Michael Hunt discusses advances in low burst pressure rupture disc technology
Fine chemical and pharmaceutical production often require the use of glass-lined reactor vessels and low-pressure fermentation vessels and separator tanks, each of which must be protected from unexpected rises in pressure.
Graphite rupture discs are often used in these applications because their inherent low burst pressures allow them to open and relieve the pressure before the vessel or pipework fails. Additionally, graphite rupture discs often appear more cost-effective than metallic rupture discs, have a variety of gasket material and attachment options, and are installed directly between pipe flanges thereby eliminating the need for a holder.
Although these features are beneficial to plant managers, graphite discs do have their disadvantages.
Perhaps most problematic is when a graphite rupture disc is called upon to burst, it fragments into black, contaminating dust, requiring an extensive cleaning process. This often results in unplanned downtime, which can severely affect production schedules, delivery dates and, ultimately, revenues.
Graphite discs are fragile. The most common cause of breakage is during installation. Clamping a fragile, graphite rupture disc between rigid flanges frequently induces a hairline crack (future leak path) into the disc, which affects its performance and ability to control pressures
Historically, metal rupture discs struggled to meet such low pressures, especially in small sizes. Although metal rupture discs are more ductile, more robust and more accurate, graphite rupture discs were the only pressure relief option for these applications.
However, recent advances in rupture disc manufacturing technologies have made possible what was once thought impossible: a metallic rupture disc with the ability to reliably perform in low-pressure environments, especially in small sizes.
For example, Fike’s newest metallic rupture disc, RD540, features burst pressures as low as 1.75 psig/0.12 barg. Metallic rupture discs able to achieve these low burst pressures will also avoid the disadvantages found in graphite rupture discs via three main characteristics.
The first is being non-fragmenting. Many metallic rupture discs are considered “non-fragmenting,” meaning the disc simply opens and remains in one piece. Unlike with a graphite rupture disc, there’s no need for clean-up; rather, the disc can be replaced, and the process may continue normally.
The next characteristic is high efficiency. A rupture disc’s “operating ratio” refers to the ratio between the process’ maximum operating pressure and the set pressure at which the disc will burst. Therefore, rupture discs with lower operating ratios require processes to run less efficiently. RD540 includes a 95% operating ratio, when most graphite rupture discs are only able to achieve an 80% operating ratio.
High performance is the final characteristic. Rupture discs are under the most stress when pressure transitions from vacuum (negative pressure or atmosphere) to a positive pressure. Reverse-acting rupture discs can withstand this pressure change without fragmentation or alteration of the disc’s performance of any kind, while graphite rupture discs are brittle and therefore unable to endure these forces
Although graphite rupture discs will remain a viable pressure relief option for many, metallic rupture discs offering the same low burst as their graphite counterparts will be ideal to solve many plant managers’ unique challenges and business objectives
Michael Hunt is with Fike