Wear rings

Wear rings provide for a close running, renewable clearance, which reduces the amount of liquid leaking from the high pressure zones to the low pressure zones in the pump. They are commonly fitted in the pump casing and on the impeller (Figure 16-1, next page).

These wear rings are lubricated with the fluid being pumped. Eventually they will wear. Tolerances open and more liquid passes from the discharge end back to the suction end of the pump. The rate of wear is a function of the pumped liquid's lubricity. When the wear is excessive, the pump suffers degradation in its performance. This is particularly true with small pumps running at high speed. The strict tolerance in the replaceable wear rings governs the efficiency of the pump. When the pump goes to the shop, these wear rings should be changed.

You can expect the pump to loose 1.5 to 2% efficiency points for each one thousandths (0.001 inch) wear in a wear ring beyond the original

Stationary Casing Ring

Stationary Casing Ring

Strict Tolerance |

Impeller Wear Ring

Strict Tolerance |

Impeller Wear Ring

Impeller /

Figure 16-1

factory setting. This setting is based on the operating temperature of the application. Let's consider how much money the lost efficiency costs. We will use some formulas from Chapter 5 in this book on useful work and efficiency.

F " Eff. Pump x Eff. Motor where: 0.000189 = Conversion factor GPM = Gallons per Minute TDH = Total Dynamic Head $Kwh = Cost per Kilowatt-hour sp.gr. = Specific Gravity 8,760 = Hours in a year Eff. Pump = Pump Efficiency Eff. Motor = Motor Efficiency

To show cost increase, consider this newly installed pump in a properly designed system. We have the following values:

GPM = 2,000 gpm TDH = 120 ft. $Kwh = $ 0.10 sp.gr. = 1 (water) Eff. Pump = 77% Eff. Motor = 93%

The electricity cost to run this pump for a year is $55,450.80.

After being in line for six months, this pump is disassembled and it is noted that the tolerance in the wear bands has opened 0.004 inch from the original factory setting. This wear represents an 8% decrease in efficiency. Now the pump is 69% efficient. Let's do the math with all other factors constant. This reduction in the efficicncy represents an annual electricity cost of $61,845.60. The additional electricity is six thousand three hundred ninety four dollars and eighty cents. Four thousandths wear (0.004 inch) has cost us almost $6,500.00 per year for just one pump. Just to mention, a new wear ring may cost up to $60.00 plus the labor to change it (this will never add up to $6,500.00).

Effective and well planned maintenance can reduce the operating cost of your pumps and other equipment as this example demonstrates. With differential pressure gauges on the pump, an amp meter and flow meter you can determine if strict tolcrance parts are worn. This indicates the need to take the pump into the shop for corrective procedures. If you don't do it, you are wasting your annual operating budget. As we mentioned in Chapter 6, the Wear Rings should be called Efficiency Rings. Now you know why.

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