Oversized Flash Vessel
Oversized Flash Vessel
(OP)
I have sized a flash vessel with operating pressure 14 bar and design pressure 21 bar. The actual vessel in the plant seems to have diameter twice of what I sized but height more or les the same. However this flash vessel have a great water carryover. A decision is made to install a separator after the flash vessel but there are still problem because the condensate of seperator will have steam with it and this is a loss to us. Can anyone suggest to me why my flash vessel have such a great carryover?





RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
Have you tried calculating the vapor velocity in the existing vessel and comparing it to the entrainment velocity of the liquid droplets? Why add a second drum instead of adding a demister or coalesser pad?
RE: Oversized Flash Vessel
Regards
Benjy
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
Steam(water) enter the flash vessel at 1.5 Metric tonne per hour(MT/hr),65bar,100% saturated liquid. Dry steam, come out at 14 bar at 0.3145 MT/hr.Liquid flow out at 1.1855MT/hr,14bar. Actual dimension of flash vessel in plant:Height=1.645m,diameter=0.8m,steam outlet,condensate inlet and outlet diameter=50NB.There was no demister pad in this flash vessel. Why serious carryover happened in my flash vessel?Why not install a demister pad but install a separator?Is it cheaper and more troublesome to install a demister pad in it?
RE: Oversized Flash Vessel
A larger diameter tank will reduce the vapor velocity within the tank and will reduce liquid carry over to the steam outlet. Your flash tank's diameter is too small and the inlet velocity is causing liquid carry over in the steam line.
RE: Oversized Flash Vessel
Presuming your numbers are correct the calculation goes like this.
The enthalpy difference of hot condensate at 65 bar and 14 bar is (295-197) kcal /kg and latent heat of steam at 14 bar pressure is 665.7 kcal/kg.
So the percentage of flash steam is (295-197)x100/665.7 = 14.72%
Total condensate flow rate is 1500 Kgs/Hr, so flash steam generation rate is 1500 x 0.147 = 220.5 kgs/hr. The specific volume of steam at 14 bar is 0.143 cu.m/kg, so the volume flow rate of steam is 31.58 cu.m/hr or
18.57 cu.ft/min. Now considering the recommended safe velocity of flash steam (which will reduce carryover)at
600 fpm (10 fps) equation for the diameter in feet is
[(18.57 x 4)/(3.142 x 600)]1/2, well I am afraid to give the answer. check it on your own.
Now you can calculate other way round to know the velocity of steam considering 0.8 m as diameter. It is coming out to be exceptionally low.
Now my questions are,
1. Is it a continuously drained system? If so check the condensate flow rate with the calculated one.
2. what are you doing with that high pressure condensate? (for a weaker soul like me 14 bar is still high pressure
3. Is your flash tank insulated?
3. For more learned gentlemen, what if the velocity of steam is very low? None of my books addressed this issue.
Regards,
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
You guys are calculating the percent flash incorrectly. By definition:
h(vl) = X*h(v) + (1-X)*h(l)
where
h(vl) = two phase enthalpy
h(v) = saturated vapor enthalpy
h(l) = saturated liquid enthalpy
X = quality
If you re-arrange the above equation, you can calculate the % flash (quality):
X = [h(vl)-h(l)]/[h(v)-h(l)] = [295-197]/[665.7-197]
X = 0.209 or 20.9% flash which gives a 313.6 kg/hr steam outlet flow.
...Not like it matters to terribly much. His vessel is plenty big. I'm not really sure what the velocities need to be, but if they are not high enough, the centripetal force will not separate the vapor from the liquid. Either a mist eliminator (between the flash inlet and the steam outlet line) or a baffle system is needed to separate the entrained liquid. If the condensate level is low and the nozzle is in the lower portion of the tank, there may be enough vapor space to keep entrained liquid from carrying over into the steam line.
jproj
RE: Oversized Flash Vessel
saxon
RE: Oversized Flash Vessel
You are correct in saying the percentage flashed is really 20% not 14% however for this vessel it really does not make any difference. You bring up a good point though about asking if the vessel has either a tangential feed nozzel or baffles above the feed nozzel to help with disengagement. That could be another helpful piece of information?
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
RE: Oversized Flash Vessel
I went through your drawing but without much result. Everything seems to be ok. but few concerns.
1. As you are pumping condensate inside the vessel almost tangentially, lighter water particles may raise and enter steam outlet. (steam also froms from the water collected below, still it is a speculation)
2. I checked for splashing of condensate due to striking on the other side of the vessel. The distance of other end of vessel from condensate inlet is 550 mm approx. (from your drawing) If I consider velocity of condensate at 2.2 m/sec (15000 kg/hr, but loalised velocity at exit may be high) it takes 0.25 sec to reach the other side of wall. So water would fall vertically and strike the wall at 307mm below the condensate inlet (opposite to it). So no problem of water carryover because of flashing.
3.You didn't tell us whether the condensate drain is continuous or intermittent.
4. Is it possible for you to provide a downward elbow at condensate inlet to the tank?
Beyond this I am exhausted.
Regards,