A large gate valve’s failed motorized operation threatens NH3 plant guarantee test completion.
| 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.
Molecular Sieve Brief Introduction
Molecular Sieves (MS) are granules of specific size and shape pores having affinity to absorb specific chemicals from fluid streams.
CO is a poison to ammonia catalysts. MS charged vessel 1 (V1) adsorbs the CO a poison to ammonia convertor catalyst from the 250-bars pressure synthesis gas (SG 75% H2, 25% N2 gas mixture) entering the NH3 Convertor for synthesis into NH3.
4-hours later a programmable logic controller (PLC) diverts the SG flow to Vessel 2 for adsorption, de-pressures V1 slowly, and admits heated small quantities low pressure SG into V1 for 2 for two hours to strip the CO from the MS;
The hot impure gas joins the fuel gas stream. V1 cools for 1½ hours and becomes ready for adsorption. The PLC alternates the adsorption and regeneration between V1 and V2 at 4-hourly intervals.
Various sized several motorized gate valves open and close according to the PLC commands to purify the gas entering the convertor.
Gate Valve Failed Motorized Operation
The 12” 1500 # butt welded to the pipe line gas inlet to V1 motorized gate valve (MGV 1) spindle bush damage prevented the changeover between V1 and V2.
The only option was continuing V1 adsorption as long as possible and devising a way to open and close MGV1 at least manually.
More Details
Mechanics identified that the valve gear toothed stem bush (GTSB) stripped threads prevented the MGV was the reason for MGV 1 inoperability and the failed V1-V2-V1 automatic changeovers. The valve vendor quoted GTSB delivery time was 3-months and price exorbitant.
The commissioning crew overlooked MGV gear boxes was the reason for the MGV1 stripped threads. Hence, the crew filled grease in all GBs and saved further damages. Fortunately the steam bush gear teeth escaped damages.
The instrument crew adjusted the PLC to hoot and flash a lamp 1 hour before MGV 1 close / open needs. Mechanics standing by responding to the operator’s telephone call reached the MGV 1 and closed it using a hydraulic jack / opened using the already suspended chain pulley block and operated the valve close / open limit switches as MGV 1 would do on PLC operation.
Thus, the PLC recognizing proper MGV1 position completed the rest of the changeover tasks. Thus, the PLC completed the V1-V2-V1 changeovers automatically aided by manual MGV1 close / open steps and the guarantee test progressed to successful completion.
During the few days it took to restore the MGV 1 motorized operation, the instrument crew set V1-V2-V1 changeovers to 8-hours instead of the usual 4-hours to avoid too frequent crew call outs.
Bush Repair
The author got the gear toothed stem bush (GTSB) repaired thus:
- A machinist made a 10-mm wall thickness CI bush (CIB), enlarged the GTSB bore and shrink fitted the CIB to the GTSB
- He cut ACME threads in the CIB bore to suit that on the MGV1 stem
- As no sample stem was available the machinist walked to V1, ascended the platform several times and walked back to the workshop to try the bush on the valve stem till the stem accepted it smoothly.
- His determined and patient efforts yielded a smoothly screwing in / out GTSB, restored MGV 1 open / close and fully automatic V1-V2-V1 changeovers and the Guaranteed Test Completion.
New Idea
The innovative idea and the machinist’s fine job immensely benefitted the company thus:
- Enabled the hard won export orders and boosted the company image in the international market
- The company’s financial gain too was enormous
- Restored Molecular Sieves full automatic operation – a boon in tightly staffed units
- Saved the enormous expenses of buying a GTSB and several months human aided V1-v2-V1 changeovers
- The valve repair saved sending back the vendor engineers to Japan and calling them back for a repeat guarantee test – an infructuous enormous expense.
Author: S. Raghava Chari
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