Blowdown line with vent

[mfqd13]

Hi,

Yesterday i visited a company witch has a big steam line.
The blowdown line has a vertical pipe opened in the top to let flash steam exit to the air.
I've never saw this, and i don't understand why this is made. Can anyone give me a tip.

I've noticed also something very interestant. When i was seacrhing for documents talking about insulation in blowdown lines, i found a text that was refering to this tipe of situations, and it was mentioned that for this kind of installations, the insulation of the line probably is not worth it.
What does this means?

Thanks!

 

[mfqd13]

PLEASE FORGET THE PREVIOUS TEXT AND CONSIDER THIS!!!

Hi,

Yesterday i visited a company witch has a big steam line.
The blowdown line has a vertical pipe opened in the top to let flash steam exit to the air.
I've never saw this, and i don't understand why this is made. Can anyone give me a tip.

I've noticed also something very interestant. When i was seacrhing for documents talking about leaks in blowdown lines, i found a text that was refering to this tipe of situations, and it was mentioned that for this kind of installations, the leak corrections, probably does not worth it.
What does this means?

Thanks!

 

[BigInch]

It would be extremely difficult to measure the steam lost to atmosphere. Since some methods of leak detection depend on comparing the volume or mass going into the system verses the volume or mass exiting the system, not having an accurate measure of the steam leaving via that route would make it impossible to determine if the steam was vented to atmosphere, or if steam was being lost by unintentional leakage.

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[pennpiper]

Marcosdias,
You did not say where you are or the kind of plant where you saw these situations. Therefore we must just take some wild guesses.

1. A vent stack such as you describe is not unusual. I have seen them used in Boiler Blow down and at some Turbine Drivers for pumps or emergency generators. When used at the pumps or generators it was normally because they were located in a somewhat remote location and deemed not economical to collect and return the condensate. The stack extended the height of the steam release to a safe elevation so as not to create a hazard for plant personnel.
The exhaust from the turbines or boiler blow down is piped into the stack near the bottom of the stack. The fluid will Flash when it reaches atmospheric conditions. The steam will rise and the condensate will settle to the bottom of the stack. The bottom of the stack is fitted with a "U" bend drain (to maintain a seal). The drain is routed to an appropriate drain system. The top of the stack is often fitted with an Exhaust Head (see

http://www.watsonmcdaniel.com/wmpdf/ss-weh.pdf).

The Exhaust Head separates additional condensate from the steam.

2. Insulation on Blow Down lines is not normally done because you are throwing the steam away so why conserve it. The reason for the insulation is not for conservation and is not continuous. The insulation is purely for safety purposes and the protection of the plant operators and maintenance people. Only install insulation where someone can touch the line during normal operation.

 

[mfqd13]

Thank you both for your answers.
Can you please take a look to my drawing to see this case.

Thanks

 

[BigInch]

Marcos,
I'm a bit confused, if you want to talk about insulation or leaks, or both. If you want to write me in Spanish, you can find my e-mail address under "contacts", at the virtualpipeline link here,


**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[vpl]

Also, if your first post is incorrect, you can "red flag" it and ask the site administrator to just remove that single post.

Patricia Lougheed

******

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[mfqd13]

Sorry BigInch,

I was talking about leaks.
I confused both problems, becaus i'm studying both...sorry.
My question is: what is the function of that "vent" line?

Thanks

 

[BigInch]

Then I believe the good Mr Pennpiper has answered your question.

<luego>

**********************
"Pumping accounts for 20% of the world’s energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies)

http://virtualpipeline.spaces.live.com/

 

[mfqd13]

So, pennpiper

I talked about how the steam is thrown away and gived the example of pumps and turbines, but i stil don't understand why this is made in the blowdown return line.

What you said about insulation only applies to this situation? In the case that is not thrown away is normal to insulate the line to recover this heat? Correct?

Thanks

 

[JJPellin]

In an oil refinery, insulation is used for three reasons. First and foremost it is used to protect personnel against injury from hot surfaces that they might come into contact with. Second, it is used to save energy. Third, it is used for equipment reliability. Insulation on a long steam line up high in a pipe rack is probably there to save energy. Insulation on the top cover of a steam turbine may be there to protect personnel and also to save energy. Insulation on a heater stack is only there for reliability. If the stack was uninsulated, water vapor in the exhaust gas could condense on the inside of the stack and corrode it over time. If a steam vent stack is insulated, I would not expect any benefit for reliability. If the vented steam is saturated, there will be liquid water present anyway. If the steam is being vented to atmosphere, there is no advantage for energy savings. All that is left is personnel protection.

If there is a line that is recovered steam, there may be energy value in insulating the line. But it depends on where that line goes. If the line goes to a surface condenser that condenses the steam back to boiler feed water, then there is no advantage. The heat saved by the insulation will just end up in the cooling water in the surface condenser. If the recovered steam is sent to some sort of a reboiler that reheats it for use in a lower pressure steam system, then the energy savings could be great. It could reduce the condensation in the system that has to be trapped off. Less condensation leaves more steam for whatever use this system supplies. You really have to understand the entire system to understand the purpose and value for the insulation.


Johnny Pellin

 

[mfqd13]

JJPellin,

Thanks for you extended reply!
1) You talked about the steam vent pipe being insulated for personnel safety. In relation to that i agree and that is verified on this installation.

2)I know that it depends of all system and it only makes sense to recover the blowdown energy if the system layout allows it. In this case, there is heat recovery from the blowdown, but the problem is that the line is not insulated.

Although your good reply, i still haven't seen my question answered. Why does exist a vent in the blowdow line? to purge air?? But isn't the air also removed in the deaerator???

Thanks

 

[ione]

Blowdown is carried out to keep the TDS (Total Dissolved Solids) in the boiler within values that shouldn’t provoke damages to the boiler.

Steam boilers are operated at different pressures: the higher the pressure the higher the enthalpy content in the blowdown water. In other terms the blowdown water is water at the saturation temperature of the boiler pressure. Water cannot exist at such saturation temperatures under atmospheric conditions (or in general at lower pressures).
Depending on the pressure of the discharging line of the above mentioned water, you will get a percentage of your blowdownwater that immediately turns into steam (flash). So you have two possibilities:

1) Recover steam and so energy for other purposes
2) Vent it to the atmosphere.

 

[mfqd13]

IONE,

thanks. You answer was clear.
So in this case is clear that there is no heat recovery. The flash steam is vented and the condensed water rejected.

So, i would like to ask one thing: where can be applied this heat? to heat the make-up water for example?

 

[ione]

First of all you have to calculate the amount of flash steam available and then evaluate whether it is opportune (from an economic point of view) to reuse it.
Usually it is easy to use flash steam in the boiler feed tank (it is located nearby) in order to heat up your feed water, but you can also use it in low steam pressure heat exchangers.

One last note: condensed water is still hot and it could be used in heat exchangers too(please consider that water coming from blowdown is usually dirt and so when using it in heat exchangers, fouling is more than an issue).

I hope the situation is clear.

Regards

 

[mfqd13]

Yes!

Thank you very much for your answers and thank you all for the replies!!!

Problem solved