Design & Simulation

Don’t Thumb Your Nose At Rules Of Thumb

Some can provide useful estimates if applied with appropriate caution

By Dirk Willard, Contributing Editor

Rough guidelines can come in handy. However, as any experienced engineer will tell you, such rules of thumb (RoT) sometimes contain dangerous assumptions. Still, RoTs do provide reasonable estimates for many engineering calculations.

A while ago, I covered some RoTs that are useful, so long as you view their results with some skepticism: “Treat Rules of Thumb Right When Measuring.”

Now, let’s look at some others that have survived the test of time:

• Size an auxiliary vessel, e.g., a surge tank or knock out drum basin, for a residence time of ten minutes.

• For gas collecting lines in oilfields, design for a maximum velocity of 65 ft/sec at the pipe outlet.

• Allow 2% hp mechanical loss in centrifugal compressor applications involving non-magnetic bearings and direct-coupling. Allocate 5% hp mechanical loss for reciprocating centrifuges.

• For petrochemicals’ simulations, choose Peng-Robinson to start or perhaps Lee-Erbar-Edminster; for simplicity at the expense of accuracy, pick Redlich-Kwong or, for more accuracy, use Benedict-Webb-Rubin if you have the physical data.

A number of rules of thumb have survived the test of time.

• Conduct a full-scale pipe stress analysis if any of these conditions apply: 1) liquid flow where temperature exceeds 650°F; 2) any fluid above 750°F; 3) lines greater than 16-in. in diameter; 4) vertical lines between equipment running more than 30 ft; 5) pipe connects to rotating equipment; 6) the corrosion allowance is less than 1/16 in.; 7) a line operates or experiences excursions below -50°F; 8) a line serves for blowdown or relief or is part of a flare system; 9) pipe consists of material softer than carbon steel undergoing a temperature change of more than 90°F; and 10) a long horizontal pipe run. (The definition of a long run is circumstance-dependent. It only may take 100 ft of pipe to pose a challenge if the fluid inside is heated to 500°F and pressurized to 300 psig.) Another reason for a full or partial pipe stress analysis is external stress.

• Spacing between supports for pipe filled with water:

Span = 7D½

where span is in feet and D is the nominal pipe size. This rule may not work for D greater than 12 in. or if the pipe rack looks like a snarled old oak, i.e., with disused pipes, dead legs, rusting abandoned pipe clamps, small pipes hung off larger pipes or slightly larger pipes hung off pipe originally intended to be ductwork. I could go on but I don’t want to trigger post-traumatic stress disorder in equipment inspectors. Add corrosion to the mix and it’s a nightmare for everyone.

Want to improve the design of procedures? Register for this free Process Safety  webinar now!

• Heat tracing life expectancy: For steam tracing, it’s five years, based on a manufacturing study, probably with copper tubing and cast iron traps. The life expectancy of electric tracing strongly depends on the temperature maintained and cycling: manufacturers say 20 years below 750°F and one year at 1,600°F but my experience is closer to five years, not 20 years — if properly maintained. The manufacturer tests probably don’t take into account neglect. Refer to: http://bit.ly/2F7wCoL. In fact, neither steam nor electric tracing will survive long if left dangling, unstrapped or uncemented to an uninsulated surface. In such circumstances, failure can occur in months, perhaps weeks, not years. Electric tracing, especially self-regulating tracing, tends to be more sensitive.

• Insulation: For personnel protection, 1½ in. of fiberglass insulation suffices at 125°F pipe skin temperature, 400°F bulk temperature and 90°F air temperature, in a shiny jacket for up to 4-in.-dia. pipe. For a dull jacket, add another inch of insulation.

The following equation (from the ASHRAE 90.1 standard for heat conservation), if rounded to the nearest decimal, estimates a reliable thickness, L, in inches for fiberglass insulation:

L = -4.162237052×10-5×T2 + 6.135299069×10-4×T×D + 5.670103093×10-3×D2 + 2.461996803×10-2×T + 2.233798132×10-2×D - 1.447022351

where T is in °F and D is the nominal pipe diameter. This equation only applies to 8-in.-dia. or smaller pipes and temperatures between 400°F and 0°F. When using calcium silicate instead of fiberglass, add another ½ in. of insulation.

• Recommended maximum liquid velocity, v, in heat exchanger tubing or pipe:

v = 15×F×(1/SG)½

where F = 1 for stainless steel, 0.67 for carbon steel and 0.6 for copper, and SG is the specific gravity of the liquid.

Many other RoT can be useful if thoroughly understood. Unfortunately, the simplifications they contain, if not appreciated, can lead to trouble, especially when engineers are grasping for solutions such as during cost estimations.


dirk.jpg

DIRK WILLARD is a Chemical Processing contributing editor. He recently won recognition for his Field Notes column from the ASBPE. Chemical Processing is proud to have him on board. You can e-mail him at dwillard@putman.net