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In this article, we talk about the value of a system curve, or system resistance curve, to understand how the elements of a fluid piping system operate together.
System curves graphically determine flows and pressures in process systems where there is typically one supply pressure or tank, a centrifugal pump, a control valve, and one destination tank or pressure. Using this graphical approach, system curves can visually determine:
In Part 1 of our Pump System Curve Series, we’ll show you how to create a system curve, calculate pipe head loss, generate a system curve with multiple pipelines, and the value of a system curve.. In the future, we’ll discuss means of controlling the flow rate, along with the limitations of a system curve, and explore ways to see how to visualize more complex piping systems.
The first step in developing a system curve is to develop system curves for each loss element found in a piping system. These elements include pipelines and components such as filters and heat exchangers. These elements are typically represented by a second-order equation based on the flow rate. Equation 1 shows the basic equation:
The Darcy equation (Equation 2) is used to easily calculate the pipe head loss in a single pipeline for a specific flow rate.
hL = head loss in ft of fluid
f = Darcy friction factor
L = pipe length
D = pipe diameter
v = fluid velocity
g = gravitational constant
By performing multiple head loss calculations for a range of expected flow rates, a curve can be developed showing the pipeline head loss for any flow rate within a defined range. See the Figure 1 below:
Figure 1. The pipeline system curve is generated by calculating the head loss in a pipeline for a variety of flow rates and graphing the results.
A single pipeline head loss graph can be used to easily determine the head loss for a given flow rate. For example with a flow rate of 240 gpm through the pipeline there is a 12 ft head loss. Using this graphical approach one can easily determine the head loss for a given flow rate without having to resort to calculations.
There is no such thing as a free lunch, because the multiple head loss calculations for the various flow rates still must be performed in order to develop the curve.
Determining the flow rate from a given head loss using the Darcy equation is a much more difficult calculation. That is because both the head loss and the Darcy friction factor are a function of the velocity of the fluid through the pipeline. Since one cannot isolate the fluid velocity term in the Darcy equation, the equation must be solved using an iterative approach. In other words a guess must be made at a flow rate in order to calculate the head loss. If the head loss for the guessed flow rate does not match the desired value, the guessed flow rate must be adjusted and the calculations performed again. This process is repeated until the head loss for the improved guessed flow rate equals the desired head loss value. This approach requires multiple calculations of the Darcy equation.
Instead of performing iterative calculations the pipeline system curve can be used to graphically determine the flow rate. For example to determine the flow rate through the pipeline graphed in figure 1 that results in a 15 ft head loss, you can enter the curve on the head axis at 15 ft and proceed horizontally across graph until you intersect the system curve. In this case the flow rate of 260 gpm provides results for a 15 ft head loss in the pipeline.
Components such as filters, heat exchangers, and orifices, have a similar second order equation to that of a pipeline.
Since a system is made of multiple pipelines, the next step is to see how to generate a system curve with multiple pipelines in a series. When the multiple pipelines (and components) are placed end to end, the flow rate through each pipeline is identical, so one can determine the head loss from multiple pipelines in series by adding the head loss for each pipeline. By graphing the system curves for each pipeline, the head losses for each pipeline can be added for a range of flow rates. The resulting curve shows the total head loss for all pipelines in series. Figure 2 show a composite curve of three pipelines in series.
Figure 2 – The head loss for a given flow rate or head loss can be determined from the pipeline system curve. The graph shows clearly the total head loss across the pipelines with a 280 gpm flow rate.
The performance of the pump, its head and flow rate, is shown on the manufacturers supplied pump curve. By superimposing the manufacturer’s pump curve on a pipeline resistance curve, a pump system curve or system resistance curve is identified. The value of the pump system curve is it shows graphically how the pump and system interact. For example the flow rate through the system occurs at the intersection of the pump curve and the pipeline resistance curves.
Figure 3 shows a typical system resistance curve, from which it can be easily determined that the intersection of the pump and system curve occurs at 403 gpm with a head of 70 ft. If there is a need to adjust the flow rate to a value other than the 403 gpm, then either the shape of the pump curve or system curve must be adjusted to achieve a different intersection point.
Figure 3- By inserting the pump curve (in red) and pipeline resistance curves (in blue) on the same graph, the flow rate through the system can be easily determined at the intersection of the two curves.
The adjustments in the flow rate (intersection point) through the system can be accomplished by:
In future, we’ll explore these various options and discuss how the flow rate can be controlled in a piping system using the system resistance curve. In addition, we’ll discuss the advantages and disadvantages of each method presented.
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