Key Considerations for Secondary Support Design

Secondary Support Design

Source: KnowPipingField.com

II JAY SHRI KRISHNA II

Secondary Supports, often referred to as brackets, are crucial components in piping systems, providing essential structural support to piping lines. While their primary function is to ensure the structural integrity of the system, several critical design considerations must be taken into account to guarantee their long-term performance and safety.

Key Considerations:

One of the most important factors is the dynamic nature of piping loads. Unlike static loads, piping systems are subjected to various dynamic forces, including thermal expansion, vibration, and seismic loads. These forces can significantly impact the design and selection of secondary supports. Therefore, it's essential to conduct thorough stress analysis and vibration analysis to accurately assess the loads and select appropriate support types.

This article delves into key considerations for designing effective secondary supports.

Support Location and Configuration

Example of Secondary Support

When designing secondary support systems, careful consideration must be given to the location and configuration of supports to ensure optimal performance and cost-effectiveness.

Consolidated Support Design:

Rather than designing individual supports for each pipe line, it is often more efficient to consolidate multiple lines onto a single support structure. This approach offers several advantages:

• Reduced Material Usage: By combining multiple lines onto a single support, the overall material usage can be significantly reduced, leading to cost savings.

• Improved Structural Efficiency: Consolidated supports can be designed to optimize load distribution and minimize stress concentrations, resulting in a more robust and efficient structure.

• Simplified Installation and Maintenance: A reduced number of supports simplifies installation and maintenance processes, saving time and labor costs.

L-Type Secondary Support: Considerations and Best Practices

L-type secondary supports, while commonly used, have certain limitations, especially when dealing with significant loads or long spans. To optimize their performance and minimize potential issues, consider the following:

Avoiding Unnecessary Projections:

• Minimize L-Dimension: When the L-dimension becomes large, it can lead to increased stress concentrations and potential instability.

• Connect the Loose End: If possible, connect the loose end "X" to a nearby structure to provide additional support and rigidity.

• Increase Member Size and Connection Details: If connection to a nearby structure is not feasible, consider increasing the member size and reinforcing the connection at the insert plate.

Example of L-Type Support

Prioritizing Two-Ended Connections:

Whenever possible, opt for support details that connect both ends of the L-type support. Even for relatively short L-dimensions (around 350 mm), a two-ended connection can significantly enhance structural integrity.

By adhering to these principles, you can design L-type secondary supports that are both efficient and reliable.

Engineer's Note on Torsion:

When using L-type (cantilever) supports, always evaluate the Torsional Load on the primary beam. A heavy pipe on a long cantilever bracket acts like a giant wrench, trying to twist the main structural column. If the twist is too high, a "Back-to-Back" support or a "Two-Point" connection is mandatory.

T-Type Support Connection Details for Beams:

T-Type Supports: T-type supports offer better load distribution and stability, especially when supporting multiple lines.

Box-Type Supports: Box-type supports provide excellent rigidity and can accommodate heavy loads. They are particularly suitable for supporting multiple lines in a compact configuration.

When erecting T-type supports on beams, proper connection details are vital for ensuring structural integrity and safe operation. Here are some basic points & examples:

Importance of Connection Details:

• Well-designed connections ensure the T-type support can effectively transfer piping loads to the beam without compromising either component.

• Clear details also facilitate proper installation and maintenance.

Examples of Connection Details:

1. Welded Connection:

• Suitable for high-load applications.

• Requires qualified welders and proper inspection procedures.

2. Bolted Connection:

• Offers good strength and can be disassembled for maintenance.

• Requires sufficient bolt size and proper tightening procedures.

3. Clamped Connection:

• A faster and simpler option for lower-load applications.

• May not be suitable for high vibration or dynamic loads.

T-shaped support for two parallel pipe lines

Avoiding L-Type Supports:

While L-type supports are a common configuration, they can be less structurally efficient compared to other options, especially when dealing with significant loads or long spans.

• Limited Load Capacity: L-type supports can be susceptible to bending moments, which can limit their load-carrying capacity.

• Potential for Vibration: Long, slender L-type supports may be likely to vibration, particularly under dynamic loads.

Example:

Instead of designing separate supports for two parallel pipe lines, a single T-shaped support can be used to accommodate both lines. This consolidated support can be anchored to a structural beam or column, providing a strong and efficient solution.

