Nitrogen purging plays an important role in the safety and functioning of various plants that are susceptible to fire hazards. In fire and explosion protection engineering, an inert (ie, non-flammable) purge gas (like nitrogen, helium, argon, etc) is introduced into an enclosed system (eg, a container or process vessel) to prevent the formation of a fuming flammable atmosphere. This inert gas helps to mitigate the risk by overriding hazardous fire/explosion-forming agents like oxygen. In this article, we will explore more about Nitrogen purging, Its definition, procedure, benefits, etc.

What is Nitrogen Purging?

Many a time people in safety engineering ask what nitrogen purging actually means. Nitrogen purging is the process of introducing nitrogen into closed vessels, pipelines, containers, etc to displace undesirable hazardous atmospheres and to clean the inner walls. Nitrogen in the nitrogen purging process pushes out oxygen and moisture and creates a stable non-combustible environment, thus reducing the potential hazard.

Why Nitrogen Purging is Required?

Chemical Industries like acetylene production plants, Oil and Gas plants require nitrogen purging on a regular basis. The main benefits that nitrogen purging provides are:

• Nitrogen purging helps in removing the oxygen % from the internal surfaces of the pipelines and equipment. So, the possibilities of sparks and fires are greatly reduced during operation.
• Nitrogen purging removes moisture that may be present which helps in lowering the dew point.
• The nitrogen purging process ensures a safe environment for the workers and nearby residents.
• It prevents chemical alteration of the product.

Uses of Nitrogen Purging System

A lot of industrial manufacturing processes use nitrogen purging to eliminate moisture or oxygen-rich air. Introgen purging equipment is sometimes integrated with oxygen-sensitive operations to avoid unfavorable conditions. Major industrial applications of the nitrogen purging process are:

• Transformers and other volatile electrical environments enhance safety by using the N₂ purging procedure.
• Nitrogen purging in the brewery industry extends the shelf life of beer.
• Ships and tankers use nitrogen blanketing to remove potentially combustible environments.
• Organic compounds generating chemical and petrochemical industries widely use nitrogen purging processes to eliminate toxic gases from process chambers.
• Atmosphere Packaging and Food industries apply the nitrogen purging process to remove moisture, oxygen, and other gaseous impurities.
• Nitrogen purging is widely used in oil and gas pipeline projects for drying, cleaning, and limiting oxygen concentrations.
• To eliminate compounds affecting weld quality, metal fabrication industries use N2 purging systems.

Why Is Nitrogen Used for Purging?

Nitrogen is dry, non-combustible, and economical as compared to other inert gases. This makes the nitrogen purging process more affordable.

Difference between Nitrogen Purging and Inerting?

The purge gas is inert. By definition, it is non-flammable, or more precisely, non-reactive. The most common purge gases available commercially in large quantities are nitrogen and carbon dioxide. Other inert gases, eg. Argon or helium can be used. Nitrogen and carbon dioxide are not suitable for purge applications in some cases because these gases can chemically react with fine dust from certain light metals.

Since an inert purge gas is used, the purge procedure can (erroneously) be called inerting in everyday language. This confusion can lead to dangerous situations. Carbon dioxide can be considered a safe, inert purge gas. Carbon dioxide is an inert gas that is not safe for inactivation as it can ignite vapors and cause an explosion.

Difference between nitrogen purging procedures from other industrial explosion prevention methods?

Fires and explosions can also be prevented by controlling ignition sources. However, purging with an inert gas (nitrogen) provides a higher level of safety because it is guaranteed that no flammable mixture is formed. Therefore, it can be said that primary prevention is relied on to reduce the possibility of a spreading explosion, and ignition source control relies on secondary prevention to reduce the possibility of an explosion. Primary prevention is also referred to as essential safety.

Types of Nitrogen Purging Procedures

There are four main industrial nitrogen purging procedures that are widely used. They are:

1. Displacement purging (Plug effect)
2. Dilution Purging
3. Pressure swing Purging, and
4. Vacuum purging

Displacement purging (Plug effect)

In a displacement purge, an inert gas is injected into an open vessel to evacuate hazardous or noxious gases. Slow flow is maintained (velocity < 10 m/s).  

The displacement nitrogen purging procedure is mainly used for high H/D (height/diameter) ratios. Ideally, the inert gas should be denser than the displaced gas on Fig. 1 shows how nitrogen is used for the displacement purge of the vessel. Gas is transported in tankers. Liquid nitrogen is vaporized in the vaporizer and gaseous nitrogen is injected into the vessel. Nitrogen pushes the atmosphere out of the vessel through an outgassing valve. The amount of nitrogen required is relatively small, typically 1.2 times the capacity of the vessel.

Dilution Purging:

A dilution purge involves introducing an inert gas to reduce the concentration of hazardous gas.

