Design Pressure

INTRODUCTION:

???????????Static Equipment Design is the Backbone of the Oil & Gas Industry, Chemical Industry, and Pharmaceutical Industry. Without the design of the equipment, none of the processes would be able to function correctly. The purpose of Static Equipment is to contain pressure and temperature so that processes can run smoothly. As a Mechanical Engineer or Static Equipment Design Engineer, it is essential to understand the Basics of Design Pressure. In this chapter, we will discuss the Basics of Design Pressure.

?

What is Pressure?

A.????The use of persuasion or intimidation to make someone do something

B.????Continuous physical force exerted on or against an object by something in contact with it

C.????Force action on a unit area

D.????It is multiple of atmospheric pressure

As a layperson, all of the options are generally correct, but it depends on whether we are approaching the topic from the perspective of an Engineer. In that context, Option C would be considered correct.

?

Knowing the conversion factors helps to quickly convert pressure units

As a Pressure Vessel Design Engineer, it is crucial not to mess up with the units. Therefore, take your time to remember these units and their conversions.

Pressure:

Definition of pressure: Force per unit Area

Pressure =

????????????????= 1 N/m2

????????????????= 1 Pa

Unit of Pressure: Pascal (Pa)

??Liquid Pressure:

Now let's consider how liquids create pressure. Imagine a bucket with three holes at different elevations. If we open all three holes at once, which one will have the highest flow of water? The bottom hole will have the highest flow because it has the greatest head pressure. Head pressure is due to the static column of liquid, which is calculated by multiplying the density of the liquid, the height of the column, and the gravitational acceleration.

So, we can derive the equation for calculating pressure by considering the weight of the liquid. We start by calculating the mass of the liquid, which is equal to the density of the liquid multiplied by the volume of the cylinder.

Mass = Density × Volume

Mass = ρ × V

The volume of the cylinder can be broken down into two elements, the area of the cylinder and the height.

Mass = ρ × Ah (V= Ah)

Hence, the Weight of a liquid Column = Mass X acceleration due to gravity = (ρAh)g

Weight is a force, and if this force is acting on an area, we can calculate the pressure exerted by a liquid column

Pressure Exerted by a liquid Column (P) = ρgh

Let us take an example of a 10 meter water column to demonstrate how to calculate pressure.

Let us assume the density (ρ) of the water is 1000 kg/m3, the acceleration due to gravity (g) is 10 m/s2, and the height (h) of the water column is 10 meters.

To calculate pressure, we use the formula

P = ρgh

P = 1000*10* 10 kg/m3 x m/s2 x m

P = 100000 Pa

??= 1 Bar (Refer to the conversion table given above)

10m water Column = 1 Bar or 0.1 MPa

In general, a 10-meter water column is roughly equal to 1 Bar of pressure. This relationship can help determine the pressure in a pressure vessel or other container filled with water.

It is important to remember this relationship, as it can affect the safety of the container and the materials within it. Therefore, it is essential to always calculate pressure, even if it seems insignificant.

?

Example of Liquid Pressure:

Drinking through a straw is a simple but fascinating phenomenon that involves principles of physics. In this, we will explore the mechanics behind drinking liquid through a straw, including the concepts of pressure and vacuum

Let's begin with a simple question - how do straws work? We use straws to drink water, juice, or coffee, but have you ever wondered how the liquid flows into our mouth? The answer lies in the principle of pressure. When we suck on a straw, we create a partial vacuum or a low-pressure area inside the straw. This partial vacuum pulls the liquid up the straw and flows into our mouth.

Now, let's discuss the basic principles behind this phenomenon. As we suck on the straw, we create a partial vacuum or low-pressure area inside the straw. At the same time, atmospheric pressure is acting on top of the liquid in the straw and the pipe. When we create a partial vacuum, we are trying to suck all the atmosphere inside our mouth, creating a small amount of vacuum inside the straw. This vacuum is not a complete absence of atmosphere but a slight, very low-pressure or partial vacuum. When we create a partial vacuum inside the straw, the pressure inside the straw is lower than the atmospheric pressure outside the straw. This pressure difference creates a force that pushes the liquid up the straw, making it possible for us to drink the liquid.

For the concept of creating a vacuum to suck water from a straw or pump, we will go through the process of how creating a pressure less than atmospheric can help us in this process. We will also understand the limitations of using a pump to create a vacuum and its maximum capacity. The process of sucking water from a straw involves creating a pressure less than atmospheric.

Now, let us assume that the height of the straw is 12 meters. If we try to suck water from the straw, we will not be able to do so because we cannot create a vacuum of more than 10 meters of the water column.

Even if we use a vacuum pump, we cannot create a vacuum beyond 10 meters of the water column. This is because the maximum vacuum pressure that can be created by a pump is less than one atmospheric pressure. We cannot go beyond zero absolute pressure. So, even if we create an absolute vacuum at this level, the vacuum will only reach up to 10 meters of the water column, after which it will be balanced with atmospheric pressure.

Atmospheric Pressure:

?Atmospheric pressure is the pressure exerted by the atmosphere surrounding the Earth. The heaviest layer of the atmosphere is around 14 to 15 kilometers from the Earth's surface, and it has the highest density, thus creating the maximum pressure. This layer is called the troposphere, and it is around 12 kilometers from the Earth's surface. The atmospheric pressure decreases as we go higher up from the Earth's surface and at the level of Mount Everest, it is around 200 millibars

The pressure we feel at sea level is 1 bar, which is equal to 1,000 millibars or 14.7 pounds per square inch (psi). If we go below sea level, the pressure increases. At a depth of 10 meters below sea level, the pressure is 2 bars. If we go deeper, the pressure keeps increasing, and it becomes difficult to dive very deep into the sea.

