Characteristics of flammable liquids for chemical plant begineers

flammable liquid
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Information on flammable liquids of Class 4 hazardous materials has been compiled.

Knowledge of flammable liquids is extremely important, and essential knowledge for designing and maintaining chemical plant equipment.

Of course, it can be said that it is the basic of the basics in driving.

By knowing the characteristics and dangers of dangerous objects, you will understand the difficulty of safe driving and maintenance for the first time.

You may know the background that people who work at chemical plants have a very high safety awareness.

Dangerous because you can’t see it

general properties

Introducing the general characteristics of Class 4 hazardous materials.


Flammability is a major feature of Class 4 dangerous goods.

Keywords related to flash point, flammability range, vapor pressure, and minimum ignition energy.

Class 4 hazardous materials, which are flammable liquids, are divided into several ranks, and it can be said that this is largely determined by the flash point.

The lower the flash point , the more dangerous it is.

Liquid specific gravity is lighter than 1.0

Liquids classified as dangerous goods are basically lighter than water .

Liquid specific gravity is lighter than 1.

When a reaction is carried out in a reactor in a batch chemical plant, it is normal for the reactor to separate into a water layer and an oil layer.

A liquid separation operation occurs here, and is used to determine where the target material should be transferred.

If the target is an oil layer, the lower water layer is discarded.

The middle layer must be returned to the oil layer side.

In addition, oil separation tanks also take advantage of the fact that the specific gravity of the oil layer is lighter than water.

It’s analog, but that’s why it’s so effective.

Gas specific gravity is heavier than 1.0

Class 4 dangerous gases are heavier than air .

Gas specific gravity is heavier than 1.

The liquid specific gravity is less than 1, but the gas specific gravity is greater than 1.

It seems like a mistake.

It is this feature that makes dykes and reservoirs for hazardous materials outdoor tank storage work.

This is also the reason why flammable gas detectors are installed around oil separation tanks and storage areas.

This property is also used when simulating how far the combustion gas is released from the chimney and spreads.

Nowadays, dedicated simulation software is used, so mechanical engineers probably don’t have a chance to use it.

does not conduct electricity

Class 4 hazardous materials do not conduct electricity .

That’s why static electricity accumulates.

It’s so basic that you might miss it.

Many people are so concerned about the bonding of equipment and piping that they don’t realize the obvious fact that Class 4 hazardous materials do not conduct electricity.

If you work with instruments, you may notice the use of conductivity meters in liquid separation operations.

Since the water layer has conductivity and the oil layer has no conductivity, it can be detected with a conductivity meter .

General fire prevention methods

Introduce general fire prevention methods.

Do not generate steam unnecessarily

Avoiding vapor generation from flammable liquids is one of the measures.

This is the basics of fire prevention.

The flash point is dependent on the temperature; the higher the temperature, the higher the flash point.

There is no way to raise the temperature for driving.

Otherwise, avoid high temperatures.

It is possible to reduce the risk by not storing it near fire or exposing it to direct sunlight as much as possible.

Seal tightly and store in a cool, dark place

Flammable liquids should be tightly sealed and stored in a cool, dark place .

If it’s sealed, flammable vapor won’t spread.

It would be nice to know the image of storing it in a drum can.

Nitrogen seals are similar to hermetic plugs in chemical plant equipment.

Exhaust to a high place outdoors

Vapors of flammable liquids are exhausted outdoors to high places .

This is because flammable liquid vapors are heavier than air.

We are aiming for the effect of diffusing with the wind from the time it is exhausted until it reaches the ground.

The chimney-like pipes that extend from the final abatement equipment at the hazardous materials manufacturing plant to high places are aimed at the diffusion effect.

Diffusion will take you out of the flammable range.

The same idea applies to areas such as walled-in areas within a hazardous materials manufacturing facility that require ventilation to prevent flammable vapors from accumulating.

A realistic countermeasure is to install a ventilation fan or not to enclose more than three sides.

