Flash tank - how it works

[mfqd13]

Hi,

I want to use an automatic blowdown system and use it's waste energy, in a boiler that produces 6.400 kg/h at 10 barg. A typical use for this kind of situations as it is showed in the image that i've attached.

My question is if the boiler feed water tank is atmospheric what is the pressure in my flash tank and that i need to perform my energy recovery calculations?
And what if i want it to generate steam at a controled pressure (for example 3 bar)? How can this be done?

Thanks!

 

[steamdog]

By the wording of your post it seems you do not have a deaerator but just an atmospheric boiler feed tank, so you are correct in that you cannot use the steam from the flash tank in the De-A. If you have use for low pressure steam local to the flash tank, then you can pipe the vent from the flash tank to the low pressure steam header. I have seen this installation with a backpressure regulator on this line and also have seen it piped directly (for both cases a safety relief valve should be installed). You use a simple BTU calculation to figure out how much flash steam you will get via the steam tables.

 

[mfqd13]

Ok.

You mentioned a De-A but i don't understand why it's not possible to use the flash stem in the De-A.
When you refer the backpressure regulator what is its application. I didn't understand...

Thanks

 

[GHartmann]

A back pressure regulator is simply a control valve or similar device that keeps a constant pressure upstream the valve.

For the system depicted the flash will occur at a pressure slightly above atmospheric. The actual pressure being determined from how much pressure drop is needed to enter the deaerator vessel and exit through any internal nozzles or pipes.

You need to make some assumptions and perform your heat and material balance.

Assume for example a flash at 2 - 3 psi above atmsopheric
Calculate your steam flash from your blowdown
Assume a temperature approach for your water preheater

Assume a boiler water temperature of 210 - 212 deg F

Perform the needed heat balance

I am not sure how much benefit you will get from the flash from a 10 bar-g boiler. Also not sure if you will have enough flash steam to heat all your return water back to the needed outlet T of 210 deg F

Good luck

 

[mfqd13]

Hi,

Thanks for the reply.

In my case i have a continuous blowdown system and a condensate tank (that feeds water to the boiler) at 50m away from the boiler room and 10m height difference.
My idea is to put the flash tank near the condensate tank, because i have saturated water at 10barg that easely can overcome those 50m, otherwise i pu the flash tank near the boiler, the flashed steam maybe could not overcome those 50m and 10m in height.
So, if i consider that the saturated water loses aprox. 1,5bar from the boiler to the flash tank near the condensate tank, i have 8,5barg at the inlet.
Now my problems arise here:

1) If my condensate tank is atmospheric, shouldn't i consider that water should flash from 8,5 to 0 barg?
2) For what i understood by the replies, if i want to control the pressure of flash and for instance i want flashed steam at 2 barg, i should put a backpressure valve in the outlet line of the flash tank?

Can you give me an opinion regarding this 2 questions?

Thanks

 

[GHartmann]

Perhaps you should sketch out your system for us.

It is not very clear to me exactly what you are trying to achieve.

General comments:

1) The pressure of your condensate tank and blowdown flash tank or not related
2) You want the final blow-down to go the sewer
3) The pressure of your flash should be dependent on its final destination
4) If your condensate tank is atmospheric, then it must be pumped to your deaerator

Good luck!

 

[mfqd13]

Thanks

With this sketch, can anyone put pressures in the diagram.
Assumptions:

Boiler pressure: 10 barg
Head loss from boiler to flash tank: 1,5 bar

Thanks

 

[hacksaw]

just make sure you have adequate steam supply to maintain D/A pressure in the event of off-normal conditions

 

[mfqd13]

What do you mean with "D/A pressure"?

 

[MJCronin]

Is anyone else becoming a bit concerned here ?...... I am....

marcos:

http://www.pennseparator.com/

http://www.spiraxsarco.com/us/

http://en.wikipedia.org/wiki/Deaerator

http://en.wikipedia.org/wiki/Boilers

Please let us know about:

- How you are sizing your equipment
- Your selection of materials
- Your use of properly certified valves for blowdown
- If this is your first assignment on designing and selecting boiler room equipment

Regards

 

[mfqd13]

MJCronin

You shouldn't be concerned. I am the one that it is concerned because no one could put values in this...

You mentioned several idead but what abou putting pressures along the lines. That is the help that i need. Talking that kind of things doesn't help...i already know them, but it seems that my "stupid" question is not so stupid....

I'm Portuguese so i don't know what means "D/A pressure"

Is there anyone that can put pressures inthe drawing?? and give some kind of explanation to my initial question?

Thanks

 

[rmw]

DA or D/A in our standard english venacular indicates Deaerator or deaerating feed heater. However, per your sketch, you don't indicate having one. I am going to ask since you show that you are going to exchange heat from the flash down fluid out of the flash tank to the make-up water, why don't you exchange the heat on the leg feeding the flash tank and then flash what is left, if any. You couold sparge (a device that distributes steam into a vessel or under a water surface) the flash steam below the level of the condensate tank. The flash tank pressure would then be at the pressure of the head of the water over the sparger nozzle, say a meter more or less.

rmw

 

[mfqd13]

Dear rmw,

Thanks for the reply and help.

1) I don't use the all the blowdown water for pre-heating the make-up water, because i have no data regarding the consumption of this water so, i am assuming that are some periods that no make-up water is entering the condensate tank, therefor, in this situation, no heat recovery is done. So, my idea is to insert it in the condensate tank to maximize this recovery.

2) So, what you are saying is that the pressure in the flash steam pipe in the condensate tank should be aprox. 0,1 barg. So, would be the pressure in flash thank that i may use to perform the calculations for ammount of flashed steam?

Thanks!

 

[GHartmann]

Perhaps before assigning pressures you should develop your best estimate of the required flows. This is the basic concept of a "material balance."

Say for example you generate the 6400 kg/h of steam

Say you have a nominal blowdown of 3 to 10% (depends on quality of make-up water)

Determine how much returned condensate you have (are there any losses from the steam system?)

You can then calculate the make-up water (equal to losses from system)

What some of the above posters were trying to say is that it wasn't clear if you had a mechanical deaerator vessel or just a tank where you sparged steam to remove the oxygen from the make-up and return condensate.

There was just too much missing information to start nominating pressures. Plus many were tyring to get you to think through the system and develop your own proposed set of pressures based on your own assumptions.

 

[mfqd13]

Ok.

I make it simple.

You have a continuous blowdown at 10barg and there is 20m long of pipe and then the flash tank. The flashed steam is 400 kg/h and ?? barg and after 5m pipe length is connected to a heat exchanger that have 0,2 bar of pressure drop and then the condensate after this heat-exchanger is thrown away.

Can anyone put pressures along this simple circuit?? Just as an example... you may assume some values to turn it simple and give me an idea...

Thanks