News
Background
Our Operations Manager from Mandate Systems attended a bio-methane site to undertake the WSE and examinations.
Conducted within the appropriate timescale, the examination identified an accumulation of media as well as serious deterioration to the wall thickness of the vessel.
Understanding the Biogas Site Operations
The Biogas site operates an anaerobic digestion plant producing biomethane gas. Gas generated from the digesters undergoes hydrogen sulphide (H₂S) reduction through controlled oxygen (O₂) dosing within the digester, facilitating biological desulphurisation.
Following treatment, the gas (at approximately 100% relative humidity) is routed through a booster compressor system and passed through carbon filtration in two stages. The gas is then received into a low-pressure receiver vessel operating at approximately four bar, before being further compressed to 12.0 bar downstream of the filtration process.
Biogas Implications on the Written Scheme of Examinations (WSE)
Due to the nature of the mixed gas and operating conditions, the receiver vessels are subject to periodic internal examinations in accordance with the Written Scheme of Examination (WSE), typically on a two to four yearly interval. The vessels are internally lined to provide corrosion protection against the wet gas environment.
WSE Identifying a Problem
Approximately one year following an internal examination, accumulation of media was identified in the base of a receiver vessel by Mandate Systems. Subsequent inspection determined that carbon filter media had migrated downstream and caused mechanical damage to the internal protective lining. This exposed the underlying mild steel substrate.
Given the combination of damaged lining and saturated gas conditions, accelerated corrosion occurred. Over a period of approximately two years, the vessel wall thickness reduced from 8.5 mm to 4.5 mm due to severe oxidation.
Regulation 10 PSSR
The defect was reported in accordance with Regulation 10 of the PSSR, and the affected vessel was removed from service.
A Regulation 10 is the identification of imminent danger. Here the competent person conducting the examination must report it immediately to the user or owner. In addition, within 14 days, it must also be reported to the enforcing authority.
Remedial Action Regulation 1o PSSR
As part of remedial actions, the client elected to upgrade the system by installing a receiver vessel constructed from 316L stainless steel, offering improved corrosion resistance under high humidity and contaminated gas conditions.
The site is further equipped with both fixed and portable gas monitoring systems, including personal gas detectors with man-down (immobility) alarm functionality.
Subsequent WSE
The Written Scheme of Examination was subsequently revised to require 12-monthly internal examinations, reflecting the increased risk associated with the relevant fluid and potential consequences of system failure.
All receiver vessels are installed externally and are therefore exposed to prevailing weather conditions, which may contribute to environmental degradation factors. Which again reinforced the need for the 12-monthly examination.