Deaerator Tank Inquiry

[mpeck1982]

We have 3 DA tanks at our steam plant. 2 of the tanks have 2 sources of steam (medium and Low Pressure) serving as a steam blanket. 1 tank has only Low pressure going to it. We have to replace a valve that will not close on the low pressure steam line. I have attached a drawing for clarity. We will have to isolate all medium and low pressure steam going to the 3 tanks. We are planning to run the plant for 5 hrs without using any steam blankets on our DA tanks while valve replacement is taking place. We will be using DA tank-2 during the valve replacement. We will increase DEHA chemical as needed throughout the outage. I am afraid of what kind of pressures we will get inside DA tank 2. If I don't have any steam going to the tank, how will I stay above 0-psig?

 

[chicopee]

Your description is really confusing.
1)"...We will have to isolate all medium and low pressure steam going to the 3 tanks...." Does that mean the 3 tanks will still be under pressure and the tanks are isolated from each other?
2)"...We will be using DA tank-2 during the valve replacement...." what do you mean by that statement?
3)"...I am afraid of what kind of pressures we will get inside DA tank 2. If I don't have any steam going to the tank, how will I stay above 0-psig?" does that mean that you want tank #2 to remain under some low pressure while the valve is being replaced?

If you want no pressure in tank #2, why not use the two other tanks to remain on line eventho there will be no steam blanketting, and assuming that they can be isosated so that no steam remain in the line on which you want to replace the valve. Also since you show no vapor release valves on the tanks about retrofitting them so that you can depressurize them when required.

 

[mpeck1982]

Sorry for the confusion. What I am trying to say is during this outage, there will not be any steam blanket for the operating DA Tank-2. The steam will be taken off since this valve has to be changed out. I will just have pumped condensate and boiler feedwater going into the DA tank and only boiler feedwater leaving. I have never seen this type of operation done before. Is this ok? Have you ever seen DA tanks operate without using any steam?

 

[dougarthur42]

You should be ok for a few hours, as long as you have a good amount of oxygen scavenger, as you have said you will. We have operated this way before for a short duration and it is fine. The Da should stay slightly positive because of condensate pump pressure and temperature, and i presume a recirc line from Feedwater pump. Another event that will occur is the water temp from the DA will be cooler as you have no steam pressure. You can hand test feedwater Disolved Oxygen levels and adjust according but should not be a problem. From the picture if you added one more isolation valve on DA #1 you could always have steam to that DA. This could be added during the outage. Just a suggestion and good luck. Keep posted here we need feedback.

 

[steamdog]

If you keep your vents open to the tank the pressure should not go below 0 when the water in the tank cools down.