The dilution nitrogen purging procedure is used when the H/D ratio of the machine is low. The amount of nitrogen required is approximately 3.5 times the capacity of the vessel. The configuration in Fig. 2 shows how nitrogen gas is vaporized and injected into the device with the outlet valve open. The diluent gas, which consists of noxious gases and nitrogen, is released into the atmosphere or further processed.

The following  equations can be used for this process:

The number of volume changes:                               i = ln [C_a ⁄ C_e]

The Volume of inert gas required:                              V_N = i · V_B   

Here,

• i = Volume change
• C_a = Initial concentration
• C_e = Final concentration
• V_N = Volume of inert gas
• V_B = Volume of vessel

Pressure swing Purging:

In pressure swing purging, the closed device is sprayed with an inert gas. When the gas is released, the dangerous or harmful gas disappears. The process (closed-injection-open-exhaust) continues until the desired concentration of harmful gases in the device is reached. Variable pressure purge is used, for example, when the inlet and outlet are close to each other. The device must also be a pressure vessel. In the pumping pressure, there is a difference between vacuum purification and excessive production. One of the main goals of the pressure swing occurs due to hazardous substances or harmful substances, such as oxygen. The residual concentration of harmful substances can be calculated in the following formula:

C_SR=(P₁/P₂)ⁿ * (C_SG-C_SI) +C_SI

Here,

• C_SR = hazardous substances residual concentration
• C_SG = Concentration of hazardous substances in mixtures
• C_SI = Concentration of hazardous substances in inert gas
• n = Number of pressure swings
• P₁ = Pressure 1 (before inerting)
• P₂ = Pressure 2 (after inerting)

Vacuum purging

Vacuum purge involves the use of a vacuum pump to remove harmful gases and then supply inert gas to the evacuated unit. This process is repeated until the desired hazardous gas concentration is reached. Vacuum purge is particularly suitable for machines with multiple dead zones.

The effective inert gas requirement  is calculated as follows:

V_N = V_B * f * n

Here,

• V_N = Inert gas requirement in m³ 
• V_B = Vessel volume in m³
• f = Pressure change ratio
• n = Number of pressure swings

The pressure change ratio equation is as below:

f=1-(P₁/P₂) (P₁/P₂)<1

Here,

• f = Pressure change ratio
• P₁ = Pressure before inerting
• P₂ = Pressure following inerting

Nitrogen Purging in Pipelines

Newly laid pipeline network and sometimes after maintenance and shutdown work nitrogen purging in pipelines is performed. This process is important to remove retained moisture, oxygen, and other impurities that may otherwise change the quality of the fluid being transported.

Nitrogen purging in pipelines is a pretty straightforward procedure. Pressurized nitrogen gas is forced through pipelines that force out all gaseous and particulate impurities present inside. However, Pipeline nitrogen purging may sometimes involve risks. Hence, To safely conduct the pipeline nitrogen purging process, the operators should take the following steps:

• Ensure proper instruments/apparatus handling
• The operation must be performed by trained personnel.
• Emergency protocols for shutdown and personnel evacuation must be well informed to all.
• Personal protective equipment must be worn by all personnel involved in purging operations

Inerting refers to the process of introducing an inert gas into an enclosed space to release the gas already present, resulting in a kind of hazard. Although this process can be used to displace toxic gases, it is most commonly used to displace oxygen or to reduce the oxygen concentration in a space when the space is completely enclosed and substitution is not possible.

An inerting system reduces the combustion potential of flammable materials that are stored in a confined space. A common example of such a system is a combustible liquid-filled fuel tank that transports gasoline, diesel fuel, jet fuel, aviation fuel, or rocket propellant. Once the fuel tank is filled and during use, a space vapor barrier above the fuel known as the ullage is created. It contains evaporated fuel mixed with air containing oxygen, the important element necessary for combustion. An inerting system is used to replace the air with inert gas, such as nitrogen, argon, helium, etc. to avoid the combustion hazard.

Reason for Inerting

There are many chemical processes where inerting is required

• to make the system or process explosion-proof
• to eliminate unwanted reactions 
• to keep food away from moisture 
• to ensure safety during maintenance work. 

Engineers often rely on inert gases and specialized inert equipment, as these goals cannot always be achieved by technology and equipment design alone. There are many situations where deactivation is the only way to meet safety standards during the production process and maintenance. In other cases, inerting is used to improve product quality.

Basic Principle of Inerting

The basic principle of inerting is to completely or partially replace air containing oxygen and flammable and/or toxic gases that often contain moisture or inert gases.

Inerting is based on the principle that a combustible (or combustible) gas can only burn (explode) when mixed with air in the correct proportions. The flammability limit of the gas determines this ratio, i.e. the flammability range. In terms of combustion technology, it can be said that the inert gas inlet dilutes the oxygen below the limiting oxygen concentration. Inerting prevents the formation of a flammable mixture that spreads by definition. Inerting introduces an inert gas to make the flammable mixture safe.

Gases Used for Inerting

The most widely used gases for inerting are Nitrogen and Carbon dioxide. Other gases such as argon and helium are applied in certain instances. Steam and exhaust gases are used in some industrial applications.

