During engineering projects, I usually face with usual fluid condition. Therefore, the piping material used is usually carbon steel. But this year my team handled fluids with high acidity, so that material selection for piping and equipment are quite challenging.
In this occasion, I want to share a post about piping material selection. The main purpose is to refresh my knowledge about piping material selection I got during 3rd year in college. I hope I find this post useful in my future projects.
For info, this post will be summary of what I read from several resources. I am not a master of piping. But I hope this post will give us a bit insight in how to select piping material as process engineers.
Purpose of Piping Material Selection
As engineers, we are taught not only to have a good design, but also economically interesting. The same goes for piping material selection. Purpose of selection are technically to achieve metallurgical stability that prevent failure from environment, normal operating time, and upset conditions, and also, to achieve economic design life by the use of appropriate construction materials.
Figure below shows the process of piping material selection:
Piping Material Selection Criteria
The first step to do in piping material selection is to understand the fluid characteristics we are dealing with and the environmental conditions to which the plant will be subjected.
Piping engineers will ask process engineers about fluid characteristics through Process Flow Diagram (PFD) and Heat and Material Balance (HMB). They will check if the fluid contains acid materials, and so on. As for environmental conditions, piping engineers will check basis of design.
Piping material selection will be based on:
- Fluid composition, especially corrosive components such as hydrogen sulfide, carbon dioxide, and chlorides. Other parameters that are usually considered are scaling potential, water content, temperature profile, pressure profile, and stresses on piping.
- Economic and practical considerations, such as market availability, constructability, and also total cost (materials and installation cost)
- Maximum normal operating pressure, temperature, pH, velocity, and fluid properties, including contaminants.
The basis of piping material selection shall be based on the following parameters:
- Service life
- Code requirements
- Allowable stresses
- Design temperature
- Design pressure
- Corrosion
- Economics
Let’s study one by one.
Service Life
Service life is the estimated duration of pipe to provide adequate performance before maintenance, repair, or replacement. For guidelines, minimum service life of alloy steel/stainless steel is 20 years. For carbon steel piping, 15 years is minimum service life.
Code Requirement
The governing code will depend on the area where the piping is installed. For example, power generation application may fall under Power Piping Code (ASME B31.1), while units installed in refinery or chemical plants may be covered in Process Piping Code (ASME B31.3).
Allowable Stresses
The Code sets the temperature and pressure limits for piping materials by tabulating the allowable stresses. Piping designers usually use these stresses to determine the size and thickness of pipe size.
Design Temperature
Design temperature of fluid flowing in the piping is generally assumed the highest temperature of fluid in the equipment connected with the piping concerned. However, it is different story when we are dealing with cryogenic piping.
Design Pressure
Design pressure of the piping system shall not less than pressure at the most severe condition of coincidental internal/external pressure and temperature expected during the service life.
Corrosion
Corrosion is the destructive attack on a pipe by a chemical reaction with the materials and the environment surrounding the pipe.
In low corrosive water where the flow is stagnant, a 1.5 mm corrosion allowance is adequate. However, 3 mm is usually used for conservatism in carbon steel piping and equipment. There are several factors that affect corrosion, which are pH, soil resistivity, chlorides, lime, and sulfates.
pH
Corrosion can occur when metal pipes are used in location where the surrounding material have excessive acidity or alkalinity, which is expressed in pH value. The closer the pH value to 7, the less potential the substance has for causing corrosion.
Soil Resistivity
Because corrosion is an electrolytic process, it has the greatest potential of causing damage in soils that have a relatively high ability to pass electric current. The ability of soil to convey current is expressed as resistivity expressed in ohm-cm. Soil with a low resistivity has a greater ability to conduct electricity.
The greater the resistivity of the soil, the less capable the soil is of conducting the electricity thus the lower the corrosion potential. For general guidelines, resistivity above 5000 ohm-cm are considered to present limited corrosion potential. Resistivity between 1000 to 3000 ohm-cm will usually require some level of pipe protection, depending of pH.
Table below shows resistivity of several materials. The values may be different, depending on moisture content and temperature.
Resistivity of material will be lower when as moisture content is getting higher.
Chlorides
Corrosion potential increases in the presence of the chloride ion. Chloride attacks reinforced steel in concrete if the concrete cover is inadequate.
Lime
Corrosion can also be caused by surrounding materials, such as lime treated base, Portland cement concrete, and other materials. It is necessary to cover the pipe from surrounding by using coatings or other means.
Sulfates
Combined with oxygen and water/moisture, sulfates can form sulfuric acid. High concentration of sulfuric acid can lower pH value, thus potentially increase material corrosion.
Economics
Economic is important. In many occasions, the usage of inferior materials with periodic replacement is beneficial against usage of superior materials, of course without sacrificing the safety of the plant.
References:
[1] Process Piping Material, CED Engineering.com
[2] Earthing Fundamentals
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