Feature Report
Control Valve Diagnostics
The proof is in the process, as exemplified in this former HART Plant of the Year winner. This process economic evaluation helps frame the business case
Gyula Ötvös and József Bartók MOL Group Refining
T
he benefits of field-device diagnostics are often acclaimed in qualitative terms, but on a much lower frequency in quantitative ones. Yet, an attractive cost-benefit analysis is necessary for almost any initiative to make it off the drawing board and into a plant. Even more rare are economic figures that come directly from the process plant engineers (as opposed to a vendor with vested interest in selling diagnostic equipment FIGURE 1. Contaminated instrument air affects the I/P converter or systems). Perhaps that state of affairs was on the judges’ minds when they used technical data from the debuta- Control valve operation selected the MOL petroleum refinery nizer equipment of the fluid catalytic In the Danube refinery described near Budapest, Hungary to receive cracking (FCC) units in the Danube here, intellegent, electro-pneumatic the 2010 HART Plant of the Year refinery and in the Bratislava refinery positioners are used to operate conAward. The application, which is the of Slovnaft, MOL Group. trol valves (Figure 1). The phrase basis for the analysis here, highlights electro-pneumatic means that the two areas where control-valve diag- EFFECTS OF POLLUTED device generates a pressure value nostics benefit the bottom line sub- INSTRUMENT AIR from an electrical signal. Meanwhile, stantially: Contamination of control- In many process plants, such as pe- the word intelligent describes the invalve instrument air and inaccurate troleum refineries, pneumatic control formation available through microcontrol-valve settings. valves are most often used as inter- controller based devices, which have In the first part of this article, the vention devices. The intervention de- arisen in the digital age. In fact, the operation and significance of control vice is part of the automatic control positioners manufactured in the past valve diagnostics is explained, using loop; in other words it is the element 10–15 years all possess intelligent an existing problem of contaminated executing the command issued by the electronics. This observation will be instrument air. The newly developed, control unit. Control valves are incor- important later on. diagnostics-based method that solved porated in pipe armatures or fittings One of the most important elements the problem, is also briefly described. to regulate the quantity of the flow- of the positioner is the device convertBased on the success of the new ing media. This way the control loop ing the electrical signal into a pressure method, it was eventually ordered can directly or indirectly influence the signal; in what follows, this element across the entire operating area of the defined parameters of the technology. will be called the I/P converter. Danube refinery. The signals received from the travel The term ”pneumatic” refers to the The second part of the article inves- auxiliary drive (energy) of the inter- sensor attached to the valve shaft — tigates the product loss caused by the vention device, that is, where a con- a potentiometer in most cases — and inaccuracy of control valve settings cerned device applies energy drawn the distributed control system (DCS) and the economic impact that diagnos- from air pressure to move mechanical are compared by the electronics of tics and preventive maintenance can elements. To drive these pneumatic the positioner, which then sends out have in reducing lost product. valves, so-called instrument air at 5 the electrical signal with the proper For the purpose of both studies, we bars pressure is used. magnitude (typically in the 1–2-mA 42
CHEMICAL ENGINEERING WWW.CHE.COM NOVEMBER 2011