With the advent of OSHA’s Process Safety Management regulations (1910.119), many businesses are focusing more closely on the safety of their flammable liquids storage and processing.. This focus has led many of them to request that CWWA design inert blanketing systems for their flammable storage and processing vessels.
While nitrogen is the most common inert gas, carbon dioxide has also been employed where appropriate. Only in very special situations are the noble gasses used due to their high cost. This Engineering tip will only address nitrogen..
The purpose of the inert blanket is to prevent an explosive mixture by excluding oxygen from the vapor space of a tank or vessel. This exclusion makes monitoring of upper and lower explosive limits unnecessary since no oxidizer is present. For most solvents stored or processed in atmospheric tanks or vessels, this is an effective means of protection.
However, we have found that the definition of "solvent" varies from company to company. We have found companies where true monomers are considered solvents. This can create a real problem if blanketing is employed.
Most monomers have a polymerization inhibitor added by their manufacturer, before shipment. This inhibitor is necessary to prevent cross linking of the monomer and the initiation of an exothermic reaction in an uncontrolled environment such as a storage tank or vessel.
There are many types of inhibitors for the various monomers, but some of them actually need oxygen to prevent polymerization in storage. CWWA engineers specifically look at the type of inhibitor used to see if nitrogen blanketing is appropriate. We also look for areas in the process where distillation can occur and inhibitors might be left behind, or where oxygen may be needed to allow the inhibitor to work.
If you do not know whether or not oxygen is required, contact your supplier before proceeding with a blanketing project. Always remember that there are alternatives to maintaining the vapor space below the lower explosive limit (LEL).
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