Example of T-Type Support

Alternative Configurations:

Example:

Instead of using an L-type support to support a pipe line, a T-type support can be used to provide additional stability and load-carrying capacity. The T-type support can be connected to a structural beam or column to ensure a secure and reliable installation.

T-type support replacing an L-type support

Optimizing Support Spacing:

Proper support spacing is crucial to accommodate thermal expansion and contraction of pipes. Insufficient spacing can lead to excessive stress and potential failures, while excessive spacing can result in excessive deflection and vibration.

• Thermal Expansion: Consider the expected thermal expansion of the pipe and ensure that the support spacing allows for free movement without inducing stress.

• Vibration: Adequate support spacing helps to minimize vibration and resonance, particularly for long spans and flexible piping systems.

Example: Supporting Hot Pipes and Addressing Horizontal Loads

Accommodating Thermal Expansion:

When designing secondary support structures for hot pipes, it's crucial to account for thermal expansion. This involves providing sufficient clearance between the pipe and the support to allow for movement without inducing stress.

• Minimum Clearance: A minimum clearance of 200 mm is generally recommended.

• Additional Considerations: Factors such as wind load, seismic loads and other dynamic forces can further influence the required clearance.

Connecting Small T-Post Structures:

For smaller T-post structures, it's advisable to connect them to nearby structures whenever possible. This connection, while often minimal in cost, significantly enhances the overall structural integrity and stability of the support system.

Addressing Horizontal Loads Due to Friction:

It is critical to recognize that horizontal forces generated by friction can be substantial. These loads occur at the contact point between the pipe shoe and the secondary support beam during thermal expansion or contraction.

To design a safe support, engineers must account for the specific coefficient of friction (μ) based on the contact materials:

• Steel-on-Steel Contact: Friction loads are typically calculated at 30% (μ = 0.3) of the total vertical load.

• Low-Friction Interfaces (PTFE/Teflon): Friction loads can be significantly reduced to 10% (μ = 0.1) or even lower.

Failure to account for these lateral forces can cause the secondary bracket to "trip," buckle, or induce unwanted torsional stress on the primary structural members.

Strategies to Mitigate Horizontal Forces:

• Roller or Ball Bearing Supports: These are highly effective for large-diameter or high-temperature lines to allow for nearly restricted axial movement.

• Low-Friction Slide Plates: Incorporating materials such as Teflon (PTFE) or graphite sheets between the pipe shoe and the support structure minimizes resistance and protects the integrity of the secondary steel.

By integrating these friction considerations, engineers ensure that the secondary support system is robust enough to handle the thermal and dynamic realities of a functioning plant.

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Dynamic Load Considerations:

• Vibration Analysis: Conduct vibration analysis to identify potential resonance frequencies and take necessary measures to mitigate vibration-induced stresses.

• Seismic Design: Design supports to withstand seismic loads, considering local seismic codes and standards.

• Thermal Expansion: Account for thermal expansion and contraction of pipes by providing adequate clearances and flexible connections.

Structural Integrity:

• Material Selection: Choose materials that are suitable for the operating conditions, including temperature, pressure, and corrosive environments.

• Stress Analysis: Perform rigorous stress analysis to ensure that the support structure can withstand the combined loads, including static, dynamic, and seismic loads.

• Fatigue Life Assessment: Evaluate the fatigue life of the support structure, especially in areas prone to cyclic loading. Consider the impact of potential vibration and thermal cycling.

Construction and Maintenance:

• Accessibility: Design supports with easy access for inspection, maintenance, and repair. Avoid obstructed areas or tight spaces that hinder maintenance activities.

• Modular Design: Consider modular support designs that can be easily assembled and disassembled, reducing installation time and costs.

• Corrosion Protection: Implement appropriate corrosion protection measures, such as coatings or galvanization, to extend the service life of the support structure.

By adhering to these design considerations, engineers can ensure the long-term reliability and safety of piping systems.

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Short Revision:

Secondary Support Design: Bridging the Gap

While primary structures (pipe racks and sleepers) provide the main framework of a plant, Secondary Supports (often called Brackets or Secondary Steel) are the localized structures that connect the pipe to the primary steel. Proper design is essential to ensure these "smaller" components don't become the "weakest link" in your piping system.

