3.4 Application Modelling of Reliability and Performance in Engineering Design 273 Fig. 3.77 Hazards criticality analysis logistics worksheet Distributed control systems are dedicated systems used to control processes that are continuous or batch-oriented. A DCS is normally connected to sensors and ac- tuators, and uses set-point control to control the flow of material through the plant. The most common example is a set-point control loop consisting of a pressure sen- sor, controller, and control valve. Pressure or flow measurements are transmitted to the controller, usually through the aid of a signal conditioning input/output (I/O) device. When the measured variable reaches a certain point, the controller instructs a valve or actuation device to open or close until the flow process reaches the desired set point. Programmable logic controllers (PLCs) have recently replaced DCSs, es- pecially with SCADA systems. A programmable logic controller (PLC), or programmable controller, is a digital computer used for automation of industrial processes. Unlike general-purpose con- trollers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and im- pact. PLC applications are typically highly customised systems, compared to spe- cific custom-built controller design such as with DCSs. However, PLCs are usually configured with only a few analogue control loops; where processes require hun- dreds or thousands of loops, a DCS would rather be used. Data are obtained through a connected supervisory control and data acquisition (SCADA) system connected 274 3 Reliability and Performance in Engineering Design Fig. 3.78 Hazards criticality analysis logistics spreadsheet to the DCS or PLC. The term SCADA usually refers to centralised systems that monitor and control entire plant, or integrated complexes of systems spread over large areas. Most site control is performed autom atically by remote terminal units (RTUs) or by programmable logic controllers (PLCs). Host control functions are usually restricted to basic site overriding or supervisory level intervention. For ex- ample, a PLC may control the flow of cooling water through part of a process, such as the reverse jet scrubber,but the SCADA system allows operators to changethe set points for the flow, and enables alarm conditions, such as loss of flow and high tem- perature, to be displayed and recorded. The feedback control loop passes through the RTU or PLC, while the SCADA system monito rs the overall performance. Using the SCADA data, a criticality ranking of the systems and their related as- semblies was determined, which revealed that the highest ranking systems were the drying tower, hot gas feed, reverse jet scrubber, final absorption tower, and IPAT SO3 cooler. More specifically, the highest ranking critical assemblies and their re- lated components of these systems were identified as the drying tower blowers’ shafts, bearings (PLF) and scroll housings (TLF), the hot gas feed induced draft fan (PFC), the reverse jet scrubber’s acid spray nozzles (TLF), the final absorption tower vessel and cooling fan guide vanes (TLF), and the IPAT SO3 cooler’s cool- ing fan control vanes (TLF). These results were surprising, and further analysis was 3.4 Application Modelling of Reliability and Performance in Engineering Design 275 Fig. 3.79 Typical data accumulated by the installation’s DCS required to compare the results with the RAMS analysis design specifications. De- spite an initial anticipation o f non-correlation of the FMECA results with the design specifications, due to some modifications during construction, the RAM analysis appeared to be relatively accurate. However, further comparative analysis needed to be considered with each specific system hierarchy relating to the highest ranked systems, namely the drying tower, hot gas feed, reverse jet scrubber, final absorption tower, and I PAT SO3 cooler. According to the design integrity methodologyin the RAMS analysis, the design specification FMECA for the drying tower ind icates an estimated criticality value of 32 for the no.1 SO2 blower scroll housing (TLF), which is the highest estimated value resulting in the topmost criticality ranking. The no.1 SO2 blower shaft seal (PLF) has a criticality value of 24, the shaft and bearings (PLF) a criticality value of 