Nitrogen blanketing is the process of supplying the N2 (or other inert gas) gas to the vapor space of a container to control its composition. It provides improved process safety and better product quality along with a longer equipment life cycle. Nitrogen blanketing practices can be applied to a wide variety of container sizes. The process is widely used for storage tanks and because of that Nitrogen blanketing is often termed as tank blanketing, tank padding, or Nitrogen padding. In this article, we will explore more about Nitrogen blanketing; Its procedure, purpose, application, and controls.
What is Nitrogen Blanketing?
Nitrogen blanketing is the process of supplying the storage tank with an inert gas (the most economical), such as nitrogen, to counteract the effect of oxygen on the storage material, which is usually liquid. When purging the vessel with inert or inert gas, the vessel material does not come into contact with oxygen. This extends product life and mitigates potentially explosive conditions.
Applications of Nitrogen Blanketing
Nitrogen blanketing is used in all industries wherever flammable or explosive products are used or generated. Some of the industries that widely use nitrogen padding methods are:
- Chemical Plants
- Petrochemical industries
- Food Processing
- Pharmaceutical industries
- Petroleum Refining plants
- Aerospace industries
- Ships and Transformers
- Chemical and Petrochemical Transportation industry, Chemical tankers.
Purpose of Nitrogen Blanketing
The blanket helps to maintain constant inert conditions for the product in the container. The goal is to prevent explosion, discoloration, polymerization, and other unwanted quality changes. Here, the flow and pressure of the inert gas stream and/or the oxygen level in the exhaust gas will control the operation. The main benefits that a nitrogen blanketing system provides are:
- Increased safety by reducing the explosion possibility by reducing the amount of oxygen in the vapor space of a chemical storage tank.
- The reduced potential of fire and explosion.
- Increased equipment life by reducing the corrosion possibility due to oxygen and moisture reaction.
- Reduced chances of product degradation by creating the blanket.
- Reduced possibility of product property alteration
Nitrogen blanketing can be applied to a variety of container sizes from a multi-million-gallon tank to a large or smaller container. Although blanketing is a factory practice, there is scope for improvement. Bad choices may add costs, increase nitrogen wastage, or increase plant emissions.
Nitrogen Blanketing Procedure
Before diving into the nitrogen blanketing procedure, let’s recapitulate some of the basics. We all know that a fire happens when all three elements; an ignition source, fuel, and oxygen are present. Removing any of these three elements will remove the possibility of fire. The space above the tank contains a mixture of air and steam from flammable materials that are being stored, eg solvents.
Mixtures of solvent vapors and air will only ignite and burn if the vapor mixture is within the flammability of the solvent and an ignition source is present. This can lead to tragic consequences such as serious injury to people.
Static charges can develop within the system or within the solvent itself, creating a source of ignition although storage tank facilities may be electrically grounded to reduce the probability of ignition.
A storage tank can be made inert in any of the following three ways:
- Limiting the Oxygen Concentration (LOC) of the vapor space
- Reducing the fuel concentration below Lower Explosion Limit (LEL), or
- Increasing the fuel concentration above Upper Explosion Limit (UEL).
The manufacturer’s Material Safety Data Sheet (MSDS) provides the values of LEL and UEL for a chemical. LOC values are often found in chemical industry handbooks and guides such as the National Fire Protection Association (NFPA) Guide. Flammability limits of mixtures of gases, combustibles, and inert substances for high temperature and pressure can be determined by computational methods.
Refer to Fig. 1, which provides a sample example of a flammability envelope for Hydrogen in Air. The combustion will not take place beyond the LOC.
When considering a new or improved blanketing design, the first factor to take into account is the type of tank. This will determine if blanketing is required or not. The following rules apply to the storage tanks for the nitrogen blanketing philosophy.
- When a fixed roof tank stores any flammable material, nitrogen blanketing must be applied.
- Floating roof tanks do not need tank padding.
- Internal floating roof tanks or covered floating roof tanks occasionally need nitrogen blanketing as there is the possibility of vapor build-up.
Fig. 2 explains the above concept of storage tanks requiring nitrogen blanketing.
In addition to the storage tanks and similar vessels, a number of sealed tanks like Non-pressurized spaces may also require blanketings, such as a pneumatic conveyor, powder and dust hoppers, or controlled atmospheric tanks.
The nitrogen blanketing procedure maintains a concentration of nitrogen within the padded tank/vessel. It also prevents the reintroduction of oxygen and other gaseous contaminants within that region. The nitrogen blanketing procedure usually employs the following methods:
- Automatic pressurized vessel purging
- Continuous pressurized nitrogen blanketing using manual controls
- Oxygen concentration measurement and variation
- Dual oxygen concentration and pressure monitoring
- Maintaining a steady pressure of gaseous nitrogen and eliminating oxygen.
Nitrogen Control by Continuous Purge
The continuous purge system uses a continuous flow of nitrogen, which is simple to do but can lead to high nitrogen consumption. Nitrogen can also remove vapor in the headspace and put more uploads factory emissions management system. In addition, air may enter empty space if the tank is empty too quickly and the liquid level drops too quickly. Despite these problems, this type of system continues to be used because It’s very simple and quick to make. Replace this method with pressure or concentration control can lead to savings.
Nitrogen Control by Pressure Control
Pressure control systems are used for closed tanks, that can hold pressure. A valve detects pressure in the headspace of the tank and supplies nitrogen accordingly. Headspace pressure control can be set quite low; less than an inch in the water column is enough. When the tank is discharged, the liquid level is dropped, pressure is reduced and nitrogen is added. When the tank filling, the pressure increases, and nitrogen escapes through the vent valve.
Nitrogen Blanketing Calculation
Simple Calculations for nitrogen requirement are shown where basically, the nitrogen requirements of blanketed tanks have two parts:
- Nitrogen as required by the flow, or material flow through the tank(NW); and
- Nitrogen is required by heat breathing or the increase and decrease of the liquid level due to external temperature (NTB). Of these two sections, usually, the NTB is significantly larger.
The equations mentioned in Fig. 4 are used to calculate the N2 requirement for the Nitrogen blanketing process.
- NT= Total volume of nitrogen required per month (ft3)
- NW = Required Nitrogen by the material flow through the tank (working throughput)
- NTB = Required Nitrogen by the rise and fall of the liquid level owing to the external temperature conditions (thermal breathing)
- VT= total gallons discharged from the tank per month
- VHS = Average empty headspace (gal)
- Thigh = Maximum temperature in the tank (°F)
- Tlow = Minimum temperature in the tank (°F)
- F = Estimated number of temperature swings per month
- 555 = A constant (˚R) pertaining to the vapor space expansion factor
- 7.48 = Gallons to the cubic feet conversion factor
Reference and Further Studies
To learn more about the subject refer to the following article: https://www.chemicalprocessing.com/assets/wp_downloads/pdf/nitrogen-blanketing-keeps-employees-and-equipment-safe.pdf