A coalescer is required to be installed in straight run kero line of the CDU. What are the requirements under OISD (Oil Industry Safety Directive ) for this? Are there any other safety directive norms that need to be considered?
Answers
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16/02/2009
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A:
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Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
You should refer to API 2003 and ASTM D4865 regarding electrostatic discharge.
The friction caused by passage of kerosene through the run down filter and coalescer media may build up an electrostatic charge which in the right circumstances can cause an explosion during vessel maintenance.
From OH&S perspective, it is important to properly ground the pre-filter and coalescer with earthing braid and follow some precautions for the housing filling rates (Safety procedures require filling the vessel a 1/30th of the rated flow).
There is typically 5 to 10 electrostatic explosion incidents a year reported at the CRC Electrostatic Committee. In almost every case in the last 20 years where "filtration" equipment was involved, the incident involved a coalescer cartridge (coalescers charge worse than filters, and water adsorbing cartridges are even worse chargers than coalescers) and in EVERY case where there was an explosion someone did not follow the safety procedures outlined in the references above.
The most common explosions are caused by either filling vessel at rated flow or not bonding and grounding the truck when fuel is being transferred either to or from the truck (the hoses charge also). The Highflow Ultipleat filter vessel (the Pall typical pre-filter used) was designed to NOT ever be a capacitor, but it is still recommended to follow the safety procedures, of course.
Pall Corp manufacture Highflow Ultipleat filters and high efficiency Aquasep and Phasesep liquid liquid coalescers which will not have a problem with electrostatic charging if it is sized properly and the API and ASTM electrostatic Safety procedures of bonding and grounding as well as vessel filling procedures are followed. Ref API 2003 and ASTM D4865.
There are two reasons why the HighFlow Ultipleat filter is much better designed than most filtration systems for electrostatic charging:
1. The medium velocity is relatively low for the Highflow Ultipleat pre-filter and consequently generates relatively low charge.
2. The permanent cage in the HighFlow Ultipleat housing effectively grounds the pack and bleeds off any charge generated to ground.
Most filter (and coalescer cartridges for that matter) have a metal core that is electrically isolated from ground by the gaskets or o-rings that seal the cartridges, and this makes the filter into an effective capacitor that can store up charge to enough voltage to arc to ground like a spark plug. If this arc happens in an explosive fuel/air mixture (such as in filling a vessel) there is an explosion. The safety procedures above require filling the vessel a 1/30th of the rated flow. The most common explosions are cause by either filling vessel at rated flow or not bonding and grounding the truck when fuel is being transferred either to or from the truck (the hoses charge also).
I might also add from a process point of view, high efficiency coalescers should be selected to minimise water and trace caustic carryover from the caustic treater to salt drier and clay tower. Inappropriate coalescer selection and resulting performance can lead to excessive salt usage and may result in unscheduled shutdowns for clay tower changeout. Pall offers coalescers that have efficiency of <15 ppmw feree water downstream of the coalescer - even for low IFT fluids such as caustic containing water. Coalescer datasheets are available on our web site www.pall.com if you want more information.
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