Use explosion-proof equipment

Explosion-proof equipment is used at factories that handle class 4 hazardous materials .

Static electricity will cause ignition.

The motor is an explosion-proof electric motor, and should be explosion-proof or explosion-proof for increased safety.

remove static electricity

Static electricity must be removed in factories that handle class 4 hazardous materials .

In addition to grounding the building itself underground and connecting metal equipment to the building,

Appropriate bonding must also be done.

Electrostatic shoes are important not only for equipment, but also for people.

Even though they have taken such precautions, they still paint the floor surface so that it does not conduct electricity.

Relationship between classification and flash point

There is a close relationship between Class 4 hazardous materials and flash points .

Flash points have a very wide range, so they are divided into several stages.

We will introduce their classification and how to handle them in chemical plants.

Special flammables -20°C or less

Special flammable substances are those with a flash point of -20℃ or lower.

Other conditions include an ignition point of 100°C or lower and a boiling point of 40°C or lower.

Since the flash point is below -20℃, static electricity countermeasures must always be considered in normal environments.

Since it only applies to special substances, there are few opportunities to handle it even in chemical plants.

It seems better to understand that it is just that dangerous.

Class 1 petroleum below 21°C

Class 1 petroleum has a flash point of less than 21°C. I also say it’s one stone.

In a chemical plant, two stones are not treated much differently.

It has a flash point at room temperature and may also have a flash point in winter, making it difficult to take countermeasures.

Gasoline and acetone, which are commonly used, are also useful.

In fact, it is better to be aware that it falls under the category of one stone and is dangerous.


Alcohols are hydrocarbons with an alcohol group (OH) attached.

The Fire Service Law applies to saturated monohydric alcohols with up to 3 carbon atoms.

As a mechanical engineer, it’s difficult.

They are methanol, ethanol and propanol.

Methanol is sometimes used in chemical plant reactions, and is also used in equipment cleaning.

Ethanol is simply alcohol .

In general, if the alcohol content is 60°C or higher, it has a flash point.

Class 2 petroleum below 70°C

Secondary petroleum products have a flash point of 21°C or higher and lower than 70°C. Also called two stones.

Many hazardous materials handled in chemical plants fall into this category.

It has a flash point at room temperature, so static electricity countermeasures are essential in the factory.

Niseki can be handled at low temperatures below its flash point, making it easy to take safety measures.

Up to the second stone, sufficient care is required, and the hurdle is lowered for the third stone.

This feeling is also important as a mechanical engineer.

In general, kerosene and diesel oil fall under the category of two stones.

Other substances used for reactions and solvents, such as xylene and butanol , are also two stones.

Class 3 petroleum below 200°C

Type 3 petroleum has a flash point of 70℃ or higher and lower than 200℃. Also called Mitsuishi.

As a chemical plant, it feels like a dangerous substance that can be handled with peace of mind.

A flash point of 70℃ means that it does not have a flash point at room temperature.

It has a similar feel to water in that it can be used without excessive consideration of static electricity countermeasures.

Generally, heavy oil falls under Mitsuishi.

There are also a certain number of other hazardous materials that fall under Mitsuishi.

As a mechanical engineer at a chemical plant, I would like to keep a certain distance from Mitsuishi’s equipment that handles hazardous materials.

Class 4 petroleum below 250°C

Type 4 petroleums have a flash point of 200℃ or higher and lower than 250℃. It is also called four stones.

For a chemical plant, knowing machine oil is sufficient.

This is a lubricating oil used in motors, which are mechanical equipment in chemical plants.

In a factory, it is common for hazardous materials to be managed in the manufacturing department or other department that handles hazardous materials.

The difference between whether the oil change work is carried out by someone from the production department or by maintenance personnel will vary depending on the company.

Machine oil is a Class 4 dangerous substance and must be managed.

Grade 1

Ichikoku is a substance with a flash point below 21°C and is actually quite dangerous.

Some people may be relieved because gasoline falls under this category, while others may want to be careful when handling gasoline.


