Rule of Thumb: How to Design a Pressure Vessel

When you work at a chemical plant, there is a great likelihood that you will need to know basic design of process equipment. Regardless whether you work in an operation, or technical service, or design office, and project office, this basic knowledge is required so that you can quickly make sense of the numbers and any peculiarity.

Let’s say you are an operation guy, an operating engineer, you see your process gets upset in the production
plant, you will then zoom it into some specific equipment, knowing the basic of the process equipment design will help the operation engineer to quickly know if the said equipment should deliver such and such capacity and it is not delivering due to fouling, damages or any other reason. This will help the troubleshooting process.

There will also be a time when the technical service engineer will be required to do a modification project or need to design simple equipment in order to ensure the chemical plant can continue to run. This will be the time when knowing rule of thumbs for process equipment design will help a lot.

Below is some of the Rule of Thumbs or Guides for Process Equipment Design:

Rule of thumb to quick design estimate of pressure vessel design is as follows:

1. A pressure vessel is usually used to hold a certain volume of fluid be it gas or liquid. For simplicity, we will use liquid holding in a pressure vessel for example. First you will need to know the flowrate of the material into the pressure vessel. Say it requires 100 m3/hr of liquid A at 1000 kPag.

2. A design engineer will need the amount of holding time required, usually will be given by the process engineer or operation engineer working in the plant. Normally for intermediate pressure vessel a 15 minutes holding time is required in order to ensure that panel man can intervene if there is a problem at the chemical plant to avoid plant shutdown or plant trip.

3. From this we will require 100 m3/hr x 15 minutes of volume equivalent to 25 m3.

4. In order to calculate the cost estimate for the pressure vessel which is usually the aim of first cut design, a process design engineer can then estimate the amount of material needed for the pressure vessel. Assuming a cylindrical type of pressure vessel, the surface are of the pressure vessel can be estimated to house the 25 m3 of liquid.

5. After having the surface area of the material needed, we need the wall thickness for the pressure vessel. The wall thickness of a pressure vessel is correlated to temperature and pressure of the pressure vessel itself. To estimate this, the following formula below can be used.

6. Once you have the thickness you can then estimate the volume of material needed to construct the pressure vessel. Multiplying the volume with the density of the material i.e. carbon steel will give you the total mass of material needed.

7. Using the industrial price of the material i.e. carbon steel industrial price, you can then estimate the material cost for the pressure vessel. This is termed as direct cost i.e. material and bulk cost.

8. In order to complete the cost estimate for the project of installing a pressure vessel, it is recommended to multiply the direct cost by two to get the total installed cost which includes the indirect cost.

9. Once you complete this exercise, you will have estimated using a rule of thumb for accuracy of plus minus 40% cost estimate of the project cost. This is good enough for first hurdle estimation to look into the economics and project returns.

Formula to calculate pressure vessel wall thickness, correlated to pressure. This formula is to estimate the thickness of cylindrical pressure vessel. If the semi-circle portion or a spherical shape pressure vessel, factor of 0.2 instead of 0.6 can be used.
where

t = Minimum required thickness (in.)
P = Design pressure (psi)R = Inside radius (in.)S = Allowable stress (psi)E = Weld joint efficiency factor, assume 1.0