Pratley’s new range of flameproof cable glands allows for soft-bedding cables like PVC and others which are prone to creep, to be fitted to flameproof equipment without any concerns about cable damage or poor long-term sealing.
The Pratley range of Taper-Tech® flameproof glands consists of four different glands in two categories. First, the Pratley Taper-Tech® gland for armoured and braided cable is a ‘double compression’ gland, meaning it has both an inner and an outer seal. Second, the Pratley Taper-Tech® single-seal flameproof gland, also for armoured and braided cable, has 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. This is designed to be fitted with flexible hosing – for example, where cables are fitted to mining machines – to protect the cable outer sheath.
The unique feature of this range of flameproof cable glands is that all versions are fitted with Pratley Taper-Tech® flame seal technology, which consists of tapered seals made from superior, Pratley developed, 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. “In other words, less torque is needed to reduce the diameter of the bush,” Sven Breedt, Electrical Research & Development Manager at Pratley explains.
One of the benefits of this 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 from PVC, which exhibits ‘cold flow’, that is, the relaxation of the cable bedding over time. “This can be dangerous for the flameproof sealing properties of the termination as well as the secureness of the cable connection to the equipment,” Breedt says.
The new range of flameproof Taper-Tech® cable glands.
Flameproof cable glands fitted to flameproof equipment are intended to seal against extremely high explosion pressures which, in many cases, can 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. Even if you increase the pressure to 40 or 50 bar, the seal will accommodate that pressure by further closing onto the cable. Once that pressure is released, the bush will relax again, and reduce the contact pressure on the cable,” Breedt explains.
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 it is needed.
The same technology is amplified in the cable gland for unarmoured cable, which has to seal against high explosion pressures and retain the cable axially against pull-out forces. “This is achieved in a similar way, but the rear outer seal has a taper which is in the direction of the cable pull-force. This too increases proportionally to the pull-force on the cable, so high contact pressures are only present on the cable when it is being pulled on. Taper-Tech® completely negates cable damage usually experienced with subpar weak-back seals, and complies 100% with the IECEx flameproof cable gland requirements,” Breedt says.
For more information visit: www.pratleyelectrical.com
