 |
|
June 2004Subscribe Share Feedback Unsubscribe www.lawrencepumps.com |
Welcome to the inaugural issue of RUN TIMES! Our goal is to provide readers with pump application and performance insights that will assist in improving operations and saving money. If you have a comment please share it with us. If you like our newsletter, please share it with a friend! Performance loss and premature outages due to worn sealing (wear) rings are costly events and often unnecessary. In this issue we'll address the advantages and disadvantages of sealing rings vs. pump-out vanes when pumping abrasive mixtures.
Dale B. Andrews - Editor Speed Kills is a saying as applicable to slurry pumps as it is to cars. Wear in a slurry pump increases as Vn where "V" is the particle velocity and 'n' is about 2.5. The primary high wear areas in a centrifugal pump are the volute cutwater, the wear rings, impeller balance holes, and the impeller vane tips.
In this issue I want to talk about pump-out vanes and how they out-perform wear rings and balance holes under abrasive conditions. Wear rings are used to reduce leakage from the impeller discharge back to the impeller eye. They form an area of tight clearance between the casing and the impeller that restricts the leakage flow. Balance holes are used to reduce axial thrust by relieving pressure on the back side of the impeller back to the impeller eye. Both wear rings and balance holes behave like orifices: they use a restriction to restrict the flow that in turn increases the flow velocity. Putting your thumb over the end of a garden hose creates a restriction that has a similar velocity effect.
Slow is the key word here. Whereas wear rings increase the fluid velocity, pump-out vanes reduce it. Pump-out vanes also reduce the pressure on the back side of the impeller, thereby reducing axial thrust and eliminating the need for balance holes.
With a relative fluid velocity of 2.5 times that of the pump-out vanes, wear rings of similar materials will wear at a rate equal to 2.52.5 = 10 times faster than pump-out vanes. If hard facings or diffusion coatings are applied to the wear rings, wear life will improve, but pump-out vanes will still usually outlast wear rings by a factor of 2-3 times.
The bottom line is that wear rings are great for clean liquids, but if abrasives are a problem for you, pump-out vanes should be seriously considered to save unnecessary downtime and expense. |
Even a properly designed pump, using the best of materials, will ultimately lose performance due to impeller vane wear. Cutwater life can be managed by design and materials, but this is a topic for future newsletter.
Pump-out vanes are small impeller-like vanes located on the outside of the front and back impeller shrouds. They create an outward pumping action to slow the recirculation between the impeller discharge and eye, and to reduce the pressure on the back side of the impeller.
This chart shows the relative velocity (the velocity you would see if you were riding on the impeller) for two pumps of similar size. One pump has wear rings, the other has pump-out vanes. Not only is the wear ring velocity 2-1/2 times as high as the pump-out vanes, but the velocity in the wear ring will actually increase as wear occurs due to a reduction in frictional losses.
The trade-off in using a slurry impeller with pump-out vanes is efficiency. A new process pump with wear rings can have five to ten points more efficiency than an equivalent slurry impeller. This is because pump-out vanes use horsepower to control leakage and wear rings do not. But how important is this in an abrasive slurry environment? A 100 hp slurry pump with a five point efficiency deficit will consume $1,500 more in electricity each year @ $0.05/Kwh. This becomes less significant in comparison to an extra $5,000 to $10,000 pump repair bill associated with a wear ring failure.