5 replies to this topic

[trungphu]

Hello everyone,

 

I am a junior operator in an Ammonia plant . It's my pleasure to learn more about Ammonia plant operations from professionals in the forum. My question here is about protection of process air preheater. When the process air to sec. reformer is too low, an amount of protection steam will be admitted to the air preheat coil to protect it from overheating. In this case, what should we do to keep primary reformer and sec. reformer not being tripped?

 

Regards,

 

Trung Phu

Edited by trungphu, 08 May 2015 - 11:10 PM.

[Atttyub194]

Please elobrate your question and your intent

[Faisal Ghafoor]

some plants have low air flow security also tripping secondary reformer; means air cut to secondary reformer, and as a consequence back end also trips. To continue primary reformer operation, you need to take care of system pressure by giving of-set to system pressure vent valve and you also need to take care of steam system; steam let down stations should be opened smoothly to avoid low S/C ratio trip of Primary reformer.

[Satyajit]

Hi Trung,

 

If I understand your question correctly, please see my response below.  I think you are talking of two things:

1. Protection of process air coil at low process air flow.
2. You intend to keep both primary and secondary reformer running with low process air flow.

Q1. Protection of the air coil.
Ans. All licensors advise to keep a small flow of 2-3 MT/hr medium pressure steam along with process air all the time.  Make sure you are doing that.  Whenever plat trips or process air compressor trips, a pre-set value of steam will go to process air preheat coil protecting it from overheating.  You need to shut off some of your tunnel burners, reduce reformer firing and also increase medium pressure steam flow to keep the skin temperature and process air temperature within your plant operating envelope.  Be careful while injecting Medium Pressure steam into the coils.  Watch secondary reformer bed temperature.  If the secondary reformer combustion zone temperature falls below auto-ignition temperature of 680 ^oC, you might have an explosive mixture and that will cause big threat to the reactor in particular, the plant and the people in general.  Please put an alarm on low air/gas ratio if there is no trip connected with low-low air/gas ratio; normally trip is connected with high-high air/gas ratio.

 

Q2. How to keep the plant running with low air flow.
Ans. What Faisal has mentioned above, you need to follow systematically.  In fact, you do have similar situation during plant start up.  I would take the following steps and few precautions.

1. Take syn loop partial trip, vent syngas at syngas compressor suction, syngas compressor on recycle; stop extraction and open High Pressure to Medium Pressure steam let down valves.
2. Import Medium Pressure steam; reduce front end load to match process air flow and steam requirement in the plant.  Never run the Reformer below 50% capacity consistently for a long time for thermodynamic reason.
3. Bypass the Low Temperature Shift Converter.
4. Stop the refrigeration compressor if available steam is not adequate.
5. Analyze why you had a situation of low process air flow; if not solved, you should shut down the plant.  Keeping low process air flow or no process air flow could lead to reverse flow of process gas into the process air line and an explosion could occur.

Ammonia industry is 101 years old now, many incidents happened in the past and you should read those articles to improve your knowledge and share with others.  Please refer to the articles published in AICHE annual safety meeting for last 60 years.  I appreciate that you raise these questions in this forum as it helps everybody.

 

One more suggestion to you: As you are working in shift in the plant, spend part of time in doing similar process safety review of plant operation at different scenario like this.  This will help to improve knowledge and safety awareness in the plant.  Hope this replies your questions.

 

Good Luck.

Satyajit

Edited by Art Montemayor, 11 May 2015 - 06:03 PM.

[trungphu]

Thank all for your valuable advice especially Mr. Satyajit. Your answer is absolutely perfect for my concern.

[engahmed1984]

why ammonia flowing in a pipeline at (pressure 20 bar / temperature 18.5 C / density 612 kg/m3 ) forms ice at its surface ? 
is this related to data from ammonia P-T chart ?