10, and the impeller (PLF) a criticality value of 7.5. Fr om th e FMECA case study extract given in Table 3.25, the topmost criticality ranking was determined as the drying tower blowers’ shafts and bearings (PLF), and scroll housings (TLF) as 5th and 6th. The drying tower blowers’ shaft seals (TLF) featured 9th and 10th, and the impellers did not feature at all. Although the correlation between the RAMS analysis design specifications illus- trated in Fig. 3.80 and the results of the case study is not quantified, a qualitative 276 3 Reliability and Performance in Engineering Design Table 3.24 Acid plant failure modes and effects analysis (ranking on criticality) System Assembly Component Failure description Failure mode Failure effects Failure consequences Failure causes Hot gas feed Hot gas (ID) fan Excessiv e vibration PFC HotgasIDfanwouldtriponhigh vibration, as detected by any of four fitted vibration switches. Results in all gas directed to main stack Production Dirt accumulation on impeller due to excessi ve dust from ESPs Reverse jet scrubber Re verse jet scrubber W/acid spray nozzles Fails to deliver spray TLF Prevents the distribution of acid uniformly in order to provide protection to the RJS and cool the gases. Hot gas temp. exiting in RJS will be detected and shut down plant Production Nozzle blocks due to foreign materials in the weak acid supply or f alls off due to incorrect installation Drying tower No.2 SO2 blower Shaft & bearings Fails to contain PLF No immediate effect but can result in equipment damage Production Leakage through seals due to breather blockage or seal joint deterioration Drying tower No.1 SO2 blower Shaft & bearings Excessiv e vibration PFC Can result in equipment damage and loss of acid production Production Loss of balance due to impellor deposits or permanent loss of blade material by corrosion/erosion Drying tower Drying tower Restricted gas flow PLF Increased loading on SO2 blower Production Mist pad blockage due to ESP dust/chemical accumulation Drying tower No.1 SO2 blower Scroll housing Fails to contain TLF No effect immediate effect other than safety problem due to gas emission Health hazard Cracked housing due to operation above design temperature limits or restricted expansion Drying tower No.1 SO2 blower Shaft seal Fails to contain TLF No effect immediate effect other than safety problem due to gas emission Health hazard Carbon ring wear-out due to rubbing friction between shaft sleeve and carbon surface 3.4 Application Modelling of Reliability and Performance in Engineering Design 277 Table 3.24 (continued) System Assembly Component Failure description Failure mode Failure effects Failure consequences Failure causes Final absorb. tower Final absorb. tower Fails to absorb SO3 from the gas stream TLF Will result in poor stack appearance, loss in acid production and plant shutdown due to environmental reasons Environment Loss of absorbing acid flo w or non uniform distribution of flow due to absorbing acid trough or header collapsing Final absorb. tower FAT cool. fan piping Inlet guide vanes Vanes fail to rotate TLF Loss of flow control leading to loss of ef ficienc y of the FAT leading to possible SO2 emissions. This will lead to plant shutdown if the emissions are excessive or if temp. is >220 ◦ C Environment Seized adjustment ring due to roller guides worn or damaged due to lack of lubrication Final absorb. tower FAT cool. fan piping Inlet guide vanes Vanes fail to rotate TLF Loss of flow control leading to loss of ef ficienc y of the FAT leading to possible SO2 emissions. This will lead to plant shutdown if the emissions are excessive or if temp. is >220 ◦ C Environment Seized vane stem sleeve due to deteriorated shaft stem sealing ring and ingress of chemical deposits Final absorb. tower FAT cool. fan piping Inlet guide vanes Operation outside limits of control TLF Loss of flow control leading to loss of ef ficienc y of the FAT leading to possible SO2 emissions. This will lead to plant shutdown if the emissions are excessive or if temp. is >220 ◦ C Environment Loose or incorrectly adjusted vane link pin due to incorrect installation process or over-stroke condition I/P absorb. tower I/PASS absorb. tower Fails to absorb SO3 from the gas stream TLF Will result in additional loading of converter 4th