Instrumentation engineering root cause analysis of Pneumatic Controllers Bumpy Manual to Auto and Vice Versa Transfers threaten indefinitely Delaying Pioneer Modern Fertilizer Plants Commissioning.
|Root Cause Analysis (RCA)
|S. Raghava Chari
Note: This root cause analysis (RCA) is from real-time scenarios that happened in industries during the tenure of two or three decades ago. These articles will help you to improve your troubleshooting skills and knowledge.
Pneumatic Controller’ Bumpless Transfer Issues
The pneumatic Controllers Bumpy Manual to Auto and Vice Versa Transfers threaten indefinitely Delaying Pioneer Modern Fertilizer Plants Commissioning.
Note: In today’s world, electronic controllers are available as physical hardware devices. We can also use PID controllers (software) in a PLC or DCS system. The pneumatic controllers were obsoleted. This article is for information purpose only.
The operators put panel and field Pneumatic Controllers’ on automatic thus:
- They turn the controller mode adjusting switch (MAS) knob to the manual mode position; in this mode the set point adjuster (SPA) output reaches the control valve (CV) and manipulates it.
- MAS in manual mode SPA turning increases / decreases the controller output and opens / closes the control valve (CV) to get the required process variable (PV). Now, turning the MAS to ‘seal’ position seals the pressure to the CV at the last value and hence the holds the PV also at the last value.
- Now turning the SPA changes the set point. They turn the SPA till the set point coincides with the PV.
- Finally, they turn the MAS to ‘Auto’. The process is now in automatic control.
The signal to the control valve (CV) remains steady during the control modes transfers. Such a transfer is called BUMP-LESS mode transfer.
Unfortunately, during control mode transfers, the South Indian pioneer Modern Technology Fertilizer Plant’s Utility, NH3, Urea, and NPK Plants panel and field controllers’ outputs to the CVs varied; hence the CV openings also and upset the process; i.e., the control mode transfer was ‘BUMPY’. It would prevent smoothly putting the process into ‘auto’ or manual modes as necessary. It threatened delaying the fertilizers plants commissioning indefinitely.
The US instrument vendor’s Indian Licensee who supplied and installed these instruments could not help. Fortunately, the author loop checking the NPK plants raw materials feeders ‘packages’ found the panel mounted weight recording pneumatic controllers (WRC) are of the same make and model as the Indian Licensee supplied controllers, but the US factory made.
THE US CONTROLLERs BUMPLESSELY TRANSFERRED MANUAL TO AUTOMATIC AND VICE VERSA! This observation elated the author; he placed the India and US made controllers side by side on a table and examined them.
He noticed an aluminum plug on both controllers at identical locations. The black paint marked the US Aluminum plug. He pulled out both the plugs and compared: the US plug was 20% longer and had two O-rings fitted, but the Indian Plug was shorter and one O-ring only fitted.
The India-made controller accepted the two O-rings fitted US plug. The encouraged author with a vigorous prayer bolted the US plug-fitted India Made controller to an India-made Receiver Rear. Lo and behold! That controller manual to automatic and vice versa transfers were bump-less! This check repeat with another India made receiver mounted controller was also a success.
The author asked the Indian Licensee, who also took part in this study to supply 350 Nos. of the two O-rings fitted plugs copying the US factory made part ASAP say 2-3 days.
But he regretted telling that involves their US collaborator’s approval; in addition, as they get the parts from the US factory and just assemble them into full instruments, they have to await the parts from the US, which might take min 2 months.
The plant workshop loaded with many more commissioning required jobs could not take up the plugs making also immediately. Though frustrated but not defeated, the author gave a US plug to each of the town’s 3 machine shops – cautioning each NOT TO LOOSE OR DAMAGE IT.
They delivered 120 plugs each in a day (total 360 plugs). In addition, a rubber parts maker delivered 400 O-rings duplicating the sample O-ring given to him – the author ordered few extra O-rings, as often O-rings making involves approx. 5% rejects.
The India made O-rings and plugs fitted India Assembled receivers + controllers Manual to Automatic and Vice-Versa bump-less transfers pleasantly surprised everyone; they heaved a relief sigh and congratulated the author. This enabled commissioning the Utilities, NH3, Urea, NPK Plants A and B on schedule at negligible additional expenses.
Author: S. Raghava Chari
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