Steam Valve Sizing Effects 1

[scoots]

I want to make a comparison calculation for the effects of increasing the voloume of steam into one of our machines. To save money we want to use the same steam valve and just increase the diameter of piping after the steam valve. The other option is to increase the size of the steam valve as well as the piping.
How can I calculate this to determine whether there is a difference?

 

[vidanes]

I cannot at this point give you the calculation you required. However, in the case of steam volume, good engineering practice require that you calculate the volume of steam required by the process and that's it. Not more because it is wasteful and not less because your process will suffer. In the case of steam valve (steam trap?)selection of size will depend on the running load (steam) multiply this by a factor of 2 to make up for the startup load and the differential pressure (inlet pressure minus any back pressure)You can then look at the sizing tables of steam trap manufacturers to come up with the right size.

 

[kyong]

This is not that simple problem.
Let me try, though.

Q=((P1-P2)/L)^0.5 x D^1.5 / f x k
Q=flow rate
P1=pressure at start point of pipeline
P2=pressure at end point of pipeline
P1,P2 are regarded as unchanged
D=ID of pipe
L=equivalent length of pipe line based on D
f=friction factor
k=proportional constant
f decreases with increasing D, but not much. So let's say f is unchanged.
Then the equation becomes
Q=(1/L)^0.5 x D^1.5 x k1(k1 is new proportional constant)
L=L1+L2
where L1=eq. length of pipe,
L2=eq. length of valve
Then the equation becomes
Q=(1/(L1+L2))^0.5 x D^1.5 x k1
L1=length of straight pipe + sum of (K x D / f ) of all elbows
Usually K x D / f is significant compared to legth of straight pipe. Let's say f=0.02, K=0.6. Then KD/f for one elbows is 30D. Now you can figure out how long L1 is.
L2=(Kv*D/f)x(D/D2)^4
where D2=ID of valve inlet(ID of outlet should be same as inlet. D2 is not the diameter of inside of valve), Kv=10 for globe valve, 0.19 for gate valve.
Let's say f=0.02, D2=10", D=12".
Then L2=(10*12/0.02)x(12/10)^4 = 12432"(globe valve)
L2=(0.19*12/0.02)*(12/10)^4=236"(gate valve)
I don't know how long L1 is. Let's just say 6000".
Then Q=(1/(6000+12432))^0.5*12^1.5*k1(globe)
Q=(1/(6000+236))^0.5*12^1.5*k1(gate)
As you can see, globe valve affects very much while gate valve affects just a little bit. So if the valve is globe type, there is little possiblity you can avoid replacing it.

Hope this helps.

 

[TBP]

Each component needs to be sized individually, using the outlet pressure from one as the inlet pressure to the next. If you're sizing a PRV associated piping, a pretty good rule of thumb is that the inlet piping to the control valve will be one size larger than the correctly selected valve. The outlet piping will be double the diameter of the inlet piping. If you can increase the steam pressure on the line feeding the control valve, you may be able to deal with your situation that way. Get a copy of Spirax Sarco's "Hook-Ups". There's a ton of good information in it. Just be aware that it looks at things from a performance perspective only. It doesn't address code issues.

Do some checking first. The biggest single problem I see with steam equipment, control valves, traps etc is gross oversizing. If you do increase sizes and/or supply pressure, be sure to check the capacity on any associated components like safety valves.

 

[TD2K]

Basically (I'm assuming some things here), you have a steam source, piping, your steam valve, more piping and the machine. You need to look at the pressure drops through this system at current rates (the valve takes enough pressure drop to match the supply pressue, the delivery pressure and the pressure drops through the line) and then do it again at the new desired flow rates.

You know the steam supply pressure and you know the pressure you want at the machine. At the new flow rate, will the control valve just open up more to pass the additional amount of steam and deliver that flow at the required pressure? If not, can you save enough pressure through bigger piping downstream of the control valve to offset the pressure drop resulting from the higher flow? Can you replace the trim in the valve (assuming it's a conventional control valve)?

Essentially, you need to look at the hydraulics through the entire sytem to made a conclusion. If the existing pressure drop through the downstream piping isn't that large, then replacing the piping isn't going to help you much but you need the specifics of the system to look at that in any detail.

 

[ChasBean1]

Scoots, I agree with TD2K - it might behoove you to repost with some more details. What is the upstream steam pressure, what is the current valve size, what is the pipe diameter (upstream and downstream), where does the valve currently control (i.e., what percent open) during maximum demand? Also, what is the peak steam demand for the new component, and what sort of percentage increase is there from the old peak demand?

If you look at it as a simple pipe size increase without considering the load, orifice steam flow data shows that your Y (flow) varies with X (diameter) squared. System flow after increasing the pipe size from 12" to 16" (133%) will cause flow to go up 178%. This does not, however, take into consideration the actual system, friction losses due to pipe length, and the demanded load. Your answer might just be to install the new component without changing anything... I say put the new thing in a see what happens! If anyone asks, tell them some guy in a forum said it was okay.