pass and final absorbing tower with possible stack emissions Environment Loss of absorbing acid flo w due to absorbing acid trough or header collapsing 278 3 Reliability and Performance in Engineering Design Table 3.24 (continued) System Assembly Component Failure description Failure mode Failure effects Failure consequences Failure causes Drying tower Drying tower Fails to remove moisture from the gas stream TLF Will result in blower vibration problems, deterioration of catalyst and loss of acid production Quality Damage, blockage or dislodged mist pad due to high temp./excessive inlet gas flow, or gas quality Drying tower Drying tower Fails to remove moisture from the gas stream TLF Will result in blower vibration problems, deterioration of catalyst and loss of acid production Quality Damage, blockage or dislodged mist pad due to improper installation of filter pad retention ring IPAT SO3 cooler SO3 cool. fan piping Inlet guide vanes Vanes fail to rotate TLF Loss of IPAT efficiency due to poor temperature control of the gas stream. Temperature control loop would cut gas supply if gas discharge temperature at IPAT cooler too high Quality Seized adjustment ring due to roller guides worn or damaged due to lack of lubrication IPAT SO3 cooler SO3 cool. fan piping Inlet guide vanes Vanes fail to rotate TLF Loss of IPAT efficiency due to poor temperature control of the gas stream. Temperature control loop would cut gas supply if gas discharge temperature at IPAT cooler too high Quality Seized vane stem sleeve due to worn shaft stem sealing ring and ingress of chemical deposits IPAT SO3 cooler SO3 cool. fan piping Inlet control vanes Operation outside limits of control TLF Loss of IPAT efficiency due to poor temperature control of the gas stream. Temperature control loop would cut gas supply if gas discharge temperature at IPAT cooler too high Quality Loose or incorrectly adjusted vane link pin due to incorrect installation process or over-stroke condition 3.4 Application Modelling of Reliability and Performance in Engineering Design 279 Table 3.25 Acid plant failure modes and effects criticality analysis System Assembly Component Failure consequences Probability Failures/ year Severity Risk Crit. value Failure cost/year Crit. rate Fail cost Drying to wer No.1 SO2 blower Shaft & bearings Production 100% 12 5 5.0 60.0 $287,400 High crit. High cost Drying to wer No.2 SO2 blower Shaft & bearings Production 100% 12 5 5.0 60.0 $287,400 High crit. High cost Hot gas feed Hot gas (ID) fan Production 100% 12 4 4.0 48.0 $746,400 High crit. High cost Re verse jet scrubber Re verse jet scrubber W/acid spray nozzles Production 100% 6 6 6.0 36.0 $465,000 High crit. High cost Drying tower No.1 SO2 blower Scroll housing Health hazard 80% 4 10 8.0 32.0 $1,235,600 High crit. High cost Drying tower No.2 SO2 blower Scroll housing Health hazard 80% 4 10 8.0 32.0 $1,235,600 High crit. High cost Drying to wer No.1 SO2 blower Shaft & bearings Production 100% 7 4 4.0 28.0 $449,400 High crit. High cost Drying to wer No.2 SO2 blower Shaft & bearings Production 100% 7 4 4.0 28.0 $449,400 High crit. High cost Drying tower No.1 SO2 blower Shaft seal Health hazard 80% 3 10 8.0 24.0 $366,300 High crit. High cost Drying tower No.2 SO2 blower Shaft seal Health hazard 80% 3 10 8.0 24.0 $366,300 High crit. High cost Drying tower Drying tower Quality 80% 4 7 5.6 22.4 $620,200 High crit. High cost IPAT SO3 cooler SO3 cool. f an piping Inlet guide vanes Quality 100% 3 7 7.0 21.0 $219,600 High crit. High cost IPAT SO3 cooler SO3 cool. f an piping Inlet control vanes Quality 100% 3 7 7.0 21.0 $215,100 High crit. High cost I/P absorb. tower I/PASS absorb. tower Environment 60% 4 8 4.8 19.2 $915,600 High crit. High cost Final absorb. tower FAT cool. fan piping Environment 80% 3 8 6.4 19.2 $216,600 High crit. High cost 280 3 Reliability and Performance in Engineering Design Fig. 3.80 Design specification FMECA—drying tower assessment of the design integrity methodology of the RAMS analysis can be de- scribed as accurate. The RAMS analysis design specification FMECA for the hot gas feed indicates an estimated criticality value of 6 for both the SO2 gas duct pressure transmitter a nd temperature transmitter. From the FMECA case study extract given in Table 3 .25, the criticality for the hot gas feed’s