Most engineers know the basic hand calculations to properly size a pump. Add the resistances from the piping, components and fittings as well as system static head, and this will provide you with the total dynamic head the pump needs to deliver the desired flow.
So, why is it important to use flow analysis software when you can size a pump by hand?
What about on large systems? Hand calculations are more difficult for complicated systems with significant looping, branching, control features, etc. What about when heat transfer is important? Often, hand calculations are a best guess that leads to an oversized system with several safety factors. What if you could size and select your pumps for these complicated systems to avoid the need for extra safety factors? When selecting pumps, flow analysis software makes pump sizing and selection more efficient.
A good flow analysis software will provide solid data and accurate models. As a result, calculation errors can be significantly reduced, and a sizing process that is easy to apply to different system configurations can be streamlined, implemented and passed on to other engineers for future expansions.
Image 1. Original piping system shown with a proposed expansion to the right. The proposed expansion flow paths through the flow control valves and heat exchangers are assumed to be the same as the original system. (Image courtesy of Applied Flow Technology)10 Things to Look for When Evaluating Flow Analysis Software
- Multiple Levels of Software Calculation Validation—This can include model comparisons to published examples, an official quality assurance program such as one that adheres to ASME NQA-1 standards for usage in safety-related systems in the nuclear industry.
- Scenario Management—An ability to model a variety of cases within a single model file where any input parameter or fluid properties can be changed to determine their effect on results. A “family-tree” like scenario structure is effective where changes in parent cases should automatically update dependent child cases. An example could be modeling varying pump speeds across scenarios that reflect different operating conditions.
- Global Editing—Efficiency is important. The ability to massively edit multiple pieces of information at the same time is key. For example, specifying the same pump curve or pump speed for multiple pumps is easier and more efficient than entering the input component by component.
- Excel Integration—Importing model changes to multiple scenarios simultaneously makes updating cases with data variances effective. Sending specific model output results, such as pump operating points and pump and system curve data, in an automated fashion leads to significant flexibility when carrying out other calculations.
- Design Alerts for Maintaining Code Compliance—Quickly bring attention to when important parameters are outside the maximum or minimum allowable boundaries.
- Heat Transfer—Full piping heat transfer calculations should be included to handle convection, buried pipes, layers of insulation, and the ability to calculate external heat transfer coefficients based on ambient temperatures and windspeeds. Heat transfer through heat exchangers is necessary, as is the ability to perform the proper energy balances to determine the mixture temperature when two streams mix.
- Goal Seeking & Transient Modeling—Goal seeking saves time by automatically changing multiple input parameters to provide the needed results. This also aids with model calibration to measured data. Perhaps users need to vary the pump speed of several pumps in order to achieve a desired flow elsewhere in the system. Goal seeking automates this process by removing time- consuming manual iteration. Transient modeling is important to consider because systems rarely stay the same, and it is necessary to determine the overall dynamics of a system and how multiple components interact with each other as things change. Simply running multiple cases as a series of steady-state runs could cause users to miss important or problematic intermediate results.
- Non-Newtonian & Slurry Modeling—A quality flow analysis software should be able to handle not only a wide variety of fluids other than water, but also other types of behavior such as paper stocks, power-law fluids, Bingham plastics or settling slurries.
- System Cost Analysis—Accounting for the material, installation, maintenance and energy costs for an entire piping system in an easy-to-read cost report is just as important as determining the system flow, pressure and temperature distributions.
- Water Hammer Analysis—Water hammer situations can lead to disastrous effects on sudden pressure surges. A quality flow analysis software should have the ability to determine the system pressure response due to various causes like pump trips, valve closures, pump starts and the ability to use surge suppression equipment.
Streamline Pump Sizing & Selection
There are many other pieces to the puzzle that must fit together to ensure a pumping system operates safely, reliably and efficiently. Flow analysis software can help to maximize uptime and profitability; minimize repairs, downtime and energy costs; evaluate the impact of system modifications; and troubleshoot operational problems to determine the root cause of an issue.
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.
Planning for a potential system expansion and sizing or selecting the pumps with this in mind would certainly be an easier challenge to overcome rather than learning later the expansion would cause the entire system to not function as needed. Not to mention all the other issues this could introduce.
Flow analysis software streamlines the pump sizing and selection process and allows users to accomplish more than typical hand calculations or spreadsheets allow.