Make Models Match

Readers explain how differences in pressure can arise.

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As part of our refinery expansion, we must confirm the capacity of a schedule-80 crude-oil-desalter wastewater line. It's a 2,500-ft pipeline consisting of two heat exchangers and a mix of 3-in. and 4-in. pipe emptying into a slop-oil decanter operating at 5 psig. We're hoping that capacity can be increased by 25%. We collected some data using existing pressure gauges. An initial hydraulic study with software using equations from Crane Technical Paper No. 410 indicated there is an "extra" 65 psi when 125 gal/min of brine flows at 160 psig. However, another study done using different Crane-based software gave about-50-psig extra pressure. How could two programs produce such different results?

TROUBLESHOOT CAREFULLY
"Why is there a difference between two programs that both use the Darcy method?" This should be expanded to a more general question: "Why isn't my system working the way I think it should?" It doesn't matter whether you are comparing the results of two different programs, calculated results against system instrumentation, or how the system works now versus two years ago. The troubleshooting method is the same.

The first assumption I will make is that difference in pressure is due to a problem in building the model used by each program rather than the accuracy of the programs, especially since both use the Darcy method for calculating the pipeline head loss.

So let's start by seeing what system items could result in a large difference in head loss or pressure difference between what you expect and what you see.

The system is described as pipeline and heat exchangers, so let's concentrate on those two elements. Items that can affect the model are:

1. differences in pipe diameter — since the system includes both 3-in. and 4-in. pipe, the diameters used in the models may not match, which can significantly impact head loss;

2. schedule — while the system uses schedule-80 pipe, some programs default to schedule 40, again resulting in a large difference in head loss;

3. elevations — if those of system boundaries and junctions between pipelines aren't the same in the models, calculated pressure will vary;

4. head loss characteristics in the heat exchangers — if not modeled comparably, pressure values will differ; and

5. pipe roughness — while the value used normally doesn't have a major impact, for a 2,500-ft pipeline it could account for the difference.

The best way to troubleshoot a model is to start at one end, and compare the pressures and flow rates in each section of the system. For example, let's assume the inlet pressures in both models are 160 psig, and the flow rate through the system is 125 gal/min. Based on those input values each program should calculate the same head loss for the first section of pipeline. If the head losses differ, check the diameters, schedule and roughness until the results match. Next, if the outlet pressures differ between the two models, check the elevations. Repeat these two steps for each pipe segment in the system. With the heat exchangers, the inlet pressures should match, as should the calculated head loss and differential pressures. If not, check the head loss characteristics used.

Engineering programs provide a rapid way of performing tedious calculations but it is important that the person using the results has the ability to troubleshoot the system and model and to determine if the difference is caused by the model or the actual system.
Ray Hardee, chief engineer
Engineered Software, Inc., Lacey, Wash.

GET SOME DATA
I would start by collecting data. To understand the pipe system, two curves are necessary — the friction-flow curve and the pump head-flow curve. Other information will also be needed, including: the elevation profile; a detailed description of all pipe and fittings as an isometric or table; the pressure drops for the heat exchangers; the pipe schedule; and the condition of the pipe inside surface (it could be rusted or scaled). This information will be required to create a pressure drop versus flow rate curve.

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