Steam Fow Question 1

[marada93]

Say you have a 1" steam drain pipe (schedule 80).

On the inlet is steam at 200 psig and 500 F. It exhausts to a condenser at approx 25" mercury approximately 60' away. The I.D. is .96" (I think).

Can someone determine from this what the flow through the pipe is? Or is there a need for more information. I'm out of my area of expertise on this, so I'd appreciate anyones input.

I/m trying to quantify what how much steam we waste with this configuration (ie...no trap, no valve)

Thanks in advance!

 

[Latexman]

marada93,

"Can someone determine from this what the flow through the pipe is?" Yes, they can.

"Or is there a need for more information"? No, all the information you need is there.

I'm glad I could answer your questions!

Good luck,
Latexman

 

[marada93]

Now thats funny right there. I don't care who you are.

Let me re-phrase...

Given this information, could someone either show me how to calculate the flow rate, OR calculate the flow rate for me then possibly, I hate to even hope...give me the answer?

In pph or kpph?

 

[TD2K]

Crane technical paper 410C would be a great reference for you, they have a very similar calculation.

As this is a gas, its density decreases as it moves towards the outlet. As a result, its velocity increases and the pressure drop per unit length increases. Also, given you inlet pressure and relatively short length, the flow will almost certainly be choked at the exit.

I estimate the mass flow as about 2400 lb/hr. That's taking the 60' as straight pipe with an entrance and exit losses. I didn't include any allowance for fittings which would reduce this flow somewhat.

 

[Drazen]

What is missing:
Quantity/outflow of condensate. If you can measure it, you can use this

http://appserver.ittind.com/software/HSapps/steamcalculators.htm.

As you can see, the calculation itself is not simple.
or
If you could measure pressure somewhere downstream of your pipe, before condensation occurs, you could estimate steam flow much simpler, on basis of the pressure loss (your 200 psig - pressure you measure downstream = pressure loss). There are tables for this estimate although at this moment I cannot find any online. Tell me if you reach this point

 

[marada93]

Yeah...I knew there was more to this than meets the eye. Immpossible to measure the condensate a it's under vacuum when we're on line, and when we're off line, theres no steam flow through the line at all. (an extraction line).

But, thanks for the input. This gets me headed in the right direction.

 

[Latexman]

marada93,

Sorry, I couldn't resist. I was at home and my software that makes short work of this problem was at work. I could just hear my Dad over my shoulder saying for the thousandth time, "if you want the right answer, you have to ask the right question", so I had to be witty.

I ran your numbers this morning and got the same flow as TD2K.

Good luck,
Latexman

 

[TD2K]

If the inlet steam to this 1" pipe is 500F at 200 psig, they are well above the saturation temperature. If the numbers are right, they will be dealing with dry steam only flowing through this line.

 

[AndreChE]

Use the following formula:


u = 44.72 * SQRT (DELTAH)

where u -> m/s
DELTAH -> Enthalpy difference from state 2 to state 1 in kJ/kg

It is usual to have this equation in the Water Mollier h,s diagram

 

[TD2K]

Andre, what's the background for this equation?

For example, a control valve has zero enthalphy change across it (as it's 'commonly' taken as an isenthalphic expansion) and yet the velocity is not zero obviously. I'm missing something here I know.

 

[Montemayor]

From one Chemical Engineer to another, I'm very interested in learning how we can extract a flow rate value (mt/sec) from the Steam Mollier Diagram that is only a plot of Enthalpy versus pressure - or the T-S Diagram, where we plot temperature against Entropy. Either way, these diagrams only yield thermodynamic values and paths - not flow rates!

As TD2K points out, we're dealing with well-superheated steam and it's going to be dry and gaseous throughout its piped trajectory. Therefore, the Darcy calculation applied to it (assuming it's withing the 10% allowable pressure drop) is valid. Additionally, measuring the resultant condensate from the downstream vent condenser doesn't necessarily account for all the steam flowing through the inlet pipe. Some steam goes out the vent and this is inevitable - especially if a vacuum is being maintained via a vacuum device (25" of Mercury is mentioned, so I assume there is a vacuum kept there).

Both TD2K and Latex are correct in their analysis and basic results.

 

[marada93]

Thanks one and all for the input, and no offense taken Latexman. I thought MY dad had somehow zoomed me once again.

Correct on the superheated steam constant. This is a full load value which we run at pretty much constantly. Same with vacuum (give or take a little depending on summer or winter conditions).

There are six 90s on the 60 foot run and one 45 before it enters the condenser. Also one valve (wide open).

As I can tell from the responses, this does play into it. I don't know whether its a negligible change or not. I would have given this info the 1st time had I known.

In the meantime, thanks again for your posts. Extremely helpful to me.

 

[Latexman]

What kind of valve? Reduced port or full port? If it's a control valve, what's the Cv?

Good luck,
Latexman

 

[marada93]

Heres what I know of the valve:

It's a 1" Copes Vulcan control valve (either full open or full shut...not modulating)

It looks to be full port, though I can't find that on the specs, it just looks that way from the diagrams.

Nameplate data shows it has a maximum capacity of 5200 pph at 240 psig and 516 F, which is slightly higher than the conditions I stated in my 1st post...nevertheless my intial conditions are accurate.

Specs show a maximum CV of 15 at full travel (1") with a maximum CV required of 14.72468

It doesn't appear that the valve at full open is a choke point. You?

 

[sailoday28]

How significantg will heat transfer from the pipe to atmoshphere? I believe the curves from Crane are for an adiabtic pipe.

 

[TD2K]

At that flow (if indeed it's dry steam flowing through a pipe at these conditions), heat losses will be insignificant in my opinion.

I doubt even if you had that pipe submerged in water (with quite a bit higher heat losses compared to air) that you are going to be much different from adiabitic.

 

[Latexman]

marada93,

It could be full port, but most control valves are reduced port. For a 1” control valve body with a Cv = 15, I’d expect about a 0.75” port diameter.

Assuming a 0.75” port diameter on the control valve, the 60’ of pipe, and the elbows, I estimate your steam flow at 1950 pph (control valve on upstream end of pipe) or 1875 pph (control valve on downstream end of pipe).

If, indeed, the control valve is full port, I’d expect 2150 pph steam flow no matter which end of the pipe it’s on.


Good luck,
Latexman

 

[marada93]

Thanks for staying with me this long through this thread Latexman.

The control valve is upstream, so It looks like 1950 pph. Even so, why would the valve rating show 5200 pph at the steam conditions mentioned...that seems way off, doesn't it?

As far as full port or reduced, the only other thing I can get from the specs is on the trim as follows:

-inner valve trim = 1"
-characteristics = modified parabolic
-style = unbal.single seat, plug CV-600
-flow direction = under the web

But still, all in all we're looking at roughly 2000 pph give or take. Thats probably close enough for the girls I go out with. As I said before, the steam conditions change slightly based on load, and condenser backpressure, etc.

for instance...as we speak the steam inlet conditions are 221 psig, and 515 F. Drops down to about 195 psig during the winter.

 

[Latexman]

maradas,

It's the total frictional effect of the piping, control valve and elbows that reduce the flow to 2000 pph. The control valve, by itself, connected to 240 psig and 516 F steam on one end and open to the atmosphere on the other will flow 5200 pph.

If you only had 60 feet of straight pipe, the flow would be about 2400. If you add the elbows, the flow would be about 2300. Then, if you add the control valve, the flow would be about 2000.

Good luck,
Latexman

 

[marada93]

I'm with you.

Thanks for biggerin' my knowledge on this subject.

Many thanks Latex!