AESSEAL®, the global specialist in the design and manufacture of mechanical seals, bearing protectors and seal auxiliary support systems, has introduced an innovative shorter API pump sealing solution for legacy pumps in the oil and gas industry that need to upgrade to dual seal technology. Shane Chester, AESSEAL’s MD in South Africa, explains.
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The traditional way of sealing legacy pumps was to use gland packing, which consists of braided rope-like material that is packed around the shaft in a ‘stuffing box’, with the packing forced into the gap between the shaft and the pump housing. Gland packing is still commonly used in many applications, including legacy oil and gas installations, but increasingly, pump users have replaced these with mechanical seals for the following reasons:
- The contact friction between the rotating shaft and the packing wears away the braided seal material over time, which leads to increased leakage until the packing is adjusted or replaced.
- The friction of the shaft means that the packing needs to be flushed with large volumes of water in order to keep it cool. This has become increasingly unacceptable.
- The packing needs to press against the shaft to reduce leakage, which means the pump needs more power to turn the shaft, making pumping more inefficient.
- The packing in contact with the shaft will eventually wear a groove into it, and shafts are costly to repair or replace.
Mechanical seals, on the other hand, are not only far more effective in preventing leakage between the shaft and the pump casing, they are also far safer. Particularly in the oil and gas industry, product leakage poses potentially catastrophic risks such as fire and explosions, along with expensive contamination and adverse health and environmental effects. In high risk applications such as process streams containing acute toxicity, aspiration hazards or flammable liquids, mechanical seals of increasing sophistication such as dual mechanical seals have to be used to minimise the potential dangers and costs of product leakage.
Mechanical seals generally contain three sealing points. The stationary part of the seal is fitted to the pump housing with a static seal, typically with an O-ring or gasket clamped between the two connecting components. Similarly, the rotary portion of the seal is sealed to the shaft, usually with an O-ring. This seal can also be regarded as static as it always rotates with the shaft.
The mechanical seal itself is the interface between the static and rotary portions of the seal. One side is static, while the other rotates with the shaft. They are both resiliently mounted and spring loaded to accommodate any small shaft deflections, shaft movement due to bearing tolerances and out-of-perpendicular alignment due to manufacturing tolerances.
This primary seal is essentially a spring loaded vertical bearing consisting of two extremely flat faces, one fixed and one rotating, running against each other. The seal faces are pushed together using a combination of hydraulic force from the sealed fluid and spring force from the seal design. This forms a seal that prevents process fluids from leaking from the stationary areas inside the pump to the outside of the rotating shaft.
The surfaces of the seal faces are ‘super-lapped’ to a high degree of flatness, typically 2-3 helium light-bands or 0.8 µm.
If the seal faces rotated in contact with each other without some form of lubrication, however, they would wear and quickly fail due to face friction and heat generation. For this reason a fluid film of lubrication is required between the rotary and stationary seal faces. This film can either come from the process fluid being pumped or from an external seal-fluid circuit.
By maintaining a precise gap between the faces – large enough to allow small amounts of clean lubricating liquid in, but small enough to prevent contaminants from entering the gap between the seal faces – a perfect balance between protecting the seal and preventing leakage can be achieved. The gap is so tiny that particles that would otherwise damage the seal faces are unable to enter, while the amount of liquid that leaks through this space is so small that it vaporises on exit, typically at a rate of around half a teaspoon per day of pumping.
In summary mechanical seals offer:
- No visible leaking. Mechanical seals leak a tiny amount of vapour as the fluid film reaches the atmospheric side of the seal faces. If captured and condensed, this would approximate to ½ teaspoon a day at normal operating pressures and temperatures.
- Modern cartridge seal designs do not damage the pump shaft or sleeve in any way.
- Day to day maintenance is reduced as seals have inboard springs, which make them self-adjusting as the faces wear.
- These seals have very lightly loaded faces that consume less power than gland packing.
- Bearing contamination is reduced in normal operation as the lubricant is not affected by seal leakage and wash out.
- Plant equipment also suffers less from corrosion because the liquid product is contained in the pump.
- Less wasted product means more money saved. Even water is an expensive commodity and less clean-up of the area will be needed.
In addition, of course, there are the massively reduced safety risks associated with the better sealing performance of mechanical seals, which are further enhanced by the use of dual mechanical seals.
Dual mechanical seal upgrades for the oil and gas industry
Many of the developed world’s refinery and petrochemical processing facilities were built over 30 years ago and the dependability of the legacy pumps being used, coupled with their high cost, often restrict their replacement. Many of these have been upgraded to use single mechanical seals, but these no longer meet current safety requirements. Upgrading to modern sealing devices that incorporate a double seal arrangement – such as the API 682 compliant dual mechanical sealing solution – has to be implemented to avoid wholescale pump replacements or upgrades.
The physical space required for many of today’s API compliant dual mechanical seals does not lend itself to their installation on old pumps with small seal chambers (stuffing boxes). In addition, the cost and inconvenience of pump modification or replacement in order to accommodate traditional API 682 dual cartridge seal designs can be considerable.
AESSEAL’s CAPI-TXS™ is an innovative solution. This dual cartridge mechanical seal has been designed specifically for legacy pumps found in abundance in the oil and gas industry. Using API 682-qualified componentry in a compact modern design has enabled the CAPI cartridge seal to fit almost all pumps without modification to the seal chamber. Key advantages include:
- The CAPI-TXS reduces lifecycle costs, upgrade costs and lead times.
- It enables users to extend the useful life of their older process pumps while simultaneously meeting 21st century safety and emissions requirements.
- The compact dual mechanical seal fits most legacy process pumps without the need to modify the pump casing, which significantly reduces upgrade costs.
- An upgrade using the CAPI-TXS also means the large capital costs associated with replacing the pump or fitting a ‘back pull out’ unit are completely avoided.
- Project execution can be both swift and efficient because a CAPI-TXS upgrade can be performed during the regular maintenance cycle. It is a simple matter of replacing the existing seal.
AESSEAL reinvests over 7% of its annual sales in R&D, and has done so for several decades. This has led to one of the most advanced ranges of sealing technology in the world. The company has long and established expertise in supplying the oil and gas industry with leading edge sealing solutions and has several thousand designs installed as retrofits to legacy pumps.
These upgrade solutions are cost effective, simple to fit, generally do not require modification to the pump and in many cases can be performed during the routine maintenance planning cycle. “Our CAPI range of API seals has been qualification tested to API 682, so customers can be confident that they are buying the very best,” concludes Chester.
