Liquid expansion is a very frightening phenomenon that can occur unnoticed in chemical plants.
Classic and easy to forget.
In a plant that is based on manual operation, the operators know the reasoning behind it and respond accordingly, but as automation progresses, their awareness diminishes.
The opening and closing operation of the automatic valve seemed to be unnecessary, and when I deleted it because it was unnecessary, the liquid seal occurred!
You can easily imagine that.
I always want to have a solid understanding of the principles.
Liquid expansion principle
Liquid expansion is a very frightening phenomenon.
When designing piping for a chemical plant, always consider the possibility of liquid sealing.
First, let me explain the phenomenon of liquid sealing.
temperature expansion
First, I will explain the natural phenomenon of thermal expansion.
See the diagram below.
The top is the object at normal temperature, and the bottom is the state where the temperature is slightly raised .
When the temperature is increased, the object expands.
For simplicity’s sake, consider it in one dimension as shown in this picture, and use the length L, temperature difference Δt, and temperature expansion coefficient α.
$$ L_1 = L_0(1+αΔt) $$
Consider the relational expression This is also called linear expansion .
In the case of 3 dimensions, the name changes to volume expansion.
The actual physical phenomenon is volumetric expansion, but the calculation is complicated.
volumetric compression
Next, let’s talk about volumetric compression . This is also a common physical phenomenon.
When you push an object, it shrinks.
As shown below.
The pushing force is also called compression force .
It is evaluated by bulk modulus in technical terms .
$$ F = β({L_1}-{L_0}) $$
You can evaluate in the world of Hooke’s law as it is.
What are the dangers of Liquid expansion?
A liquid seal can be thought of as a phenomenon that occurs when the temperature rises while a liquid is confined .
- A liquid expands when its temperature rises
- Even if you try to inflate it, it cannot inflate because it is trapped
- The liquid receives force from its surroundings to suppress expansion
- At the same time, the liquid exerts an expansion force on its surroundings
If this state is in equilibrium, there is no problem.
The problem is that the surrounding side loses with a relatively low temperature rise .
Losing the surroundings means that the liquid inside cannot be confined and is released to the outside.
It’s a leak . It’s hard.
Thermal expansion is explained as a phenomenon called thermal stress in mechanics of materials.
simple calculation
Let’s do a little math about the liquid seal.
- water
- Increased from 20°C to 25°C
- Thermal expansion coefficient: 0.2×10 -3 1/℃
- Bulk modulus: 2,100 N/ mm2
Let’s use this data.
When the temperature of water rises by 5°C to 25°C, the amount of expansion of water is
0.2×10 -3 ×(25-20) = 1×10 -3
The force applied to the water from the outside to oppose this elongation is
1×10 -3 ×2,100 = 2.1N/mm2 = 2.1MPa
In batch-type chemical plants with many JIS10k (1 MPa) flanges, the piping and equipment are not capable of withstanding a force of 2 MPa.
In other words, if the temperature rises by as much as 5°C , the liquid seal will easily damage the equipment.
It’s normal to have a temperature difference of about 5°C between day and night.
Liquid sealing can occur even under normal conditions.
It’s scary.
automatic valve
Now that we know how easily liquid sealing occurs, we will explain where liquid sealing occurs.
Automatic valve first.
When sending a liquid somewhere with a pump, the case with two automatic valves is a point to be careful of.
Let’s look at the steps to stop the pump.
- Confirm that a certain amount has been sent by looking at the flow meter and liquid level gauge
- Close automatic valve B
- Close automatic valve A
- stop the pump
It should look something like this.
A liquid seal will occur between automatic valves A and B.
As a countermeasure, it will be a means to let the liquid escape, so the following means can be considered.
Liquid sealing measures for automatic valves
- Open automatic valve A or automatic valve B for a few seconds after stopping the pump
- install a safety device such as a relief valve
It’s as easy as opening and closing a valve, right? You may think that, but people have to worry about it, so it may be forgotten.
When it comes to long-distance piping, it is difficult to check the line where the automatic valve is attached.
Since the damage is relatively large, I want to eliminate the risk.
Even if it is automated with an automatic valve, it will not work unless the process for liquid sealing countermeasures is included in the sequence.
Even if the sequence is put together, the problem remains.
When releasing by opening the automatic valve B side
When releasing by opening the automatic valve A side
Depending on the installation position of the flow meter, a certain amount that is not counted by the flow meter is additionally sent.
If the tank liquid level is higher than the pipe height, the liquid will not fall to the pump side.
It will be a countermeasure that depends on the driving conditions.
Not versatile.
In that case, the idea would be to add an analog but reliable safety relief valve .
Relief valves are relatives of safety valves.
ball valve
A liquid seal can also occur in the ball valve itself.
Let’s show a schematic diagram of a ball valve.
A ball valve consists of three parts: a ball , a disc , and a seal .
The part of the red frame is hollow and the liquid enters.
In spite of that, it is covered with a blue sticker and there is no escape route, so it becomes a liquid seal.
Let’s think about mechanical strength and think about where it will leak when the temperature rises.
- Gland seal around valve stem
- Valve seat seal ( blue seal )
- body gasket
About these three.
If the ground seal (1) and main body gasket (3) are in their weakest state, and the liquid seal is raised, the process liquid will leak into the atmosphere.
Therefore, one idea is to design so that the seal part of 2 is weakened .
This method is actually used.
The website of HISAKA WORKS CO., LTD. clearly states that it prevents internal pressure from rising.
It seems that the site has a structure with an escape port for the valve body itself.
Freezing is also a kind of liquid expansion
Freezing of water actually occurs on the same principle as liquid sealing.
It basically applies only to water.
This is because water expands when it becomes ice .
In the explanation of thermal expansion, I wrote that it is normal for the volume to expand as the temperature rises.
But water is different. Special.
It has the property that its density is maximum and its volume is minimum at 4°C .
This means that if the temperature is lowered below 4°C, the volume will increase.
Ice floats on water because its density is low.
When you boil a bath, hot water collects on the top and cold water collects on the bottom, which is another matter of density.
When it freezes, you don’t immediately notice it.
This is because even if the surroundings are destroyed in the state of ice, the leakage will not be noticed because it is ice.
There are many patterns that freeze and break down at night or dawn, and melt and become water in the daytime.
Freezing alone is powerful enough to break steel pipes.
Freezing and liquid sealing are based on the same principle, so I would like to have a sense that both freezing and liquid sealing are scary.
reference
Liquid sealing is simple in principle, but that is why it is often overlooked in actual equipment.
Many problems related to piping have occurred in the past, so let’s learn systematically with the following books.
Related information
Related information
lastly
A ball valve has a simple structure, but that causes the problem of liquid sealing.
Liquid seals are very dangerous even though they can happen without you noticing.
I explained from the perspective of automatic valves, ball valves, and freezing.
When designing piping, always keep in mind the possibility of liquid sealing.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (The comment section is at the bottom of this article.)
*I will read all the comments and answer them seriously.