Multistage Flash Drums Erratic Level Readings

Instrumentation engineering root cause analysis of multistage flash drums erratic level instrument readings.

Article Type:Root Cause Analysis (RCA)
Equipment Type:Sensors
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.

Multistage Flash Drums Erratic Level Readings

High Pressure (HP) side extended seal diaphragm differential pressure transmitters (DPT) never developed pinholes, unlike the previously discussed urea plant seal DPTs, shop calibrated, and static pressure tests passed before field installation.

Yet the 5-stages flash drums erratic level readings stopped the 750 T/d NH3 commissioning.

Author RCA

Because of drift-free calibration, all differential pressure level transmitters (DPLT) erratic outputs and the excellent brand name the author’ attributed root cause was:

HP, LP or both representative signals did not reach the differential pressure level transmitters (DPLT).  Therefore, he studied the high-pressure (HP) and low-pressure (LP) taps carefully from the vessel drawing and noted the following

  1. The 4” flanged HP extended diaphragm immerses fully in tranquil liquid NH3 contained in each stage chamber. Hence, the DP senses representative HP signal
  2. Welded to the previous stage bottom a 300 mm long extension skirt forms a 150 mm radial width annulus (below figure)
  3. Liquid NH3 from the previous higher pressure stage enters the annulus tangentially – a process requirement
  4. The DPLT LP tap senses the annulus gas pressure
Multistage Flash Drums Erratic Level Readings

Root Cause Identified

He considered these observations and concluded:

  1. The tangential entry and liquid evaporation cause unsteady pulsating annulus pressures
  2. The DPLT LP port senses this pulsating unrepresentative pressures – the erratic liquid NH3 level readings Root Cause disturbances could be causing the DPLTs outputted erratic level signals..

Root Cause Removed

½ hour thought flashed solution was: insert a supplementary nozzle via the LP tap penetrating ½‑extension skirt (above figure) to feed the skirt tranquil pressure to the DPLT LP port.

The diagnosed root cause impressed management ordered, “Go ahead; modify all stages LP taps”, to avoid a second shutdown if this step succeeds.

Root Cause found solution implemented

A crew took the below-listed steps:

  1. They inserted a shop made 100 mm long guide bush in the LP tap and bolted a flange over it
  2. A long drill passing through the guide bush bore drilled 20 mm dia hole in the skirt wall
  3. The crew inserted the shop made supplementary nozzle secured it bolting a flange
  4. Instrument crew connected the supplementary nozzle to the DPLT LP port
  5. Mech and Inst crew did these for all stages

Root Cause Solution Benefits

The DPTs read correctly, as expected, and commissioning progressed rapidly.

Author: S. Raghava Chari

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