demonstrated by the spark as the excess charge is neutralised when brought close to an electrical conductor (e.g. a path to ground), or a region with an excess charge of the opposite polarity (positive or negative). The familiar phenomenon of a static shock, more specifically, an electrostatic discharge–is caused by the neutralisation of charge.
The most common generators of static electricity are processes involving flammable liquids. Static electricity is generated by liquids flowing through pipes and in mixing, pouring, pumping, filtering or agitating liquids or solids. The rate of generation is influenced by the conductive of the liquid or solids, the amount of turbulence in the media, the interfacial surface area between the flowing material and other surfaces, liquid velocity and the presence of impurities.
Some specific locations where static electricity is generated are:
- Material transport system: In piping systems the generation rate and the subsequent accumulation of static charge is caused due to the function of the flow rate, velocity, pipe diameter and pipe length.
- Filling operations: The turbulence experienced in filling operations, caused by large flow rates, splashing or free-falling liquid and solids, greatly increases the charge accumulation above the level generated in piping systems.
- Separation: There are many separation operations being implemented. Vibrating system is one among them which causes the generation of static electricity.
- Filtration: Filters, because of their large surface area, can generate as much as 200 times the electrostatic charge generated in the same piping system without filtration.
Pharma manufacturing have come across many operations in line with the above categories wherein it is prone to generate static electricity for e.g.
- Liquid handling systems.
- Solvent dispensing
- Vibro-sifters
- Fluidised bed drier
- Powder transmission
- Mixing and processing reactors
Some of the processes are discussed below
Solvent dispensing
The various process including the transportation of the liquid from one process to other or solvents storage yard and dispensing of the same as and when required is the major area where this static current can cause even an explosion. So better interlocking is necessary because of the flammability nature of the solvent.
Static charge accumulation is a function of the resistance of the path by which charges dissipate within a liquid. The dissipation of static electricity is dependent on a property of the liquid known as ‘conductivity.’ Some flammable liquids have very low conductivities and tend to accumulate static charges.
The chances of static build-up may lead spark explosions, fires, property damage and injury to workers. Occupational Safety and Health Administration (OSHA) has regulation for these liquid dispensing systems to ensure the processes being safe enough for men at work and the environment.
While OSHA does not prescribe how, but it dictates when and where static grounding and bonding procedures must exist. For e.g. when unloading or loading bulk carriers such as tanker or tank trucks or transferring flammables in small barrel, OSHA 1910.106(F)(3)(iv) talk about the static protection.
Sec. 1910.106(f)(3)(iv) (a, b &c) Static Protection.
- Bonding facilities for protection against static sparks during the loading of tank vehicles through open domes shall be provided:
- Where Class I liquids are loaded, or
- Where Class II liquids or Class III liquids are loaded into vehicles which may contain vapors from previous cargoes of Class I liquids
- Protection as required in (a) of this subdivision (iv) shall consist of a metallic bond wire permanently electrically connected to the fill stem or to some part of the rack structure in electrical contact with the cargo tank of the tank vehicle.
- Such bonding connection shall be made fast to the vehicle or tank before dome covers are raised and shall remain in place until filling is completed and all dome covers have been closed and secured.
Although the generation of static electricity cannot be eliminated, its rate of generation and accumulation can be reduced by adopting a proper dissipation system with real time control will eliminate the possible explosion. There are various manufacturers who provide such interlocking systems which monitor and control the rate of generation and dissipation of the static charge. A typical block schematic diagram (Figure 1) of such system is being indicated here, courtesy GE Advanced Sys-tek, Vadodara.
Vibro sifter
Vibro separators are circular gyratory screens used to separate solids from solids and liquid from solid. Vibro sifter consists of specially designed motor mounted vertically at the center of the base plate of the screen. The screen is in between feeding hopper and bowl. The material is fed on to the centre of top screen. The undersize material passes rapidly through the screen during its travel to the periphery. The oversize material gets continuously discharged through a tangential outlet. This is achieved by specially designed vibratory motor along with eccentric top and bottom weights.
Rotation of the top eccentric weights creates vibration in the horizontal plane which causes material to move across the screen cloth to periphery increasing the horizontal throw, causing oversize material to discharge at a faster rate. The bottom eccentric weight rotates below the centre of mass creating tilt on the screen giving vibration in vertical and tangential plane. Adjustment of the quantum and relative position of the top and bottom weight permits control of amplitude and material flow patterns. Rugged springs placed over the circular motor base amplifies the vibration.
Due to the vibration the friction between the screen and the product material static current is being generated in the body of the shifter. Unless the product is flammable in nature the it will not lead to an explosion but it has a great impact on the operator around it, once the operator gets in contact with any conductive part of the shifter he/ she gets the electric shock. To avoid such unpleasant situation it has to be properly grounded and on real time it should be monitored and controlled. The static current grounding device should be integrated for monitoring and control of the process.
Fluidised bed drier
Drying is a process of removal of liquid from solid products with the help of heat. Various process of drying is being implemented. Drying is a function of contact area, between the drying media and the product. Currently, fluidised bed drier is popular across powder drying application because of it high efficiency and the fast process. Normally an initial moisture content upto 20 per cent is ideal but materials with higher initial moisture content can be dried successfully.
In fluid bed drying, heat is supplied by the fluidisation gas, in some cases the heat also being supplied by heating plates or tubes which are immersed in the fluidised layer.
These fluid beds are designed to dry powder particles where the residual moisture content is higher than what is wanted in the final powder. Hot drying air is distributed through specially designed perforated plates on which the powder particles/ agglomerates are resting.
Static charge generated due to the powder or granules which gets fluidised inside the dryer unit should be dissipated with proper grounding system otherwise it will also create unpleasant situation as explained in the above application like operator safety etc.
Powder transmission
Powder transmissions are being done by any of the following methods like conveyors, gravitational flow or flow through a pipeline with dry air. In case of gravitational flow and pneumatic conveying the friction caused between the powder and wall generates the static electricity at both ends i.e. at pipeline and the product. So the pipeline and the collecting bin should be properly grounded so that the static current generated is dissipated properly. Same grounding system (used in case of Vibro sifter) can be deployed to control the static charge generated in the system.
Conclusion
Static charge generation is a common phenomena for any flowing or vibrating mechanism. Where there is friction between two independent surfaces there is static charge generated. Generation of static charge can’t be fully eliminated but can be controlled. Best way to handle such static charge is by planning a better dissipation methodology with controlled automation to it.
Really helpful information!
Thanks!