Consider various operating conditions and system expansions in your decision.
by Ben Keiser
February 21, 2019

After pumps are sized and selected, the work does not stop. Different system scenarios should be analyzed. Users can determine how pumps operate in conditions such as varying tank levels, changes in control valve set points, differing valve positions, parts of the system being online or offline at different times, etc. Another important reason for using the software is when the configuration changes, like during a system expansion.

For example, a refinery of an oil and gas company had recently put two new heat exchangers into service for a small expansion. A senior engineer asked upper management if they had conducted a flow analysis beforehand. Upper management had decided against it. When they opened the valves to the new heat exchangers, they received little flow because the pumps were operating at full capacity and serving the rest of the system demands.

This is an example of why it is beneficial to use flow analysis software—not only to make it easier to size and select the right pumps, but also to see if the pumps can operate under various system conditions.

If this refinery had performed a flow analysis, it would have determined well before the system was back online if the new heat exchangers would get the flow required.

What To Look For

A good flow analysis software makes it easy to account for heat transfer and the associated thermophysical property changes throughout the system.

A good software also easily handles non-Newtonian fluids and slurries. If viscous fluids are pumped, then viscosity corrections should be considered.

The software should allow a user to experiment with operating conditions and run multiple scenarios all in one file rather than several independent files that are not connected together in any way.

For example, in a system of a piping network of decent complexity with control valves and heat exchangers in addition to typical fittings and piping, the desired system flow rate is fixed through the pumps, as is an assumed efficiency. When the model is run, the total dynamic head is determined for the pumps.

After the operating point for flow rate, pump head and net positive suction head available (NPSHa) are known, a pump from a manufacturer can be selected. Once selected, the pump curve can be imported, rather than using fixed flow rates. Operators can then verify how the selected pump curve meets the original operating point.

Using flow analysis software with sophisticated scenario management capabilities, users can easily modify the system with an expansion.

To keep things simple in this example, assume that the expanded system has the same piping characteristics as the existing system. Running the model, one can validate if the system requirements will be met.

The results would show if the heat exchangers are getting the proper flow rates, where the pumps would be operating in relation to their best efficiency points (BEP), if they have any NPSH issues, or if the control valves in the system can meet their set points.

Analysis Results

With a quick analysis of the results from the system expansion, several issues can be determined that demonstrate the system will not operate as expected. Flow velocities increased significantly due to higher system flow demands. Pumps are operating significantly above the BEP. Heat exchangers receive about 30 percent less flow than that required. None of the control valves can meet their set points and would open all the way to allow as much flow through them as possible.

What do all these severe issues indicate? The pumps, as well as the piping system itself, are massively undersized for the system with the proposed expansion. It simply will not work, and therefore it is important to take the time to carry out the flow analysis. When the piping system is modeled, users can understand the changes needed in the system to get it to operate correctly.

Users might use the model of the expansion to re-size the pumps with the system fully expanded. But this would be expensive and not practical. That said, many speak about ways to “optimize” the system operation, but they only focus on the pumps. The system model would quickly show that focusing on the pumps would not provide an optimal solution. The piping system and its complicated interactions matter significantly.

Another approach can be accomplished with flow analysis software. Start with a system model that includes a proposed expansion from the beginning of the pump sizing and selection process for the original system design. Certainly, the details and thought of an expansion may not be on the table at this point. However, if the pumps are sized and selected to accommodate larger system flows early on, operators can model scenarios where the expansion flow paths are turned off and perhaps not as many pumps would be needed initially.

Variable speed drives (VSDs) could also be included to operate the pumps at different speeds to handle lower flow demands more efficiently. This would reduce the need for control valves that can waste significant amounts of energy and cause the pumps to operate away from the BEP, which could lead to reliability issues, expensive repair costs, downtime and other negative impacts.