Inst ToolsInst ToolsInst Tools
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Search
  • Books
  • Software
  • Projects
  • Process
  • Tools
  • Basics
  • Formula
  • Power Plant
  • Root Cause Analysis
  • Electrical Basics
  • Animation
  • Standards
  • 4-20 mA Course
  • Siemens PLC Course
Reading: Steam Letdown Station Problems and their Solutions
Share
Notification Show More
Font ResizerAa
Inst ToolsInst Tools
Font ResizerAa
  • Courses
  • Design
  • PLC
  • Interview
  • Control System
Search
  • Courses
  • Automation
    • PLC
    • Control System
    • Safety System
    • Communication
    • Fire & Gas System
  • Instrumentation
    • Design
    • Pressure
    • Temperature
    • Flow
    • Level
    • Vibration
    • Analyzer
    • Control Valve
    • Switch
    • Calibration
    • Erection & Commissioning
  • Interview
    • Instrumentation
    • Electrical
    • Electronics
    • Practical
  • Q&A
    • Instrumentation
    • Control System
    • Electrical
    • Electronics
    • Analog Electronics
    • Digital Electronics
    • Power Electronics
    • Microprocessor
  • Request
Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Inst Tools > Blog > Root Cause Analysis (RCA) > Steam Letdown Station Problems and their Solutions

Steam Letdown Station Problems and their Solutions

Root Cause Analysis (RCA): Steam turbine 100 to 40 bar steam letdown station 3-year long persistent flange leaks stopped.

Last updated: September 28, 2022 3:19 pm
S Raghava Chari
Root Cause Analysis (RCA)
No Comments
Share
7 Min Read
SHARE

Root Cause Analysis (RCA): Steam turbine (K-601 T) 100-40 bar steam letdown station 3-year long persistent flange leaks stopped.

Contents
Background InformationLetdown Station DescriptionLetdown Station ConfigurationSteam Letdown Station ProblemsBenefits of the RCA based solution
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.

Background Information

1500 T/d NH3 plants 2 Nos. fired boilers start first. Each feed 100 T/H 40 bar superheated steam (SHS) into the 40‑bars SHS header. The frontend plants start using this SHS.

They stabilize in two days and deliver Synthesis gas (SG 75% H2 and 25% N2 gas mixture) and their waste heat boilers around 360 T/H 100-bars 530o C SHS. The plant vents the SG till the required purity is achieved and letdown the 100-bars SHS.

into the 40-bars SHS header. The plant DCS adjust the firing to hold the 40‑bars SHS pressure constant.

K-601 starts compressing the pure SG and venting stops. Its drive turbine K‑601 T admits the entire 360 T/H 100-bar SHS. 330 T/H extracted SHS from it joins the 40-bar SHS header. K-601 T condenser condenses the balance 30T/H SHS.

As before the DCS adjusts firing to hold the 40‑bar SHS pressure constant. Thus, the letdown station avoids wasteful SHS venting and noise pollution during the two after front end start and stabilizing days.

The paragraph letdown station’s configuration describes its other benefits.  

Letdown Station Description

Vertically downwards 10” pipe tap off from 100 bar SHS header branches into 3 Nos. zero elevation floor supported 6” pipe branches.

Steam Letdown Station Problems
Fig 1

The branches combine into a 10” vertical takeoff joining the 40‑bar SHS header (fig 1). Each branch consists of:

  1. 6” both ends welded inlet block valve
  2. 6” 1500 # RTJ flanged control valve (CV)
  3. CV downstream 5 Nos. 8” 1500 # RTJ flanged restriction orifice assemblies (ROA). The 5 ROAs absorb around 80% letdown pressure for the below given reasons:
  4. Enables selecting line size control valve for high pipeline strength
  5. The low across CV ▲P minimizes valve plug and seat erosion and prolongs their lives
  6. ROA downstream 6” 1500# butt weld end block valve
  7. Connecting piping and pipe supports

Letdown Station Configuration

Below is the system configuration:

  1. One Manual Station outputs 70% (15.2 mA) open signal to Branch 1 (B1) letdown CV. This signal passes via K-601 T Trip Throttle Valve (TTV) close limit switch (LS)  
  2. On K-601 T trip the LS conveys the 15.2 mA B 1 CV; it opens 80% within 3 secs, lets down bulk of the K‑601 T consumed steam into the 40-bars steam header.
  3. The DCS controls the 100-bars header pressure by operating B2 and B3 CVs in split range control and also the 40-bars SHS header pressure by adjusting the boilers firing
  4. Thus, the letdown system controls the 40-bar and 106-bar steam pressures rapidly to prevent safety valves blowing and for continued waste heat boiler runs  

The above mentioned are letdown stations other benefits referred previously.

Steam Letdown Station Problems

from day-1 estimated 20 T/H 1 huge SHS leaks wasted enormous money. Repeated ring gasket changes at available plant shutdown (SD) opportunities, online hot bolting, and online leak sealing were futile.

The author replaced the flanged ROs with shop pre-made butt weld end Restriction Orifice Assemblies eliminated 15 flange joints and their leaks. However, leaks to a much smaller extent developed at the three Nos. CV flanges.

The author rejected his colleagues’ insistence to weld the control valves also to the pipelines, as he now became certain that pipeline stresses warping the flanges is the unstoppable flange leaks’ root cause.

Welding the CVs also to the pipe removes the only stress relief point; hence, the pipe could burst and the resulting huge quantities of steam spills would be unmanageable and shutdown the plant for several days. The only benefit was former leaks 20 % only now, which paid the welded orifice assembly installation costs in a week. 

