Sizing Flash Tank (Coal Power Plant)

[Boghi1990]

Hi,

I am trying to size a flash tank. There are 19 drain lines (sizes varying from 3/4" to 10") coming into the flash tank. From the flash tank there are a 3" line and a 6" line running to a condenser(pressure in the condenser is 1.5Hg inch, vacuum).

Is anybody out there who knows what methodoly should I use to size the flash tank? What are the steps to be followed? Is there any textbook, Guideline, Code or Standards which refers to flash tanks sizing?

Any suggestion and advices would be much appreciated.

Thank you,

One Point

 

[MJCronin]

Flash Tanks are a specialized piece of equipment, best left to the people who do this everyday

I would contact these fine people:

http://www.pennseparator.com/

Please be respectful and let us know about your final design.....

 

[rmw]

So why don't you just run the drain lines directly to the condenser?

rmw

 

[MJCronin]

rmw...

I assumed that at least some of these "drain lines" were for periodic boiler blowdown.

If that is true, then some of the lquid (with solids) should be sewered and not directed to the condenser.

Flashed steam can be vented or sometimes directed to the deaerator.

With only incomplete information from the OP, we can just guess...

 

[rmw]

MJC,

From the description he gives, this sounds like the myriad of drains and vents associated with a steam turbine. What does not make sense is that the 3" and 6" lines from the flash tank both go to the condenser. One would be the flashed down liquid in the drains and vents and the other would be the flash steam. That doesn't make sense. Flashed steam should go to a LP FWH where it can be utilized in the thermodynamic cycle. Even though the DA is by definition a LP FWH, it is usually at too high of a pressure. Pre flash for a condenser should be at a pressure below atmospheric (if there are LP FWH's available at that pressure to take the steam.)

In the absence of mention of using the flash steam for any practical purpose, and the mention of the 2 lines going to the condenser, I framed my question. If he/she isn't going to use the steam, why go to the bother of a flash tank?

Nothing in what he stated indicated to me in any way that boiler blow down was involved and if it is, then you are certainly right, that doesn't belong in the condenser. Normally the two never meet and generally aren't even in the same vicinity of the plant.

rmw

 

[Boghi1990]

Hi rmw,

I cannot run those drain lines to the condenser, because hot condensate flowing through those drains has got high temperature.

The upstream operating pressures and temperatures in those drain lines ranges from 1813 psia to 110 psia and from 1050 0F to 693.5 0F. I am not sure though if these are the presures I need to consider when estimating the mass flow rates through the drains.

I am in the process now to calculate/estimate the flow rate flowing through these drains during the start up of the system. Whomever sized those lines, did noot use the flow rates as the basis of sizing. So I have to determine myself the flow rates of the hot condensate. I know that the condensate will flash along the drain lines before it gets to the flash tank. So there is a two phase situation occuring in these drain lines, therefore the procedure of estimating the flow rates is not quite easy.

Regards,

One Point

 

[rmw]

You still haven't answered some basic questions. You did give a few more details in the thread you hijacked in the Chemical Engineering forum, however.

You still haven't answered what you are going to do with the flash steam (which is only asked to try to determine what pressure the flash tank might operate at).

The 6" vent and 3" drain seem WAY undersized for what you describe.

Your turbine vendor will have furnished heat balances for various operating conditions of the turbine. Vent and drain flows are typically shown for the operating condition.

Typically most of these vents and drains just function during start up, warm up, and trips. In those cases, the fluids are not at the temperatures you mention, BUT THEY CAN BE if the valves, vents and drains have faulty equipment and leak.

What is being done with these drains now before you add this flash tank?

Most large surface condensers I have had experience with (and that number is more than just a few) had the turbine vents and drains straight into the condenser. Some were under the hotwell water level, and some were above. Some were above the Hx tube bundles, some were below. Some were straight in, some had baffles and diverters. Some had spray nozzles but most didn't.

I am curious where you are starting from with respect to where you are trying to get to now. Based on what I have seen these drains do (erode condenser floors that were theoretically protected by hotwell water, knock 1" diverter baffle plates full penetration welded to condenser walls completely off - or peeled back like a tuna can lid, condenser internals cut to ribbons, etc) the drain and vent line sizes from the flash tank you quote seem woefully small unless, of course your unit is something less than ~30 MW.

