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Forensic Engineering:

The Story of the Blown-out Steam Header

A long time ago an accident occurred at a steel plant powerhouse.  The turbine lead steam header of a LP back pressure type steam turbine had blown off from one end, releasing steam under pressure. We heard that two of the plant’s operators died and more were injured. We were called in to investigate.

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The initial finding by the owner’s engineer revealed that after 30 years of operation, there was a shear failure at the welding between the header and the endplate. We were not fully satisfied that this was the core reason for the failure. We decided to delve in deeper.

Gathering Evidence

To begin with, we collected drawings of the piping and the building. We asked for the MDR, operations data and maintenance data, but none was available. However we did get an important piece of information from the plant operators, who told us that they had heard hammering noises in the turbine lead header. But besides that we had to rely on our first-hand observations, which were:

  • The curved endplate was made of the same material as the pipe (DN250 ASTM A106 Gr.B)
  • Shearing was from the welding line
  • Insulation was tattered in some places allowing heat transfer to the atmosphere
  • There was no steam trap drain line to take the condensate away from the main line
  • The piping was spring supported throughout, but one of the springs at the top, near the safety valves was locked – maybe in order to prevent the line from vibrating

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Analysing Evidence

It was clear that the welding had given way. But, it is important to note that the welding and the endplate survived for 30 years. So, the root of the problem must have been deeper.  It seemed to be something to do with the high pressure. Surprisingly the endplate was cut from a similar pipe to the header. This was a non-compliant use of material and the thickness of the cut pipe was less than that was needed for an end plate under ASME B31.1 for a design condition of 2000 kPa at 215° C.

Although hard to prove in the absence of process data, it seemed that the hammering sound indicated the presence of saturated steam and condensate in the line, possibly creating a steam hammer. The condensate was being formed due to the tattered insulation, which was causing heat loss. We found through pipe stress analysis that the lowest point of the line, due to thermal movement, was near a spring, which was bottomed out. The locked spring support, acting as a vertical rigid one, near the safety valves, was causing the piping to grow all the way downwards as it expanded with heat. This was bending the elbow so far that the condensate began to collect in a pool, restricting the flow of steam, causing a hammering noise. One day, when there was too much condensate, the steam escaped by blasting through the weakest point in the line – the endplate.

Providing Solutions

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Our technical solutions were:

  • Replace the end spool of the turbine lead header with a new pipe and a BW cap

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  • Repair and provide proper insulation
  • Provide steam traps and drains at the lowest points, determined by pipe stress analysis under operating conditions
  • Move the location of the safety valves
  • Provide spring support at the first support on the roof top
  • Modify support locations and change some of the springs
  • Do non-destructive tests of new joints and old ones

But our solution did not stop at the design-installation. The incident was also clearly preventable through proper maintenance and good organisational practices. So, we advised the owners to:

  • Maintain operations process data, keep a maintenance log
  • Establish an early reporting protocol of any deviation in process parameters or anomalies observed by the operators
  • Engage owners’ engineering specialists for any modification and organise third party review at regular intervals for health check
  • Averting Failure

There were a lot of lessons to be learnt in failure aversion from the endplate incident at every stage of the project. We have summarised a few underneath from each phase.

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Project Management

  • Close and conservative material management

Design Phase

  • Proper design and analysis considering all failure modes
  • Appropriate design foresight
  • Using standard components design as much as possible (do not deviate under duress)

Executions Phase

  • Construction contractor should follow design more closely
  • Quality inspector should be more thorough

Commissioning

  • Checking Design Review and other safety review reports and ticking off all punch list items to satisfaction
  • Proper review of MDR

Operations Phase

  • Early heads up
  • Operators need to have more detailed knowledge of handled fluid
  • Retain and analyse operations & maintenance data
  • Proper maintenance regime
  • No modification without designers consent

The methodology of doing a project has been established over hundreds of years. The process is tedious but needs to be followed meticulously. Any shortcut or variation unapproved by the design group can spell catastrophe, if not now, then in the future.