Gasoline is a familiar item used in cars.

However, I don’t know much about its nature.

  • There is a particular odor
  • Specific gravity is around 0.7
  • Combustion range is 1.4-7.6vol%
  • Flash point -40℃ or less
  • not soluble in water
  • does not conduct electricity
  • rubber. dissolve fat

You can see that it pretty much covers the general characteristics of dangerous goods.

Characteristic properties of gasoline are “special odor” and “flash point -40℃”

There are surprisingly few substances with flash points below -40℃.

Even if you handle gasoline in Hokkaido, there is a risk of it catching fire, so it is essential to take precautions against static electricity.

Since the flash point is too low, a smell is added to detect leaks.


Let’s look at the properties of benzene in the same way.

  • There is a particular odor
  • Specific gravity is 0.88
  • Combustion range is 1.3 to 7.1vol%
  • Flash point -20℃
  • Insoluble in water but soluble in organic solvents.
  • good at dissolving organic matter
  • is toxic

Benzene is not talked about much as a hazardous material.

You can think of it as a degraded version of gasoline .

On the other hand, when handling benzene in a chemical plant. You have to worry about “toxicity”.

It is rarely used in batch chemical plants.


Let’s look at the properties of toluene in the same way.

  • Specific gravity is 0.87
  • Combustion range is 1.2 to 7.1vol%
  • Flash point 5℃
  • Insoluble in water but soluble in organic solvents.
  • toxic

Toluene is a degraded version of benzene.

Due to its toxicity, it has been used less and less in chemical plants in recent years.

I think there are factories that use it casually, but the times are changing.


Let’s look at the properties of acetone in the same way.

  • Specific gravity is 0.79
  • Combustion range is 2.2 to 13.0vol%
  • Flash point -20℃
  • Soluble in both water and organic solvents

The biggest feature of acetone is that it dissolves in both water and organic solvents.

Also called a hydrophilic solvent.

This is very useful in chemical plants.

It can be used to clean equipment, but it can also be used to remove general oil stains in factories.

For example, if the paint gets on your clothes.


When you think of alcohol, you mean alcohol.

Many people may feel close to it.

However, it is actually dangerous and may have some unknown properties.

We will introduce the properties of alcohol within the scope of Dangerous Goods.


Let’s check the properties of methanol.

  • There is a pungent odor
  • Specific gravity is 0.79
  • Combustion range is 6-36vol%
  • Flash point 11℃
  • Soluble in water and organic solvents.
  • good at dissolving organic matter
  • used as fuel for cars
  • toxic

Methanol has a simple structure that should be called the basis of alcohols.

As a hazardous material Otsushi, it has a relatively wide flammable range and a low flash point like Issoku.

As a chemical plant, it is also used for equipment cleaning, taking full advantage of its characteristic of being soluble in both water and organic solvents .

It is also used as fuel for cars.

Since ethanol is alcohol, can you drink methanol as well? If you think so, you are wrong! ! !

Another feature is that even when it burns, the color of the flame is pale and difficult to see. It’s scary that you won’t notice even if it burns.


Let’s check the properties of ethanol,

  • There is a pungent odor
  • Specific gravity is 0.79
  • Combustion range is 3.3-19vol%
  • Flash point 13℃
  • Soluble in water and organic solvents.
  • Main ingredient of alcoholic beverages

Ethanol is very famous as an alcoholic beverage.

If the alcohol content exceeds 60 degrees, it will burn.

If you bring a fire close to the surface of alcohol that is over 60 degrees, it will burn.

The flammability range is slightly smaller than methanol, but the specific gravity and flash point are almost the same as ethanol.

Because it has a hydroxyl group (-OH), it is soluble in both water and organic solvents.

Ethanol has a lower degree of solubility.

Ethanol has more CH groups than methanol, and CH groups are insoluble in water.

The more water-insoluble CH groups there are, the less water-soluble it is.