The atmospheric pressure is due to the static column of the atmosphere, like the static head for water or any other liquid. Air behaves like a fluid and follows the same principles as other fluids.

In absolute terms, atmospheric pressure is the pressure relative to the vacuum of space. In terms of pressure units, it is measured in bars, pounds per square inch, or millibars.

To calculate the pressure created by the atmospheric column, we can use the formula ρ g h. The pressure at sea level is 14.7 psi, which is equal to one bar. So We experience pressure at sea level because of the atmospheric column.

?

Concept of Gauge Pressure:

So far, we discussed local atmospheric pressure, starting with the concept of absolute pressure. At sea level, the atmospheric pressure is 14.7 psi, which is what an instrument giving measurements in absolute terms would read. It is important to note that in absolute pressure terms, there is no such thing as negative pressure, as the pressure cannot go below zero. Any pressure below 14.7 psi is considered a partial vacuum and 0 psi will be a Full Vacuum (Absolute Zero)

Gauge Pressure: ?Any pressure above or below Local Atmospheric Pressure is measured in terms of Gauge Pressure. If we replace the local atmospheric pressure with a Gauge Pressure system, then any pressure above is considered positive and below is considered a negative (partial vacuum).

?Now, let's look at an example of a Gauge Pressure. If we take a Gauge Pressure System and measure the pressure at sea level, it will read zero because the pressure is the same as the local atmospheric pressure. If we create a full vacuum inside a vessel and measure the pressure, the Gauge Pressure will indicate minus 14.7, which is the Absolute Zero (Full Vacuum

So the relation between absolute and gauge pressure can be given by:

Pabs = Patm + Pgauge

It is important to remember this formula.

Static Equipment:

Static Equipment, specifically pressure vessels, requires a clear understanding of pressure to avoid failures. The equipment is designed to contain pressure, and our job is to provide the appropriate thickness to ensure that it can contain the desired pressure at the required temperature. Failure to do so will results in equipment failure, something we want to avoid.

?To ensure that pressure vessels can contain pressure safely, we must consider several factors. Firstly, we must understand the design code requirements for pressure vessels and provide the minimum thickness required for the specific process. Secondly, proper welding procedures must be followed to ensure the weld quality and strength of the pressure vessel. Thirdly, we must select the appropriate material for the specific process based on its properties and strengths. Lastly, proper inspection and testing must be conducted before the pressure vessel is put into service to detect any flaws that could compromise its ability to contain pressure.

By considering these factors, we can ensure that our pressure vessels can contain pressure safely and avoid any potential failures.

?

Q1. Let us consider a pressure vessel. The atmospheric pressure outside the vessel is 1 bar absolute, while we are maintaining 10 bar absolute pressure inside the vessel. The question is, for which internal pressure will we design this equipment? (write your answer before reading the answer) (no need consider margin on design pressure)

Ans. The resultant pressure acting on the vessel will be the internal pressure minus the external pressure. Therefore, in this case, the resultant pressure acting on the vessel will be 10 bar minus 1 bar, which is 9 bar. Thus, we will design this pressure vessel for internal pressure of 9 bar absolute.

Q2. Now, let us consider the question of which external pressure we need to design this equipment for? And further, do we need to design it for external pressure or not? (write your answer before reading the answer)

Ans. The answer is that we do not need to design this equipment for external pressure unless there is a chance that the pressure inside the vessel may go below 1 bar. External pressure will start acting only when the resultant pressure acting on the vessel goes lower than 1 bar. It is important to note that external pressure will act only when there is an offset condition, which is not intended and is the result of some failure.

For example, one possible failure could be the closure of the inlet valve while the pump is still running and not receiving an indication to shut down even though the liquid level has dropped below the low level. In this case, the pump will suck down all the liquid and try to suck some air, and the external pressure will start acting.

Therefore, to conclude we need to design for external pressure only if there is a possibility of the pressure inside the vessel going below one bar due to some failure.

Q3. Now, let us assume a pump is connected to the outlet of the pressure vessel of earlier case. Any change in design pressure?

Let's see this case, so the question is what will be the design pressure for the vessel with a pump added to it? ?The internal pressure will remain the same as before, which is 9 bar. This is because the operation is happening at 10 bar, and the resultant pressure is 9 bar.

The second question is about the external design pressure? ?We need to design for external pressure in case there is an offset condition that may exist because of the pump getting connected. In this case, the external pressure will be 1 bar. It is essential to note that we only need to design for external pressure if there is a chance of a vacuum getting created due to the pump being connected.

To be on the safer side, we should design for external pressure for this offset condition. However, if there are additional measures taken, and the client assures that a vacuum will never happen, then there is no need to design for external pressure.

?

Q4. Consider the service meduim is steam. What will happen then?

If we have a steam application with an internal pressure of 10 bar absolute and an external pressure of 1 bar absolute, we need to determine for which internal pressure will we design this vessel? Without considering any margin, we will use the internal pressure that is 9 bar absolute. This means we will have to design for 9 bar absolute pressure inside.

When it comes to the external pressure of a steam application, there are chances that steam will condense, creating a vacuum, so it's better to design for that offset condition. We don't want the vessel to collapse if it's not designed for this condition. Therefore, it's better to design for the offset condition so external pressure will be 1 bar absolute.

Thank You….

**************************************************************************

Express your appreciation with a thumbs-up, share, or thoughtful comment. Let's join forces to amplify the reach of knowledge together!??"

We provide very in-depth training in very simplified manner on ASME SEC VIII Div 1 & Div 2,? To Learn more about our training program sign up with the below link:

https://forms.gle/8mVVZraVHPcnFft49

Go through our YouTube channel as well where you will find such valuable information in video content. Click on the link below:

https://www.youtube.com/@ScootoideLearning