Choosing materials for pipes that carry liquids is a detailed job. It’s similar to picking the right ingredients for a tricky recipe. Each part needs to come together perfectly for the final result to be just right. Here are some important things to think about:

1. Fluid Properties

• Viscosity: This describes how thick or thin a liquid is. Thicker liquids create more friction, so you need materials that can handle this.
• Temperature: The temperature of the liquid matters. If it’s very hot, you might need special materials that won’t break down.
• Chemical Composition: Knowing what's in the fluid is key. Some liquids can be harsh and might need special coatings or materials to avoid damage.

2. Flow Characteristics

• Type of Flow: There are two main types of flow: calm (laminar) and rough (turbulent). A key number called the Reynolds number ($Re$) helps us figure this out. Calm flow has $Re < 2000$ and rough flow has $Re > 4000$. Depending on the expected flow, the choice of material can change.
• Flow Rate: This is how much liquid needs to move through the pipe in a certain time. Higher flow rates need bigger pipes and stronger materials.

3. Pressure Considerations

• Operating Pressure: The pressure in the pipe is crucial. Materials must handle different pressure levels to avoid breaking. Thicker and stronger pipes are needed for high-pressure situations.
• Surge Pressure: If the flow changes suddenly, it creates surge pressure that materials must withstand to avoid serious problems.

4. Environmental Conditions

• External Environment: Things like humidity and exposure to chemicals can affect how materials perform. Materials that resist rust or damage are better for wet areas.
• Installation Environment: Where the pipes go matters too. Underground pipes might need materials that stop soil or water from getting in.

5. Mechanical Properties

• Tensile Strength: This is about how well a material can resist being pulled apart. Stronger materials are essential where pipes carry heavy loads.
• Fatigue Resistance: Pipes often experience repeated stress, so materials should resist fatigue, especially in high-stress situations.

6. Economic Factors

• Cost: The budget for the project is very important. It's best to balance good performance with cost. Sometimes, pricier materials can save money in the long run by needing less maintenance.
• Availability: The materials chosen must be easy to find in the amounts needed. If they aren’t available, project timelines could be delayed.

7. Longevity and Maintenance

• Durability: Materials should last a long time to minimize how often you need to replace them. Characteristics like resistance to rust and wear are important.
• Maintenance Requirements: Some materials need more upkeep than others. Picking materials that need less maintenance can save time and money later.

8. Regulatory and Safety Standards

• Compliance: Sometimes, materials must meet certain rules and safety standards. Specific jobs may need certifications to make sure they're safe and effective.
• Compatibility with Standards: Materials should fit with standards from organizations like ASTM or ISO, making sure they work well.

9. Joining Methods

• Weldability: How materials can be joined, like through welding, matters. Some materials are easier to weld, affecting how strong and well the pipe system will be.
• Compatibility with Fittings and Valves: The materials chosen must work well with the fittings and valves in the system to prevent leaks.

10. Aesthetic Considerations

• Surface Finish: For some projects, especially where pipes are visible, how they look is important. This could impact the choice of materials that not only work great but also look nice.

In short, picking materials for pipes is a complicated decision. It involves considering many things, like the properties of the liquid, the materials’ strength, the environment, and your budget. These factors all relate to one another, making the choice of materials really important for how well the pipes work. Just like making a great dish, paying attention to every detail can lead to a strong and lasting result.