Pumps are found on every mining and minerals processing site and on just about every one of these sites the engineering and maintenance teams report pump problems. Here are 10 practical tips which could help avoid pump down-time:
1. Suction valves
An isolation valve should obviously always be fitted so the pump can be isolated from the supply for maintenance purposes. However, the choice of valve is critical. Always use a low head-loss (i.e. “high recovery”) valve such as a knife-gate, ball or butterfly valve on the suction side. Never use a high head-loss valve such as a globe valve as you are much more likely to cause pump cavitation which will lead to accelerated wear and reduced pump performance. The final choice of valve depends on solids concentration, solids particle size/shape and materials compatibility issues.
2. Pressure tapping points
A tapping point for a pressure gauge or transmitter should always be located at the pump discharge for critical process pumps. Ideally a gauge should also be located on the pump suction for high head pumps. The difference between the pump discharge pressure and the pump suction pressure will give you the approximate total dynamic head of the pump (i.e. when pressure in kPa is converted back into metres of liquid). If the discharge pressure or pressure difference drops to an unacceptable level over a period of time, maintenance action can be planned and the pump can be restored to its original output.
3. The use of non-return valves
One should always be employed at the pump discharge to: (i) prevent damage to the pump by reverse rotation when high static heads are involved or (ii) to prevent back-flow through the pump when parallel pump setups are used, or (iii) to prevent back-flow when pumping into other pressurised lines or pressure vessels. Not all non-return valves are the same so the final choice depends on solids concentration, solids particle size/shape, materials compatibility and whether “slamming” will occur due to column separation due to pipeline transients. Consult your valve supplier for selection advice.
4. Ideal centrifugal pump operating point
Always run a centrifugal pump as close to the Best Efficiency Point (BEP) as possible so that radial thrust is reduced and power usage is optimised. However, some centrifugal slurry pumps are best operated at 80 percent of BEP so that a compromise between radial thrust, power usage and wear rate is found. It is best to check with your pump supplier to find the best range of operation but typically, if you run at less than 50 percent of the BEP flow rate or more than 120 percent of the BEP flow rate your centrifugal pump will experience excessive radial thrust leading to bearing, seal, wear ring and impeller failure/wear issues.
5. Pump-drive alignment
Always check the alignment of a pump-drive combination prior to start-up for the first time after installation (even if the unit has been supplied on a common baseplate). Misalignment will probably have occurred during prior transit and handling. Expect seals and bearings to fail prematurely if you don’t adhere to this tip.
6. Mechanical seals and runout
Mechanical seals cannot tolerate shaft runout whereas gland packing can. If a mechanical seal has failed prematurely, check the shaft runout prior to installing another seal. If runout is excessive, replace the offending bearings prior to installing a new seal.
7. Pressure relief and bursting discs
Positive displacement pumps keep increasing pressure in a “closed system” until some piping component or pump housing ruptures. Pressure relief valves or bursting discs should be used to prevent this. Use a pressure relief valve unless the liquid being pumped can block, plug or damage the valve otherwise use a bursting disc.
8. Gland water pressure
As a rule-of-thumb, if gland water is to be used it should be supplied to the stuffing box at 70 to 100 kPa higher than the highest expected pump discharge pressure. This will maximise your chances of eliminating pumped fluid from the stuffing box thereby increasing seal or packing life.
9. Minimising gland water usage
Gland packing will give you a perfectly acceptable wear life if you ensure that leakage occurs at a rate of one to two drops per second. Any more than this is unnecessary. For critical pumps, “set-and-forget” flow control valves such as those supplied by Maric Flow Control P/L are the best option. Alternatively, injectable type packings which require no gland water can be used for low solids concentration pumpage.
10. Issues with variable speed drives
Positive displacement pumps don’t experience any major issues when run via variable speed drives unless the pump is run at low speed which may cause motor overheating. If this is the case, an independent cooling fan can be installed on the motor. A centrifugal pump should only be run in its recommended head versus flow range otherwise radial thrust issues will develop leading to premature failures or reduced performance.
KASA Redberg Pty Ltd