Flash tank design temperature

[carletes]

Dear all!! I am designing an atmospheric flash tank that receives the drains of the steam lines from the different pipes of a power plant. My question is the following: I know the temperature and pressure of the drains at the take-off point (i.e 22 bar, 300ºC)and that the tank is vented to the atmosphere, so, which would be the design pressure and temperature inside the tank? And just in the nozzles? I would like to know how the pressure and temperature changes from the pipe up to the tank. Any ideas?

Thanks for any help.

 

[davefitz]

For the maximum flowing case, the pressure in the steam superheaters or main system piping (upstream `of drain valves) is its full design value, and one assumes the drain valves are wide open.

The flow area minimum is usually in a valve throat, so one assumes the flow is acoustically choked in the 2nd downstream valve. However, if the small downstream piping is long, then the flow can also be choked in the downstreampiping, but the flow determination is usually based on choking in the second valve.

For the downstream piping, the enthalpy at the pipe outlet ( tank inlet nozzzle) is at mach =1.0, and the pressure profile upstream of this nozzle is based on the Fanno curves or relationships. If you know the casualty flow ( based on choking at 2nd valve) , the enthalpy, and pipe diameter, you can figure out what the pressure P* is in the nozzle at Mach=1.0 .

The pressure at the outlet of the 2nd valve Po is based on the Fanno relationahsips and the ratio Po/P* is:

Po/P* = {(k+1)/(2+(k-1)M^2)}/Mo
Mo= mach number at outlet of 2nd valve, per :

fL/d=(1-M^2)/kM^2 +(k+1)/2k * ln{(k+1)M^2/(2+(k-1)M^2)}

typical values for k=Cp/Cv = 1.3 are :
Mo= 0.3, fL/d=5.75
Mo= 0.2, fL/d = 15.73

 

[MJCronin]

Carletes,

In my opinion, the tank should be specified desgined and inspected as an ASME pressure vessel ( or the international equivalent). This type of tank is commonly called a "blowoff tank" if there is no valve on the vent to atmosphere

Design the flash tank for the maximum temperature (300C)of the boiler connection.

With regard to the MAWP for the tank, regardless of the actual developed steam pressure as explained above, use 50 psig as the minimum value......this will ensure a tank with a reasonable nominal wall and head thickness.

If there is a valve in the vent to atmosphere, specify the MAWP to be the largest value that can be developed by the system (typically this is the boiler MAWP)

See this website:

http://www.pennseparator.com/pbs6.htm

My opinion only....

MJC

 

[sailoday28]

davefitz (Mechanical) Jun 28, 2004
You have stated"For the downstream piping, the enthalpy at the pipe outlet ( tank inlet nozzzle) is at mach =1.0,"
This is an assumption and is only true if fl/d =fl/d MAX
In addition, an upstream valve may not necessarily be choked and the tank inlet nozzle may be at M=1.

 

[davefitz]

If the valve is at the flash tank nozzle, then the choking will be at the valve vena contracta. If the valves are back at the boiler , there is typicaly a long enough pipe run that the choking is frictional choking ( Fanno flow) in the piping. It is also possible that both occur, ie, the flow is choked at the valve vena contracta while the pipe discharge is at Mach 1.

Usually there are 2 similar sized valves in series and a significant pipe run downstream of the 2nd valve. You will have to iteratively solve for the pressure vs location to determine the choked flowrate, and the discharge pressure and outlet mach number.

The pressure vs location in the outlet piping is as per the Fanno relationships if the outlet is at M=1 , or as per standard pressure drop relationships if not frictionally choked. The pressure at the inlet and outlet of each valve is as per the ISA Control Valve handbook, in the general case viz:
W= 63.2 Cv*Y * SQRT ( X*PI/sv,i)

The determination of whether the valve flow is choked occurs when Y reaches its min value of Y=0.667 which occurs when X(=Dp/Pi) reaches its max value of X=Max{Dp/Pi, Xt*Cp/(1.4Cv)}