Chemical tanks are often “padded” or “blanketed” with nitrogen. Nitrogen serves as an inert atmosphere that prevents oxidation of the chemical to prevent fire or explosion. A popular and convenient method of supplying nitrogen is to produce it on-site from standard compressed air using a nitrogen generator. Typically, two questions come to mind when considering blanketing with a generator: The first is “how do I size a nitrogen generator to ensure I have enough gas to blanket my tank?”, and the second is “how do I physically blanket the storage tank?”. This blog post will answer both of these questions.
When determining the amount of blanketing gas, you must consider both liquid losses during pump out and condensation of tank vapors during atmospheric thermal cooling. The maximum flow rate and desired purity determine the size of the nitrogen generator required.
Below are the steps for sizing a blanketing generator:
Determine the gas flow rate due to the pump out. See table 1.
Find the gas flow rate due to atmospheric cooling. See table 2.
Add the requirements of 1 and 2 to get the flow rate capacity of the nitrogen generator needed.
Determine the nitrogen purity level required for the application.
Use the flow rate and purity to determine the size of the nitrogen generator needed for the application.
When the tank is not being pumped out, it is only necessary to purge for leaks to prevent the ingress of air. This maintenance flow can be quite low and is easily maintained by the nitrogen generator.
Table 1: In-Breathing Rate Due to Pump-Out
Table 2: In-Breathing Rate Due to Thermal Cooling
One way to blanket a tank is to reduce the oxygen content in the vapor space to a value lower than the minimum concentration that supports combustion, i.e., the limiting oxygen concentration (LOC). Another way to blanket a tank is to keep the fuel concentration in the headspace to a value lower than the minimum concentration that supports combustion, i.e., the lower explosive limit (LEL) or lower flammability limit. A third way is to increase the fuel concentration in the headspace to a value greater than the maximum concentration that supports combustion, i.e., the upper explosive limit (UEL) or upper flammability limit. A material’s flammability envelope is bounded by its LEL, UEL, and LOC. The values for specific chemicals are found in material safety data sheets, the National Fire Protection Association’s NFPA 69: Standard on Explosion Prevention Systems and in chemistry handbooks.
How nitrogen is controlled usually depends upon the type of tank used. Methods include continuous purge, pressure control and concentration control.
Continuous purge blanketing provides a constant flow of nitrogen and is probably the easiest and most common method because it is simple to set up and a control device or control loop is not required. However, nitrogen consumption can be high.
In pressure control blanketing, a valve and pressure regulator allow the addition of nitrogen when the liquid level drops while a vent enables the release of nitrogen when the liquid level rises.
Concentration control blanketing relies on a feedback loop from an oxygen analyzer to the nitrogen generator to cycle the generator on or off. This method economizes the use of nitrogen because it shuts down the nitrogen supply until enough outside air infiltrates the tank headspace to raise the concentration of oxygen above the acceptable limit.
In each case using a nitrogen generator is desirable as generators can deliver nitrogen on a continual basis without the need for replenishment or monitoring. Maintenance consists of simple filtration replacement every six months and valve replacement annually.
This post was contributed by David Connaughton, nitrogen generation systems product manager, and Judy Silva, gas generation technology blog team member - Parker Hannifin.