Designing a steam system requires careful consideration of
pipes and their sizing. This crucial aspect ensures optimal performance and safety. This guide provides comprehensive advice on standards, materials, and sizing for various steam duties, whether saturated or superheated.
Numerous piping standards exist worldwide, but two of the most prominent are from the American Society of Mechanical Engineers (ASME) and the American Petroleum Institute (API). These standards categorize
pipes into schedule numbers, which correlate with the pressure rating of the piping.
For nominal pipe size (NPS) 6 and smaller, Schedule 40 (often referred to as 'standard weight') is typically the lightest option suitable for steam applications. However, for NPS 1/2 to NPS 24, only Schedules 40 and 80 cover the entire range and are widely used for steam pipe installations. These schedules ensure reliability and efficiency in steam systems.
Pipe materials play a critical role in steam system reliability and performance. Carbon steel ASTM A106 is a common choice for steam system pipes due to its durability and high temperature resistance. While copper tubing is preferred in certain industries for condensate lines due to its corrosion resistance.
For high-temperature superheated steam mains, alloys with additional elements like chromium and molybdenum are often utilized. These elements enhance tensile strength and creep resistance, ensuring the piping can withstand the extreme conditions associated with high-temperature steam.
When it comes to piping lengths, they are typically delivered from the factory in standard sizes:
1. Single random length pipes typically measure around 5-7 meters.
2. Double random length pipes have lengths ranging from approximately 11-13 meters.
These standardized lengths facilitate easier handling and installation, contributing to the efficiency and convenience of steam system construction.
Pipeline sizing is essential for ensuring that steam is delivered to the point of use at the appropriate pressure. Therefore, minimizing pressure drop throughout the distribution system is crucial.
D'Arcy Thompson (1860-1948) emphasized that fluid flow requires a higher energy level at Point 1 compared to Point 2 (as depicted in the image below). This energy disparity is essential for overcoming frictional resistance between the pipe and the flowing fluid, facilitating efficient fluid transport.
In practical applications, whether for water or steam systems, striking a balance between pipe size and pressure loss is crucial.
Oversized pipework results in:
1. Increased costs: Pipes, valves, fittings, and associated components become more expensive than necessary.
2. Higher installation expenses: Additional costs are incurred for support structures, insulation, etc.
3. Enhanced condensate formation: Larger pipes lead to greater heat loss, resulting in increased condensate formation in steam systems. This necessitates more steam trapping or results in the delivery of wet steam to the point of use.
In a specific scenario:
1. The installation cost of NPS 3 steam pipework was 44% higher than that of NPS 2 pipework, which would have provided sufficient capacity.
2. Insulated NPS 3 pipework experienced approximately 21% higher heat loss compared to NPS 2 pipework. Additionally, non-insulated sections of NPS 3 pipes would lose 50% more heat due to the increased surface area for heat transfer.
Conversely, undersized pipework leads to:
Lower pressure at the point of use: This can hinder equipment performance by providing only lower-pressure steam.
1. Risk of steam starvation: Insufficient steam flow may occur.
2. Increased risk of erosion, water hammer, and noise: Higher steam velocity in undersized pipes can lead to these detrimental effects.
