Steam Flowrate Conundrum 5

[HopDr]

I have a plate heat exchanger designed to consume 2533 lbs of steam per hour at 249 deg F (roughly 14.5 psig). My boiler (under ideal conditions) delivers steam to this HX at ~338 deg F, 100 psig, through a 1" sch. 40 line. I'm trying to determine how many pounds of steam are actually being delivered to the HX per hour, as its symptoms indicate that it may be starving for steam. I don't know definitively the steam velocity at the HX, but Spirax-Sarco tables indicate that it should be no more than 131 fps. At this velocity & 100 psig, the same table indicates that the 1" line can deliver no more than 736 lbs/hr.

Another engineer feels that frictional losses (total equivalent length) in the 1" line will introduce roughly 20 psi pressure drop between boiler & HX, which will somehow cause flowrate to double to ~1400 lbs/hr & the velocity to increase to near-sonic. I've tried mightily to cipher out the physics for this to be true, as he has more experience at this than I, but I'm boggled!

Perhaps someone can address a question basic to my friend's position:
1. In a closed system such as this, does steam accelerate between its constant-pressure source -- the boiler -- & the point of lowest pressure -- condensation inside the HX -- or does it move at a constant velocity for a given pipe diameter?

Any feedback is greatly appreciated!

 

[TBP]

Your line is too small. You likely require something like a 2" pipe, with an 1-1/2" temp control valve, with a 4" line after the TCV, going into the heat exchanger.

 

[chicopee]

I have an equation from which you can estimate pipe sizes based on steam velocities; please see the .JPEG attachment

 

[chicopee]

OOps! forgot the attachment

 

[hacksaw]

if you are delivering that much steam, you'll hear it, I get 550 fps at 100#/338 F, with a pressure drop of 25 # in 10 feet of straight pipe.

with such an undersized line, the pressure & velocity are not constant as you move along the line. Since you are delivering saturated steam, it is very possible that you are delivering condensed steam to the exchanger and you might be risking damage to the hex tubes.


you should have a gage on your heat exchanger, what does it read? and also is the 1" pipe insulated?

 

[HopDr]

Hacksaw,

Thanks for the valuable input. I know the steam is quite wet by the time it reaches the HX, as this appliance is the furthest from the boiler: nearly 300 feet away. The condensate load post-HX is very heavy & usually waterlogs the lower portion of the HX until pressure is sufficient to push it through the steam trap.
The gauge at the HX's steam inlet cycles between 80 psi & 100 psi, in response to the stalling described above.
The 1" stainless steel line is uninsulated, & is surely contributing a lot of condensate.

 

[ione]

With the following data

Steam pressure at boiler 100 psig
1” schedule 40 pipe 300 ft long
Steam pressure at HX: 80 psig

Assuming a straight run from the boiler to the HX (quite unrealistic) your steam flow rate should be approx 520 lb/hr. As pointed out above lack of insulation further affect the performance of the line as you’ll certainly deal with two-phase flow. To have an idea of the amount of steam you can carry out a condensate measure downstream the heat exchanger. Disconnect the steam trap from the condensate line and allow condensate to discharge in an atmospheric pot. Measure the weight of condensate produced in a given time. In order to have a more precise idea repeat measurement. Please anyway consider that the measurement will give you just an estimate of the steam flow rate in working conditions as the steam trap performance improves as differential pressure across it increases.

 

[zekeman]

I have a problem with these calculations. Maybe somebody can enlighten me.
Where do we get a pressure drop of 20 lbs. I get an overall pressure drop of 100-14 or 86 lbs which makes all of the weight flow calculations dubious.
If 80 psig enters the HX *( which I doubt) then it would choke to get it down to atmospheric.

I think the proper way of doing this is to assume a flow rate to the final conditions at the HX and itertively change the flow rate until the pressure drop along the pipe ends with a match at the HX.

My guess is the answer is closer to the design value then 550Lb/hr.

 

[quark]

As suggested by TBO, if you go for a 2" Sch 40, you will have 3.12 bar pressure drop(i.e 44 PSI). Regarding the pressure drop for the existing system, your friend is wrong and I agree with Hacksaw's numbers. 530 lb/hr is the flowrate you can get, with present set up for 100 psi drop. So, this is the maximum value.

Flow doesn't choke because of pressure drop due to friction. Secondly, if you trap the condensate, it is very less likely that volumetric velocity increases all along the pipe line.

 

[TBP]

If you go to Spirax Sarco's website - spiraxsarco.com - there are calculators there for sizing (among other things) steam lines.

