Off the Curve Definition

What Does Off the Curve Mean?

A pump that is operating “off the curve” is operating at a point that does not fall along the pump performance curve.

There are a variety of circumstances that can cause it to appear that a pump is operating at a point that does not fall along the pump’s performance curve. Some of the most common causes of a pump running “off the curve” include:

Instrumentation Calibration Issues

Probably the most common cause of alarm over a pump running “off the curve” is caused by improperly calibrated instrumentation. Before assuming there’s something wrong with the pump, make sure that the signals being sent from instrumentation associated with the pump system are being properly scaled and converted by the pump controls.

Power Supply Issues

Pump motors are designed to operate a specific voltage – such as 460v – but can operate at any voltage that falls within 10% of the design voltage – anywhere from 414v to 506v for a 460v motor.

As the voltage increases or decreases, motor speed also increases or decreases. If the actual voltage on site is 10% different from the rated voltage, then motor may be found to be operating at a considerably different speed than is expect. The affinity laws predict that is motor speed changes by just 10%, flow produced by the pump will also change by 10% and head produced will change by more than 20%.

This will make it appear that the pump is operating “off the curve” when it may be operating exactly as one would expect based on the actual operating speed of the motor due to the voltage of the power available on site.

Inadequate NPSHa At the Impeller Inlet

If NPSHa at the impeller inlet is insufficient the pump will cavitate excessively and performance may drop off precipitously. Compare the suction pressure to the pump to the NPSHr of the pump. Ensure that you are considering absolute pressure (psia, not psig) when looking at the pump suction pressure. If you find that NPSHa is very close to or more than the pump’s NPSHr find a way to increase NPSHa, or reduce the pump’s NPSHr by reducing the pump’s operating speed.

Debris Clogging One or More Impeller Vanes

If debris has entered the pump it’s possible that it may have been caught in the impeller vanes thereby reducing the flow path through the impeller. If no other culprit can be found, when the pump is disassembled carefully inspect the impeller vanes and remove any debris that is encountered.

A Problem with the Pump Itself

If the pump in question is a new pump and it did not receive a factory performance test, it’s possible that there are some irregularities in the impeller or casing casting that are causing some unintended hydraulic issues. The only way to determine if this may be going on is to eliminate all other possible causes and then have the manufacturer test the pump to determine if it is performing as intended. However, this can be a costly ordeal and if the pump is found to be within design and test standard tolerances the owner will probably end up footing the bill for a costly testing procedure.

If the pump has been in service a while, wear to the impeller, casing, and wear rings can cause the pump operating characteristics to change over time. So when dealing with an older pump, after other simpler causes have been ruled out, the best option is to open the pump, inspect it for wear, and replace worn components with new parts supplied by the original equipment manufacturer.