Please fill in some more details so we can try to help you. From what I know now it seems like you are headed for some real grief.

And on the other hand, I could be reading much more into this than is really there. Only you know.

rmw

 

[Chuckybos]

Is your flash tank design for supercritical or subcritical coal plant?

 

[Boghi1990]

Hi Chuckybos,

It is for a sbcritical coal plant. I do not have much knowledge in the field of power plants systems. I have opened the thread hoping that I get some suggestions on how should size this flash tank. As I explained before in my threads,There are 19 drain/vent lines (sizes ranging from 1" to 3",Sch 40,80 & 160),running into the flash tank. They want to replace the old flash tank (40 years old) with a new one for there have been some upgrades to the system. Unfortunately, I do not have the flow rates. Therefore,I am in the process to calculate the flow rates at different conditions( start up/warm up and trip).

For the startup/warmup situation, I am considering the hot condensate flowing through the drains and flashing before it reaches the flash tank. These calculations take into consideration the choking flow condition where the max flow rate occurs at sonic velocity. I am struggling now to get the operating parameters when the motor operated valves (valves installed on these drain lines) are turned on and off. Once I calculate these flow rates I can size the flash tank. I have found some examples how to size the flash tank once you have the total mass flow rate flwing through into the tank.

The other case(trip), involves a different type of calculation.I have used a method described in Flow of Fluids, Crane for Compressible Flow, to estimates the flow rates through the drains. I have used the maximum operating paramaters at superheated steam, for if trips occurs , there will be only superheated steam flowing through those drains.

I am not sure how could I size the flash tank once I get the flow rates for the superheated steam condition.




Regards,

One Point

 

[curtis2004]

Boghi1990,
Why as a starting point with size of existing flash tank? It lasted 40 years, which seems not bad at all. If you haven't had any operational or maintenance problems, I wouldn't change anything and would order the exact copy of flash tank. You probably have other problems in your life to solve, so one less problem. I would just check design against latest lastest codes and regulations to make sure that rules are followed.
Why would you invent bicycle if old one served good and long?
Regards,
Curtis

 

[davefitz]

For Information Only

The vast majority of coal fired US boilers are over 30yrs old, and the design procedure used prior to 1990 went approximately along the following lines:
- procedure was published by the either the nat'l board of boiler inspectors or an equivalent organization
-assumed only 1 steam drum on site- this procedure became invalid when the first construction bubble of combined cycles began in 1990.
- water volume in flast tank related to the main steam drum's water volume between hi-hi trip and lo-lo trip
-flash tank steam vent to be sized to be large enough to vent all steam generated when the saturated liquid from the steam drum is flashed in the tank , when the drum is at ( 100 psig ???) and the drain lines to the FT are wide open, and this vent size will ensure the FT is below 15 psig at this case.==> use Fanno curves for sizing this line.
-flow thru drain lines to tank as per the flash drain line curves continained in the old printed copy of the ASME steam tables , appendix in the back of that now forgotten book.

 

[MJCronin]

Based on my experience with power plant steam cycles, there was always a HP drain/flash tank (for turbine drains etc) and an LP tank(for everything else.

As I recall, the HP drains tank flashed to the DA with liquids drining to the LP tank....which flashed to the main condenser.

Could there be a problem with the OP trying to route all drains into a single tank ?

My opinion only

 

[davefitz]

IN most simple cycle Rankine plants, the reheater operates at vacuum during stg startup, so connecting the RH drains to a tank that may operate at pressure presents a risk of forcing drains liquids up into the reheater transfer piping. Likewise any other heater drain that may operte at vacuum at startup.

Recent changes to ASME sect I recognize this issue for combined cycle plant HRSG's ( although most of these plants use a bypass sytem that maintains a pressure on the RH) and requires separate tanks for RH vs HP systems, but a similar risk exists for most US rankine plants- usually there is enough gravity head from the bottom of the RH inlet header to the FT to prevent beckflow- but not always. Most waterhammer failures of the CRH Xfer pipe can be traced back to this issue.