Automated Hanger Design

When a pipeline operates at high temperatures, the thermal expansion often causes the pipe to lift off or push down heavily on its supports. If a rigid support is used in these locations, it can generate catastrophic localized stresses or lift the pipe entirely off adjacent supports.

To mitigate this, engineers use Variable Spring Hangers.

The Pipe Stress FEA engine features an integrated Two-Pass Spring Sizer that evaluates the system kinematics and automatically calculates the optimal spring rate and cold preload in accordance with MSS SP-58 guidelines.

4.3.1 The Two-Pass Algorithm

When the engine detects one or more nodes with their restraint type set to SPRING and autoSizeStiffness enabled, it intercepts the standard solving sequence and executes a specialized two-pass routine.

Pass 1: Rigid Weight Analysis (The Hot Load)

The goal of the first pass is to determine how much the pipe weighs at the exact location of the spring.

1. The engine temporarily converts the spring into a Rigid Support (locked in the vertical axis).
2. The solver runs a purely static Sustained (SUS) load case, applying only gravity and fluid mass.
3. The vertical reaction force at the locked node is extracted. This becomes the required Hot Load ($L_H$)—the force the spring must exert to perfectly balance the pipe's weight during operation.

Pass 2: Free Thermal Analysis (The Travel)

The goal of the second pass is to determine how far the pipe will move due to thermal expansion.

1. The engine removes the rigid support, leaving the vertical axis completely Free.
2. The extracted Hot Load ($L_H$) is applied to the node as a constant, upward external point load.
3. The solver runs a full Operating (OPE) load case, combining gravity, fluid, and thermal expansion.
4. Because the weight is perfectly balanced by the upward point load, the resulting vertical displacement at the node represents the true thermal Travel ($\Delta$).

4.3.2 Sizing the Spring Rate ($k$)

Industry standards (like MSS SP-58) mandate that the variability of a spring hanger should not exceed 25%. Variability is defined as the change in load caused by the pipe's travel, relative to the operating load.

$\text{Variability} = \frac{k \times |\Delta|}{L_H} \le 0.25$

The engine automatically calculates the maximum allowable spring rate ($k$) to satisfy this 25% threshold:

$k = \frac{0.25 \times L_H}{|\Delta|}$

Physical Clamping

To ensure the mathematical spring rate matches physically available commercial hardware, the engine automatically clamps the calculated stiffness to a minimum of $2.0 \text{ N/mm}$ and a maximum of $300.0 \text{ N/mm}$. If the thermal travel is practically zero ($< 1.0 \text{ mm}$), the engine defaults to a stiff rate of $50.0 \text{ N/mm}$.

4.3.3 The Cold Load ($L_C$)

Once the spring rate ($k$) is determined, the engine must calculate the Cold Load ($L_C$)—the physical preload that must be dialed into the spring hanger at the factory before installation.

The engine calculates the Cold Load based on the Hot Load and the directional travel vector:

$L_C = L_H + (k \times \Delta)$

(Note: The engine uses the raw directional vector for $\Delta$. If the pipe moves UP during operation, $\Delta$ is positive, and the Cold Load will be mathematically higher than the Hot Load, ensuring the spring relaxes to the correct operational tension).

Final Matrix Injection

After calculating $k$ and $L_C$, the engine permanently overwrites the node's JSON properties with these specific values. The system then seamlessly proceeds to the final Code Compliance run, injecting the sized spring rates into the Global Stiffness Matrix and applying the Cold Loads as external preloads.