Steam Flow Control 2

[MaNaTMoS]

What is the best way to control low-pressure steam (3.5 barG) flow to the air preheaters or steam reboilers channel head? Is it correct to install FRC on the steam inlet side or condensate outlet side?
If control valve is on the steam inlet side, the pressure of steam condensation may be changed (which changes temperature of condensation) and thus allowing operators to control heat input. If control valve is on the condensate outlet side, the way of changing heat input to the preheater is achieved by varying condensate level, i.e. varying condensing/subcooling surface area ratio.
What is the preffered method when using low-pressure steam in such services?

I have one particular control scheme on my mind, it looks to me rather interesting and flexible when using low-pressure steam for heating purposes ("Lesson learned by watching home radiators").

Heat input is controlled by varying condensate level in preheater's channel head, but not directly - there's a separate condensate pot, connected to the lower part of channel head (below pass-partition baffle) by pressure-equalizing line. Since both are at the same pressure, the level in the pot corresponds to the level in the channel head. By varying condensing/subcooling heat exchange surface area ratio, the overall heat duty is being changed. Another good part of such arrangement is prevention of CO2 or air pocketing in the channel head (which would promote CO2 corrosion and reduced heat duty by lowering steam partial pressure).
Additional protection is "level override": LC on the condensate pot assumes control at high & low level values, in order to prevent excessive condensate backup or blowing out the condensate seal. Steam line, as usually, has an ordinary gate valve which could be used anyway for changing steam condensing temperature (by changing pressure in the channel head), but it's not of much importance in low-pressure steam services, because there's too little space for varying condensing pressure between the steam header and condensate header pressure.

Again, I think this applies only for low-pressure steam services.
Please comment.

 

[25362]

When the heat input must be varied, there is a third option of control, and this is by-pass control. A fraction of the stream to be heated bypasses the exchanger blending with the fraction that has passed through. The fluid that passes through the exchanger will experience the full temperature of the steam.

Maximum steam P/T is provided to the exchanger tube side, condensate is removed by a trap or by a pot on LLC, while the fluid is partly by-passed. Two CVs or a three-way CV on the fluid side acted upon by temperature control may be used. For air flows use low pressure drop duct dampers.

A change in the setpoint of the fluid temperature controller is equivalent to a load change.

Systems where the fluid may polymerize, coke, or suffer other damage from steam temperature, as well as reboilers, cannot use this form of control.

I haven't seen air preheaters working on this type of control but in special cases it shouldn't be ruled out in advance. Any combination with the two other ways of control you mentioned (steam pressure and flooding of tubulars) can be envisaged.

Kindly comment.

 

[25362]

BTW, if can get hold of it, there is an article in the ChE, Sept. 3, 1973 issue, titled "Performance of Steam Heat Exchangers" by Jimmy Mathur (Brown and Root, Inc.) that may interest you.

 

[rmw]

I recall this topic being discussed in other threads in this forum, but I don't know what words to recommend you do a site search on, sorry.

rmw

 

[Montemayor]

MaNaTMoS:

For an excellent source of information on how process equipment is controlled by a recognized and respected pro in the business, go to:

http://www.driedger.ca/

There, you will find a treasure trove of articles that you can download, specifically detailing out the following:

1 CONTROLLING CENTRIFUGAL PUMPS
First published in Hydrocarbon Processing , July 1995

2 CONTROLLING POSITIVE DISPLACEMENT PUMPS
First published in Hydrocarbon Processing , May 1996

3 CONTROLLING SHELL and TUBE HEAT EXCHANGERS
First published in Hydrocarbon Processing , March 1998

4 CONTROLLING STEAM HEATERS
First published in Hydrocarbon Processing , November 1996

5 CONTROLLING FIRED HEATERS
First published in Hydrocarbon Processing , April 1997

6 CONTROLLING VESSELS and TANKS
First published in Hydrocarbon Processing , March 2000

I am not the only one on this Forum that is keen on Walter Driedger and the expertise he has shared through his great articles. His knowledge and experience is well known in Canada as well as the US. The answer to your query can be found in "Controlling Steam Heaters".

I know you will enjoy Walter's writing - as well as his "Kojak" hairdo.


Art Montemayor
Spring, TX

 

[MaNaTMoS]

Thanks,

I have seen this web site before, it's very good. And Mr Kojak is dangerous dude, I can say that for sure)

 

[katmar]

I asked a similar question recently, and received very helpful comments from Art Montemayor snd others. In summary, it seems that controlling via the condensate level works very well if the load on the reboiler or exchanger is relatively steady, and you have good quality (non-corrosive) condensate.

The thread where I asked the question was thread124-87417. You will also find several other interesting discussions by doing an advanced search on "reboiler steam control" and selecting the "all words" option.

regards
katmar