Acceptance Testing of Control Systems – Part # 2

Continued…

Back to Prelude: In the long list of Project Accomplishments, Acceptance Testing of Control Systems and Emergency Shutdown Systems for New Plants had been one of the most enriching and satisfying endeavors of the author. As a young Engineer, alacrity to inspect finer details of complex control system architecture unravels technology in layers of design. With time and experience, the finer details give way to checking of system performance and process design implementation while mentoring young engineers to explore finer details!

(Please see Part#1 for Sl Nos. 1 to 7)

8. Getting Site Ready: Fire Resistant Raceways inside panels not only provide Neat Segregation of wiring depending on its functionality (Signal, IS/Non-IS, Power, Communication) but also makes a visual treat working at the site. These raceways are designed to accommodate quantum of wiring that usually terminates on the series of Terminal Blocks, Barriers, Relays etc. Field cabling data is mostly available with Panel Designers; however interface cables from MCC are different gauge than field cables. Hence it is essential that Electrical System designers provide core diameter of cables from MCC terminating at the DCS Marshalling Panel. Hence it is essential that adequacy of Cable Raceways is ascertained in totality. Same is also applicable to Terminal Blocks receiving cables from Interfaces and should be suitable for incoming cables.

9. Periodic Thermal Imaging during Burn Test can provide any abnormal rate of change of component temperatures. Thus eliminating the possibility of any abrupt failure during System Acceptance Testing. 

10. Load Sharing of Power Supplies: In Power Distribution Cabinets where Bulk Power Supplies i.e. 24VDC to users is installed, it is a good idea to provide load sharing indicator Diodes. However if the system design has been selected to provide single diode module for each power supply powering a bus bar, it is indeed necessary that physical testing of the rate of change of current is witnessed during failure of redundant power supply. All Settings as found and as corrected should necessarily be noted at the time of FAT so that any abnormal rate of health deterioration during SAT is noted and suspect Power Supply is immediately replaced before plant commissioning.

11. Processor loading should be considered with the wiring of installed spares and spare space available in marshaling cabinets. Though it is unlikely that addition of future I/Os will be linear loaded on processors but gives a fair idea on present logics and i/o assignments.

12. Historian capability can be exploited through monitoring of the rate of increase of data size on the hard disk when all fast trends are configured with sample tags. This provides a brief idea of data accumulation over the period of Factory Acceptance Testing and vindicates if the selection of Hardware is adequate for Historian requirements.

13.Licenses: In legacy systems not from distant memory, I/O Assignment, Loading of Controllers & Routines with I/Os determined Population of Tags. Though Industrial Processors evolved exponentially with Digital revolution, licensing standards for Windows based systems sneaked in to restrict Operator Stations and various DCS functionalities on these stations. Though Competitive evaluation on desired specifications and I/O Counts brings best prices for customers, there remain certain pitfalls that can be avoided and clarity on such licenses should be carefully obtained. Determine Licenses for the individual I/O Types and whether restrictions apply for the same (Analog, Digital, Memory Tags & Serial Data). In Client Server Architecture, Primary Licenses reside on One Server. It is a good idea to check If the backup server doesn’t issue license errors when it takes over this responsibility from the Primary one. It can also be ascertained whether the licenses cater to the installed spares in I/O Sub-system and Spare Space. It remains a good idea to carry out a dummy exercise to obtain licenses if license manager is corrupted. This can provide good information on your OEMs lead time to respond to lead time from distant shores, where such authority resides of major OEMs. Remote login of stations within a Control System network should not give license errors. For e.g. if Engineering Station accesses Operator Window, it should not get inhibited on the pretext of over use of say Operator View License. If an OEM insists on licenses linked to Machine ID, same should be taken care with Mandatory Spare Station as it would render existing license useless when the need utilizes for installation of this spare station.

14. Implementation of Management of Change: Logic Descriptions and Interlock Cause & Effect diagrams through P&ID provided by Licensor FEED form basis for Logic Diagrams and ultimately Alarm Set points by Detailed Engineering Consultant. Simulation during Acceptance Testing includes simulation of Inputs and intermediary steps for realization of desired results. Once the Logic Testing is completed for a functional part of a process, the same should immediately be put into Standard MOC Process of the customer. It is seen that during further checking of logics which are functionally independent but affect other logics, revisit of earlier checked logics is required. It also happens during site interlock checks that site conditions have undergone some changes which were not present during original interlock design. In such cases, care should be taken in revisiting the logic and MOC clearance should be demanded by the group implementing site testing. This not only ensures timely and correct inputs from DEC at the time of FAT but also ensures intervention of domain experts during site interlock testing which demands deviations from original inputs. This brings efficiency and elements of safety in the process of Interlock Clearance.

15. Descriptions of Alarms, First Out and SOE Stampings provide analysis during plant interruptions, incidents, and operator Actions. In many cases, Operator Guide Messages are also provided specifically in batch processes. During the course of IO Checking and Logic Simulation, it is essential that such messages are coherent and relevant to the situation. On several occasions, it becomes a travel through obscurity while decoding sequence of events during plant disturbance and analysis by stakeholders other than Automation specialists becomes a nightmare. Alarm Rationalization and Monitoring Packages are also dependent on original content design. Hence their utilization is also affected by irrelevant descriptions of IO and Memory Tags. Present considerations require configuration of all inputs and outputs in SOE Recorders and in the case of intra-logic memory tags, a configuration of such tags provide logic flow during conditions of aberrations. Customer along with Licensor should devote adequate time while logic checking to ensure stamping of alarms is relevant to the flow of logic and not left on the Automation Vendor. While Licensor can provide a correct description, cultural inputs of understanding can be utilized for improvisation of Event Stamping. 

Planning and Execution of Effective Acceptance Testing are essential for safe and reliable Automation System when installed on site. Identified points mentioned in the article are no way comprehensive but provide next level of engagement beyond typical Control System Testing routines identified through generalised FAT/SAT procedures. The article is also open to share and learn from experiences atypical to routine Control System Testing from first-hand users.