6 replies to this topic

[wil ma]

Hi,

I'm a plant operator (neither a chemical engineer nor student, so please go easy).
We have a heat exchanger (bfw preheater) where we assume a problem.
Sketch: see attach.
Because we noticed a decrease in available power to our turbine (witch consumes all the steam generated in a downstream steam drum) and a (over a long time) decrease in temperature (from 328°C to now 318°C) from the bfw out, the general thought is that the heat exchangers tube side has problems with fouling.
A difficulty for interpretating the data is that you cannot see the temperature decrease as long as the heat exchanger still produces some steam (will always be the steam saturation temperature).
During next major shutdown we are planning to clean them.
My concern is (because we see no improvement after a not planned short stop, witch is as good as a tube-side steam flushing) that something is wrong on the shell side (baffle corrosion - erosion, if that is possible).
The bundle cannot be pulled.
My questions:
Is it possible that because of higher gas temperatures (below design) and therefore bigger steam production in the heat exchanger something at the shell-side went wrong?
The same question but then for rather quick depressurisation of the shell-side.
Are there other possibility's to explain this phenomenon?
What options do we have if the problem is on the shell-side?

Kind regards.

 preheater.xls   19KB   79 downloads

[Art Montemayor]

Wil Ma:

First and foremost, you have no need to qualify yourself. Your communication skills speak highly of you and that is a quality that is sadly lacking in some of our degreed and veteran engineers. I can frankly tell you that it is a pleasure and a point of pride to have a legible, clear, concise, and detailed query on this Forum. I commend you for displaying your skills and knowledge.

I am addressing some comments back to you in the attached Rev1 form of your excellent communications tool: your workbook sketch.

I would add the following as responses to your direct questions:

You sketched out a BFW (Boiler Feedwater) preheater. Yet, you refer to a steam generator. I assume that the produced, heated BFW is converted to saturated steam further downstream in a steam drum heated by another source. It is the BFW preheater that is giving you the problem by not producing a constant, 328 ^oC heated water stream. By producing a design outlet temperature of 328 ^oC, the BFW preheater is feeding hot water that only needs an additional 2 ^oC to reach the saturated steam conditions of 129 bara and 330 ^oC. This is a very close, but optimized condition for the preheater in order to maximize the efficiency of the steam generation downstream.

If you are witnessing the outlet BFW temperature on the preheater dropping in value, this can be the result of various problems – all based on the BFW flow rate, temperature, and pressure being constant:

1. the temperature reading is erroneous;
2. the 3-way valve is leaking and by-passing more cooler BFW than it should;
3. the shell side of the preheater is fouling and not allowing design heat transfer to take place;
4. the tube side of the preheater is fouling and not allowing design heat transfer to take place.

Based on your description, I would suspect that the shell side is fouled. This is normally expected with the BFW stream. Regardless of how low your solids and chemical content is in the BFW, sooner or later there will accumulate sufficient solids in the preheater so as to start affecting the heat transfer. That is yet another reason for expecting the BFW to be put on the tube side. The tube side is much easier to clean or rod out than the shell side.

I don’t understand your questions: “Is it possible that because of higher gas temperatures (below design) and therefore bigger steam production in the heat exchanger something at the shell-side went wrong?

The same question but then for rather quick depressurisation of the shell-side.”

If gas temperature are higher, how can they be below design? Also, NO steam production should be taking place in the heat exchanger; if there is, you have much worse problems. I don’t understand what is your concern with “depressurisation of the shell-side.”

If the problem with the BFW preheater is on the shell-side, I’m afraid you don’t have any options except to try to effect a chemical clean-out of the shell side. Since you obviously have a fixed tube sheet and can’t remove the tube bundle, outside of cutting into the shell, I don’t know of any other option. As I mentioned above and inferred in my sketch comments, that is one of the reasons why BFW is normally inserted into the tube side of preheaters – in my opinion.

I hope this experience helps you out.
 BFW_Preheater_Rev1.xls   31KB   109 downloads

[wil ma]

Hi Art,

Thanks for your knowledgeable reply.

I'll try to clarify some points.
The produced, heated BFW is indeed converted to saturated steam further downstream in a steam drum heated by another source. In the past, the temperature of the saturated steam and the bfw to the steam drum were nearly the same.
We also assumed that there was some steam production in the preheater; hence its design where the mixing of cold bfw takes place in the middle of the exchanger and not after (mixing steam and water) and the slope of the pipe to the steam drum.
The controlled value for the 3-way valve is the cold gas out temperature.
We excluded the possibility that the 3-way valve is leaking by closing the manual valves in the bypass.
It is possible that the temperature reading is erroneous but as I said above we also have a loss in power for our steam consumer.
There is some pressure drop over the 3-way valve but we don't have the indicators for that.

"If gas temperature are higher, how can they be below design?"
>> The gas outlet temperature is controlled, so if gas inlet is 20°C higher (but below design witch is 480°C), more heat is exchanged, bypass of the 3-way valve is closed, and there will be (i.m.h.o.) some steam production.

"I don’t understand what is your concern with “depressurisation of the shell-side.”"
>> My concern was some mechanical deformation of the baffles or other "stuff" on the shell-side.

Sorry that I haven't put some of the need to now facts in my first post.
Chemical clean-out of the shell side seems indeed the best (and may be only) solution.
Thanks.

Kind regards.

[AA Mishra]

Good to see a plant operator.

You are having difficulty to put on the paper.

Good.

Your post is Plant President.

You are neither chemical engineer nor student.

No problem.

Why?

You with Chemical Engineer find out the problem areas.

No Problem.

Why?

You assume a problem.

BFW exchanger.

Good.

Your concern is you see no improvement.

No Problem.

You see the life of heat exchanger.

More than 10 yrs old, buy new exchanger.

New exchanger installation is with slope.

The lower slope is with drain provision.

During normal operation, every 15 days, heavy blowdown operation.

The new exchanger is having tube pulling facility.

Old exchanger, sell.

During next shutdown, plan to install new exchanger.

Plant operator is President.

You face difficulty in writing the same.

No problem.

Regards

[Art Montemayor]

Wil ma:

If you are a Belgium - particularly from Flanders - then I could never be surprised to see how much know-how and operating skills you have and demonstrate.

I was priviledged in 1969 to manage a grass-roots Furfuryl Alcohol Plant project in Geel, Belgium and it turned out to be not only a unique and super successful venture for me and my company; it also turned out to be a first-class learning lesson for me in having first-class plant operators that not only were fully prepared and educated in 3 languages, they were skilled in the sciences and math as well. That facility is still in operation today and producing product at a profit - after going through more than one production expansion project through the last +35 years.

I think I suspect something about your very capable and professional outlook and know-how. You need very little help in evaluating your potential problem and I commend you on your engineering capabilities. If you are in Flanders, then enjoy a Triple Trappist Beer for me. And if its not available, then a Stella Artois will do.

Best of luck in your remedying of the situation. I think it is in very good hands.

[wil ma]

Hi Art,

Thanks for the kind words and advice.
Next time, I will enjoy a Westmalle Triple Trappist for you.

Kind regards.

[wil ma]

Hi AA Mishra,

Buying a new one is at the moment no alternative, although your ideas are very interesting.

Kind regards.