2. Different H-Q curve shape:
When the replacement pump is a different model type, speed, and/or has a different specific speed (September 2009 column), the shape of the new pump H-Q curve could be either steeper or flatter than the old pump. The higher the specific speed the higher the shut-off head, and steeper the H-Q curve, while the lower the specific speed the flatter the H-Q curve, which could droop toward shut-off. If the shape of the replacement pump H-Q curve is flatter or steeper than that of the old pump, any system operation at higher or lower than the design system head point will yield a higher or lower than expected flow rate. This means that, in order to avoid an unpleasant surprise, the impact of any change in H-Q curve shape on the full system performance range should be fully evaluated ahead of a new pump purchase.
3. NPSHR too high:
If the replacement pump NPSHR (requirement based on a 3% head drop) is higher than that of the old pump (over any portion of the operational flow region), it could cause the system NPSHa (available) to be less than the new pump NPSHR. This could result in a throttling of the pump flow rate, should changes in the system curve push the pump further out on its H-Q curve, as shown in Figure 2. To avoid this cavitation caused flow loss (when ordering a replacement pump), make certain that, not only does the new pump meet the H-Q curve of the old pump, but that the replacement pump NPSHR is equal to or less than that of the old pump, over the full actual flow range. However, selecting a new pump with a lower NPSHR than the old pump could create a different problem, if it results in a “High Suction Energy” pump. Care should, therefore, be taken to insure that the “Suction Energy” of the new pump has not been moved into the “High of Very High Suction Energy” regions, in order to avoid possible cavitation damage, see October 2008 column.
4. Actual system H-Q curve not known:
The actual current system H-Q curve may be different than the original system design. Once a plant is commissioned and the plant is put in service, the system head begins to change. In the short term, levels change in the tanks and wells, valves open and close, and filter screens become clogged. As maintenance occurs, pipe schedules are changed, equipment is changed and new equipment is added into the system. In the long term, equipment loses efficiency, scale forms on the internal pipe walls and the plant undergoes expansion and contraction. Even when new, the original calculated system curve may differ from the actual system performance due to the assumptions used in the calculation, such as 10 year old pipe. Any pump change should, therefore, start with the development (confirmation) of the true current pumping system “Head-Capacity” curve, as detailed in the writer’s January 2009 Column on: “Creating an Accurate Pumping System Head-Capacity Curve...“ A field test of the pump total developed head at one or more measured flow rates can help determine the actual (current) pump and system H-Q curves. By developing the true system head-capacity curve, an accurate determination of the current and new pump operating conditions can be established.
5. Different local pump pipe fittings:
