Software companies use equations to optimize simulation performance speed while maintaining nearly universal application of their products. These equations normally convert a vendor's pump curve into a fourth order quadratic equation. The pump curve approximations are accurate for most hydrocarbon liquids and for operations near the design points of the pump curves. However, these equations breakdown when they are used on highly compressible fluids, heavy crude oils, or away from the design point. This paper describes conditions where results from approximated pump curve and calculations need to be scrutinized. Solutions to deal with inaccurate pump curve approximations or calculations will also be presented.
Hydraulics engineers are constantly vying to model their pipeline systems as accurately as possible. One source of hidden errors is the pump curve. Errors can be introduced into pump curves from the following sources:
Incorrect pump curves supplied by vendors
Product flowing through the pump is different than the design fluid
Pump internals have changed over time
Modeling software oversimplifies pump behavior
Modeling software curve fitting
Centrifugal pumps will be focused on within this paper. Many of the concepts described are valid for all types of pumps. This paper aims to explain errors introduced by simulation software and vendors. Methods to minimize the errors will also be provided.
Centrifugal pumps can move through a four quadrant operation from positive head and positive flow, positive flow negative head, negative head negative flow and negative flow positive head. The relationship is graphed on the Knapp curves. Depending on the pump it can operate in any of these modes. Water pumps often operate in more than one quadrant.