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Drum feed water

Monday, June 12, 2017   By: Tushar [2153] 0 Stars
Sir,

I wonder, what if we put water having higher approach point to drum?

What are the maximum limits for approach point temperature?

Please guide.

Regards,
Tushar

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Tuesday, June 13, 2017   By: Murphy [21] 4 Stars
If I understand you right, you want to increase the steam drum temperature to the maximum allowed. Of course the maximum allowed is stamped on the boiler and I would hope at the manway opening. So, this is the saturation steam pressure - temperature of the boiler should we be talking about a subcritical boiler.

Can you clarify what you mean?

Can you include a picture of the nameplate?

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Tuesday, June 13, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Mr. Tushar:

I wonder if you mean to increase the feedwater temperature, for a design working pressure. If it were so, as Mr. Murphy stated, the limit is the saturation temperature. As far as I understand, there is always a certain margin, say some 10 to 15C, to avoid the risk of steaming in the economizer, because then the pump capacity may be impaired, for it is designed to pump liquids, not a gas.

If steaming were marginal, the pump might still deliver the biphase flow, but compare with centrifugal compressors, they need a much higher rpm to compress gases than it is required for pumps, this is because of the low density of the gas as compared to a liquid.

Reducing the margin will certainly increase the efficiency of the plant, but consider and evaluate it with caution, I feel it is risky.

Claudio Molanes

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Sunday, June 18, 2017   By: Jim Watts [764] 5 Stars
Hi Claudio,
I finally decided to comment on your answer even though you have probably realised by now the feed pump is not in the steaming zone and only pumps dearator water at dearator temperature with a static head.
The only consideration with economiser steaming is the distribution of flow in the economiser tubes as steaming will reduce flow in some if not all are steaming at the same rate. The pump capacity is able to deal with the small head increase.
However if a wide variation is possible as sometimes occurs in HSRG the feedwater control valve is moved to be between the economiser and the steam water drum.
This supresses steaming by increasing the economiser saturation temperature slightly.


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Sunday, June 18, 2017   By: Jim Watts [764] 5 Stars
Ha Ha I just realised I have been typing HSRG instead of HRSG

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Wednesday, July 05, 2017   By: GwenPayne [2157] 0 Stars
Hello I am new here

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Thursday, July 13, 2017   By: Jim Watts [764] 5 Stars
HI, Just join in and ask questions or make comments or answers as much as you like.
The more you put in the more you get out.

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Tuesday, June 20, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Hullo Jim!

Thanks for your comments, due to your experience you can give much more specific data and practical solutions... placing the valve downflow of the economizer will certainly place the pressure fall in the valve after the eco, so it will work at a higher pressure and reduce the possibility of steaming.

I had no idea that some steaming is acceptable... steam will be a less efficient cooling medium than water for the economizer tubes, so some overheating of the tubes might be a nasty consequence if there be too much steam, but as you speak of 10 to 15 percent, it must be still safe working conditions.

Mr. Tushars problem apparently was different as I had first thought, as I realized after reading his last posting. He has to deal with a low pressure boiler, and feed water temperature may not be so strict a factor in my opinion, specially if for a short period...

Regards

Claudio Molanes

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Tuesday, June 20, 2017   By: Tushar [2153] 0 Stars
Mr. Claudio Molanes,

I understand what you explained. Thank you for that!

My concern is, what will happen if I put condensate return of say 45 Deg. C directly to drum having pressure of 0.75 MPag saturation temp of 173 Deg.C approx.. i.e. higher approach point from usual 10-15 Deg.C

Will there be any operational problem?

Kindly advice.

Regards,
Tushar

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Tuesday, June 20, 2017   By: cmolanes@itba.edu.ar [50] 4 Stars
Oh, Mr. Tushar!

From your data I now realize that you are speaking of a low pressure unit, 7,5 barg. In this case I imagine you have an industrial boiler, perhaps obtaining saturated steam for industrial processes as this is quite common. As you mention a drum, it may be a D-type boiler... you speak now of introducing feed-water at 45C, that is, with no preheating.

This will impair somewhat the boiling process so you will certainly loose efficiency for your cycle, but I believe will not bring serious consequences unless the feed pipe is too close to the steel walls of the drum. In any case you should have thermocouples to check the temperture differentials.

If your needs are to replace or repair an existing economizer, or existing preheaters, then it should be considered as temporary procedure and acceptable, but not in the long run, specially from economic point of view. If you can preheat the water using steam, hot condensates, flashed steam or the like, both the boiler and the purse will be happy.

As for your last paragraph, I do not know if I understood properly, with 173C saturation temperature, usual feed water should be, as I spoke of 10 to 15 lower, so in many plants would work at 160 C or so. Feed water at 45 means much lost efficiency.

Claudio Molanes

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Wednesday, June 14, 2017   By: Jim Watts [764] 5 Stars
As has already been stated the feedwater is limited to saturation temperature in the drum because it is impossible to be higher. However steaming in the economiser of conventional fired boilers is not uncommon with some being designed to do so and others occuring due to fouled superheaters or other firing issues. 10 to 15 percent steaming would be the maximum however.

It is a different matter in HSRG however as the relative low gas temperatures and low steam and water flows require a higher differential between the gas and steam temperatures to cool the exhaust gas temperature. approach temps are highly optimised even to the extent of running three drum pressures and of course no feed heaters after the dearator.

So your answer is its probably impossible to raise on a normal boiler and extremely undesirable on a HSRG.

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