Problems with Demister Pads

Thanks to innovative ideas a resources and time constraints ridden seemingly impossible job done in less time than given and averted 1500 T/d Urea Plant indefinite outage.

Article Type:	Root Cause Analysis (RCA)
Category:	Mechanical
Equipment Type:	Pipelines and Miscellaneous Problems
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.

Table of Contents

A 10-m elevation duct conveys fan blown air to the urea granulator. Demister pads contained in a flanged spool removes the air carried over dust and moisture.

Operations overlooked the demister cleaning and six months after plant commissioning raised a work order for starting the demisters cleaning within a week and finishing it in less than 8‑hours to avoid the entire urea NH3 complex plant shutdown (SD).

This would save orders in hand not delivering heavy penalties. NH3 plant alone cannot run as the plant had regretted NH3 export enquiries, as the urea export was more lucrative.

The job posed constraints are:

The surge tank can hold max 8 hours upstream synthesis section produced dilute urea solution
Biuret an undesirable impurity forms in longer than 6-hrs stored dilute urea solution; it makes urea unsalable. Its reprocessing is impossible; in addition, pollution regulations prevent its dumping also.
Scaffolding, unbolting the over 250 Nos. of 10 mm dia DS flange bolts, and DS crane lowering and assembling back are min 16 hours tasks
The rented crane to lower the duct would be available 15 days later only

Still the author took up the scaffolding immediately to save time hoping he or someone else would get an idea as the scaffold progresses to solve the constraints ridden seemingly unsolvable problem (SUP) easily and quickly.

SUP solved; Two hours after the scaffold work start his below given idea flashed:

Pre-Granulation Shutdown Tasks

Complete the already started scaffolding
Unscrew alternate duct bolts, clean the threads, smear grease and retighten back mild
Unscrew the other others clean the threads and save. This way rust frozen fasteners caused delays would be absent.
Lower the duct spool taking shutdown after crane availability and complete the job as fast as possible.

New idea of Cleaning the Demisters in-situ Fflashed

Three hours after scaffolding start, the below given new idea of cleaning the demisters in-situ flashed

Complete the scaffolding
Omit the above method other steps
Workshop to make a 550 x 550 x 50 duct stub, weld a 25 mm wide collar all around at one end using 6-mm thick plate. Cover the collar with a 600 x 600 x 6 thick bolted plate.
After the preparatory task’s completion, operators would shut down the granulation section and issue work permit
Field Crew to flame cut a 550×550 mm opening in the duct bottom close to the demisters
Operators enter via the opening and hose clean the demisters. The dirty water would drain through the existing drain
After they exist the duct, maintenance crew would weld the readied nozzle and tighten the blind flange
After the first tightening round operators start the granulation

The below given still simpler idea flashed an hour later

Omit tasks 2, 3 & 4 as the flashed idea eliminates duct spool lowering and entry nozzle making. Instead do the following
The workshop to make 600 mm x 600 mm x 6 thick cover and weld a 25x25x16 resting lug at 27 mm away from lower and left edge and another likewise on the right side.
Weld a 16 mm x 50 long SS stud at door CL 40 mm away from top edge and another like that from bottom edge (below figure)
Weld 50x10x12 steel piece to each nut each side to convert the nuts in to wing nuts for easy hammer tightening and opening
Cut a 750 x 50 x 12 mm strip and drill 20 mm dia holes in it to pass through the cover studs. Call it the ‘door retainer’.
Insert a 600 x 600 x 6 mm thick gasket passing through the cover studs; cut the gasket out a conveyor belt piece

Shutdown Jobs

Field crew to flame cut a 550 x 550 mm opening on the duct wall near the demisters pad and near and duct bottom
Operators to enter through this opening and hose clean the demister pads and position back
After coming out they, will insert the cover along the opening diagonal, and parallel it to the opening and rest it on the duct opening at the lugs. Inserting the door retainer and tightening the wing nuts holds the door. Air pressure on fan start would make it still more leak tight
Granulator is ready for use (RFU)

This way, the operation and maint team cleaned the demister pads in < 5 hours SD,

The innovative solution benefits are

Operators themselves clean the demisters in an hour during periodic ½ shift long granulator cleaning shutdowns without maintenance help.
They use the maintenance provided permanent cat ladder, cleaning water, and drain connections and working platform at DS
More frequent demister pads cleaning maximizes airflow and thus granulation efficiency

All welcomed the author developed no separate shutdown (SD), crane and maintenance help requiring easy demister cleaning method.

Author: S. Raghava Chari

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Problems with Demister Pads – Root Cause Analysis (RCA)