Understanding the Software Options

More documents

Recommendations

Info

Feature Report the process, the more effective one can assess the quality of these models. In the end, a P&ID that reflects the nature of the process is essential to a successful process control application. P&IDs are also developed largely based on someone’s understanding the process. A steady-state model formulated from basic process mechanisms is another way to express the behavior of the process. But as previously noted, we have no technology to translate these relationships into a P&ID. The traditional view is that the dynamic nature of a process determines the appropriate process control configurations, that is, the loops as represented on the P&ID. When developing the P&ID, this translates into the following rule: Control every variable with the nearest valve. Developers of P&IDs usually deny that this is the guiding principle, but if one examines enough P&IDs, it seems to turn out that way. Probably a more accurate statement is that what one sees on P&IDs is based largely on considerations pertaining to process dynamics. This leads to trouble, and here is a “war story”. The process in Figure 3 is a batch chlorine vaporizer. The chlorine is delivered in a vessel equipped with an internal coil through which heat can be supplied from low pressure steam or hot water. The vaporizer is a pressure vessel, so a pressure relief device is required. The process objective is to deliver the desired flowrate of chlorine vapor to the process, but in doing so, do not exceed the pressure setting on the relief device. The variables to be controlled are the following: • Chlorine vapor flow • Vaporizer pressure The final control elements are: • Control valve on chlorine vapor line • Control valve on the steam supply The P&ID in Figure 3 reflects thinking based on process dynamics. The chlorine vapor flow measurement is the flow through the chlorine vapor valve, so a customary flow controller is proposed. The vaporizer pressure is controlled via the steam valve, which influences the rate of heat input. There is a fundamental problem with this control configuration. What Steam 38 CHEMICAL ENGINEERING WWW.CHE.COM AUGUST 2011 PC PT Chlorine vaporizer FT Condensate actually determines the chlorine vapor flow? The rate of heat input to the vaporizer. To vaporize a kilogram of chlorine, some kilocalories (the latent heat of vaporization for chlorine) of heat are required. If you line out the process, switch both controllers to manual, and increase the opening of the chlorine vapor valve, what happens? • The chlorine vapor flow initially increases • The vaporizer pressure and the vaporizer temperature decrease • The heat input decreases • The chlorine vapor flow decreases Once the transients have passed, the chlorine vapor flow returns to a value very near its initial value. At a fixed steam valve opening, the steady-state sensitivity of chlorine vapor flow to chlorine vapor valve opening is essentially zero. These cause-and-effect relationships are the basis for the following logic: • To increase the chlorine flow, the flow controller increases the chlorine vapor valve opening • This decreases the vaporizer pressure • The vaporizer pressure controller increases the steam valve opening, which in turn increases the chlorine vapor flow The conclusion: the configuration in Figure 3 is perfectly satisfactory. This is not necessarily the case. The chlorine vapor flow controller is totally dependent on the vaporizer pressure controller — if the vaporizer pressure controller is on manual, the chlorine vapor flow controller will drive the chlorine vapor valve either fully open or fully closed. The output of the chlorine vapor flow controller has only a nominal effect on the chlorine vapor flow. When Loop A is totally dependent on Loop B, the following statements can be made: • The configuration can only function when Loop B is on auto • Loop B must be significantly faster than Loop A, preferably by a factor of about five FC Chlorine gas These statements also apply to cascade configurations, where the outer loop is totally dependent on the inner loop. The configuration in Figure 3 does not have the required