Enegineering consulting – technical knowledge and experience as tools for effective reduction of industrial emissions
Phd Radoslaw Sieczkowski, Head of Technical Department in SPETECH
Eliminating leaks and ensuring the tightness of pressure equipment not only increases work safety but also provides a straightforward path to reducing uncontrolled industrial emissions. But what if several attempts to fix the leaks have failed, the problems persist, and the equipment is critical to the technological process? The SPETECH® Engineering Consulting Team successfully solved such a problem at one petroleum refinery plant located in South Pacific region.
Shell-and-tube heat exchangers are a type of pressure equipment commonly used in the chemical and petrochemical industries. Their reliable sealing can be a significant technical challenge. This is due to both high operating parameters and frequent cycles of pressure and temperature changes. Users of this type of equipment expect long-term, failure-free operation while maintaining a safe level of emissions of hazardous substances.
The subject of this study is a “reboiler” type heat exchanger operating at the plant located on the shores of the Tasman Sea in Oceania. Operator of the “reboiler” had been struggling with recurring leaks after shutdowns and restarts of the installation at the tube sheet flange joint connection, where two serrated gaskets were installed (Figure 1). The key operating parameters of the heat exchanger are listed in the table below.
Diagnosis of the Problem and Technical Expertise
After several unsuccessful attempts to solve the problem with their own maintenance staff, the heat exchanger user decided to order an external expert assessment to diagnose the causes of the leaks and propose possible solutions.
Utilising its knowledge and experience, the SPETECH® Engineering Team determined that the issues stemmed from the serrated gaskets used, which had insufficient elastic recovery. This meant that the gaskets were unable to cope with the excessive range of movements and deformations caused by thermal expansion of the connection, elongation of the double-ended bolts, or flange bending.
Figure 1: The considered “reboiler” type heat exchanger with leak points marked at the tube sheet connection
In the case of the heat exchanger in question, the double-ended bolts used at the connection had a relatively small cross-section in relation to their extended length, resulting in significant elongation during operation. An optimally selected gasket should accommodate these displacements with high elastic recovery. However, initial analysis showed that the gaskets used basically in the installation did not have these properties.
First Phase of Engineering Consulting
The first phase of the expert assessment involved performing calculations on the connection with the existing serrated gasket using the EN1591-1:2013 algorithm and EUROPARTNER® engineering software. Based on the calculations under various operating conditions, it was estimated that the maximum possible axial deformation, measured on the double-ended bolts, was 0.26 mm. This value is difficult to compensate for with standard gaskets, such as the serrated gaskets used in this case.
In the next step of the consulting work, two alternative solutions were proposed to ensure the compensation of the occurring displacements while providing the highest available level of connection reliability:
• SPETORING® RM2 metal membrane seals welded to the joint (“Weld ring gasket”).
• SPETOGRAF® GUS660 metal-core gaskets with graphite layers (“Carrier ring gasket”).
Figure 2: Weld ring gasket (left) vs Carrier ring gasket (right) in the tube sheet joint, with necessary assembly length calculated
Both solutions were presented to the user, highlighting the benefits and requirements of each.
In the case of Weld ring gaskets, the main advantage was the reliability and tightness of the connection, which could not be achieved by other types of solutions. However, the challenge would be the installation, requiring a total of 6 weld seams, as well as the need to extend the bolts (each gasket typically has a thickness of 30 mm). For these reasons, the client rejected this option, citing concerns over high costs and significant time requirements during repairs.
On the other hand, the use of Carrier ring gaskets did not require bolt extension, and the installation could be carried out according to standard procedures. At the same time, this solution provided high resistance to varying operating conditions, including external loads and thermal shocks. The client's high expectations for the reliability of the solution necessitated further design and optimisation work. Consequently, after initial approval, the SPETOGRAF® GUS660 gaskets underwent additional analytical work and verification procedures.
Second Phase of Engineering Consulting
Following the decisions made, optimisation of the material and geometric features of the SPETOGRAF® GUS660 gaskets was carried out.
Such gaskets consist of solid metal core and two graphite layers:
- For the 6 mm thick metal core there were two material options selected. First option was austenitic steel AISI 316L and another was high-strength nickel based ALLOY 625 material. The final choice of core material was to be determined later on by the required level of core mechanical strength.
- For the layers SIGRAFLEX® HOCHDRUCK PRO material was selected. This expanded graphite is provided by company SGL CARBON® and ensures very high tightness (reaching up to 10^-7 mg/(s*m)). With a single graphite layer thickness of 3 mm - in total for two gaskets in one connection amounted to 12 mm of soft filler material. This thickness successfully compensated for the estimated displacements.
Figure 3: Diagram of the heat exchanger showing external stresses considered during the second phase of consulting work
Next, using the finite element method (FEM), an analysis was performed on the most stressed cross-section of the gasket, shown in Figure 4.
Figure 4: Critical cross-section of the GUS660 gasket marked with red lines
The stress analysis indicated that the maximum stresses in the critical cross-section exceeded 230 MPa. This value exceeded the conventional yield strength (Rp0.2) for AISI 316L steel. Consequently, the Alloy 625 material, which offers much higher mechanical strength compared to AISI 316L, was ultimately recommended.
Figure 5: Stress distribution in the core of the GUS660 gasket determined for various operating states – one of the technical analysis elements conducted using FEM
Thus, the consulting phase in terms of selecting the optimal solution was completed. The final report included full design documentation with complete analytical calculations and FEM analysis. It also provided essential guidelines for the reliability of the connection, including surface preparation, installation methods, and operation recommendations, along with the suggestion for necessary training of the personnel operating the installation.
Supply of Seals and Achieving Emission Reduction
The scope of consulting services ordered from SPETECH® was ultimately expanded to include the production and supply of the recommended seals, along with a complete set of required calculations, including bolt torque moments and quality control documents. A SPETECH® representative was also involved during the supervision of installation work and during the final commissioning. The gaskets were successfully installed, and the heat exchanger was restarted. The installation has been operating without failure for over three years – a clear indicator of the technical success in solving the problem, coupled with a reduction in uncontrolled emissions to the atmosphere.
Figure 6: The optimal solution for the presented case study. SPETOGRAF® GUS660 gaskets with Alloy 625 core and Hochdruck Pro layers
Thanks to over 35 years of technical and production expertise, SPETECH® is the rare example of company acting in the industrial sealing sector that offers comprehensive technical consulting. Recently, we have focused especially on the development of engineering and consulting, which is becoming increasingly important in the context of emission reduction.
Thanks to our experience, we can offer our clients the best solutions in terms of gaskets selection, engineering calculations, seals supply and installation supervision. Our products and services are already delivered to over 100 countries worldwide.