Constant thinking and several site studies finally revealed to the author the flange leaks Root Cause viz. the piping stresses elimination:

He concluded pipe supports welded to the pipes at the top and their foot plates grouted to concrete floor prevented the pipes’ temperature change travels i.e. zero pipeline flexibility to put in pipe designers’ parlance – the root cause. Remember 530o C steam flows through the pipe!

Below listed the author got done online piping flexibility improving remedies stopped the 3-yearlong solution defying steam leaks. 

  1. Crew flame cut the support foot plate bolt holes oblong along pipe travel direction i.e. along pipe length (fig 2)
  2. They inserted shop made spacers (S) on the bolts protruding 1‑mm above foot plate
  3. The foot plate to ground securing nuts bottom on the spacers; this way B1, B2, B3 travel freely with temp changes dragging their supports sliding on the floor
  4. I.e. B1, B2 & B3 have enough thermal travel flexibilities.
SHS Pipe Support Foot Modification Details
Fig 2

Benefits of the RCA based solution

Below listed are the realized benefits:

  1. The three yearlong flange leaks vanished as if magic confirming the diagnosis and effectiveness of the solution
  2. Estimated 20 T/H flanged orifice assembly leaks during the first year and 1 T/H control valves flange leaks during the subsequent 2 years and its enormous costs vanished

Stopping the leaks by eliminating the high stresses due to flexibility lack eliminated the previously described sudden unexpected potential pipe ruptures, associated catastrophes and long plant outage.

Author: S. Raghava Chari

Do you face any similar issues? Share with us through the below comments section.

If you liked this article, then please subscribe to our YouTube Channel for Instrumentation, Electrical, PLC, and SCADA video tutorials.

You can also follow us on Facebook and Twitter to receive daily updates.

Read Next:

  • Leaking Plug Valves
  • Flange Joint Problems
  • Pipe Elbow Sudden Burst
  • Fan Motor Journal Bearing Failures
  • Fired Boiler Induced Draft Fan Failures
Don't Miss Our Updates
Be the first to get exclusive content straight to your email.
We promise not to spam you. You can unsubscribe at any time.
Invalid email address
You've successfully subscribed !

Continue Reading

Instrument Steam Pressure Taps Primary Block Valves Glands and Bonnets Leaks
Flanged Rotameter Caused Pipe Weld Cracks and Gives Wrong Readings
Transformers Coolers Weld Joints Oil Oozes Were Eye Sores
Multistage Flash Drums Erratic Level Readings
Cooling Water Circulating Pump Low throughput Resolved
Compressor IP Case Discharge Temperature Gradually Increases
Share This Article
Facebook Whatsapp Whatsapp LinkedIn Copy Link
Share
Leave a Comment

Leave a Reply Cancel reply

Your email address will not be published. Required fields are marked *

Stay Connected

128.3kFollowersLike
69.1kFollowersFollow
210kSubscribersSubscribe
38kFollowersFollow

Categories

Explore More

Wrong Operation Internals Destroyed Demineralization Mixed Bed Vessel
Pipeline Shakes Torn apart Valve Positioners
Compressor Cracked Shaft Spacers
Repeat Failures of Letdown Valves
Many Bucket Elevator Wrecks rob crew morale
Bucket elevators Chain and teeth wear Problems
Pressure Correction Pneumatic Instrumentation
Primary Reformer – Each Turnaround done expensive Tubes Repairs

Keep Learning

Rotary Granulator Drive

Rotary Granulator Drive Root Cause Analysis (RCA)

Compressor IP Case Discharge Temperature Gradually Increases

Compressor Anti-Surge Control near-miss incident

Pipe Elbow Sudden Burst

A Pipe Elbow Sudden Burst Fatalities and Wrecked Panel Instrumentation

On Off Valve Root Cause Analysis

Root Cause Analysis for FRP Line Damaged due to On-Off Valve

Totally Enclosed Fan Cooled (TEFC) Motor

Totally Enclosed Fan Cooled (TEFC) Motors Rampant Winding Burnouts

Process Air Compressor Overhauls Eliminated

Process Air Compressor too Frequent & too Lengthy Overhauls Eliminated

CO2 Stripper Old Differential Pressure Transmitter Stopped Working

CO2 Stripper Old Differential Pressure Transmitter Stopped Working

Solving Critical Control Valve Hunting

Solving Control Valve Hunting: Instrument Troubleshooting

Learn More

Methods of Measurement

Methods of Measurement

Connect PLC with SCADA System via OPC UA

How to Connect PLC with SCADA System via OPC UA?

PLC Raw Count Calculation formula for Pressure Transmitter

PLC Raw Count Calculation for Pressure Transmitter

Capacitive Field Effects on Conductors-4

Capacitive Coupling Effects

PLC Program for Forward and Reverse Motor control

3 Phase Motor Control using PLC Ladder Logic

Difference Between Wifi and Industrial Wireless

Difference Between Wifi and Industrial Wireless

What is a function call in TIA Portal

Difference Between FC and FB in Tia Portal

advantages-disadvantages-of-ac-and-dc-power-transmission

Advantages & Disadvantages of AC and DC Power Transmission

Menu

  • About
  • Privacy Policy
  • Copyright

Quick Links

  • Learn PLC
  • Helping Hand
  • Part Time Job

YouTube Subscribe

Follow US
All rights reserved. Reproduction in whole or in part without written permission is prohibited.
Welcome Back!

Sign in to your account

Username or Email Address
Password

Lost your password?