Eldon Kruger discusses the role of flameproof cable glands in the oil and gas industry
Cable glands for the oil and gas sector are deployed in some of the most challenging environments imaginable. They play a key role in minimising explosion risk for operators and their significance should not be underestimated.
A recently introduced flameproof cable gland range from an industry expert allows for soft-bedding cables such as PVC – which are prone to creep – to be fitted to flameproof equipment without any concerns regarding cable damage or poor long-term sealing.
The Taper-Tech flameproof gland range from Pratley consists of four separate glands. First is a gland for armoured and braided cable. This is a ‘double compression’ gland, meaning it has both an inner and an outer seal. The second is a single-seal flameproof gland, also for armoured and braided cable, with a durable rubber shroud. The range also includes two ‘double compression’ glands for unarmoured cable, one being a hose-tail version that features a spigot on the compression nut.
The novel feature of this range is that all versions are fitted with Taper-Tech flame seal technology, consisting of tapered seals made from high temperature and low compression-set elastomers. The seals all have a taper on the leading edge that abuts against the gland nipple taper on the inside. The taper on the bush and the one on the cable gland’s nipple are angled differently.
This means that, under thrust or pressure from the front, when it is tightened, this differential taper allows the bush to slide down the taper easily, reducing the force needed to achieve radial compression.
A major benefit of the technology is low initial radial pressure or a small amount of contact pressure on the cable. This is important because most cables in the field are made of PVC, which exhibits ‘cold flow’, meaning relaxation of the cable bedding over time. It can be dangerous for both the flameproof sealing properties of the termination and the secureness of the cable to the equipment.
The Issue Of Pressure
Flameproof cable glands fitted to flameproof equipment are intended to seal against extremely high explosion pressures, which may be as high as 30 bar. For the seal to function properly, there must be adequate pressure between the seal and the gland bedding.
Imagine the explosive gases travelling down the cable and pressing against this seal. This seal is now being pressed on its taper which, due to the taper action, forces the bush to press down onto the cable, increasing the seal force. As the pressure rises, the contact pressure between the bush and the cable increases proportionally.
As the leading edge of the seal forms a taper in the direction of the applied explosive pressure, when this applied pressure increases, so too does the contact radial seal pressure on the cable bedding. Once the pressure is relieved, the bush returns to its initial low-pressure state, so the high seal pressure is only on the cable when needed.
The same technology is amplified in the cable gland for unarmoured cable, which not only has to seal against high explosion pressures, but also retain the cable axially against pull-out forces.
The rear outer seal has a taper that is in the direction of the cable pull-force. This, too, increases proportionally to the pull-force on the cable, meaning high-contact pressures are only present on the cable when it is being pulled on. Taper-Tech completely negates the cable damage that is frequently experienced with subpar weak-back seals, while complying 100% with the stringent IECEx flameproof cable gland requirements.
Eldon Kruger is with Pratley Electrical
