Summary Notes

Topics

  1. Troubleshooting
  2. Monitoring
  3. Does it fit the observation?

Troubleshooting top

Troubleshooting Guidelines

  1. Gather information.
  2. Apply solid engineering fundamentals.
  3. Separate observations from hypotheses or conjectures.
  4. Independently verify data using field measurements and observations, when possible.
  5. Make rigorous comparisons with satisfactory operations and compare the data obtained under normal operation with that obtained under faulty operating conditions.
  6. Spend time in the unit making direct observations - even if you are not sure what to expect.
  7. Consider the entire system related to the problem.
  8. Practice good listening skills.
  9. Do not reject serendipitous results.
  10. Brainstorm all the things that could explain the fault.
  11. Use other troubleshooting strategies to deduce what happened during the faulty run. Present an analysis in the form of a table or chart.
  12. Choose the most likely cause or set of conditions that produced the data and run the equipment at these conditions to attempt to reproduce the data to verify the hypothesis. Do not fall in love with a hypothesis - seek to reject, as well as to accept.
  13. Suggest a new troubleshooting scenario. After supervisor approval, collect data and describe how the problem should be approached.

Woods' Troubleshooting Worksheet

5 W's and an H

What: What do we know? What is the purpose?
What was observed? What don't we know?
What is a related problem? What was not observed?
What are the constraints? What is unrelated?
What is expected? What is not a constraint?
What is the same? What is unexpected?
What is the importance? What is different?
What resources are needed? What is not important?
What are the criteria?

When: When was the difficulty first noticed? When were the instruments calibrated?
When must it be solved? When was everything OK?
When were changes made?

Where: Where in the plant did the problem arise? Where do the products go?
Where do the inputs come from? Where in the plant is everything OK?

Who: Who do I contact to get more information? What said what?
Who will help me? Who should not be contacted?
Who did what? Who cannot help?

Why: Why is this an important problem? Why is this an unimportant problem?
Why doesn't it work?

How: How is the problem related for past experiences? How did we arrive at this result?
How do we know this for sure? How is the problem different from past experience?

Troubleshooting Procedure

  1. Compare the data obtained under normal operation with that obtained under faulty operating conditions.
  2. Brainstorm all the things that could explain the fault.
  3. Use other troubleshooting strategies to deduce what happened during the faulty run. Present an analysis in the form of a table or chart.
  4. Choose the most likely cause or set of conditions that produced the data and then run the equipment at these conditions to attempt to reproduce the data to verify the hypothesis.
  5. Suggest a new troubleshooting scenario. After supervisor approval, collect data and describe how another engineer should approach the problem.

Troubleshooting: The Boiler Feedwater Heater Case #1 Marlin and Woods

Example of In-class Exercise from T.E. Marlin and D.R. Woods Case History based on a case from P.L. Silveston. Case 28 from Woods, D.R., Successful Troubleshooting for Process Engineers, WILEY-VCH VERLAG DARMSTADT GERMANY (2006)

Waste flash steam from the ethyl acetate plant is saturated at slightly above atmospheric pressure. It is sent to the shell side of a shell and tube heat exchanger to preheat the boiler feed water to 70°C for the nearby boiler house.

Condensate is withdrawn through a thermodynamic steam trap at the bottom of the shell. The water flows once through the 3/4" nominal tubes. There are 1000 tubes. "When the system was put into operation 3 hours ago everything worked fine," says the supervisor. "Now, however, the exit boiler feed water is 42°C instead of the design value. What do we do? This difficulty is costing us extra fuel to vaporize the water at the boiler." Fix it.

Fundamentals

Tout = Tsteam - (Tsteam - Tin) exp(- UA/mCP)

The overall heat transfer coefficient U is related to the individual heat transfer coefficients inside (hi) and outside (ho) by the equation

1/U = 1/ho + 1/hi
ho = 20,000 W/m2-°C (outside: shell side)
   hi = 1500W/m2-°C (inside: tube side)
If air were present instead of steam the shell side heat transfer coefficient would be about
    ho = 10 W/m2-°C (outside: shell side)

Brainstorming Possible Faults

  1. The steam trap is blocked causing liquid condensate to back up in the heat exchanger so the steam does not contact the pipes in the exchanger.
  2. The entering water is sub-cooled.
  3. The steam pressure and temperature have dropped.
  4. The heat exchanger has become fouled.
  5. The steam is dirty, i.e., contains non condensable gases.

Monitoring top

If I make this measurement or take this action, what will it tell me?

Measurement: Inlet Temperature Reason: Sub-cooled inlet
Measurement: Water flow rate Reason: Higher than normal flow rate could cause the fluid not to reach 70°C

Action: Check to see if the steam trap is closed, and not functioning properly. If it is functioning, it should open and close periodically as condensate is formed in the shell. Reason: Water may be filling up the shell side of the exchanger reducing the condensing steam heat transfer coefficient.
Action: Check to see if the filter is plugged Reason: Would give same symptoms as a closed steam trap
Action: Carefully open the vent Reason: If non-condensable gases have accumulated in the shell, the steam side heat transfer coefficient would be decreased, reducing U.

Action: Check to make sure the drain valve is open Reason: If someone has closed the drain valve, water may be filling up the shell side of the exchanger reducing the condensing steam heat transfer coefficient.
Action: Check the inlet steam temperature and pressure Reason: If either of these has decreased, the enthalpy of the entering steam will be less than expected, reducing the outlet water temperature.

Does it fit the observation? top

Cause Result Does it fit the Observation or Measurement? Steps needed to check cause Feasibility
Fouling/scale on water side, or on steam side. Decrease in heat transfer coefficient. Does not account for a temperature drop over short period. Instrumentation and measurements to calculate H.T. coefficient / inspection of tubes. Inspection of the tubes Time consuming and costly if instruments are not available.
Malfunctioning steam trap / clogged condensate valve. Rise in water level and consequent loss of area. Water build-up wouldn't account for temperature drop. Observation of water level in condenser shell. Easy: Shut down condenser and remove drain.
High inerts in steam / clogged bleed valve. Decrease in heat transfer coefficient. Inerts build-up may account for temperature drop. Shut down, vent inerts and restart, bleed gas analysis Availability of skilled technician and equipment?
Inaccurate temperature reading. No actual malfunction in the boiler. Does not account for drop over short period. -- --
Steam superheat too high I water flow too high. No malfunction. Does not account for drop over short period or large drop -- --
Drop in steam pressure due to a steam side leak. Change of condensation temperature. No visible / audible signs of steam leak. -- --