separation in dynamics. In fact, the separation is probably the opposite of what is required — the chlorine vapor flow loop is likely five times faster than the vaporizer pressure loop. The result will be major tuning problems with the chlorine vapor flow loop. This loop will only function if tuned very conservatively — so that it is five times slower than the vaporizer pressure loop. But then the resulting performance of the chlorine vapor flow loop is unacceptable. Basically, the configuration in Figure 3 does not reflect the fundamental nature of the process. The chlorine vapor flow is determined by the heat input from the steam flow; the chlorine vapor valve must basically be used as a back-pressure regulator. You have to understand the process. And when it comes to P&IDs, understanding the steady-state nature of the process is the crucial part. ■ Edited by Gerald Ondrey Author FIGURE 3. Shown here is a P&ID for a chlorine vaporizer, which is discussed in detail in the text Cecil L. Smith has been providing consulting services on a full-time basis since 1979. (Cecil L. Smith, Inc., 2034 Pollard Parkway, Baton Rouge, LA 70808; Phone: 225-761-4392; Fax: 225-761- 4393; Email: cecilsmith@cox. net) His consulting practice is devoted exclusively to industrial automation, encompassing both batch and continuous processes. About a third of his time is spent teaching a variety of continuing education courses on process control and related subjects. with a focus on the process aspects of the subject, and not the systems aspects. His other current efforts are directed at developing and enhancing educational materials that utilize computerbased training (CBT) technology for continuing education. He has B.S., M.S. and Ph.D. degrees in chemical engineering from Louisiana State University. He is a registered Professional Engineer licensed in Louisiana and California, and a fellow of the AIChE. This article is based on his experiences, which also provide the basis for three books (“Practical Process Control: Tuning and Troubleshooting,” “Advanced Process Control: Beyond Single Loop Control” and “Basic Process Measurements”) published by John Wiley and Sons.
A SINGLE FAILURE ON A BOLTED JOINT CAN STOP PRODUCTION! TM CHALLENGE Get Started with a FREE Survey! 24/7 Service: 1-800-FOR-HYTORC info@hytorc.com | www.Leakage-Zero.com Circle 16 on p. 54 or go to adlinks.che.com/35068-16
Page 1 and 2: www.che.com August 2011 CAPITAL COS
Page 3 and 4: Hitting a wall discharging solidifi
Page 5 and 6: August 2011 In ThIs Issue Volume 11
Page 7 and 8: Winner of Eight Jesse H. Neal Award
Page 9 and 10: Calendar Calendar NORTH AMERICA ACS
Page 11 and 12: Edited by Gerald Ondrey August 2011
Page 13 and 14: Green honors In June, the U.S. Envi
Page 15 and 16: This new membrane bioreactor cuts e
Page 17 and 18: Lozowski FIGURE 2. The �at-plate
Page 19 and 20: Newsfront Thermo Fisher Scientific
Page 21 and 22: W process gases at very high levels
Page 23 and 24: A Department Editor: Scott Jenkins
Page 25 and 26: A fine way to separate solids from
Page 27 and 28: E20001-F218-P280-X-7600 How far can
Page 29 and 30: and also for more intensive deagglo
Page 31 and 32: Bosch Packaging Technology Italvacu
Page 33 and 34: People WHO’S WHO Büchner Broders
Page 35 and 36: TABLE 3. PURCHASE COST (C PCS) OF H
Page 37 and 38: TABLE 5. PURCHASE COST (C PCS) OF P
Page 39 and 40: TABLE 7. PURCHASE COST (C PCS) OF V
Page 41 and 42: TABLE 8. PURCHASE COST (C PCS) OF O
Page 43 and 44: Operator interface node Transmitter
Page 45 and 46: only, but quickly gained a reputati
Page 47 and 48: Circle 14 on p. 54 or go to adlinks
Page 49: Circle 15 on p. 54 or go to adlinks
Page 53 and 54: Solids Environmental Processing Man
Page 55 and 56: PLANT DESIGN STAGE - SOME IDEAS ON
Page 57 and 58: Solids Processing Preventing Self-H
Page 59 and 60: Temperature, °C Self-heating test
Page 61 and 62: proportional venting of overpressur
Page 63 and 64: Intelligen Suite ® The Market-Lead
Page 65 and 66: Advertisers’ Index Advertiser Pag
Page 67 and 68: Economic Indicators PLANT WATCH Out
Page 69 and 70: See you in The largest water qualit

Understanding the Software Options

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?