1-propanol, 2-propanol

Propanol, like methanol and ethanol, is an alcohol.

Propanol has almost the same sensation as ethanol and methanol.

Propanol differs in the number of CH groups.

  • One methanol CH
  • Two ethanol CHs
  • 3 propanol CHs

Due to the large number of CH groups, its solubility in water is rather low.

The flammability range is also rather narrow.

Grade 2

The most famous two stones are kerosene and diesel oil.

Familiar Substance Alongside Gasoline

It has a flash point of 21 degrees to less than 70 degrees, so it is a dangerous substance that can become flammable depending on the environment in which it is used in everyday life.

Kerosene/light oil

Check the properties of kerosene and diesel oil.

  • Specific gravity is 0.8-0.85
  • Combustion range is 1-6vol%
  • Flash point 40-45℃
  • Not soluble in water.
  • dissolve fat
  • Kerosene is fuel for stoves.
  • It becomes flammable when mixed with gasoline.

This is a typical hazardous material.

Kerosene has a flash point of over 40℃, so it is generally safe as long as it is handled in winter.

Light oil, which is diesel oil, can be used outdoors in the summer in places where temperatures can exceed 45 degrees Celsius, so it is somewhat dangerous.

It’s not the temperature of the air, but iron pipes and car bonnets that can reach temperatures exceeding 45 degrees Celsius when exposed to direct sunlight.


Let’s check the properties of xylene.

  • Specific gravity is 0.86-0.88
  • Combustion range is 1-6vol%
  • Flash point 27-33℃
  • Not soluble in water.
  • Frequently used as a solvent in chemical plants

Xylene is also a typical Otsushi substance.

The second stone can be interpreted as the average of the fourth stone.

Xylene is a commonly used solvent in batch chemical plants.

I used to use toluene a lot, but now I use xylene a lot.

It may be recognized as a so-called “standard”.

acetic acid

Let’s check the properties of acetic acid.

  • Specific gravity is 1.05
  • Combustion range is 4-19.9vol%
  • Flash point 41℃
  • Solidifies below 17℃
  • Soluble in water/ethanol
  • Mild acidity
  • Vinegar when diluted

Acetic acid is also a familiar substance in the form of vinegar.

However, it is actually a flammable dangerous substance.

Is it surprising?

Since it is a water-soluble hazardous substance, you may not think it is dangerous.

It’s probably the same as ethanol, which is the raw material for alcohol.

What is different from the average for Dangerous Goods Otsu 4 is:

  • Specific gravity is heavier than 1
  • solidifies at 17°C

There are two points.

Specific gravity is less noticeable because it is water-soluble, but coagulation is somewhat troublesome.

It solidifies in winter, so you need to take measures when handling it.

Since vinegar is diluted with water, the freezing point is naturally lower.

Grade 3

Mitsuishi is a substance that can be considered as a safe zone for chemical plants.

Instead, it has a peculiar feature that was not found in Futseki.

heavy oil

Let’s check the properties of heavy oil.

  • Specific gravity is 0.9-1.0
  • Flash point 60-150℃
  • Not soluble in water.
  • Obtained by atmospheric distillation of crude oil
  • It is divided into A heavy oil, B heavy oil, and C heavy oil according to kinematic viscosity.
  • Contains sulfur as an impurity

You might think that heavy oil is a petrochemical issue.

In fact, batch chemicals do not use heavy oil as a raw material.

Some old boilers use heavy oil as a raw material.

Recently, city gas and LNG have become mainstream.

This is because heavy oil contains impurities and pollutes the atmosphere and equipment.

Even if this sulfur is mixed with impurities in various raw materials, it has a negative effect.

Why does this ingredient contain sulfur?

When we had this problem, it was because we were putting raw materials into a drum that originally handled heavy oil.

Heavy oil is obtained by atmospheric distillation of crude oil.

Distillation yields gasoline, kerosene, light oil, etc. at the same time, and heavy oil and asphalt are obtained from the bottom of the column.