induced draft fan (PFC) ranked 3rd out of the topmost 15 critical items of equipment, whereas the design specification FMECA ranked the induced draft fan (PFC) as a mere 3, which is not illustrated in Fig. 3.81. The hot gas feed’s SO2 gas duct pressure and temperature tran smitters, illustrated in Fig. 3.81, h ad a criticality rank of 6, whereas they do not feature in the FMECA case study extract given in Table 3.25. Although this does indicate some vulnerability of accuracy in the assessment and evaluation of design integrity at the lower levels of the systems breakdown structure (SBS), especially with respect to an assessment of the critical failure mode, the identification of the hot gas feed induced draft fan as a h igh failure critical and high cost critical item of equipment is valid. The RAMS analysis design specification FMECA for the reverse jet scrubber indicates an estimated criticality value of 6 for both the RJS pumps’ pressure indi- cators. From the FMECA case study extract given in Table 3.25, the criticality for 3.4 Application Modelling of Reliability and Performance in Engineering Design 281 Fig. 3.81 Design specification FMECA—hot gas feed the reverse jet scrubber’s acid spray nozzles (TLF) ranked 4th out of the topmost 15 critical items of equipment, whereas the design specification FMECA ranked the acid spray nozzles (TLF) as 4.5, which is not illu strated in Fig. 3.82. Similar to the hot gas f eed system, this again indicates some vulnerability of accuracy in the assessment and evaluation of design integrity at the lower levels of the systems breakdown structure (SBS), especially with respect to an assessment of the critical failure mode. The identification of the reverse jet scrubber’s pumps as a high failure critical item of equipment (with respect to pressure instrumentation), illustrated in Fig. 3.82, is valid, as the RJS pumps have a reliable design configuration of 3-up with two operational and one standby. The RAMS analysis design specification FMECA for the final absorption tower indicates an estimated criticality value of 2.475, as illustrated in Fig. 3.83, which gives a criticality rating of medium criticality. The highest criticality for components of the final absorption tower system is 4.8, which is for the final absorption tower temperature instrument loop. From the FMECA case study criticality ranking given in Table 3.25, the final absorption tower ranked 15th out of the topmost 15 critical items of equipment, whereas the design specification FMECA does not list the final absorption tower as having a high criticality. 282 3 Reliability and Performance in Engineering Design Fig. 3.82 Design specification FMECA—reverse jet scrubber Similar to the hot gas feed system and the reverse jet scrubber system, this once more indicates some vulnerability of accuracy in the assessment and evaluation of design integrity at the lower levels of the systems breakdown structure (SBS). How- ever, the identification of the final absorption tower as a critical system in the RAMS design specification was verified by an evaluation of the plant’s failure data. b) Failure Data Analysis Failure data in the form of time (in days) before failure of the critical systems (dry- ing tower, hot gas feed, reverse jet scrubber, final absorption tower, and IPAT SO3 cooler) were accumulated over a period of 2 months. These data are given in Ta- ble 3.26, which shows acid plant failure data (repair time RT and time before failure TBF) obtained from the plant’s distributed control system. A Weibull distribution fit to the data produces the following results: Acid plant failure data statistical analysis Number of failures = 72 Number of suspensions = 0 . high Quality Loose or incorrectly adjusted vane link pin due to incorrect installation process or over-stroke condition 3.4 Application Modelling of Reliability and Performance in Engineering Design 279 Table. stem sealing ring and ingress of chemical deposits Final absorb. tower FAT cool. fan piping Inlet guide vanes Operation outside limits of control TLF Loss of flow control leading to loss of ef ficienc. SO3 cooler’s cool- ing fan control vanes (TLF). These results were surprising, and further analysis was 3.4 Application Modelling of Reliability and Performance in Engineering Design 275 Fig.