Mon, 30 September, 2019
Problem definition
On a typical oil and gas platform, heat exchangers are critical components for the proper operation of any hydrocarbon separation process plant. After initial processing in the separators, the oil and gas streams are generally heated or cooled in heat exchangers (depending on the process design of the topside system). In direct heat transfer, the process fluid pipe is directly heated or cooled against hot flue gas or seawater, respectively. It is common practice for seawater to be used untreated apart from dosing with hypochlorite in cooling systems (‘a once through seawater system’). Free chlorine can be continuously dosed at a concentration of less than 1ppm into the seawater in order to prevent biofouling. There are cases, especially in relatively warm waters, where the chlorinated seawater enters and leaves the coolers at approximately 20°C and 40°C respectively. However, it is likely, for example in the case of a shell-and-tube type cooler, that the water temperature where the hot fluid enters the tube bundle can reach higher values as a consequence of the process design. This may mean that the local water temperature may approach the gas inlet temperature, which could exceed 80°C. Therefore the operation of the cooler presents a dynamic system, where the structural materials of the component can experience very harsh conditions. Thus, selection of materials must ensure both economical design and reliable performance.
Work carried out by TWI
To test the suitability of materials for these kinds of service conditions, TWI built a set up to simulate the complex in-service environment of a shell-and-tube cooler. TWI then carried out testing of a Ni-based alloy (C276), typically used for this component, in order to assess its resistance to crevice corrosion in the simulated in-service conditions.
The material subject to testing was taken from a shell-and-tube cooler, including parent tubes, weld overlay from the tubesheet, and tube-to-tubesheet welds. The crevice assemblies were made with overlapping metal coupons to form combinations, e.g.: weld overlay-weld overlay specimens, that provide both a metal to metal crevice and typical castellated washer crevice. A very high torque was applied in order to represent as closely as possible the loading conditions the structure experiences in service.
The test was carried out in artificial seawater (ASTM D1141) with 0.8-1ppm free chlorine and 7ppm dissolved oxygen (DO) at 80°C. 7 ppm is the dissolved oxygen content in the seawater as it enters the heat exchanger. The operation of the cooler presents a dynamic system that allows insufficient time for the dissolved oxygen concentration to decay to the significant lower equilibrium concentration (1-2ppm) at 80°C. A lab scale experimental set up was developed to be able to simulate a dynamic system as that of a shell-and-tube cooler and monitor its complex service environment. The scale experimental set up allowed simulation and settting of the dynamic operation of the shell-and-tube cooler at lab scale and to monitor and sustain its complex service environment throughout the long term duration of the test.
Conclusions
- A lab scale experimental set up was developed in order to be able to simulate a dynamic system as that of a shell-and-tube cooler and monitor its complex service environment, typically involving seawater with the heat exchanger design. The test approach was successful in an environment consisting of seawater with high dissolved oxygen concentration and free chlorine at +80°C
- The increased susceptibility of the C276 weld overlay specimens to localised corrosion in the representative service environment was noticed. The corrosion resistance of the material is significantly influenced by the complexity of the structure’s geometry, the selection of the welding procedures and the surface passivation techniques
- Determining a maximum safe residual chlorine level in the seawater is considered to be quite problematic due to the complexity of the parameters influencing crevice corrosion and the different test procedures used by various researchers
- The flow regime (velocity and pattern) and the duration of the exposure should also be taken into consideration in the evaluation of the material’s resistance to localised corrosion.
- In general, it is recommended that safe operating temperature limits are established for materials selected for service in offshore heat exchangers/coolers via appropriate testing that reflects the dynamic conditions experienced within the heat exchangers
EUROCORR 2019
The work described above was presented by Dr Maria-Eleni Mitzithra - Senior Project Leader from the TWI Materials and Structural Integrity Group, at EUROCORR 2019, Seville, Spain in the Marine Corrosion Session. The work was under the title: ‘Crevice corrosion testing of C276 material in chlorinated seawater at high temperature for a heat exchanger system’, paper proceedings no. 192545. Positive feedback was received by oil and gas operators (TWI Members) based on their experience relevant to the specific topic/issue. TWI was also approached by alloy manufacturers/suppliers who wish to confirm the performance and suitability/compatibility of their materials for service in the specific environmental conditions.
A potential joint industry project which brings on board all relevant parties which allows to establish safe operating temperature limits for materials selected for service in offshore heat exchangers/coolers via appropriate testing that reflects these dynamic conditions within the heat exchangers, is of interest.
Figure 1. Assemblies of the test specimens that represent as closely as possible geometry and loading conditions that the heat exchanger experiences in service
Figure 2. Image of the lab scale experimental set up that sets, maintains and monitors the dynamic service environment of a heat exchanger (high temperature, high dissolved oxygen level and chlorinated seawater)