Desuperheater system design 1

[axx1]

thread124-198506

I am responsible for designing a spray water system for a desuperheater that utilises steam condensate. We are currently using boiler feed water but it is causing caustic embrittlement on the steam piping, hence the change.

I am sizing a pump and control valve for the new system and my questions are:

1) What minimum pressure is required of the spray water at the point of injection into the steam (relative to steam pressure)?

2) Is there a minimum pressure drop that is required over the spray water control valve to ensure proper desuperheating operation?

Thanks in advance.

 

[JimCasey]

Rule of thumb that was taught to me in desuperheater training was 50 psi above steam pressure for a non-steam atomizing nozzle. That's pretty arbitrary and would depend upon the nozzle design. The real concern is imparting enough energy to the flow so that the droplets are uniformly small and evaporate quickly. 0.2 seconds was the target time for full evaporation. If you have a source for steam-atomizing the attemperation flow, then the water just has to dribble in and the atomizer does all the work. What you DON'T want is to dribble in water without atomizing it and just having it run along the bottom of the pipe. It'll take forever to evaporate, the temperature control will be impossible, and you'll cause thermal shock and probably damage to the pipeline.

Valve pressure drop: a different Rule of Thumb is that one-third of syste pressure loss should occur across an open control valve. So if you have a 1000 psi source feeding a 900 psi steam header, That represents 100 psi system loss. So the valve needs to drop 33 psi in its wide-open configuration. Again, that depends upon the nature of the source pressure, pump curve, etc. but it's a place to start.

 

[CJKruger]

axx1, we recently had to change the nozzle in a desuperheater to impove performance and used 100 psi at rated flow and 25 psi at turndown flow.

I have had bad experiences with desuperheaters, and in future will buy the best desuperheater, with the highest turndown, place the TI as far away as possible and keep to the recommended steam velocities.

 

[TD2K]

I like the Yarway templow desuperheater

http://www.yarway.com/tlsd.asp

You don't have a separate control valve as the desuperheater stem moves up or down as needed to achieve the required steam temperature. You just size your pump for the required head based on the range of steam pressures (Yarway prefers to have the water minimum 50 psi above the steam pressure).

Depending on the flucations in steam flow you may need to provide minimum flow protection for your pump.

 

[rmw]

This desuperheater is new to me. I notice that they refer to water turndown, but not steam turndown. Steam flow turndown is what is critical. The prose overall throughout the piece makes it seem that it was either written by a fifth grader or translated (badly) out of another language.

Still, the concept should work well if you have adequate water pressure over (maximum) steam pressure at minimum pump discharge pressure (out on the curve). I am with JimCasey as part of the 50 psig school of thought.

Is your current system steam or mechanical atomized? What type of turndown do you expect? How much superheat are you trying to get rid of. Is 10F over sat good enough for your application? Is the rest of your system by the book; adequate downstream distance to next elbow, proper velocity in steam pipeline, etc?

rmw

 

[MJCronin]

Pay attention to the piping configuration required by the
deduperheater mfr.

Some require a certain piping "straight length" both upstream and downstream.

I have always liked the Copes Vulcan products. This company has been around forever...... has been in powerplant applications for many years.

http://www.copesvulcan.com/products/desuperheaters/ma.asp

Talk with them about process requirements and turndown.

Please complete this thread and let us know more details about your specific application and your final choice..

-MJC

 

[JimCasey]

In one of my other lifetimes, I was in a job where we were developing a desuperheater. Basic, single-nozzle type. We put flow to it in the lab at low pressure. It made an impressive cone if spray. I photographed it with an electronic flash: the droplet size was far from uniform or small until we got the pressure up.

 

[UHoustonMSME]

I was at a users group meeting last year and several memebers spoke well of the CCI design. At least one plant manager said he wanted to try the CCI cool mist nozzles. CCI has a cool mist video posted at:

http://www.ccivalve.com/index.html

Do any of you have experience with CCI desuperheaters or with this cool mist nozzle?

 

[Longinthetooth]

Regarding the Yarway rising stem design, sometimes you have only one source of HP BFW suppling all users. If you are desuperheating low pressure steam there is a huge DP across the desuperheater. We found they all developed internal leaks after about 1- 2 years and were uncontrolable.

 

[athomas236]

I would be asking "what is so different between condensate and feedwater that is causing caustic embrittlement that will not occur with condensate". May be chemical controls should be examined before installing pumps, valves and pipes.

Regards,

athomas236

 

[unclesyd]

Here is the Komax desuperheater that we have had very good sucess with.

http://www.komax.com/products/desuperheater.html

One thing to watch for is the presence of water droplets down stream of the desuperheater. These droplets can cause differential thermal cracking if they hit the wall.