Variability in blanketing gas header pressure is a common problem that I have seen while consulting end users on their tank blanketing systems. The impact that variable inlet pressure can have on a tank blanketing regulator is not well understood and can be a concern, especially on tanks where oxygen ingress can impact the stored product or operations.
The lack of understanding is primarily because we are using a pressure control device to ensure proper flow during tank operations such as pumping-out. For example, while drafting a tank the blanketing regulator must have enough flow capacity to fill the tank vapor space with the inert gas at a sufficient rate to avoid a high negative pressure resulting in O2 being pulled in through a pressure vacuum relief valve (PVRV).
Capacity tables inside blanketing regulator bulletins are organized by inlet pressure. You can see from the table below if our inlet pressure varies by as little as 10 PSIG there can be a significant impact to the achievable flow rate of the regulator.
A way to avoid this problem is to install a step-down regulator upstream of the blanketing regulator. The step down regulator eliminates the impact of variability by providing the blanketing regulator with a constant inlet pressure. In the example below, the N2 header pressure varies from 98-80 PSIG, therefore the step down regulator is set to an intermediate pressure of 60 PSI. By providing this constant inlet pressure you can be confident that your blanketing regulator will be able to meet the required flow capacity into your tank to avoid O2 ingress.
At most sites, header pressure variability is not noticed by plant operations due to the lack of continuous pressure monitoring. If you are experiencing issues due to oxygen in product tanks, consider first a nitrogen pressure survey at multiple times during the day to determine the lowest N2 pressure and (where applicable) install step down regulators to limit the impact on your tank blanketing systems.
If you have additional questions or comments on the impact of inlet pressure on tank blanketing regulators feel free to post below.