In batch chemical plants, they are treated as impurities.


Let’s check the properties of nitrobenzene.

  • Specific gravity is 1.2
  • Flash point 88℃
  • Combustion range 1.8-40vol%
  • Not easily soluble in water.
  • Soluble in ethanol.
  • It has the properties of the fifth class.

Since nitrobenzene has a flash point, it is classified as a Class 4 hazardous material.

There are also aspects of class 5.

Because it’s a nitro compound with a nitro group attached.

If you think that because it is the third stone, it will not burn unless heated, you are wrong.

Since it has the properties of the fifth class, it must be treated more carefully than the fourth class.

ethylene glycol

Let’s check the properties of ethylene glycol.

  • Specific gravity is 1.1
  • Flash point 111℃
  • soluble in water

Ethylene glycol is also a tertiary stone.

You can think of this as a typical tertiary stone.

It is also commonly used as brine in refrigerators.

However, it is rarely used in batch-type chemical plants.

This is because it has a flash point of 111 degrees Celsius, so if it receives too much heat it could potentially catch fire.

I was trying to cool down a liquid at about 120℃ with ethylene glycol brine, but the temperature exceeded 111℃.

It’s scary when this happens.

It is quite possible to use ethylene glycol as brine as long as you can ensure an environment where the flash point never reaches 111°C.

This means that ethylene glycol should be avoided in otherwise dangerous locations.

Grade 4

Dangerous substances are completely unknown at the chemical plant operation and production management level.

However, it is also used and managed in some chemical plants.

Since it is a four-stone stone, it is a safer classification than a three-stone stone, but it is important to understand its properties.


Let’s check the properties of the fourth stone.

I will introduce it in general terms without discussing its individual properties.

  • lighter than water
  • not soluble in water
  • high viscosity
  • High flash point and no volatility

Although it has a flash point, it is not flammable at room temperature and is characterized by being lighter than water.

Once it starts to burn, it becomes very hot, and even if water-based extinguishing agents are used, it will evaporate quickly.

There are cases where it becomes difficult to extinguish the fire.

Just be careful here.


A typical example of a fourth stone is lubricating oil.

There are various types of lubricants.

Let’s look at the relationship between flash point and lubricant.

Refrigerating machine oil150-250℃
machine oil80-340℃
turbine oil200-270℃
gear oil170-310℃

Substances with flash points below 200°C are, of course, tertiary stones. Check it out!

Machine oil has a slightly lower flash point, but it is still 80 degrees Celsius.

This is within the acceptable range for an environment where lubricating oil is used.

But you need to be careful.

This is because pumps use machine oil as lubricating oil, and the temperature of the oil may reach 60 to 70 degrees Celsius in the summer.

This is because the temperature of machinery and equipment during operation is approximately +20 to 40°C higher than the environmental temperature.

Considering the summer temperature of 35 degrees Celsius and +40 degrees Celsius, it is just at the flash point.

Since it has a flash point and is an oxygen-filled environment, there is a risk of forming an explosive atmosphere.

There is no ignition source inside the oil box, so there is little chance of it burning.

It is a good idea to check the properties of the lubricant you are using.


In addition to lubricating oil, plasticizers fall under the quaternary category.

Plasticizers are also called additives.

It is added to plastics, rubber, etc. to give them plasticity.

Basically, there is no opportunity to meet with a batch-type chemical plant.


Knowledge of the four types of hazardous particles is very important for people working in chemical plants.

There are various books for self-study, but almost all of them should be fine.

created by Rinker
¥1,800 (2024/02/29 19:31:08時点 Amazon調べ-詳細)


I explained the properties of Class 4 dangerous goods.

We explained general properties such as flash point, specific gravity, and electrification, as well as grade 1, alcohol, grade 2, grade 3, and grade 4.

This is an important knowledge to know as a mechanical engineer.

Please feel free to post any concerns, questions, or concerns you may have regarding the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)

*We will read all comments received and respond seriously.