The Story of a Leaking Heat Exchanger

Stories of leaking heat exchangers arise in old plants like ghost stories in abandoned houses. But this story is worth telling because it happened in a new plant. This is a story of engineering behind the scenes.

In the beginning …

It was a fairly new gas plant. But, the Amine Plate Heat Exchanger started leaking, causing the plates to corrode. The plant was shutdown. The plates were cleaned. The gaskets were changed. The plant was started again. Within months the heat exchanger started to leak again. The tiresome and expensive process of shutting down, cleaning, changing was repeated.

We got a call

The plant was restarted once more, only to shut down once again. Each shutdown cost the operator millions of dollars. The operator contacted the vendor demanding a resolution to the issue caused by the malfunctioning heat exchanger. The vendor said that they were not at fault. That model of heat exchanger has been around for a while, used widely and no one else was facing a problem.

The operators were wise. They took the sensible option of engaging independent third party engineers. As independent third party engineers, our focus was mostly on looking into the defects of the plate heat exchanger and to proceed to FEA if needed to simulate and prove those defects. Time was of the essence as we were engaged during the 4th shutdown. A quick solution was needed to stop the recurrence of leaking incidents and loss of revenue.

We went to have a look

  • We visited the site to examine the heat exchanger and the condition of the plates and gaskets. We collected first-hand information and looked at the process of running the plant as a whole.
  • We found that the Plate HX was dismantled, the plates were corroded and the gaskets were torn
  • Installation MDR was showing that everything was in order
  • The maintenance log was showing right torqueing of the heat exchanger bolt
  • The exchanger plates were made from stainless steel and the gaskets were made of rubber as per the MDR
  • No significant pressure or temperature spikes or excess flow condition was noted in the operation log
  • We gathered, after interviewing operators, that the leaks start within a few days of startup and the quantity continues to increase as the days go by
  • The gaskets were replaced to the specification of the OEM. A few plates were also changed to the OEM specification
  • We walked the line armed with the piping layout in our hands and discovered that the first support of the pipes in front of the heat exchanger was restrained on all sides. This looked a little suspicious.

Our Reasoning 

We thought long and hard about what we found. We came to these conclusions:

  • The heat exchanger selected and installed as per API 662 was fit for the purpose from the process point of view
  • The materials used were certified as per EN10204
  • The process data showed that there was no flow induced forces
  • So, we thought, the problem must be with the piping. The pipe stress analysis as per ASME B31.3 for the piping configuration showed induced forces and moments on the heat exchanger nozzles. These loads were being transmitted in its entirety to the HX body, thereby distorting and compromising its integrity. The liquid was finding its capillary path through the gaskets, corroding the faces not designed for exposure. The liquid was eventually making its way out through the HX plates’ path.
  • Restraining the pipes on the axial direction, was inducing loads beyond the allowable limits of HX nozzles
  • Why was this? Well, the earlier stress analysis did not get the confirmation for the loads from the HX vendor

Our Solution

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We provided the following solutions to ensure a fully functioning, hassle-free plant:

  • Remove the guides on the pipe supports in the axial direction to the heat exchanger nozzle from all first supports in front of the HX
  • Provide a Teflon pad under the pipe support fixed with a counter sunk screw to the base plate on the steel member
  • Put a position mark for the pad to monitor the pipe movement. Keep regular vigil on the condition of the Teflon pad
  • Modify a few support locations and type of supports at the heat exchanger and air cooler end to satisfy allowable nozzle requirements of the equipment as per API 662 standards

A Happy Ending

In short, the story had a happy ending. We helped reduce unplanned shutdown hours. The availability of the plant was increased due to our investigative and consultancy work. The owner was saved millions of dollars. We were happy to help.

Sketches and pictures are for visualisation and not actual site photos

		
100 years of the BPVC code You can read more of the ASME article in ASME link given below

https://www.asme.org/getmedia/1adfc3df-7dab-44bf-a078-8b1c7d60bf0d/ASME_BPVC_2013-Brochure.aspx