Troubleshooting | top |
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? |
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Condensate is withdrawn through a thermodynamic steam trap at the bottom of the shell.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.
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)
If air were present instead of steam the shell side heat transfer coefficient would be abouthi = 1500W/m2-°C (inside: tube side)
ho = 10 W/m2-°C (outside: shell side)
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. | -- | -- |