We will introduce the actual plate thickness calculation for atmospheric storage tanks such as outdoor tanks.
JIS B 8501 Steel oil storage tank structure is used.
Detailed calculations can be done by the equipment manufacturer, but it is better for the user to calculate the main part directly related to the overall plate thickness.
If you do what the manufacturer says, there may be cases where the cost is high due to excessive board thickness, so it will also lead to cost reduction.
The thickness calculation looks complicated, but the idea is the strength calculation of a thin-walled cylinder itself.
I really want to be able to use it.
Model of atmospheric storage tank
The model of the outdoor tank, which is the theme of this time, is shown below.
Since the tank is used under atmospheric pressure, it is also called an atmospheric pressure storage tank.
The purpose of the strength calculation is to determine the plate thickness.
Thickness has the greatest impact on tank manufacturing costs.
- Plate thickness ∝ Weight ∝ Material cost
- Plate thickness ∝ Welding cost
That’s the relationship.
body plate
When it comes to strength calculations, the torso is first assumed.
Internal pressure calculation
Calculate how much pressure the torso can withstand when it has internal pressure.
Use the strength calculation formula for thin-walled cylinders.
$$σ=\frac{PD}{2t}$$
- σ: Circumferential stress on the body 100MPa (tensile strength 400MPa divided by safety factor 4)
- D: Body diameter
- P: Pressure applied inside the barrel
- t: shell thickness
For example, when D=2,000mm and t=4mm, the allowable pressure that can be used safely is
$$ P=\frac{σ×2t}{D} =\frac{100×2×4}{2000}=0.4$$
I feel that 0.4 MPa is quite durable.
The torso is strong against internal pressure , but weak against external pressure.
Since the calculation of the external pressure is complicated, it is omitted.
Outdoor tanks are not actually designed in the direction of increasing the plate thickness to withstand external pressure.
It would be preferable to follow the 3.2mm thickness specified for the No.20 tank.
Calculation considering liquid depth
The above formula is for an empty tank.
However, when driving, the tank is actually filled with liquid.
Let’s consider this hydraulic component.
First, let us consider the case of a tank with a height of 2m.
Let the specific gravity be 1.
If it is a Class 4 hazardous material tank, the liquid specific gravity is less than 1, but in this case as well, it is good to consider the specific gravity of water as 1.
Since the hydraulic pressure is defined by \(ρgh\),ρg _his defined by
$$ρgh=1,000×9.8×2/100,000≒0.02$$
of pressure is added. The unit is MPa.
Liquid pressure is 0.02 MPa for a tank that can withstand internal pressure of 0.4 MPa.
Considering this, you can see that the liquid pressure has almost no effect on the pressure resistance of the tank.
That’s why I don’t think much about hydraulic pressure when calculating the strength of the torso .
Bottom plate
The calculation of the body plate can be derived from the strength calculation formula, but the roof plate and bottom plate are different.
Let’s look at the bottom plate first.
Since the bottom plate is clinging to the tank foundation, you may wonder what the strength calculation is about.
A simple design that does not fail, the bottom plate is one size larger than the body plate .
This is the basic idea.
It is not derived from strength calculations.
It’s more of a welding issue.
If the body plate is 4mm, consider the bottom plate as 6mm.
Since the body plate and the bottom plate are connected by fillet welding , there should be no problem if the bottom plate is the same thickness as the body plate.
It is safer to make the bottom plate thicker because the portion of the bottom plate that is not affected by heat increases.
Of course, there is also the idea of suppressing deformation of the bottom plate by increasing the thickness of the bottom plate that directly receives the load of the body plate .
More simply, if the bottom plate breaks, the liquid will completely leak out and there will be nothing to do.
shingle
The roof board should be the same size as the body board.
The load applied to the roof plate is small, but it takes courage to make it smaller than the body plate.
In that case, the idea that the same board thickness would be fine.
Although there is a load on the piping and work floor, it is insignificant.
For example, assuming that the total load on the shingle is 1 ton (pipes and work floor), the pressure on the shingle with a body diameter of 2m is
$$\frac{1,000×9.8}{3.14/4×2^2}\frac{1}{100,000}=0.03$$
is a small pressure. 0.03 MPa is about the same as liquid pressure.
In practice, the pressure applied to the shingle will not be applied to the entire shingle, but only to the area where the roof and legs are attached.
For example, if a □200 part receives a weight of 1 ton at 5 locations,
$$\frac{1,000×9.8}{0.2^2×5}\frac{1}{100,000}=0.5$$
A pressure of about 0.5 MPa will be applied to the leg.
This is a level that makes you a little uneasy or a level where deformation occurs in the tank.
There are many cases where a backing plate is added, thinking that the board thickness should be increased if it makes it uneasy .
Even so, the tank will definitely deform at the part where the backing plate is attached.
Fundamentals of Pressure Vessel Design
When studying pressure vessel concepts, let’s start with the basics of pressure vessel design.
This book is useful.
I would like to make it my motto.
In the case of outdoor tanks, the formula itself is simple, but this book is very useful in terms of knowing the big picture.
Related information
Related information
lastly
Introduced the plate thickness calculation of the atmospheric pressure storage tank of the outdoor tank.
The body plate should be calculated from the strength calculation of the thin cylinder, the bottom plate should be one size larger than the body plate, and the roof plate should be the same size as the body plate.
It will be easier later if you create a design standard in your company.
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.