STEP 1: The basic pump curves are no different than reading any other head - flow curve. For a known head value, follow the head over to the pump curve then drop down to the capacity axis and this will be the flow rate. What you are trying to figure out here is what diameter impeller is needed to get the required head and capacity.

STEP 2: The next thing to figure out is what motor is needed to drive this impeller without overloading. To do this use the dashed horsepower lines. To the right of the horsepower line is overloading and to the left is non-overloading.

STEP 3: The last thing to determine is at what pump efficiency the pump will operate. Look at the U-shaped lines and interpolate to get the efficiency.

Now let's try an example using ZM1570, Performance Data for Models 6650-6671 (5-15 BHp 4" discharge units). For the example we will size a pump for 400 GPM at 54 feet of total dynamic head.

STEP 1: Locate the point of 400 GPM at 54 feet on the pump curve. This point is slightly above the 8.31" impeller but well below the 8.63" impeller so I would go with an 8.38" impeller to hit the duty point.

STEP 2: Next, draw a new pump curve that passes through the duty point and is parallel to the existing pump curves. This will give you a close representation of the actual performance the pump will deliver. Look to see where this curve crosses the horsepower line to the right of the design point. In this example the pump curve crosses the 10 BHp curve at about 48 feet and crosses the 12.5 BHp curve at about 21 feet. We will not oversize an impeller on a pump if the overload point on the pump curve is greater than the static head for the system. So for this example, if the static head is greater than 48 feet then we can use the 10 BHp unit. If the

static head is between 21 feet and 48 feet, use the 12.5 BHp motor. If the static head is less than 21 feet then use the 15 BHp motor.

STEP 3: Now let's figure the pump efficiency we can expect. The design point is about half way in between the efficiency lines of 60% and 63%. So, for the design point of 400 GPM at 54 feet, we would expect about 61.5% pump efficiency.

As you can tell from the above example, we would consider oversizing an impeller on a unit and not overload the unit due to engineering the right pump for the system. If this were the case we would also able to provide a more competitively priced unit since pricing is based on motor size (i.e. smaller motors cost less). The only exception to this rule is a single-phase unit. ZOELLER COMPANY DOES NOT SELL SINGLE-PHASE UNITS WITH OVERSIZED IMPELLERS because we feel that this will compromise the life of a single-phase unit.