Downstream steam pipe pressure

[pch1]

Can anyone point me in the right direction for the following:

I have steam in a 6" pipe at a pressure of 64 barg and a temperature of 400 C with a mass flow rate of 100 tons per hour.
The 6" pipe is connected to a 10" pipe by means of a reducer and the 10" pipe is open to atmosphere.
What pressure can I expect to see in the 10" pipe?
What diameter would I need to increase the 10" pipe to see a pressure of 30 barg?

Any help greatly appreciated.

 

[zekeman]

Need the lengths of 6" and 10 " pipe plus any elbows.

 

[pch1]

The 6" pipe is connected directly onto the 10" pipe.
There is 1 x 6" elbow, 20 metres of 10" pipe and 1 x 10" elbow

Thanks
Paul

 

[Latexman]

On the other end of the 6" pipe is the source of your steam (I assume). What length of 6" pipe and number and type of fittings (valves, elbows, line run tees, branch run tees, etc.) is between the steam source and the 10" pipe?

Good luck,
Latexman

 

[pch1]

Yes, the 6" pipe is the steam source.
There is only 1 elbow on the 6" source, then a reducer to get to 10".

 

[Latexman]

I'm confused. What's controlling the flow rate at 100 tons/hr? With the system as described, it should flow at 5X what has been stated. Was some resistance to flow omitted? Was the inlet pressure pulled down by the 200,000 lb/hr?

Good luck,
Latexman

 

[pch1]

The system I have described is the blowdown from a boiler.

 

[Latexman]

Now that we know the application, we know there is at least one valve in the line. What else did you leave out? Every component adds resistance to flow and these problems are not simple, so please get your facts right before you ask for help.

Good luck,
Latexman

 

[zekeman]

Where does the 100 T/hr come from? If it is a design goal, shouldn't your question be " how do I design a blowoff system with x pressure in the boiler to yield a 30 barg pressure in the final piping"?
Or has it already been designed and controlled. Please explain.

 

[Latexman]

A blowdown system that passes only steam is not going to work. Removing steam leaves solids behind. The objective of a blowdown system is to reduce solids in the water in the boiler to an acceptable level. Blowdown systems remove water which contains solids. As the liquid loses pressure in the blowdown system due to friction it flashes to a two phase mixture.

Good luck,
Latexman

 

[ione]

Latexman got the point.

Usually blowdown is related to a TDS control and it is scheduled at precise time and for a set duration (if the valve is suitable for the purpose 5-10 seconds each 8-10 hours of working can be enough).
If the boiler works at such a pressure 64 barg, and blowdown valve discharge to atmosphere, flash is undoubtedly an issue. It is a quite common practice to use a blowdown vessel to separate flash steam (reusable in low pressure applications) and dirty condensate.

Take a look at the link below to get further info.

http://www.spiraxsarco.com/resource...utorials/the-boiler-house/bottom-blowdown.asp

 

[davefitz]

If it is steam with a base inlet pressure of 64 barg, then you will have 2 choke points.

The discharge at the 10" outlet to atmosphere would be choked at the speed of sound; the pressure immediately upstream of the 10" discharge is caculated as with safety relief valve exhaust stacks- see the Bechtel Laio mehtod defined in the asme B31.1 code appendix II. Knowing the pipe diamenter, flow, and ehtnalpy, the pressure can be caclualted.

The flow is defined by the choked conditions at the smallest open flow area betseen the base reservoir at 64 barg and the 6" pipe, likely a valve. You can use the ISA handbook of valves to compute the choked flow thru the valve knowing its Cv, Xt, and upstream fluid properties.

 

[Latexman]

As I Googled steam blowdown valves, I saw references that lead me to believe they are limited in size by some Codes. It appears ASME boiler and pressure vessel section 1 codes and ANSI B16.31 power piping codes limit them to 2.5" max. If that is true (boilers are not my main focus), pch1 will need to consider their local Codes on this.

Good luck,
Latexman

 

[pch1]

Thanks for all the input lads.
I used the Spirax Sarco website, and together with a colleague we calculated the expected pressure drop through the piping system.

 

[ione]

I really do not know the way the OP has arrived to the conclusions the blowdown flowrate is 100 tonne/hour.

Below the procedure I follow to evaluate an intermittent blowdown:

B = (Q * S)/(Ca – S)

B = flow rate of water for blowdown [kg/h]
Q = boiler capacity [kg/h]
Ca = admissible water conductivity [?S/cm]. It depends on boiler working pressure
S = Feed water conductivity [?S/cm]

Once B has been calculated there are sizing charts available for blowdown valves that show the discharge capacity of hot water [kg/s] vs boiler pressure [barg].

For a boiler operating at 64 barg the TDS should not exceed 625 ppm (ABMA Recommended Feedwater source).
Even if a precise conversion factor to turn ppm of TDS into ?S/cm of conductivity requires the chemical composition of TDS, it is possible (acceptable from an engineering point of view) to use the following correlation:

(TDS) ppm * 0.67 = Conductivity µS/cm

 

[ione]

pch1,

The blowdown flowrate you’ve quoted goes on puzzling me. For automatic blowdown, managed by water conductivity controllers, there are available valves (2”) capable to discharge up to 30 ton/hour (with 100 bar pressure differential), which is less than one third the value you've quoted. Would it be possible to know the way you’ve calculated 100 ton/h of hot water blowdown?