An existing fiberglass firewater piping system was expanded to enhance its capacity, necessitating validation for the newly added components. The client, engaged in the RDCG Rotterdam capacity growth project, specializes in producing renewable fuels through the chemical conversion of vegetable and animal oils. The firewater system at this facility is constructed from fiberglass piping supplied by a specialized vendor. The current scope focuses on the newly added piping integrated into the existing firewater system, for which a static stress analysis is being performed to ensure its integrity and reliability.
This study aims to validate the GRE piping in accordance with the design standard ISO 14692. The model is partially buried underground to assess the interaction between the piping and the surrounding soil, utilizing the American Lifelines Alliance soil model for this purpose. This case study outlines the methodology, findings, and recommendations derived from the analyses conducted.
The technical challenges identified in this project include:
- Potential overstress due to loading conditions.
- The newly added piping is located at an existing refinery site that experiences significant differential settlement of 75 mm due to tank settlement profiles. The ability of the piping to accommodate this settlement has been considered by FPI.
Methodology
Pipe Stress Analysis
Pipe stress analysis was conducted using CAESAR II, with the piping layout modeled according to the received isometrics. To ensure accuracy, continuation piping, which can influence the pipe sections within the scope, was also included in the CAESAR II model. This continuation piping may affect the overall system through its flexibility or by exerting additional loads on the piping.
Loading Conditions and Analysis
The system was analyzed under various loading conditions resulting from different design scenarios, such as hydrotesting and maximum design pressure. The structural wall thickness specific to each component was utilized in the model. It is important to note that Glass Reinforced Epoxy (GRE) piping exhibits different structural thicknesses across its components; for example, the wall thickness of a straight pipe is less than that of a tee or a bend.
Material Properties and Stress Envelope
GRE is classified as an orthotropic material, while steel is isotropic. Consequently, the allowable stresses for GRE are defined using an allowable stress envelope, which is constructed from multiple relevant data points. This envelope correlates the allowable axial stress to the hoop stress, incorporating a safety factor to accommodate potential long-duration failure scenarios.
GRE Flange Assessment
The assessment of GRE flanges in the system was conducted using the equivalent pressure method, also known as Kellogg’s method. This approach converts the moments and axial loads acting on the flange into an equivalent pressure, which is then compared to the allowable pressure for the flange at the relevant temperature. The equivalent pressure is calculated using the following equation:
The equivalent pressure from axial loads and bending moments are combined with the line pressure, pin, to form a combined pressure loading, pload, for the connection. Here Faxial is the axial load acting on the connection, and Mbending is the resulting external bending moment acting on the connection. In this equation, G is the gasket-sealing circle.
The qualified pressure is twice the pressure class of the GRE flanges multiplied by a part design factor f2.
Miscellaneous Modelling Aspects
The isolation valves in the firewater system, which consist of a valve stem and a butterfly valve, have been incorporated into the model. The valve stem is partially above ground, and the isolation valves have been modeled as rigid elements with soil restraints applied to accurately capture the interaction with the buried soil. To adopt a conservative approach for the analysis, the valve stem has been excluded from the model, as its presence would increase the bearing area of the valve. This decision ensures that the analysis remains focused on the critical aspects of the valve’s performance without overestimating its load-bearing capacity.
Findings
- Pipe Stress Analysis: The stress levels in the piping system conform to the allowable limits specified in ISO 14692. Longitudinal stresses in the pipes remain below the permissible threshold of the stress envelope. Notably, the highest stresses across all subsystems were observed during the hydrotest case. The geometry of the received isometrics does not necessitate any additional mitigations.
- Flange Load Verification: The flanges have been assessed using the equivalent pressure method, taking into account the appropriate gasket diameter and pressure class. The evaluation indicates that there are no excessive loads acting on the flanges.
- Loads on Valve Stem: To ensure the structural integrity of the valve stem, the displacement of the isolation valves was examined for any excessive movement, which could result from thermal expansion. The maximum displacement recorded was 7 mm, occurring in the horizontal plane. While this level of displacement is not a cause for concern if the stem is free to move, it could lead to significant bending moments if the stem is directly embedded in soil. Therefore, it is essential for the client to confirm the magnitude of this displacement.
Recommendations Following the Analysis
To address the identified issues, the following solutions are proposed:
- The static stress analysis indicates that the piping stress and flange loads are within allowable limits and remain significantly below the thresholds. Therefore, no alterations to the geometry are necessary.
- Extra caution should be exercised regarding the calculated 7 mm displacement of the isolation valves to ensure it does not impose excessive loads on the valve extender stems.
- It is recommended to carefully execute the fitting-to-fitting joints within the system. This process may be challenging due to the varying wall thicknesses of the connection fittings, which complicates the creation of joints with the required strength.
