This article was printed in CONTROL's March 2009 edition.
Ask the Experts is moderated by Béla Lipták, process control consultant and editor of the Instrument Engineer’s Handbook (IEH). Preparation of the 5th edition of the IEH will start shortly. If you are qualified to write a chapter or update an existing one or if you would like answer questions for this column, please send your resumé to firstname.lastname@example.org.
Q: Reading about the first solar-hydrogen power plant in the world ("Controlling the Post-Oil Energy Economy"), I would like to know when and where will it be built? I would also be interested in the economics of solar roofs which you describe in the chapter on “zero energy homes.”
Tarek Ben Salir, Algeria
A: I would like to build this 1000 MW solar-hydrogen power plant on a 3x3-mile area in Tunisia. There the government is stable, education is good, a deep sea port is available for liquid hydrogen (LH2) tankers, and the yearly insolation is around 7,000 KWh/m2/yr. The plant cost is estimated at $3 billion (+ LH2 tankers) which compares favorably with the $5 billion investment required for the same size “safe nuclear” or “clean coal” plant. I asked Mr. Zine Al Abidiene Ben Ali, president of Tunisia to contribute the land needed for this joint project.
As to “when,” I can only say that when the Japanese shut off our natural rubber supply in 1941, it took 25 months to put synthetic rubber tires on the American trucks. If President Obama follows President Roosevelt’s example, the conversion from our present exhaustible, import-based and climate-changing energy economy to an inexhaustible, independent and clean one can be in high gear by the end of his first term. This effort would not only eliminate “energy wars,” but the construction of the required infrastructure would create an economic boom.
As to the economics of “energy free” homes, I will use our home as an example: We consume a yearly average of 39 kWh/day of electricity for which we pay $0.18/kWh. If this electricity were generated by solar panels in the Southwest (insolation about 7 kWh/m2/day), and if the efficiency of the collectors was 20%, the collector area needed would be 39/(0.2x7) = 28 m2 (300 ft2). (In New York, where the insolation is about 2 kWh/m2/day, the area required would increase to about 1,050 ft2/home.)
The installed unit cost of a grid connected collector system in the New York area is about $1,000/m2 (total about $100,000 minus a $30,000 federal tax credit). In New York, assuming a yearly 4% increase in utility rates, the break even point is 19 years. In the Southwest, the cost is reduced by a factor of 3.5. Once the capital investment is paid off, the fuel is free, so the cost of the collected energy is only the cost of maintenance.
Q: What is the use of vent hole in the orifice plate?
A: In liquid service, small drain holes have been specified for use in horizontal meter runs to permit the liquid to drain when flow is stopped. In gas flows, drain holes have been used to permit entrained liquids to flow through rather than build up and suddenly be carried over all at one time causing a flow upset. In dirty services, any small hole will most likely plug over time and become ineffective.
In the orifice calculation, one should correct for the area of the drain or vent hole as part of the specified orifice area.
In any service adding a drain/vent drilling increases the flow uncertainly, because the calculations are not standardized for this, and the future state (clear or plugged) cannot be known. Neither is permitted by the ASME 3M and other orifice standards. What is permitted is a small bypass line which can be opened to permit draining. Use in custody transfer is not normal practice.
Uncertainties will increase in smaller pipes (less than maybe four inches, 100 mm), and with larger beta ratios, over maybe 0.50. In either of these cases, drains and vents are less useful anyway.
We gray beards have discussed this for many years. The usual conclusion is that in process flows where smooth flow indication is desired, consider drains or vents as appropriate, only in horizontal lines and then only where the flowing fluid is likely to have either bubbles (in liquids) or entrained liquids (in gases). In either case, add a hole for drilling is easier in the future than plugging it up. In every case, please document what you did and why. The future will ask.
A: Vent holes are rarely used, as gas bubbles are easy to entrain in the liquid flow. Drain holes are somewhat more common, as they help liquid to pass a gas flow orifice without changing the effective pipe bore. The V-cone is a solution if there is a significant second phase present. A Rosemount 4-hole plate is also useful, though wet-gas flowmeter theory can come into play for both.
When considering the use of vent holes on orifice plates, the following should be kept in mind:
a. Not recommended on pipes with D < 100mm.
b. ISO5167 does not recommend it regardless (but it, somewhat unreasonably, suggests closing a bypass around the plate when about to take a measurement).