Q & A > Question Details
Currently our client makes all jet A-1 fuel from a conventional Merox treating process. There is a project under consideration where hydro-treated jet will be produced through a Hydro-desulphurization Unit. The hydro-treated jet with have anti-oxidant injected into the rundown to storage and will co-mingle with straight run Merox treated jet before entering the same storage tank.
Quality assurance/control team has indicated that the jet product needs to be within 0.5 API of each other in the top/middle and bottom sections of the tank. To achieve this tank mixers or a re-circulation system is being considered. Based on API 2003, section 4.5.5 indicates that conventional low-speed propeller mixing has been in use for many years without evidence of problems from static generation.
Other important considerations are the co-mingled jet will be stored in fixed roof style tanks. Secondly there is no anti-static additive injected in the tank at this point as it gets added at the truck rack and marine terminal.
My questions are:
1) Do other refineries typically use mixers or a recirculation system on jet tanks when co-mingling different types of jet?
2) Do they use a nitrogen blanket system as a safeguard to protect against static buildup in the tank when mixers are applied?
3) Is it true that as long as the jet liquid level in the tank does not drop below the mixer elevation in the tank that there should be no concerns with static buildup in the tank? (With all product tanks on-site the low pump out level is always above the mixer elevation and a low alarm ring in when approaching low pump out.)
4) Is the only concern on start-up or shutdown when a tank is being de-inventoried for maintenance or initially filled?
 
Answers
16/07/2012 A: keith bowers, B and B Consulting, kebowers47@gmail.com
1)The degree (intensity of mixing) needed to achieve reasonable uniformity depends of relative rates of production. Using a common run-down line (say the last 50 feet into the tank) will achieve a good degree of mixing as the co-mingled products enter the tank. IF however, production of one or the other product is intermittent, then in-tank mixing may be desirable. Mechanical mixers are used in such mixing services with good predictability. 'Eductors' and circulating pumps are well used to mix one fluid into another, especially if the new fluid is of low volume. Motor powered mechanical mixers, with 'square pitch marine propellers' are probably the best choice in this case. You should ask mixer vendors to help size and specify the particular mixer for your needs.
2)Nitrogen blanketing is not generally used for jet fuel storage. If the run-down temperature is below 100F, the vapor concentration in the tank will typically be lower than the lower explosive limit. You MUST confirm this for the particular products you are making as front-end volatility is a big factor. Typically, jet is water saturated leaving the production units and is electrically conductive enough that no static charge can develop, but again this is specific to YOUR products. Dielectric properties of the two streams must be measured to confirm this factor.
3) Nonsense. Operating a mixer partially submerged will generate a lot of spray, foam, mist which will possibly increase the hydrocarbon in vapor to or above the lower explosive limit. However, electro-static charge can develop in the bulk fluid from any type of movement, whether from mixers or fluid entering/exiting the tank.
4) Nonsense. Potential for electrostatic discharges are increased when the product is moved around. Typically, the jet fuel has enough electrical conductivity from moisture to safely dissipate these charges to the tank metal. If the product is DRY and inherently has high dielectric strength (is a good electrical insulator), then enough static charge can develop to arc through the liquid surface to the tank roof (rather than to tank walls and floor). Such a surface arc could ignite any hydrocarbon vapor if the concentration were high enough to support combustion.
Historically static arc caused fires/explosions in refinery tankage has not been a problem. However, past manufacturing practices resulted in jet rundown to tankage being water saturated at somewhat warmer than ambient, with water dropping out in the rundown tank. Wet jet typically will not accumulate static charges. DRY jet CAN. whether or not your specific situation will or has the potential for static discharge arcing is dependent on the innate dielectric properties of the two products.
It depends.