Make Models Match

Readers explain how differences in pressure can arise.

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If it is an existing pipeline, it probably employs a control valve. In the days of low energy costs, some companies specified the valve dP = 50% of the line friction loss for "better control." This could be one cause of the unexpectedly high pressure drop.
Art Krugler, president
Krugler Engineering Group, Whittier, Calif.


"Some field data was collected?" Anytime you use field data, including those from wired instruments, the results will be conflicting and open to interpretation. The refinery may not even be in steady state; inflow of fresh water may not match outflows. A brine line like this tends to be the waste dump for the entire refinery — so establishing inflows into this network may be a challenge, especially when many streams run in flow-meter optional mode. No wonder the models are misleading. The best solution is to redo the measurements after the refinery has been running in steady state for a few days. Isolate as many stray flows as practical.

First, if you have the time, consider the anomaly of extra pressure. The explanation could come from either the calculation or the measurement.

The calculation in the models could differ for several reasons: 1) fittings are often unique to each model — Crane, Miller and others have developed friction coefficients (Ks) for fittings; 2) assuming the lengths are the same in each model, the roughness factor (ε) could differ — use commercial pipe with an ε = 0.0018 in., not hydraulically smooth with ε ≈ 0.0001 in. or corroded (iron) pipe with an ε = 0.02 in; 3) temperatures drop after an exchanger increases friction factor, viscosity, density and pressure drop — without taking this into account the pressure drop could be lower than actual; 4) some models can adjust for the effect of flow rate on pressure, leading to an obvious and embarrassing error if programmed badly; 5) look for common errors like using the wrong r/d ratio for elbows — 1.5 is correct for welded pipe; 6) not including Ks, or the right ones, for entrances and exits; and 7) Crane fitting Ks tend to underestimate pressure drop (there was an attempt to improve this approach in the 1980s but Crane is still used widely).

Now, let's evaluate sources of experimental errors: 1) relying on old field gauges could result in higher errors than the percent of full-scale allowed by ANSI; 2) during unsteady state, level control valves could be closing, yielding erroneous rates if used to establish flow by Q = Cv (ΔP/SG)½; 3) usually it's wise to use a single instrument for measuring pressures and a single instrument for measuring flows; 4) there are probably additional flow streams you can't locate in the pipe rack (most plant isometrics are a mess) — these streams could come from a known source or from a leak; and 5) an unexpected pressure drop such as a fouled heat exchanger, a spectacle blind or even misaligned gaskets.

If you don't have the option of a second test then, rather than chasing down streams, adjust the models to estimate the maximum flow rate through the pipeline. If you can't specify the source of the fouling, which is the most probable cause of the pressure drop, consider reducing the cross-sectional area and attributing the drop to scale. This approach meets the requirement of establishing the capacity of the pipeline and leaving it to the refinery to sort out its errant flows or fouling.
Dirk Willard, senior process engineer
Middough Consultants, Holland, Ohio

We neutralize a waste product with calcium carbonate prior to processing for disposal. The neutralization set-up consists of a weigh-feeder, twin-impeller agitator in a pressure vessel, recirculation pump, heat exchanger (cooler) and centrifugal exhaust fan. The weigh-feeder adds the carbonate to the baffled agitated tank. The reaction is complete when the spike in solution temperature dissipates and the volumes of hydrogen gas decrease. The process poses several problems: the slurry butterfly feed valve normally operates at 30% open but was designed to operate at 50%, leading to severe erosion; the pump often fails because of either gas blockage or suction fouling; the reaction frequently is incomplete, causing safety problems from the erupting hydrogen (the agitator is designed for about 0.5 hp/1,000 gallons with a impeller-diameter/tank (D/T) ratio of only about 0.4); the heat exchanger isn't performing well, although it checked out during commissioning. What can we do to make the process run smoothly? How might we limp by until the next available outage? Can you explain possible causes of the equipment failures?

Send us your comments, suggestions or solutions for this question by September 12, 2011. We'll include as many of them as possible in the October 2011 issue and all on Send visuals — a sketch is fine. E-mail us at or mail to Process Puzzler, Chemical Processing, 555 W. Pierce Road, Suite 301, Itasca, IL 60143. Fax: (630) 467-1120. Please include your name, title, location and company affiliation in the response.

And, of course, if you have a process problem you'd like to pose to our readers, send it along and we'll be pleased to consider it for publication.



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