Use of pressure letdown for dry steam generation

[jamesbanda]

Dear all,

I am after some general views on this..We are installing a new waste heat boiler shell next year. We can rate the new shell to a higher design pressure and would like to ensure dry stream..

We are considering raising steam at higher pressure and letting the pressure down to ensure dry steam rather than using a demister pad or steam seperator...or internal dry pipe inside the generator

Would this be a recommendated way to ensure dry steam ?

 

[25362]

Before considering the efficiency of a pressure letdown, may I ask whether you have checked if the TDS of the entrained water droplets may, at the end, surpass the max. allowable TDS of the steam ? I believe there are standards limiting the TDS of boiler water, and of steam, depending on pressure. See, for example, thread794-87628 and the links therein.

 

[Joesteam]

Although reducing the steam pressure will evaporate SOME moisture in the higher pressure steam, it depends upon the inlet & outlet pressure and the total amount of moisture in the steam.
I think you will get better results with a steam separator, because even if you get a minor upset in the boiler (ie: foaming and carryover) the separator will remove this liquid safely, while simply reducing the pressure will only illiminate a very small amount of liquid and you will still have the potential for poor quality steam and water hammer after the boiler.
Also, the separators are static devices needing little or no maintenance, and PRV's require maintenance, replacement, etc.....

 

[abcmex]

Have you ever heard about Boilers with integrated superheater. This is quite standard in some contries.

http://www.loos.de/loos/pdf/broschueren/Dampfkessel_en.pdf

rgs

 

[jamesbanda]

Thankyou abcmex and joestream,

I tend to aggre a steam seperator or internal demister or external demister does seem the way to go.. but I want peoples advice to validate it...

But i'm not sure how applicable this is to my case which uses a heat transfer fluid in a waste heat boiler.. steam generated on the shell side.. my consern with an integral superheater is i've seen lots of articles which refer to superheater damage from steam impingement and suggest that a steam demister is required upstream..

 

[Joesteam]

With a waste heat boiler as you have, you would have to get a separate fired (electric, gas, oil, etc.) superheater, and yes indeed this could NOT handle boiler water carryover, even in a mist...

 

[katmar]

Letting the steam down in pressure will of course help to dry the steam, but I would not use this as an alternative to steam separators and demisters. You want to feed the let down station with as dry steam as possible because the let down can only evaporate a relatively small (but easily calculated) amount of water. Rather finish up with a small amount of superheat in your steam than leaving water there.

Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JimCasey]

It's pretty easy to calculate the quality of the downstream steam. Throttling expansion of the steam is adiabatic/isenthalpic. So, take the enthalpy of the upstream steam/water mixture, and check that at the downstream pressure to see if the enthalpy value corresponds to dry or even superheated steam. Or you can go backwards: find the saturation enthalpy of your downstream steam, then evaluate what the maximum permissable moisture content upstream would be.

Example: take 150 psi steam at 98% quality (2% moisture) and the enthalpy is 1178.41. Expand that out to 125 psi and the quality only goes to 0.983.

 

[rmw]

If your steam has enough moisture in it to be concerned about then it will cut a let down valve to ribbons. I vote for the demister with their being several types to choose from. They are constructed to be able to handle the wet steam. Be careful of your turndown ratios when selecting the type you end up with.

rmw

 

[MedicineEng]

In our system that also is a secondary product boiler we have a steam separator and steam is generated in shell side.
Our vapour comes out quite dry (at least I don't get compaints from the users)