12 replies to this topic

[Luka2]

Hello everybody!

I have been give the assignment to regarding a deaerator we are using in our company.

 

We are using an old (1973) cascade type pressurized deaerator (1.2bar 105°C) that has a diameter of 1200mm and height of 1671mm. When it was built the deaerator was designed for the normal capacity of 16m³/h and max capacity of 20 m³/h and it worked fine.

 

Since then our boiler has been upgraded several times and now the deaerator has to work with 23m³/h. We are finding out that the dissolved oxygen level in the boiler water is too high.

 

I have been given the assigment to find out if this deaerator is still good or do we need a new one.

 

Are there any empirical equations that will give me the minimum dimensions the deaerator has to be for a given flow of water 23m³/h.  I would like to find out if our deaerators is too small.

 

Are there any ways of upgrading the current deaerator, like adding more trays (it currently has 6)?

 

How can I find out the minimum time required that the steam has to be in contact with the the water to properly deaerate it.  My boss said that the temperature of the water that is coming from the deaerator is 105°C (though it is not measured regularly), so it seems that steam gets enough time to heat the water.

 

Any suggestions welcome.

Thank you.

Edited by Luka2, 09 December 2013 - 07:41 AM.

[Bobby Strain]

Best to contact a vendor who supplies deaerator internals.

 

Bobby

[S.AHMAD]

Hi Luka2

1. In theory, YES by adding more tray shall reduce O₂ concentration. Similarly by increasing stripping steam flowrate and by increasing the incoming water temperature probably by heat exchange with boiler blow down stream, as an example. Of course the maximum temperature it should be lower than the saturated temperature at the deaerator pressure.

2. As you are aware, the Henry's constant is related to temperature and the partial pressure is related to concentration and the total pressure.

3. By adjusting these variables, you may get the O₂ level right and no need a new deaerator since the difference between current operating capacity and design capacity is small. Unless it is under design at initial design stage.

Edited by S.AHMAD, 10 December 2013 - 04:28 AM.

[Luka2]

Hello again,

 

thank you for your answers and suggestions.

 

Is there a rule of a thumb to calculate how many trays we should put inside or how much should we increase the steam flow.

We are using steam at 2bar and at 160°C to dearate. The temperature of the feedwater is around 50°C

[PingPong]

A P&ID of the deaerator is required to properly answer your questions.

 

It also depends on the control systems of the deaerator, such as:

- how is the stripping steam supplied: on flow control or on pressure control ?

- how is the vent gas controlled: on pressure or simply via an orifice ?

- et cetera

 

The air is to be vented together with some steam. If the vented steam quantity is not enough then the air partial pressure in the vent gas is too high and consequently more air will remain dissolved in the water.

[curious_cat]

What is your target DO level and how high is it currently? 

[Dazzler]

Hi Luka2,

Another option to explore is the dosing of "oxygen scavenger" chemical.  This is often dosed to a dearator, and if O2 is higher than target perhaps due to operating higher than nominal capacity, perhaps more of the dosing chemical or an alternative chemical may help somewhat.

Dazzler

[Luka2]

The feed water at 50°C is getting in the deaerator from the top. The water is not sprayed, the distribution of the water is achieved only with the trays.

The flow of feed water is controlled based on the level of water in the storage tank.

 

The stripping steam (2bar, 160C) is spryed into the deaerator from the bottom and it is supplied on pressure control (the deaerator is operating at 1,2bar).

 

The vent gas is controlled with an orifice. I think that to low vented steam quantity is not the problem here. We are using quite a large vent rate (around 44 kg/h), because we are not venting directly into the atmosphere but are reusing the vented steam for heating of the feed water.

 

As for the increase dosing of oxygen scavenger, that is how we are solving the problem for now.

We are using ammonia and hydrazine and are feeding them into the storage tank.

 

[S.AHMAD]

Hi,

1. Changing to spray nozzle is one option you may consider. But you need to consult the deaerator supplier/vendor

2. Increasing the orifice size at the vent is another option. By doing this, you can increase the steam stripping rate while maintaining at the same pressure. As a matter of fact, you can size the orifice so that the pressure can be reduced and the stripping can be increased. Reducing the pressure will also reduce the partial pressure of the O₂ and helps in removing the O₂ in the feed water. Increasing the stripping steam will also improve the removal of O₂ from the feed. By increasing the steam also will increase energy input and hence increasing the temperature of the BFW. This higher temperature will reduce the O₂ concentration in BFW

3. Increasing O₂ scavenger is normally used as short term solution.

[curious_cat]

We are using quite a large vent rate (around 44 kg/h), because we are not venting directly into the atmosphere but are reusing the vented steam for heating of the feed water.

 

I've a possibly naive question. 

 

If you reuse vent steam to heat the feed water (by direct injection?) where exactly does the oxygen leave the system? Or is this an indirect heater with the condensate then dumped to drain?

[Luka2]

Yes its not a direct injection, the vented steam is condensed by heating the feedwater for the deaerator, the condensate is the reused so only  O₂ and CO₂ are then lead into the atmosphere.

[curious_cat]

Yes its not a direct injection, the vented steam is condensed by heating the feedwater for the deaerator, the condensate is the reused so only  O₂ and CO₂ are then lead into the atmosphere.

 

Interesting. Is this common practice to use deaerator vent steam this way?

 

How much do you gain? My calculations show hardly 1 C if at all. I could be wrong.  44 kg/hr of steam to heat 23m³/h of water? 

 

Is that worth it?  Just curious. 

 

PS. What's your orifice size? 

Edited by curious_cat, 11 December 2013 - 08:41 AM.

[Luka2]

Hi,

 

thank you all for the suggestions.

 

I am still searching for some rule of thumbs or empirical equations, to calculate the minimum dimensions of deaerator or how much trays are needed to give steam and water enoguh residence time to properly deaerate.  Seems like deaerator design is not well docuemnted.

 

 

Interesing. Is this common practice to use deaerator vent steam this way?

How much do you gain? My calculations show hardly 1 C if at all. I could be wrong.  44 kg/hr of steam to heat 23m³/h of water? 

Is that worth it?  Just curious. 

PS. What's your orifice size? 

 

 

No it is not common practice. You are right the temperature gain is not much, but that’s not the main reason why this was built plus we are also saving the condensate. It has to do with the location of our deaerator and the feedtank in the factory. To cut a long story short, we have a lot of not enough space related problem so we had problem with vented steam condensing too fast and then water was dripping down some roof and ice forming on unwanted places, especially in winter.