Design pressure is an important factor in facility design, but it is surprisingly rare to be conscious of it.
When I became a class on first-class pressure vessels and high-pressure gas, I finally felt like thinking about it.
For other general specifications, specifications are determined based on standardized dimensions, so engineering can be done without being conscious of design pressure.
However, it does not mean that the design pressure should be neglected, and the concept will be explained by focusing on the points that novice engineers tend to fall into.
conditions
This time we are thinking about tanks.
Conditions will change with “without jacket” or “with jacket”.
| no jacket | with jacket | |
|---|---|---|
| body | FV～99kPaG | FV～99kPa |
| Jacket | – | FV～299kPa |
For the main body, consider a container that can withstand from a vacuum to a slight pressurization.
The jacket considers steam of constant pressure.
design pressure for unjacketed tanks
The design pressure is the pressure used for strength calculation.
The working pressure is the pressure used in actual operation.
Of course, the design pressure must be equal to or higher than the working pressure.
What do you have to think about when thinking about this design pressure?
The hint is atmospheric pressure.
A design pressure of Full Vacuum to 100kPaG is considered here.
Relative pressure (gauge pressure) is often used in drawings, but it is more convenient to think in terms of absolute pressure here.
Full Vacuum～100kPaG → 0kPaA～200kPaA
against external pressure
There are different ways of thinking depending on whether the tank is used under negative or pressurized conditions.
Especially in batch-type chemical plants, there are cases where the same tank is used under negative pressure in one production and under pressure in another production.
It must be designed to withstand both conditions.
First, let’s consider the external pressure side.
When the pressure inside the tank is negative, the tank receives external pressure called atmospheric pressure.
Under full vacuum conditions, the tank receives the force of atmospheric pressure itself.
- Tank pressure: 0kPaA
- Atmospheric pressure: 101kPaA
- Differential pressure: -101kPa (design pressure)
As you can see, we will consider the design pressure of a tank that receives an external pressure of 101 kPa.
relative to internal pressure
Similarly, let’s consider the design pressure when the pressure inside the tank is 99 kPaG.
The idea is the same as for external pressure.
- Tank pressure: 200kPaA
- Atmospheric pressure: 101kPaA
- Differential pressure: 99kPa (design pressure)
As you can see, we will consider the design pressure of a tank that receives an internal pressure of 99 kPa.
Relative pressure is the absolute pressure minus the atmospheric pressure , so you won’t feel any discomfort.
design pressure with jacket
Design pressure without a jacket is a natural result, but with a jacket it’s a little different.
for body external pressure
Let’s look at the design pressure against the external pressure of the main body even with a jacket, just like without a jacket.
The upper mirror part does not have a jacket.
With the same idea as without a jacket, it receives an external pressure of 101kPa.
A jacket is attached to the torso and lower speculum.
The condition that the inside of the tank is the maximum external pressure that the body receives from the jacket.
- Tank pressure: 0kPaA
- Jacket pressure: 400kPaA
- Differential pressure: 400kPaA (design pressure)
As you can see, we will consider the design pressure of a tank that receives an external pressure of 400 kPa.
Just replace the atmospheric pressure with 400kPaA, so there will be no big confusion.
relative to the internal pressure of the body
The design pressure for the internal pressure of the body considers a case where the jacket is a complete vacuum.
The maximum internal pressure is when the main body pressure is 200kPa and the jacket pressure is 0kPa.
- Body pressure: 200kPaA
- Jacket pressure: 0kPaA
- Differential pressure: 200kPa (design pressure)
We will consider the design pressure of a tank that receives an internal pressure of 200 kPa.
You wouldn’t think the jacket could be a vacuum.
It is possible.
It can happen if you tighten the jacket while the steam is on.
As the temperature of the steam cools in the atmosphere, it will condense.
It may not be a perfect vacuum, but it will be a negative pressure.
relative to jacket external pressure
Let’s look at the jacket as well as the body.
The external pressure of the jacket is the same idea as the external pressure of the non-jacketed body.
