Letters To The Editor - April 2009
April 1, 2009
Valves For Design ProblemsI have had an interest in control valves for a long time and enjoy articles on this topic. I recently re-read “Selecting Valves to Solve Unusual Design Problems” (Jan. 2008) and feel it’s necessary to make some comments on this article.
First, except for on/off valves I find it hard to envision any control valve not used as a restriction. The valve recovery coefficient is Xt. It applies also to compressible flow. It is defined at the pressure drop where choked flow begins. The basic reference is ANSI/ISA-75.01.01 (IEC 60534-2-1 Mod)-2007 - Flow Equations for Sizing Control Valves.
Second, Not all equations provided are helpful (page 22). Equation 1 is not exactly wrong, but it applies only for differential pressure less than the critical pressure drop as limited by liquid vapor pressure. And Equations 6 and 7 are from older literature and are no longer in use. Again, see ISA 75.01.01.
Third, in any discussion of pressure recovery it should be noted that the factor is a function of the valve stem position, and thus, will vary as the control loop responds to the situation.
Fourth, a user who suspects a possible problem application is well advised to present the information to one or several suppliers and to consider the information received. The established manufacturers are a good source of knowledge. The accuracy of information provided is important. A small error in liquid vapor pressure or operating temperature might be the difference between destructive cavitation or problem-free operation. It is likely that any service approaching choked flow will experience this at least part of the time. Damaging cavitation may occur even at modest water temperatures if the inlet pressures are low.
Finally, control valve flows may be considerably reduced at low Reynolds numbers with higher viscosity liquids and at very low pressure difference. Fittings adjacent to the valve can reduce the effective flow coefficient. In liquids, flashing and choked flow without cavitation is possible but rare. Precision of flow or pressure control is limited by the precision of the actuators and operating conditions. For larger size valves (4 inches and up) and high flows of compressibles, noise may become an issue. Long pipelines and fast moving valves may experience destructive water hammer.
Cullen Langford, P.E.
Ask the Expert: Drainback vs. GlycolOverall, I see a solar drainback system as being superior to a glycol system. Yet it seems a lot of installers prefer glycol systems. I wonder if John Siegenthaler, Solar Design Notebook columnist, can give me his opinion on why that is?
Steve of Shelter Products Northwest
John Siegenthaler responds:
The advantages / disadvantages of drainback versus closed loop anti-freeze-based systems have literally been debated for at least 30 years. My own preference is for drainback systems from the standpoint of simplicity, stagnation protection, and better thermal performance.
Still, many manufacturers want systems that are as “foolproof” as possible.
Glycol-based systems allow almost any type of piping path between the collectors and the storage tank, and, thus, piping does have to be pitched.
As simple as pitched piping seems, it does get installed incorrectly, and the result is a very expensive freeze up. I suggest getting a copy of the publication idronics#3 from www.caleffi.us for a more detailed look at these two options.
Wrong Acronym?In reading the Codes column by Julius Ballanco in the Feb. 2009 edition of pme, I was amazed to find a new engineering organization with the familiar ASPE acronym: the American Society of Professional Engineers. The Web site listed was www.aspe.org. Upon visiting the Web site, I found that this is not the American Society of Professional Engineers, but the American Society of Plumbing Engineers, of which Mr. Ballanco is President. I am of the opinion that this fact should have been pointed out in the article.
Migual L. Purdy, P.E.
Associate, VCP&A, Inc.
Editor’s note: You are correct. The acronym stands for the American Society of Plumbing Engineers. We also want to clarify that the column described the IRC Fire Sprinkler Coalition as the International Residential Code (IRC) Fire Sprinkler Coalition. They are not affiliated with the ICC, which owns the rights to “International Residential Code.” We regret both errors.