Dear fellow chemical engineers,
A friend of mine forwarded me a copy of an industrial incident of a storage tank explosion at Glenpool, Oklahoma which describes the danger of static electricity in the petroleum industry. After going through the report I felt the need to do some more searching of the subject. One aspect of static electricity build up is in loading of road tankers with white oil products such as gasoline, kerosene etc. which is a very common activity in petroleum refining installations.
My searching led me to some guidelines for loading flow rates for road tankers as a function of the electrical conductivity of the product being loaded. I would like to share this with the readers of my blog.
Please note that loading rates and velocities mentioned are the maximum in the subsequent write-up. Loading rates in practice may be much lower than the ones mentioned in the write-up.
Here is how it goes:
White Oil Products are generally defined as Gasoline, Naphtha, Kerosene and Gas Oil, i.e. products from the high or light end of the crude distillation process.
Static electricity is generated during loading. When non-conductive liquids (e.g. common white oil products without anti-static additive) are loaded, static electricity may build up resulting in sparks. Therefore if a flammable atmosphere may be present (e.g. when loading flammable products), the loading velocities should be determined using the following guidelines: 1. Determine the conductivity range of the product to be loaded (≤50 pS/m, >50 pS/m, unknown).
pS/m is the unit of conductivity and is defined as Pico siemens / meter
Lower the electrical conductivity of a flammable liquid, greater the possibility of a static charge built-up and resultant explosion hazard.
Fluids that have low electrical conductivity (below 50 pS/m), are called accumulators. Fluids having conductivities above 50 pS/m are called non-accumulators. In non-accumulators, charges recombine as fast as they are separated and hence electrostatic charge accumulation is not significant. In the petrochemical industry, 50 pS/m is the recommended minimum value of electrical conductivity for adequate removal of charge from a fluid.
2. Determine the sulfur content of the product to be loaded (≤50 mg/kg, >50 mg/kg)
Sulfur content in the product >50 mg/kg inhibits static electricity build-up by acting as an anti-static agent.
1. If it is determined that the product to be loaded can cause a flammable atmosphere, the conductivity determined as ≤50 pS/m or unknown and sulfur content ≤50 mg/kg then the loading rate shall be such that V*d ≤0.5 m2/s where V is the velocity of the product in the pipe in m/s and d is the internal diameter of the pipe in m. In flow rate terms this amounts to a maximum limit of 144 m3/h in a 4" pipe.
2. If it is determined that the product to be loaded can cause a flammable atmosphere, the conductivity determined as ≤50 pS/m or unknown and sulfur content >50 mg/kg then the loading rate shall be such that V*d ≤0.75 m2/s. In this case the flow rate is governed by a maximum velocity of 7 m/s in the pipework which translates into a maximum flow rate of 207 m3/h for a 4" pipe.
3. If it is determined that no flammable atmosphere can be caused by the product to be loaded, the conductivity determined as >50 pS/m then also the maximum velocity of 7 m/s shall not be exceeded in the pipework.
Some industrial incidents due to static electricity have been very well documented. One major incident which resulted in the explosion of a storage tank at Glenpool, Oklahoma is very well documented and provides a deep insight in the dangers of static electricity build up at facilities handling white oil products & the measures to prevent them.
The detailed report of the incident is attached for the benefit of the readers.
1. Wikipedia – Static Electricity http://en.wikipedia....hem_industry2-8
2. Shell Design Engineering Practice – DEP 31.06.11.11-Gen.- Loading Facilities for Bulk Road Vehicles
NTSB Tank Fire.pdf (680.76K)