Abstract
This paper focuses on the components of a typical flare system, including the knockout drum, seal drum, and flare itself. It highlights the critical role of a properly operating flare system in preventing plant disruptions from turning into disasters.
A case study of erosion-corrosion damage found in the water seal drum of a flare system after 4.5 years of operation is presented. The causes of the damage, including the corrosive element CO2/H2CO3, are analyzed, and a repair solution involving the upgrade to 304L stainless steel is proposed. The success of the repair is confirmed through five years of subsequent operation.
Introduction
A flare system is provided in a refinery or petrochemical plant to ensure the safe and efficient disposal of relieved gases or liquids. The disposal fluids are collected in a flare header and routed to the flare. It is extremely important in the event of a plant emergency such as safety valve openings or power failures. A properly operating flare system is the critical component to prevent a plant disruption from turning into a disaster.
A typical flare system consists of a knockout drum, seal drum and flare as shown in figure 1. A flare knockout drum separates liquid from gas in the disposal fluid and holds a specified amount of liquid that can be relieved during an emergency situation. Flare liquid seal drum purposes include:
- Prevent any flashback originating from the flare burner.
- Maintain a slight system pressure to ensure no air leakage into the flare system.
- Liquid seals are located after the knockout drum and before the flare itself.
In this case study, the water seal drum, which utilizes water as a sealing medium, is examined.
Figure 1. Typical drawing of flare system
Description of Damage
After 4.5 years of operation, erosion-corrosion was discovered in various parts of water seal drum, including the water seal pipe, perforated baffle, and shell. Details are as below (figure 2):
- Corrosion primarily affected the submerged and water level portions of the drum with an average corrosion rate of 0.4-0.7 mm/year.
- The areas closer to the water seal pipe exhibited more severe corrosion compared to those farther away.
- The portion of drum above the water level did not show significant corrosion.
Figure 2. Corrosion on the water seal drum
Cause of Damage:
To determine the cause of damage, three water samples from the seal drum at three distinct times were subjected to chemical analysis. Results are presented as below:
Test parameter | Test method | Unit | Sample 1 | Sample 2 | Sample 3 |
pH | ASTM D 1293 |
| 6.18 | 6.18 | 6.36 |
Conductivity | HTAS 1191 | µS/cm | 1412 | 1412 | 2540 |
Cl– | IC | mg/L | None | None | None |
SO42- | IC | mg/L | < 0,1 | 0.114 | 0.117 |
CO32- | qualitative |
| present | present | present |
NO3– | IC | mg/L | < 0,1 | < 0,1 |
|
The chemical analysis confirmed the presence of CO2 and a lowered pH due to the formation of carbonic acid.
According to design data, disposal fluid contains 5.67-19.02 %mol of CO2, which dissolves into the sealing water and forms carbonic acid (H2CO3) as follows:
The cause of the damage can be attributed to multiple factors. Firstly, the construction material of the seal drum components was carbon steel, which is susceptible to H2CO3 corrosion. The corrosive element CO2 present in the disposal fluid dissolved into the sealing water, forming carbonic acid (H2CO3). This acid then corroded the carbon steel surfaces of the water seal drum. Additionally, the operating conditions, such as startup, shutdown, and emergency scenarios, resulted in an increased flow rate of disposal fluid, causing it to dissolve into the water and form two-phase fluid. This accelerated erosion-corrosion, leading to more severe metal loss.
It is worth noting that the selection of carbon steel as the construction material for the seal drum components contributed significantly to the susceptibility to H2CO3 corrosion. Carbon steel is known to be vulnerable to acidic environments, and the continuous exposure to the corrosive nature of carbonic acid further exacerbated the damage.
Repair
Based on the above finding, corrosion is due to CO2/H2CO3. The repair solution involving the upgrade to 304L stainless steel was implemented to mitigate the corrosion issue and ensure the long-term integrity of the water seal drum. Details are as follows:
- Lining the internal surface of the shell and head of the drum with 304L stainless steel, as shown in figure 3.
- Replacement of the perforated baffle and water seal pipe with new components made of 304L stainless steel, as depicted in figure 4, 5.
Figure 3. Lining the internal surface of shell and head with 304L stainless steel
Figure 4. New perforated baffle |
Figure 5. New water seal pipe |
The repair was successful, and no damage was found after five years of operation.
Conclusions
This study emphasizes the significance of a well-functioning flare system in maintaining plant safety and preventing potential disasters. The case study highlights the vulnerability of the water seal drum to erosion-corrosion caused by the presence of CO2 in the disposal fluid. The use of 304L stainless steel for upgrading the internal components of the water seal drum has proven to be an effective repair solution, resulting in five years of operation without any damage. This case underscores the importance of periodic inspection and maintenance to identify and address potential issues in flare systems to ensure their continued reliable and safe operation.
