May 2021, Vol. 248, No. 5
Features
Enhancing Safety with Integrated Rupture Disk Assemblies
Special to Pipeline & Gas Journal
For decades, rupture disks have served the oil and gas industry as an effective, passive safety mechanism to protect against overpressure or potentially damaging vacuum conditions.
The disk, which is a one-time-use membrane made of various metals, including exotic alloys, is designed to activate within milliseconds when a predetermined differential pressure is achieved.
Because oil and gas equipment operational reliability is essential, high integrity from the pressure relief technology used is critical to protect low-and high-pressure original equipment manufacturer (OEM) systems.
As a result, oil and gas OEMs are turning to integrated rupture disk assemblies with all components combined by the manufacturer, as opposed to loose-rupture disk and holder devices that leave much to chance. These assemblies are being tailored to the application, miniaturized and available in a wide range of standard and exotic materials, as required.
This approach ensures the rupture disk device performs as expected, enhancing equipment safety, reliability and longevity while simplifying installation and replacement.
The oil and gas industry uses rupture disks 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 the flushing fluid, as well as the nature of the conveying drilling fluid. Although application-specific, hydraulic operating pressures are typically in the range of 5,000 to 20,000 psi (345 to 1,379 bar).
“A three-plunger pump is continuously cycling, so the disk must be able to withstand high pressures with 1 million pressure cycles or more,” said Geof Brazier, managing director of BS&B Safety Systems, Custom Engineered Products Division.
In the oil and gas industries, which depend on hydraulic systems to store energy and smooth out pulsations, standard system components like accumulators require rupture disks. By definition, accumulators hold hydraulic fluid under pressure. If the pressure spikes too high, without a rupture disk, there is a risk that the system, or even accumulator, could experience a catastrophic failure.
Similarly, rupture disk use is also important for safety when used with pneumatic pumps in oil and natural gas production. Pneumatic pumps are commonly used for chemical injection to limit processing problems and protect equipment.
Separate vs. Integrated
Traditionally, rupture disks began as standalone components and are combined with the oil and gas manufacturer’s separate holder device at the point of use. The installation actions of the user contribute significantly to the function of the rupture disk device.
When installed improperly, the rupture disk may not burst at the expected set pressure. There is a delicate balance between the rupture disk 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 disk 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 disk is activated.
The assembly includes the rupture disk and housing and is custom-engineered to work with the user’s desired interface to the pressurized equipment. The devices are typically threaded or flanged, or even configured for industry-specific connections.
The rupture disk 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 solutions to replace an activated rupture disk to save a few dollars or rush equipment back online.
“Oil and gas OEMs are driven to deliver the longest life and lowest cost of ownership to their customers, said Brazier. “The use of an integral assembly maximizes the longevity, proper function and trouble-free service of the pressure relief technology.”
Integrated Assemblies
According to Brazier, the most important considerations in oil and gas rupture disk 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 oil and gas OEM involves selecting the right disk 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 disks, the devices can be manufactured from metals and alloys such as stainless steel, nickel, Monel, Inconel and Hastelloy.
“Where economics is the driver, reverse buckling disks are typically made from materials such as nickel, aluminum and stainless steel. Where aggressive conditions are required, more exotic materials like Monel, Inconel, Hastelloy, titanium and even tantalum can be used,” he said.
In almost all cases, “reverse buckling” rupture disks are utilized because they outperform the alternatives with respect to service life.
In a reverse buckling design, the dome of the rupture disk 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 disk 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 disks for decades. Now the technology is available to oil and gas OEMs in miniature form as small as 1/8-inch (3.175-mm) burst diameter from BS&B. Until recently, obtaining disks of that size and performance was impossible,” says Brazier.
However, miniaturization of reverse buckling technology presents its own unique challenges. To resolve this issue, BS&B created novel structures that control the reversal of the rupture disk to always activate in a predictable manner.
In this type of design, a line of weakness is also typically placed into the rupture disk structure to define a specific opening flow area when the reverse-type disk activates and prevents fragmentation of the disk “petal.”
“Reverse buckling and, therefore, having the material in compression does a few things. Number one, the cyclability is much greater. Second, 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-sized rupture disks are sensitive to the detailed characteristics of the orifice through which they burst. This requires strict control of normal variations in the disk holder.
“With small-sized pressure relief devices, the influence of every feature of both the rupture disk and its holder is amplified,” explains Brazier. “With the correct design of the holder and the correct rupture disk selection, the customer’s expectations will be achieved and exceeded.”
Due to cost, weight and other considerations, Brazier says that BS&B has received more requests for housings that are made 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 increasingly several other connection types are being used to attach the rupture disk 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 disk 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,” explained Brazier. “When welded into an assembly, the rupture disk is intrinsically leak-tight and the set-burst pressure fixed.”
While oil and gas OEMs have long relied on rupture disks in their hydraulic and pneumatic equipment, high-pressure, high-cycling environments have been particularly challenging.
Fortunately, with the availability of integrated, miniaturized rupture disk solutions tailored to the application in a variety of standard and exotic materials, OEMs can significantly enhance equipment safety, compliance and reliability even in extreme work conditions.
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