1. Primary vs. Secondary Steel

It is important to distinguish between the two:

• Primary Steel: Large structural members (Beams/Columns) that support multiple lines or heavy equipment.

• Secondary Steel: Smaller members (Angles, Channels or Beams) added to primary steel to provide a resting point for a specific pipe that doesn't reach the main beam.

2. Common Types of Secondary Supports

• Cantilever Brackets: A single horizontal member supported at one end, ideal for lines running alongside a column.

• T-Type Supports: Used for supporting lines at a specific elevation from the floor or sleeper.

• Goal Post Supports: Two vertical legs with a horizontal cross-member, used for heavier loads or wider spans.

• Dummy Legs: A piece of pipe welded to an elbow to provide support from a nearby beam.

3. Structural Integrity Considerations

When designing brackets, several mechanical factors must be checked:

• Deflection: The bracket must be stiff enough so it doesn't sag under the weight of the pipe and fluid.

• Gusset Plates: For cantilever brackets, adding a triangular "gusset plate" significantly increases the load-carrying capacity.

• Welding vs. Bolting: While welding is more common in fabrication shops, bolting is often preferred for field-added supports to avoid "hot work" in operating plants.

4. Accessibility and Clearance

Secondary supports must be placed such that they do not block:

• Walkways: Headroom must be maintained (typically 2.1 meters minimum for Walkways/Access Ways, but for Road Crossings, the clearance must be 7 meters (or as per project spec).

• Valve Operation: Brackets should not obstruct handwheels or maintenance access to flanges.

• Expansion: Ensure the pipe can slide on the secondary support if thermal growth is expected.

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Fabrication and Field Best Practices

• Standardization of Steel: For cost-effective fabrication, secondary supports should utilize standard hot-rolled steel sections (such as ISMC channels or ISA angles). This ensures availability and simplifies structural calculations.

• Friction and Sliding Surfaces: When a pipe shoe rests on a secondary support beam, engineers must account for the coefficient of friction. In some cases, a stainless steel or PTFE plate is welded to the secondary steel to allow for smooth thermal movement.

• Common Configurations: Beyond goalposts, the use of T-posts (inverted T-shaped steel) and Brackets (triangular wall supports) are essential for supporting lines running alongside walls or equipment structures.

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Frequently Asked Questions (FAQs)

1. What is the main difference between primary and secondary piping supports? 

Primary supports are those that are directly attached to the pipe (such as shoes, clamps or hangers) to hold its weight. Secondary supports are the structural steel members (like goalposts, brackets or cantilevers) that bridge the gap between the primary support and the main building structure or pipe rack.

2. Why is "Structural Rigidity" a priority in secondary support design? 

If a secondary support is too flexible, it will deflect under the weight of the pipe, causing the pipe to sag and potentially changing the slope of the line. This can lead to improper drainage, liquid pockets, and unintended stress on equipment nozzles. Secondary steel must be sized to handle the "point load" of the piping without significant bending.

3. How do engineers decide between a "Goalpost" and a "Cantilever" secondary support? 

A Goalpost (two vertical legs with a horizontal beam) is used for heavy, large-bore pipes or where high stability is required. A Cantilever (a horizontal beam supported at only one end) is a space-saving option typically reserved for smaller, lighter pipes or utility lines where access is restricted.

4. Why must secondary supports account for "Lateral Loads" and not just vertical weight? 

Pipes don't just push down; they also move sideways due to thermal expansion, wind or seismic activity. Secondary supports must be designed with diagonal bracing or sufficient thickness to resist these lateral forces, preventing the support from buckling or twisting during plant operation.

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Conclusion:

The design and implementation of secondary supports are critical factors in ensuring the long-term performance and safety of piping systems. By carefully considering factors such as support location, configuration, dynamic loads, structural integrity, and maintenance accessibility, engineers can design effective and reliable secondary support systems.

Adhering to industry standards and best practices, as well as conducting thorough analysis and design calculations, is essential to minimize the risk of failures and optimize system performance. By prioritizing proper secondary support design, engineers can contribute to the overall safety, efficiency, and longevity of piping systems.

By carefully considering these factors, engineers can design secondary support systems that are both structurally sound and cost-effective.

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