Selecting the Inert Gas

There are various criteria that influence inert gas selection. Some of these are:

Dangers of fire and explosion and/or their effects or actions 

Nitrogen and carbon dioxide are not completely inert, but they are the best gases at room temperature. At high temperatures, nitrogen reacts with very few substances such as lithium, which forms lithium nitride. 

6 Li + N₂ → 2 Li₃N

Nitrogen can react with magnesium, Silicon at temperatures between 400 ° C and 1800 ° C for nitriding, titanium, and other metals:

2 Ti + N₂ → 2 TiN

The reaction with oxygen occurs only at very high temperatures. In comparison, carbon dioxide reacts with a longer list of substances containing strong bases. Amine, anhydrous ammonia, lithium, potassium, sodium, magnesium, beryllium, aluminum, chromium, manganese, titanium, uranium, and acrolein. The decomposition temperature of Carbon dioxide is 2000° C, but this is rarely a problem. However, increasing the pressure increases the solubility of this gas.

Effect on product and exhaust gas

When steam is used for inactivation, the condensate produced can sometimes be hazardous. inactivation. When shipping, readily available exhaust gases are used in the oil tank, so the fuel-to-air ratio must be carefully controlled. 

Cost

Inert gases are generally too expensive to use for inertization and are not as effective as inert gases. nitrogen. However, they are often used to inactivate light metal dust. For example,  Aluminium dust explosions can be prevented by using argon and that is why it is used in fire extinguishing systems for such explosions. 

Specific heat capacity of inert gas

 The specific heat capacity of a given inert gas determines its effectiveness. The higher the specific heat capacity, the higher the inert efficiency. Table 1 shows the corresponding values.

Inerting Agent	Specific Heat* (Btu/(lb mole/0F)	Specific heat* (kJ/kmol/K)
Nitrogen	7	29.308
Carbon Dioxide	8.8	36.844
Helium	5	20.934
Argon	5	20.934
Steam	17	71.176

The reason for this is that diluting with a high specific heat inert gas reduces the areas at risk of ignition. According to this logic, carbon dioxide is again favored over nitrogen because less inert gas is required.

Molecular structure of inert gas

The molecular structure of an inert gas directly affects its efficiency. Carbon dioxide is a triatomic molecule, nitrogen is a diatomic molecule, argon, and helium are monoatomic molecules. Polyatomic molecules can absorb more energy because they contain more free molecular vibrations. Therefore, the inert effect decreases in the following order:

CO₂ → N₂ → He, Ar

Figure 1 shows the effectiveness level of the inactivating agent, using the flammability of methane as an example.

The density of inert gas

Density also plays a particularly important role. If the density of the Inactive gas is greater than the density of the replacement gas, the free space above the Inert gas injection point may not be deactivated.

Table 2 shows a comparison of the density of the selected gas to that of air.

Types of Gas	Density in Kg/m3 at 1013 m-bar and 00C	Density relative to Air
Air	1.293	1
Nitrogen	1.2505	0.967
Hydrogen	0.0899	0.0696
Argon	1.7837	1.38
Oxygen	1.429	1.105
Carbon dioxide	1.9767	1.529

Application of Inerting in Chemical Industries

The chemical industry uses a wide range of materials, technologies, and devices. Inactivation or inerting is especially used for: 

• Reactor, Mixer 
• Centrifuge and vacuum filter 
• Wheat mixing plant 
• Tank farms, ships 
• Dryer, silo 
• Gas station 
• Oil line and fuel line 
• Industrial service

Objectives of Inerting Activities

The predominant objectives of those inerting activities are to

• Prevent explosive atmospheres from forming in equipment along with reactors
• Ensure secure begin-up and shutdown of flora and equipment 
• Avoid explosion dangers in the course of garage and delivery of flammable substances
• Protect merchandise in opposition to atmospheric oxygen whilst oxidation reactions might impair  fine 
• Protect in opposition to atmospheric moisture, both to keep product fine or to make sure optimum  downstream processing, as an example in grinding
• Prevent fitness and protection risks in the course of the renovation of flora, equipment, and pipelines.

Different Modes of Inerting Application

Continuous Applications:

• Protection of the production process to prevent fire, explosion, and oxidation (for quality assurance) 
• Inert solvent containers and transport equipment to prevent fire and explosion

Intermittent applications:

• Purge pipeline
• Tank purging
• Deactivation of the filter system
• Inactivation of silos
• Deactivation of grinding equipment
• Smoke evolution in the mine

Types of Inerting

Depending on the application, inerting can be classified into two groups as mentioned below:

Partial inerting:

The oxygen concentration is reduced to a level low enough that the mixture will no longer explode.  In the case of partial inertia, the goal is to reduce the oxygen concentration in the mixture to a level where it will no longer explode.

Complete inerting:

Complete inertia involves increasing the ratio of inert gas to a combustible material to a level at which the addition of any amount of air cannot produce an explosive mixture.