Miniature Rupture Disc Revolution

Online Editor

Custom-engineered, miniature rupture disc assemblies designed for high cycling at low to high pressure are ideal for applications involving hydraulics and gases

For decades, rupture discs have served the process industry as an effective passive safety mechanism to protect against overpressure or potentially damaging vacuum conditions. The disc, which is a one-time-use membrane made of various metals including exotic alloys, is designed to activate within milliseconds when a pre-determined differential pressure is achieved.

Because process equipment operational reliability is essential, this demands high integrity from the pressure relief technology used to protect low-and high-pressure OEM systems. As a result, many OEMs are increasingly turning to integrated rupture disc assemblies with all components combined by the manufacturer, as opposed to loose rupture disc and holder devices that leave much to chance. These assemblies are being tailored to the application, miniaturised and use a wide range of standard and exotic materials, as required. This approach ensures the rupture disc device performs as expected, enhancing equipment safety, reliability and longevity while simplifying installation and replacement.

The oil and gas industry, for instance, uses rupture discs on triplex pumps for many field applications including oil extraction and well servicing operations. Triplex pumps are positive-displacement pumps configured with three plungers. Commonly referred to as “mud pumps,” the devices typically can handle a wide range of fluid types, including corrosive fluids, abrasive fluids and slurries containing relatively large particulates.

The pressures the pump must endure depend on the depth of the drilling hole and the resistance of flushing fluid, as well as the nature of the conveying drilling fluid, although application specific, hydraulic operating pressures are typically in the 5,000 to 20,000 psi range.

“A three-plunger pump is continuously cycling, so the disc must be able to withstand high pressures with 1,000,000 pressure cycles or more easily experienced,” says Geof Brazier, managing director of BS&B Safety Systems Custom Engineered Products Division.

Separate components or integrated assemblies?

Traditionally, rupture discs began as standalone components that are combined with the manufacturer’s separate holder device at the point of use. The installation actions of the user contribute significantly to the function of the rupture disc device. When installed improperly, the rupture disc may not burst at the expected set pressure. There is a delicate balance between the rupture disc membrane, its supporting holder and the flanged, threaded or other fastening arrangement used to locate the safety device on the protected equipment.

For this reason, an integrated rupture disc assembly is often a better choice than separable parts. Available ready-to-use and with no assembly required, integrated units are certified as a device to perform at the desired set pressure. The one-piece design allows for easier installation and quick removal if the rupture disc is activated.  

The assembly includes the rupture disc and housing and is custom engineered to work with the user’s desired interface to the pressurised equipment. The devices are typically threaded or flanged, or even configured for industry specific connections. The rupture disc and holder are combined by the manufacturer by welding, bolting, tube stub, adhesive bonding or crimping based on the application conditions and leak tightness requirements.

There are additional advantages to this approach. Integrated assemblies prevent personnel from using unsafe or jury-rigged solutions to replace an activated rupture disc to save a few dollars or rush equipment back online. The physical characteristics of increasingly miniaturised rupture discs as small as 1/8in can also make it challenging for personnel to pick up the disc and place it into a separate holder.

“OEMs are driven to deliver the longest life and lowest cost of ownership to their customers, says Brazier. “The use of an integral assembly maximises the longevity, proper function and trouble-free service of the pressure relief technology.”

According to Brazier, the most important considerations in rupture disc device design are having the right operating pressure and temperature information along with the expected service life, which is often expressed as a number of cycles the device is expected to endure during its lifetime. Since pressure and cycling varies depending on the application, each requires a specific engineering solution. “Coming up with a good, high reliability, cost-effective and application specific solution for an OEM involves selecting the right disc technology, the correct interface (weld, screw threads, compression fittings, single machined part) and the right options as dictated by the codes and standards,” says Brazier.

Because user material selection can also determine the longevity of rupture discs, the devices can be manufactured from metals and alloys such as stainless steel, nickel, Monel, Inconel, and Hastelloy.

According to Brazier, for various industries it can be important for rupture discs to have a miniaturised reverse buckling capability in both standard and exotic materials.

“Where economics is the driver, reverse buckling discs are typically made from materials such as nickel, aluminium and stainless steel. Where aggressive conditions are required, more exotic materials such as Monel, Inconel, Hastelloy, Titanium and even Tantalum can be used,” he says.

In almost all cases, “reverse buckling” rupture discs are used because they outperform the alternatives with respect to service life.

In a reverse buckling design, the dome of the rupture disc is inverted toward the pressure source. Burst pressure is accurately controlled by a combination of material properties and the shape of the domed structure. By loading the reverse buckling disc in compression, it can resist operating pressures up to 95% of minimum burst pressure even under pressure cycling or pulsating conditions. The result is greater longevity, accuracy, and reliability over time.

“The process industry has relied on reverse buckling discs for decades. Now the technology is available to oil and gas OEMs in miniature form. Until recently, obtaining discs of that size and performance was impossible,” says Brazier.

However, miniaturisation of reverse buckling technology presents its own unique challenges. 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. In this type of design, a line of weakness is also typically placed into the rupture disc structure to define a specific opening flow area when the reverse type disc activates and also prevents fragmentation of the disc “petal’.

“Reverse buckling and therefore having the material in compression does a few things. Firstly, the cyclability is much greater. Secondly, it allows you to obtain a lower burst pressure from thicker materials, which contributes to enhanced accuracy as well as durability,” says Brazier.

Small nominal size rupture discs are sensitive to the detailed characteristics of the orifice through which they burst. This requires strict control of normal variations in the disc holder. “With small size pressure relief devices, the influence of every feature of both the rupture disc and its holder is amplified,” explains Brazier. “With the correct design of the holder and the correct rupture disc selection, the customer’s expectations will be achieved and exceeded.”

Due to cost, weight and other considerations, Brazier says that BS&B has increasingly received more requests for housings that are made out of plastics and composites.

Because customers are often accustomed to certain types of fittings to integrate into a piping scheme, different connections can be used on the housing. Threading is popular, but BS&B is increasingly using several other connection types to attach the rupture disc assembly to the application. Once the integral assembly leaves the factory, the goal is that the set pressure cannot be altered.

“If you rely on someone to put a loose disc in a system and then capture it by threading over the top of it, unless they follow the installation instructions and apply the correct torque value, there is still potential for a leak or the disk may not activate at the designed burst pressure,” explains Brazier. “When welded into an assembly, the rupture disc is intrinsically leak tight and the set-burst pressure fixed.”

Although OEMs have long relied on rupture discs in their hydraulic and pneumatic equipment, high-pressure, high-cycling environments have been particularly challenging. Fortunately, with the availability of integrated, miniaturised rupture disc solutions tailored to the application in a variety of standard and exotic materials, OEMs can greatly enhance equipment safety, compliance and reliability even in extreme work conditions. 

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