Level control valve of steam condensate pot 1

[Instr13]

Hi,
For controlling condensate level in a LP steam condensate flash drum we are using a globe valve 1-1/2" in a 4" pipe. Calculation shows that flashing occures inside the valve. Any suggestion for retrofitting the existing trim and or changing the piping layout. Process conditions are P1=40.6 psig, P2=18.99 psig, Flow= 6500 kg/hr, Temp=141 deg C.
The valve is about 9 feet below drum outlet nozzel and the valve outlet pipe line runs UPWARD toward the condensate header.
Thanks.

 

[Montemayor]

Instr13:

It's not possible to making any useful suggestions on retro-fitting the existing trim because we don't do the actual valve and trim sizing; the piping essentially does next to nothing as far as alleviating any flashing.

Your application is one of probably hundreds of thousands of similar applications on condensate level control. There will always be some degree of flashing when you throttle saturated condensate - it comes with the territory: saturated liquid going to a lower pressue; the higher the saturated pressure and the lower the condensate header pressure, the more pronounced the flashing. Fisher Controls issued a complete hardcover control valve application and design manual on this subject because of this universal effect and the importance of taking into consideration the flashing effect (which can affect sizing because of potential sonic flow) and the criticallity of selecting the proper trim and valve type. I'm not at my home library where this information is in my immmediate grasp, so I can't comment any further with specifics; but it should be sufficiemt to indicate to you that there are practical and efficiient solutions and methods to handle this type of problem. With the right type of valve and trim, the problem is only one of simple flashing - and not one of cavitation, which is another worse problem and I won't go into it because a lot of engineers don't understand the difference and it doesn't enter this application if the right valve is selected.

You are going to have flash steam vapor downstream of the throttle valve; this, you can't avoid. But you can design for it by alloy (sometimes SS piping for a nominal length) change and velocity slowdown (pipe size increase). I would not leave any low spots downstream where stagnant condensate can collect and cause corrosion. I've used this type application many times in the past without any problems - I can't remember ever going back to do any repairs or retrofits - except the usual inspections every turn-around. Your dP across the condensate valve is typical of countless reboiler and power station applications all over the world. It should work like a piece of cake for years without any problems or need of repairs.

Art Montemayor
Spring, TX

 

[25362]

This is probably a 3% mass flashing. Inst13, can you tell us why do you think retroffiting the valve trim may be needed ?

 

[rmw]

Instr13,

What Art says are the bare facts. The laws of physics say that the flashing is going to be there, short of subcooling the condensate, which normally is not desirable.

One secret that will help minimize this problem is to locate the valve as close to the condensate header as physically and thermodynamically possible, so that the two phase flow caused by the flashing is minimized. You did not state if the condensate header was in the vicinity of the flash drum, but most systems I have seen had the valve fairly near the drum, and the header some distance away.

The second is to properly size and run your downstream piping so as to minimize the effects of the erosion that will be caused by the two phase flow.

Go to

http://www.processassociates.com

and find their tool on two phase flow, and let that be your guide as to how to size the downstream pipe. This will let you know if the 4" line is the right size. The very fact that you have a 1-1/2" valve in a 4" line says to me that someone already may have given this some thought. I will leave the number crunching to you.

If this is a steady state process, you do have enough pressure differential between the drum and the header pressures to 'push' the liquid leg up closer to the height of the condensate header, if the actual height difference does not exceed the head that the pressure differential can give you. Don't forget the specific gravity change of water at even this small temperature difference, and... don't forget that you still have to account for the rated pressure drop through the control valve as your total available height consideration. If it is not steady state, I wouldn't recommend it.

At least you can gain the 9 ft back. Was there once a pump on this drum??? The 9 ft. below the drum is curious to me.

rmw

 

[Joesteam]

The above posts are right on. The condensate will flash, it is a case where you have cavitation in a control valve and there is nothing you can do about it. Many valve manufacturers make valves that will last longer than a globe valve in these conditions. They have a staged trim that has many small pressure drops over it to lessen the effect this flash will have on the valve internals. I know the big boys like Fisher et. al. make these types of valves.
Also, the valve below is designed also for continuous boiler blowdown, which is also saturated water that will flash through the valve. Look At:

http://www.gestra.co.uk/PRODUCT DATASHEETS/BA36.pdf

 

[Instr13]

Thanks for all your valuable postings. We are retrofitting the trim because of changing the process load. That is the reason I tried to figure out if any of you may provide me a guide line for new trim selection and or changing the pipe route.