20 replies to this topic

[premjagan]

dear sirs,

we maintain deaerator pressure of 1 kscg by using PCV in steam line.i eager to raise the deaerator of 1.5 kscg, Then i can get advantage of increase in head and raise in BFWP suction temperature also.But the disadvantage is to demand of steam input to deaerator is more when to raise the system pressure.

please let me clarify if i raise the pressure, can i get overall advantage?

Edited by premjagan, 16 December 2012 - 04:23 AM.

[premjagan]

Anyone pls reply ur valuable thoughts for my question.

[thorium90]

If im not wrong, the reason why bfw pumps are always below deaerators is to take advantage of the extra head. So why is there a need to add more head? If the pump can already work with the current pressure, why is there a need to spend more steam to do it? You need to clarify the reasons as to the need to increase the pressure.

[Hamid_r_2003]

The deaerator temperature (and consequently the corresponding pressure) should be set based on required oxygen content at outlet. Increasing the pressure will increase the boiling temperature in addition to the required steam to reach that temperature. Since the NPSHA is always the design issue for BFW pumps, these pumps will be installed at the lower heights in comparison with deaerators (or deaerators to be installed at higher elevations) to provide the required head. In higher boiling temperatures the available NPSH is less than your primary set and it can not help to the operation of your pumps. I suggest you to decrease the pressure loss at pump suction.

[Bobby Strain]

So, what are you wanting to achieve?

 

Bobby

[DB Shah]

Hamid_r_2003

Deareator is at saturation hence varying of pressure (increasing boiling pt) will not alter NPSHa

[premjagan]

Dear bobby,

 

             The steam input to deaerator is at 2.5 kscg pressure.Regarding steam balance , In our plant 2.5 kscg steam is excess in amount.So we are in position to consume somewhere.Thats why i have an idea to consume the amount of steam in deaerator which lead to raise the pressure.sir let me know that is overall afvantage or not?

[Hamid_r_2003]

Dear DB Shah,

you are true and i meant it. Increasing the pressure will increase the boiling temperature which makes the NPSHa remains at its previous value (Pressure increasing effects on NPSHa will be neutralized by increasing the term of "vapour pressure" at that point). so this method has no advantages.

[Bobby Strain]

You should first direct efforts to minimize excess low pressure steam. When you have done that, then use all the steam that you can in the deaerator.

 

Bobby

[KLP]

The only advantage increasing the deaerator pressure might have is that by raising your feedwater temperature it will raise the flue gas exit temperature and reduce the possibility of corrosion in the back end of the boiler, but on the other hand increasing feedwater temperature could cause boiling in the economizer if far above the design parameters for the unit.  Other possible effects could be an decreased superheater outlet temperature.  It is possible that you get better O₂ removal at the higher temperature/pressure it would most likely depend on current load of the deaerator.
 
Ken

Edited by KLP, 01 February 2013 - 03:07 PM.

[Atttyub194]

Dear All

 

Good day!

 

Generally reducing the pressure is always benefical , as it removes the oxygen mechanically and reduces the requirment of oxygen excavanger, reduses steam demand and improves preformance of the economizer and reduces stack gas temperature sonme times. However, every plant has its specific limitation and need follwing data for further sugesstion

1. What plant you are operating with a sketch or preferably PFD of the steam system
2. What steam quality you require
3. What oxygen exchavanger you are using
4. Please give 10 good reasons why do you want the change other than one you mentioned in your reply with your perspective

Waiting for your reply

 

Best regards and God Bless You

[kkala]

Following notes may be useful
1. Met local deaerators operate at ~ 0.10 barg, 102 oC (design pressure= 2 barg). Boiler feed water (BFW) pumps are located underneath, so that difference NPSHa - NPSHr = 3 m. Oxygen scavenger is injected just downstream of deaerator surge drum.
2. By increasing deaerator pressure, NPSHa remains same (as clarified in previous posts). Suction head of BFW pump (thus total head) also increases correspondingly, which may not be significant for a boiler producing e.g. steam of 40 barg.
3. Main advantage of introducing waste steam into deaerator is fuel saving, since BFW is heated to some extent there, to rest extent in the economizer (and BFW heater, if any). I cannot say that Q (kW) offered by extra steam into deaerator results in savings of Q (kW) of fuel, based on NHV.  Probably fuel saving is less, but there is.  That is why flash steam from continuous blow down is directed into deaerators in a local refinery.
4. A supplementary advantage of hotter BFW is less risk of acidic corrosion from flue gases at economizer tubes (to get their temperature higher than acid dew point of flue gases). If there is a steam BFW heater for this purpose, BFW out will get hotter. 
I vaguely remember a case where BFW heater could be eliminated by increasing deaerator pressure (1999).
5.  Maximum temperature of hot water leaving economizer should be at least 25 ^oC below the saturation temperature at the steam drum, to avoid any vaporization in the economizer tubes, according to local practices (point mentioned in post no 10 by KLP).

 

Edited by kkala, 07 February 2013 - 03:05 AM.

[Fajri]

dissolved oxygen in dearetaor discharged through venting, is there a standard of how many openings deaerator venting valve is allowed to be wasted all the oxygen in the air?

[kkala]

It is noted that a manually adjusted valve on deaerator venting pipe  is more common, see <http://www.cheresour...team-vent-flow/>. Once adjusted, it remains open as is for long time; some stripping steam also escapes through it. The operation is continuous.

I have not met an automatic valve on this service, but I suppose it would be modulating (not on/off). Others with experience can hopefully clarify this point. Have you installed (or plan to install) an on / off venting valve that will be automatic? A small sketch of the controls would be useful.

[thorium90]

Hmm, all my plants have manual valves for the vents. Opening is set during startup or commissioning of the plant. Parameters to determine how much to open are: Consistent steam flow from the vents, pressure in the deaerator. Most plants also have oxygen scavengers dosed in to supplement the removal of O2 so its actually not really a major concern to calculate exactly how much opening is needed..

[Hamid_r_2003]

I have experienced three different plants with installing orifice plate on vent stream to control in normal operation and a HV (manually Adjustable globe valve) for start-up and other operational cases. The vent stream could be low enough to be just visually available (around  0.5 to 1 wt % of incoming steam).

Edited by Hamid_r_2003, 07 February 2013 - 06:41 AM.

[Fajri]

I do not use chemical oxygen scavenger but using chemicals for filming inside the boiler tube so I had to make sure that all of the oxygen will be wasted in the deaerator venting.
I currently use a manual valve for venting deaerator

[thorium90]

There will always be a limit to mechanical deaeration, thats why chemicals are used. What are the chemicals currently used in your bfw?

[Fajri]

I agree that there are always limits mechanical separation. I am using polyamine-based chemical that serves to coat the internal tube for the rest of the oxygen is not in direct contact with the metal. Is this method is widely used in industrial steam generation

[RATNADEEP]

In one of our plant of 60 MW condensate is coming out of deaeartor,

This is spray cum tray type dearator. The steam is injected through sparger pipe in feed water tank.

The design pressure of deaearor is around 4.5 bar. But this is happening around 1.1 bar a steam pressure.

What could be the reason ?

[fallah]

Ratnadeep:

 

 

Hi,

 

Lack of the schematic diagram of the system...

 

Anyway, it appears the 4.5 barg design pressure relates to the steam source pressure would be imparted to the deaerator during failure of the steam control valve leading to be in wide open position...