Centrifugal process pumps used in critical mining applications have downtime and safety implications. Pump integrity is therefore, a key factor in minimising downtime and ensuring worker safety.
Jacques Visser of Morgan Advanced Materials examines the various technologies available for ensuring pump integrity, safety and performance through the use of modern materials and designs.
Centrifugal pumps are widely utilised in the mining industry, primarily for the removal of slurry as well as for other tasks such as water removal. Any interruption can result in the undesired build-up of liquids in working areas, with the potential to compromise both process efficiency and worker safety. However, the demanding conditions in which the pumps operate expose them to build-up of pressure when suction or delivery valves become closed or operate incorrectly, or lines become blocked due to the presence of solids such as sand, grit or small pieces of rock.
Rapid increase in pressure can lead to the failure of a pump and, even worse, a pump explosion with the potential for severe injury or even death to operatives working nearby. The mining operation is consequently not only exposed to the cost of a replacement pump as well as downtime but also damage to reputation resulting from a workplace fatality, not to speak of the financial repercussions of heavy fines and risk of imprisonment for key personnel.
To help maximise pump life and minimise risk to life and property on the mine site, a variety of technologies have been developed over the years designed to shut off operation before pressure within the pump reaches dangerous levels.
For instance, current detection equipment, which is based on the premise that current drops when pump valves are closed, has questionable value in many applications. Use of pressure sensing equipment raises questions about the relationship between pressure at immediate delivery before and after valve closures or the occurrence of blockages. Additionally, any rise in pressure is only likely to be detectable at the point when the fluid starts to boil, which is too close to the point of explosion.
Temperature monitoring equipment is considered a more reliable option than pressure sensing as the temperature increases immediately when valves are closed; however, this option is again expensive as probes or thermocouples not only have to be hard-wired to the breaker but are also subject to the effects of slurry build-up, impacting on their performance and ability to provide the rapid, reliable data needed to trigger a shutdown.
Strain gauges, often used by specifiers and system designers, are not always found to be reliable, while fusible plugs are reliant on the same technology as temperature and pressure sensing and do not overcome the issues associated with these technologies. Pressure relief valves again add to cost while their presence may compromise the leak-tight design of the system and may not react quickly enough to relieve a rapid pressure build-up. Safety valves, usually a part of most pump designs, require regular testing and maintenance to ensure they will come into action at the desired pressure.
Concerns over the effectiveness and cost of these technologies, as well as the need for a truly maintenance-free solution led to the development of the first bursting discs or rupture discs. A rupture disc is a sacrificial part containing a domed membrane, which fails instantly within milliseconds at a pre-determined pressure and cannot reseal itself.
Many of the early discs were made from foil, an ideal material but also relatively delicate that made the disc susceptible to damage such as bending and scratching, when the disc was inserted into its holder. This could lead to compromising the performance of the disc, with the most likely scenario being that it will ‘burst’ at too low a pressure, resulting in unnecessary downtime and the cost of a replacement, which can be very expensive.
The issues with foil discs drove leading materials companies to seek a more cost-effective and practical solution using a more robust material, which did not require a specialist holder, and was less prone to damage during the installation process and so to premature failure resulting from accidental alterations to the disc’s shape or surface profile.
First introduced in the market in the late 1980s, graphite discs are designed to fit between the bolts within the standard ANSI flanges found on most pumps, eliminating the need for a separate holder and simplifying installation. With graphite being a harder and tougher material, the discs can withstand a certain amount of scratching with no compromise to their burst pressure, and can operate at a broad range of temperatures between -50°C and +250°C. The use of a PTFE material bonded to the disc optimises resistance to any alkalis present and ensures their presence does not impact on the disc’s service life or performance.
The flexibility and versatility of graphite allow the production of discs to very precise customer parameters. Rather than relying on calculations using pump casing pressures, burst pressure is typically calculated by taking the working pressure and adding 75 per cent, meaning a pump with a working pressure of 1 bar will require a disc with a burst pressure of 1.75 bar. Physical destructive testing of sample discs from individual batches is carried out before they leave the facility guaranteeing that the discs will operate to the agreed customer parameters, with test certificates provided to ensure that local and international safety regulations are satisfied.
The sophistication of modern manufacturing techniques even allows the production of discs that can cope with negative or vacuum pressure. Discs can be manufactured in diameters of between 0.5” and 16” in a variety of thicknesses to suit virtually any application. If fitted correctly, graphite rupture discs have an almost unlimited service life, require no interim maintenance, and can typically be changed within a matter of minutes. While they are frequently used in conjunction with a safety valve, a rupture disc on its own is more than capable of ensuring that pre-determined pressure within a pump cannot be exceeded.
New ‘centreline’ discs have been recently introduced to completely negate scratch damage issues. The versatility and cost-effectiveness of graphite discs have seen them tested against other methods specified by many of the leading international mining companies as the preferred system for protecting pumps from excess pressure build-up.