Plugging in the numbers provided, you should likely be going with a 3" line, rather than a 1".

 

[zekeman]

Chicopee,

"I have an equation from which you can estimate pipe sizes based on steam velocities; please see the .JPEG attachment "

Where did you get this gem ? Could you please document?

It is the simple mass flow equation with some broad data on expected flow velocities. I'm puzzled.

 

[zekeman]

HopDr,

" I don't know definitively the steam velocity at the HX, but Spirax-Sarco tables indicate that it should be no more than 131 fps."

Could you tell us where this comes from? And if you believe it, wouldn't that put an upper bound on the flow rate?

 

[HopDr]

Zekeman,

Thanks for taking time to reply. The information comes from Table 10.2.4 "Saturated steam pipeline capacities in kg/h for different velocities (Sch. 40 pipe)", The Steam & Condensate Loop, Spirax-Sarco, 2008. Throughout the preceding chapter, they refer to 40 m/s as the practical limit for saturated steam. Having no means of accurately measuring velocity in my application, I assumed (danger!) this value to be close, as there's no reason to believe the velocity's any higher (i.e. erosion, noise, HX damage). In fact, the sound of steam flow is barely perceptible at the HX. At 40 m/s & 100 psig, the table shows 334 kg/h (736 lb/h). Certainly, if one adheres to this speed limit as a design rule, flowrate is limited accordingly.

TBP,
I'm coming up with the same conclusion: 3" line needed to provide the design flowrate of my HX. Thanks for the link!

 

[zekeman]

HopDr,

Never mind, I found the website and I also get around 500lb/hr and since the flow is linear with the pipe area, 2" Dia will give 2000lb/hr so I guess you need 2-1/2 " diameter.

I also found that there is a "critical" pressure for long pipes carrying 2 phase flow like wet steam similar to the choking condition for gases and it is

Pcr/P=.52

which says you cannot use a socalled "pressure drop" to get the pressure at the end of the pipe; this is very similar (but not the same since acoustic velocity is not involved) to air in choking conditions.

So in using the Spirax-Sarco tables,

the terminal pressure to use before it jets into the HX is

p2=.52*p1

 

[zekeman]

Guess our posts crossed, but the only (I get slightly higher flow rates) difference in my results is that I use .52p1 for the terminal(end of pipe) pressure and the author's problem limits the terminal pressure to 6 Bar, since his application is for heating and yours is for dumping into a 1 bar chamber. I used the author's Fig.10.2.7 nomograph for my calculations.

In any event 2-1/2 or 3 inch diameter should handle the job.

You might want to think about insulating the line since the heat loss as you noted can cause significant problems with steam quality,corrosion, etc.

 

[zekeman]

correction,

From my previous post
"............... but the only (I get slightly higher flow rates) difference in my results is that I use .52p1 for the terminal(end of pipe) pressure and the author's problem limits the terminal pressure to 6 Bar, since his application is for heating ........."

From the Spirax-Sarco nomograph Fig.10.2.7 , for a pressure drop of 20 psi over 300 ft of piping, I get a flow rate of only 100Kg/hr or 220lb/hr which is less than 1/2 the actual flow'

Given that you are measuring that drop, it is an understatement that you are starving steam.

I did a rough thermal analysis to get an idea of the steam quality and estimated that you are losing about 1000BTU per pound which means that your steam quality at exit is about 15% or you have mostly water by this calculation.
I think this should mandate insulating the line.

I also did a quick calculation

 

[chicopee]

Zekeman, the info. was obtained from a boiler book sometimes in the early 80's when I took my National Board exam. Unfortunately I did not write down the reference at the time but If I do come across it, I let you know since I see you name on many of the posts.

 

[zekeman]

Chicopee,

Thank you for responding, but since I see no dependence on pipe length, I am a little skeptical. If you look at the Spirax-Sarco nomograph Fig.10.2.7 that the OP cited, you will see what I mean.
Also, the critical flow pressure,
Pcr=0.52*p1

is not accounted for. (OK to end with a preposition?)

 

[steamdog]

You mentioned something that is bothering me, and that was the flooding of the heat exchanger.... If your piping AFTER the steam trap is not atmospheric, this may be causing a large problem in the heat exchanger. If you have a modulating control valve, you may get what is called 'Stall'....

Also, a go to web application I use to size piping I have attached.

 

[MJCronin]

I have a question.... what is the inlet nozzle size on the heat exchanger ?

I am guessing it is about a 4"NPS.

When you ran that long 1" steam supply line and you attached it to a 4" nozzle (I am guessing here)......ummmmm wasn't that kind of a clue that there would be trouble ?