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How Important is Piping Symmetry?Designers should consider the impact of unequal flows in branches.
by Andrew Sloley, Contributing Editor
When we build multiple parallel exchangers we assume an ideal world -- each exchanger getting the same flow. One way of achieving equal flow to every exchanger is making inlet and outlet piping symmetrical. So, my first thought on seeing the piping layout proposed for connecting a tower overhead to an air-fin condenser (Figure 1) was "That's not symmetrical." Tower overhead enters in the large line from the upper left and goes to the condenser; condensate then leaves through the line to the lower left. Both the inlet and outlet piping clearly aren't symmetrical, despite the designer being instructed to provide a symmetrical layout.
My second thought was "Is symmetry important here?" To evaluate this, let's look at the inlet piping because non-symmetry in the outlet piping here only has a minor impact.
Figure 2 shows a schematic of proposed inlet piping. It has a line of symmetry at the first flow split. As far as the piping designer was concerned this was a symmetrical layout. However, to get the exact same resistance to flow in each path, every split must be symmetrical to every other one at the same level of branching. For four inlet lines, a symmetrical layout requires two levels of branching: the first sends flow to AB and CD, the second splits that to A and B and C and D.
Figure 1. Both inlet and outlet piping have non-symmetrical layout.
To analyze the system, we start with one flow fundamental -- pressure drop in parallel paths must be equal. Flow and level, if present, will distribute to equalize pressure drop. For the proposed layout, this translates to 107% of design flow going to inner bays (B and C) and 93% to outer bays (A and D), or inner bays receiving 115% of the flow of outer ones.
While this seems like a lot, what's the real consequence? Do we really care about the maldistribution? What really counts is its impact on exchanger duty. Bays with low flow will tend to pinch against air temperature (reducing duty). Bays with high flow may be limited by surface area or may compensate due to increased temperature difference (because their outlet temperature rises).
A detailed analysis of exchanger performance shows duty in the high-flow bays (B and C) goes up, duty in the low-flow bays (A and D) goes down, and total duty drops slightly, by 0.4%.
The exchanger was being purchased with 25% more duty capability than required. Additionally, the non-symmetrical piping minimized structural height and reduced overhead-of-tower-to-condenser-drum pressure drop, which was important as well. Overall, the non-symmetrical layout was a better design. So, the answer to my second question is "No, symmetry's not important here."
Figure 2. Original layout has one line of symmetry but three are needed for truly symmetrical design.
Not every flow-splitting problem will have the same answer. How important is symmetry? You only can tell through a detailed evaluation of the specific situation.
This case had several key factors making symmetry less important:
• relatively high fraction of system pressure drop in exchangers compared to piping;
• large difference between outlet temperature of process stream and air stream from condenser; and
• ample over-design in exchangers.
Symmetry is more important in overhead systems when you must deal with factors such as:
• close temperature pinches between process and cooling medium; and
• low system pressure, as this magnifies the impact of pressure drop on the condensation curves.
Symmetry in piping is a good first step to achieve even flow and maximum equipment performance. However, in some systems, like the one here, it may not be worth the price. But don't decide without careful analysis.
THE history of piping design and engineering can be traced back to early civilizations. In Indus valley civilization the art and science of piping design and engineering was evident in the colonies. There was a network of pipes to transport water. In China bamboo pipes were used to transport water. Later they used bamboo pipes wrapped with wax to carry natural gases.
FACTORS WHICH LED TO GROWTH OF PIPING DESIGN AND ENGINEERING
The piping design and engineering saw fast growth after 19th century because of following factors:
Piping design and engineering is a key element in various streams of engineering. Piping and accessories constitute over 25% of the total capital investment in the chemical process industry, petroleum and petrochemical industry, pharmaceutical industry, power plants, long distance LNG/LPG/CNG Piping systems and Irrigation systems.
It helps students to design mechanical, packaging and manufacturing engineers to accurately design, route document and produce complex piping an tubing system while optimally reusing designs for dramatic time savings and improvements in productivity.
A variety of global challenges like corrosion prevention, pipeline maintenance, leak detection in long distance pipelines, pipe coating, concrete coating, bending, welding and deep water services pipeline cleaning, pipeline operating efficiency etc in itself demands a specialized stream in piping design and engineering.
With the increasing emphasis on transportation of petroleum products, natural gas, corrosive hazardous chemicals. through underground and long distance pipelines the requirement of skilled piping engineers is increasing day by day.
Indiana Man Operates Oil Well in Backyard, Producing Three Barrels of Crude a Day
Published May 19, 2008 SELMA, Ind. – It's just a drop in the global oil bucket, but an eastern Indiana man is operating an oil well in his backyard in an effort to capitalize on soaring crude prices.
Greg Losh's rig produces three barrels of crude oil a day, though he told FOX News that he hasn't started selling it yet. For now, he and his partners are keeping it in storage containers.
He declined to say how much oil they've collected in the two weeks they've been pumping.
But as oil is going for about $127 a barrel on the international market, three daily would yield just under $400 a day for Losh on the global spot market — or 1/100,000 of the daily production increase the Saudis agreed to earlier this month.
Still, in spite of those returns and the $100,000 it costs to drill a well, it's worth it to Losh considering the current price of oil, he told WISH-TV in Indianapolis.
The oil his well produces comes from the Trenton field that fueled the growth of east-central Indiana cities more than a century ago, he told the station.
He expects to drill four more wells soon on his property in the town of Selma about 55 miles northeast of Indianapolis.
"It's a money maker. It is paying off," Losh told FOX.
The oil is stored in a tank and transported to Ohio for sale, he said. His oil well also produces natural gas to heat his home and several others.
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