Pressure Switch Frozen Screwed Cover Extends the Plant Shutdown

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Instrumentation engineering root cause analysis of pressure switch (PS) frozen screwed cover which extends the plant shutdown time.

Article Type:Root Cause Analysis (RCA)
Category:Instrumentation
Equipment Type:Switches
Author: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.

Pressure Switch Frozen Screwed Cover

K-901 NH3 Refrigeration Compressor tripped on Low-Low lube oil pressure though the oil header pressure was the set 1.4 bars. A large Al screwed cover housing contains the pressure switch (PS). The crew identified the PS’s micro-switch as defective and proceeded to change it.

The corrosion and oxides frozen stubborn Aluminium (Al) cover defied two hours long all possible violent unscrewing efforts including not permitted.

Finally, the crew took the below-listed steps:

  1. Cut the cable entering the housing
  2. Chisel out the 6 mm Carbon Steel mounting bolts
  3. Send the entire assembly to the shop

Shop Efforts

Shop efforts are:

  1. Shop crew too failed cover unscrewing after an hour long more violent ‘expert’ efforts
  2. Finally the ‘experts’ machined off the cover, cleaned the body threads and took necessary measurements and sent the coverless casing to install in the field 
  3. They turned out a CI cover and fitted a cut from automobile windshield rush bought window
  4. They delivered to the field instrument crew
old Pressure Switches
This is an example image, not related to this article.

Field Instrument Crew Done Jobs

While the above shop tasks progressed, field Inst crew done jobs are:

  1. They bolted the pressure switch (PS) assembly less cover at its location using threads greased SS bolts & nuts
  2.  They replaced the defective micro-switch and set the PS for proper trip pressure
  3.  They connected the cable
  4. Using this opportunity, the simulation tested the trip system
  5. As soon as the shop delivered the new CI cover, they greased the threads profusely and screwed it in .

Thus, a simple ½-hour task extended to 12 hours of lost ammonia production, despite well-coordinated done several tasks.

Recurrence Prevention

The above hard-way learning prompted taking the below-given recurrence prevention steps:

  1. Grease all fasteners and screwed covers before bolting / screwing in
  2. Check all field instruments
  3. Unscrew / unbolt one stand fastener at time, grease and screw back to avoid future stubborn fasteners caused delays
  4. Spray WD-40 or other loosening sprays on stubborn fasteners and complete step-a. Chisel out still stubborn fasteners and fit new fasteners. Use stainless for ≤ 6mm and CS for larger all greased
  5. Unscrew and grease screwed covers
  6. Free stuck instrument door hinges and latches, and oil them

Thanks to these pro-active steps stuck screws and covers became history.

Author: S. Raghava Chari

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