Protecting bioprocessing from overpressure events

Protecting bioprocessing from overpressure events: pressure safety management of bioreactors protects personnel and the high value product they are working with

Over the past 30 years or so, bioprocessing has grown at exponential rates from the early days of research-based companies springing from the Boston, USA area life sciences educational establishments to global organisations that rival traditional fine chemical pharmaceutical producers. Indeed, many global players in the drug manufacturing industry practice both fine chemical and bioprocessing. Bioprocessing activities cover a growing range of market sectors ranging from cell and tissue engineering, vaccine development, agricultural seed and plant treatments, nutraceuticals, and biochemical engineering, both at a research and manufacturing level.

At the heart of bioprocessing, whether research, development or production, processes are supported in bioreactors, which are the vessels in which controlled cell growth occurs, finished product is stored, and from which ingredients such as pure water are delivered. Bioreactors provide a sterile environment in which controlled processes to produce high purity, often injectable, products occur. Whether the process itself requires pressure or heat, or both pressure and heat, that sealed bioreactor will require periodic sterilisation to maintain purity. When sealed, the bioreactor is a pressure vessel under ASME Boiler & Pressure Vessel Code rules and shall be fitted with an ASME certified pressure relief device when its design pressure exceeds 15 psig.

The primary means of application for rupture disc pressure relief devices to bioreactors is within so called tri-clamp fittings that are the connection of choice for bioreactors due to their sterility and quick connect/quick disconnect capability. "This quick-release, sanitary style of fitting has been popular in bioprocessing since the industry's inception," explains Geof Brazier, director of development, BS&B Safety Systems, a major supplier of rupture disc devices for a wide variety of process applications. "Developing rupture disc technology that can be installed on tri-clamp fittings that are conveniently mounted and dismounted for cleaning and inspection purposes was a key design challenge that BS&B tackled in the late 1980s when bioprocessing was gaining respect at a research level."

Bioreactors are pressure vessels used for sterile processing and storage that comply with ASME (American Society of Mechanical Engineers) Pressure Vessel Code requirements, which define an absolute requirement for pressure relief. To meet that requirement users have the choice between rupture discs and pressure relief valves. Rupture discs feature a much cleaner construction than relief valves and occupy minimal installation space – limited real estate for instrumentation and safety device connection is a challenge on commonly used small volume bioreactors. The superior cleanliness of rupture discs means that this is the technology of choice where sterility is concerned. The ASME Code permits pressure relief valves to be used in combination with rupture disc devices, typically downstream of the rupture disc device so that the valve does not come into direct contact with the process.

Choosing the right rupture disc

Rupture discs are non-reclosing pressure relief devices that protect vessels, such as bioreactors, from damaging over-pressurisation (or vacuum) conditions. They are available in various designs, sizes, shapes and set pressures.

While rupture disc sizes and pressure settings vary according to the needs of different industries, most bioprocessing applications require rupture discs that are 1-inch to 4-inches in nominal size, and provide pressure relief in the 30- to 60-psig range. The dimensions of tri-clamp fitting compatible rupture disks are unique compared to those applied to traditional pipe flange arrangements.

Experts advise that processors consider rupture discs of the "reverse buckling" design. Unlike traditional forward-acting discs, where the load is applied to the concave side of a dome, in a reverse buckling design, the dome is inverted toward the source of the load. Reverse buckling discs are typically sturdier than forward-acting discs, which can be thin and difficult to handle, and as a result have greater longevity, accuracy and reliability over time.

Reverse buckling rupture diskc are designed for non-fragmentation upon activation, and are recommended for use with downstream pressure relief valves in order to isolate them from processes, ensuring leak tightness, reduced valve maintenance expenses and often allowing the use of lower-cost valve trim.

Important design features

Other design features may be important to various users in industries where bioprocessing is accelerating. For example, discs should provide a fail-safe response to an overpressure condition for both gas and liquid media. In the field of bioprocessing, rupture discs designed for CIP/SIP (clean/steam in place) service, that provide for minimum "dead space" between process fluid and the disc, and offering a sanitary alert sensor option to announce rupture disk activation are important to the process designer.

The recommended material for most rupture discs used in bioprocessing applications is stainless steel, although the option of Hastelloy and even Tantalum can be of value for extremely corrosive environments.

In addition to employing the appropriate rupture disc, many users prefer to use an outlet fitting with industry standard tri-clamp connections that is keyed to the proprietary rupture disc gasket and ensures installation of the reverse buckling disc onto the bioreactor sanitary inlet ferrule in the right direction! This type of connection and gasket arrangement, developed by BS&B serves to mistake-proof the direction of the rupture disc installation. "Many of our customers would not use a rupture disc that does not include that device," Brazier explains. "These users range from the bioprocessing companies to equipment manufacturers, including companies that make storage systems for high-purity water for injection into process vessels, which is a primary resource in the bioprocessing industry."

Meeting miniaturisation challenges

In some instances, bioreactors and related storage vessels are becoming more miniaturised. To accommodate such cases, BS&B offers a welded ¾-inch rupture disc assembly with 1-inch tri-clamp connection compatibility.

Miniaturisation of rupture discs presents unique challenges that are best met by utilising reverse buckling technology, says Brazier. "As burst diameters decrease it becomes challenging to design a reverse buckling disc," he explains. "In many ways it can be like trying to fit a camel through the eye of a needle."

To resolve this issue, BS&B has created novel structures that control the reversal of the rupture disc to always activate in a predictable manner. This includes, for example, a hybrid shape that combines reverse buckling and forward bulging characteristics that are pre-collapsed. In this type of design, a line of weakness is typically placed into the rupture disc structure to define a specific opening flow area when the reverse type disc activates.

For more information visit www.bsbsystems.com