Chemical Processing - September 2007 - (Page 13)

FROM THE READERS << Carefully consider pressure testing fluid In “Choose the right fluid for your pressure test” (http://www. chemicalprocessing.com/articles/2007/062.html), columnist Dirk Willard says compressed gas often makes the most sense for checking a process. He neglected to include explosion hazard of using compressed gas. I believe there was a monthly article written by CCPS (AIChE) Process Safety Beacon dealing with a facility pressure testing a vessel and the cover blew off and went quite a way. As you may or may not be aware, the force from a vessel bursting apart is very high for a gas as the force remains as it is coming apart. However, once the pressure is removed from a compressed liquid the force remaining while expanding is diminished right away. That’s why one should use a low flow pump with high pressure. If one used a device with high flow one could run into much the same problems as with a compressed gas. Barry Bershad, senior process engineer Clariant Corp. Editor’s note: Several readers expressed the same concern as Mr. Bershad. Dirk Willard responds: As an engineer, I have had the sad duty of working on a fatal accident review. I have personally known several people who died while working in our profession. I don’t take safety lightly. I have used pneumatic testing several times and with proper precautions believe that it is the method of choice, especially in those situations in which hydrotesting could have dire consequences. These situations include: contamination; complicated piping or vessel design; or risk of water reaction with product. As with all engineering decisions involving safety, all factors must be carefully assessed. If the test pressure is well above the pressure of the utility gas, then hydrostatic testing is required. Most gas utilities are supplied at pressures above 150 psig, which allows testing for a number of routine vessels. Obviously, if a test must be made at 450 psig, then hydrostatic is your best choice. Just remember, that drainage is always a problem. I’ve been involved in pressure tests for over 25 years. I’ve yet to see anyone figure out how to get all the water out of the system. The problem gets worse if the system is complicated. It gets dangerous if the product reacts with water, or something in the water, like oil. Your hydrotest could get very expensive, especially in pharmaceuticals, if water from your hydrotest contaminates a pharmaceutical product worth twice as much as gold. Using gas requires some thought. A good start is a solid test skid. I require that it include a relief valve, two pressure regulators ( a small one for fine-tuning), a ball valve for a tight www.chemicalprocessing.com shut-off and two high-quality bourdon tube pressure gauges, one upstream and one downstream. The pressure should be regulated using the smaller PRV. I recommend keeping a set of PSVs; most plants have a set of pressures they work at. These should be freshly tagged and perhaps even tested prior to use. The PSV goes downstream of the small PRV. Ideally, as per code, the PSV should be on the vessel or system being tested with all block valves open. Pnuematic pressure testing isn’t a panacea. Sometimes, in engineering, it comes down to a choice of troubles. I prefer those associated with a gas. Students lack a grasp of fundamentals Mark Rosenzweig hit one nail on the head with his editorial “Curriculum change isn’t enough” (http://www.chemicalpro cessing.com/articles/2006/131.html), but he didn’t mention one aspect of current education that I have found troubling about new graduates. Because of the use of computers and “canned” programs and other pieces of available software the students haven’t achieved a grasp of the fundamentals. They typically don’t have a “feel” for the numbers and cannot look at a result and know instinctively that they must have input a wrong value in the computation. I have talked about this with a department heads and they do recognize the problem, but are having a challenge dealing with it. Going back to slide rules isn’t the answer, although that would probably do the job. Name withheld by request How can we really talk up the profession? In reference to your on-line article, “Engineers should talk up the profession” (http://www.chemicalprocessing.com/articles/2006/074.html), how can we “talk up” the profession with a straight face when we (chemical engineers in E&Cs) are always looking over our shoulder for the “bilibility police?” After 30 years, all I feel like is a necessary evil. Forget loyalty, forget a career, they turned it into a job; minimum time off, minimum benefits and maximum stress. Phil Leckner, process engineer Aberdeen, N.J. Errata In “Find the real cause of pump gas up” (http://www.chemicalprocessing.com/articles/2006/058.html), the equation for NPSH available shown in Figure 1 uses a datum line of the liquid surface in the drum. With relatively low liquid velocities in the drum the equation for NPSH available is: NPSH available = P surface - P vapor + ∆Z - hƒ Thanks to the two readers who noted this. Andrew Sloley, troubleshooting columnist September 2007 • 13 http://www.chemicalprocessing.com/articles/2006/062.html http://www.chemicalprocessing.com/articles/2006/062.html http://www.chemicalprocessing.com/articles/2006/131.html http://www.chemicalprocessing.com/articles/2006/131.html http://www.chemicalprocessing.com/articles/2006/074.html http://www.chemicalprocessing.com/articles/2006/074.html http://www.chemicalprocessing.com/articles/2006/058.html http://www.chemicalprocessing.com/articles/2006/058.html http://www.chemicalprocessing.com

Table of Contents Feed for the Digital Edition of Chemical Processing - September 2007

Contents From the Editor Field Notes In Process Energy Saver Compliance Advisor Succeed at Simulation Rethink Your Approach to Process Safety Avoid Blending Blunders Get the Right Cartridge or Bag Filter Wireless Proponents Take HART Membrane Boasts Material Benefits Process Puzzler Plant InSites Chem Show Product Preview ISA Product Preview Equipment & Services Product Spotlight/Classifieds Ad Index End Point

Chemical Processing - September 2007

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