- Jacket pressure: 0kPaA
- Atmospheric pressure: 101kPaA
- Differential pressure: -101kPa (design pressure)
As you can see, we will consider the design pressure of a jacket that receives an external pressure of 101 kPa.
relative to the internal pressure of the jacket
The design pressure for the inner pressure of the jacket considers the condition that the jacket is 400kPaA and the atmospheric pressure is applied.
- Jacket pressure: 400kPaA
- Atmospheric pressure: 101kPaA
- Differential pressure: 299kPa (design pressure)
As you can see, we will consider the design pressure of a tank that receives an internal pressure of 299 kPa.
summary
Let’s compare the design pressure without jacket and with jacket.
Think in terms of differential pressure.
| no jacket | with jacket | ||
|---|---|---|---|
| body | internal pressure | 99 kPa | 200kPa |
| external pressure | -101kPa | -400kPa | |
| Jacket | internal pressure | – | 299 kPa |
| external pressure | – | -101kPa |
The external pressure is marked with a “-” sign.
When the pressure of the jacket is added, it becomes a little complicated.
It’s not difficult if you calm down and divide it by place.
The final design pressure is shown below.
| no jacket | with jacket | |
|---|---|---|
| body | FV～99kPaG | FV～99kPa |
| Jacket | – | FV～299kPa |
Based on the design conditions, the following calculation is considered as the most severe pressure condition.
| no jacket | with jacket | |
|---|---|---|
| body | 101kPa external pressure | 400kPa external pressure |
| Jacket | – | 100kPa external pressure |
Simply put, the tank is vulnerable to external pressure, and it feels like looking for the highest external pressure.
Should I not consider anything other than the internal and external pressure of the tank?
When calculating the strength of a tank, we often discuss the internal pressure and external pressure of the process, but should we not consider other factors?
Realistically negligible.
dead weight of tank
The self-weight pressure on the legs of the tank seems to be the most influential. Let’s examine this.
The empty weight of the tank + liquid weight are added to the legs of the tank.
Of course, it also works on tank barrels and mirrors.
I calculated (empty weight of tank + liquid weight) / (cross-sectional area of leg) as self – weight pressure.
Let’s look at this with an index called tensile strength.
- Tank strength calculation : about 10% of tensile strength
- Self-weight pressure of tank : about 3% of tensile strength
It seems that it is decided to design with this degree of pressure as an implicit rule in strength calculation.
The effect of self-weight pressure will be about 30% on the strength calculation.
This can be negligible by distributing the pressure by attaching a backing plate or changing the size of the legs.
Person/raw material load
People and raw materials are temporary loads.
Think of this as a load on the building or work platform.
Generally, it is considered on the order of 100kgf/m2.
in units of pressure 1 kPa.
Even if such a load is applied to the equipment, it is almost negligible.
A pressure of about 1 kPa has only a 1% impact on equipment that can withstand a pressure of 100 kPa, so it means that it can be ignored.
Piping load
The piping load is about 1kPa per 1m2.
Consider 10 pipes of 50A per 1m2.
The weight of one 50A pipe is 5 kg per 1m, and it is about 8 kg when it is full, so 80 kg for 10 pipes.
Considering the weight of the flange and heat insulation, 10 pieces weigh about 100 kg.
100A per 1m2 is 100kg, so 100kgf/m2
in units of pressure 1 kPa.
The contribution is about 1%, which is almost the same as the load of people and raw materials.
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.
lastly
I explained the design pressure necessary for calculating the pressure vessel of a chemical plant.
Consider the negative pressure and pressurization conditions for the main body and jacket, and also consider the influence of atmospheric pressure.
Influences other than process pressure are self weight, piping, people and raw materials, but they can be ignored in terms of strength calculation.
It is easy to forget the influence of negative pressure, so be careful.
Please feel free to post your worries, questions, and questions about the design, maintenance, and operation of chemical plants in the comments section. (Comments are at the bottom of this article.)
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