AESSEAL cartridge mechanical seals work perfectly in the highly abrasive application of sand slurry transfer, but only if the correct pumps are installed at the outset. If operating parameters have not been correctly defined, and if the pump is forced to operate off its performance curve, then the pump will be inefficient, leading to possible shaft vibration, cavitation and pipe stress, all of which can lead to mechanical seal failure. It is important to understand that while the seal is vital to pump operation, it is also the weakest point in the pump. The seal is as critical to efficient pump performance as the bearing, and because it protects the bearing from contamination by fluid and slurry ingress, it must be looked after with the same level of care. Seal support systems are an essential part of this care. Without them, any mechanical seal in an abrasive environment will not last, no matter what type or make of mechanical seal it is.
At Ngqura deep-water harbour (Coega), just outside Port Elizabeth, Warman pumps in the sand by-pass system’s booster pump stations have been transferring sand-seawater slurry faultlessly for nearly four years. But this was not always the case, and through extensive fault finding and various root cause analysis, the team at Coega eventually realised that the only way forward was to fit AESSEAL’s 7” CDPH heavy-d uty double-cartridge mechanical seal and support systems.
The timeline leading to this effective sealing of the Warman 12/10 GG H W/L pump shafts was a long one; with problems in the pump stations being detected almost immediately after the sand by-pass was commissioned in 2006.
The magnitude of the problem was far-reaching and great cause for concern:
- Standard seals were designed for slurry processing and not intended for sand by-pass applications, this resulted in constant pump leakage;
- Pump leakage was so bad that the cable trenches in the pump stations would fill up with sand;
- Further, entire pump station floors would periodically be covered in sand as well;
- The leakage discoloured all the equipment (bearing assemblies, base plates, platforms and all the surrounding secondary pumps and plinths);
- Station maintenance was extensive and on-going
Before (left) AESSEAL mechanical seals were fitted: leaking booster station; and After (right) AESSEAL mechanical seals were fitted sealed booster station pump with seal support system (blue) in foreground
The Ngqura Harbour Engineers, Weir Minerals Africa and sand by-pass project consulting engineers, Prestedge Retief Dresner Wijnberg all worked together to determine the root cause of this immense problem. These specialists considered various fault finding methods in an effort to address the problem. From inspection of variable speed drives, stresses caused by the stop/start procedure, to fitting temporary steel guides to divert the sand-water slurry into the cable trenches were all part of the possible solution.
After the pumps were stripped, it was discovered that the shaft sleeves were worn as a result of all the leakage. Because of this, the decision was made to change the gland packing every 3 months and the shaft sleeves every 6 months. However, even after doing this, it only proofed to be a brief solution, and something more lasting had to be considered.
Both Weir Minerals and Prestedge Retief Dresner Wijnberg had one final, but guaranteed solution: They proposed the fitting of mechanical seals and that AESSEAL should be called in for the job. Tenders were put out in 2009 and were awarded to AESSEAL based on technical, guarantee and price considerations. AESSEAL has established a solid reputation around the world for effectively sealing Warman slurry pumps in sand slurry applications. Some customers include numerous platinum mines in Rustenburg, coal mines and a steel manufacture in the Witbank area, diamond mines in Botswana and uranium mines in Namibia as examples of installations where CDPH heavy duty double-cartridge mechanical seals have delivered effective long-term sealing of the installed slurry pumps.
AESSEAL’s product for this application, the CDPH rotary double cartridge seal, delivers the following benefits:
- Shrink-fitted faces, designed for medium to high duty slurry applications
- Multiple springs, which energize the rotary seal face, positioned out of the process media (non-clogging)
- Robust seal face drive mechanism
- Optional abrasion-resistant wetted components
- Hydraulic balanced seal faces for reduced heat generation and improved seal life
“It is absolutely the best possible cartridge mechanical seal for sand-slurry pumping applications,” Rob Adam, director of AESSEAL South Africa concluded.
The final results meant that the mechanical seals have worked faultlessly for five years in a highly abrasive environment! The results included:
- Absolutely no leakage, even at operational pressures of 550 to 600 kPa
- Achieving a transfer rate of 26 500 tons of sand per month, which is substantially more than the environmentally minimum requirement of 16 000 tons of sand per month
- The Return on Investment is easily justified by eliminating the need for new gland packing every 3 months and new shaft sleeves every 6 months
- New bearing assemblies would also have been required if these mechanical seals were not installed
Scheduled Plant Maintenance
During 2014/2015 the port authority allowed seven days for the planned pump maintenance, shutting down the booster station and trucking sand to the bypass system outlet point to maintain coastal sand flow throughout this period.
This seven-day window necessitated implementation of a carefully planned program to complete seal inspection and refurbishment in time.
As soon as the pumps were shut down, AESSEAL removed the four mechanical seals and sent them overnight to the company’s main refurbishment facility in Johannesburg, where they were inspected for wear. Three of the four seals were refurbished and pressure tested to allow AESSEAL to guarantee their performance, while the fourth was replaced with a new unit.
All four mechanical seals were re-installed in the pumps within the seven-day timeframe allowed. A certified technician from AESSEAL was on site throughout this maintenance period to ensure adherence to the schedule.
The unusually long lifespan of the seven-inch mechanical seals at this booster station have been attributed to a combination of very high product quality, correct seal support systems installed to flush the seals, and several positive operating parameters implemented by port authority management such as continuous vibration analysis, on-going bearing temperature monitoring, and reduced motor speed.