Compressible Flow Idiot 2

[KernOily]

Hi guys. I need a QUICK, IMMEDIATELY USEABLE reference on compressible flow in a long pipe for the PRACTICING engineer.

I don't have time to draw Fanno lines.

I don't have time to do integration, solve differential equations, and deal with equations that have differential elements.

I don't have time to fiddle around with real vs. ideal gases.

I am fed up with trying to use my college compressible flow texts as useful references for the practicing plant engineer. They are useless. Great for drawing oblique shock lines across a supersonic airfoil, and that's about it.

Can you tell I'm frustrated?

Here is the issue. I am a practicing engineer. Therefore, I have meetings to attend, work plans to generate, clients to meet, bills to pay, junior engineers to mentor, mud to stomp through, PHAs to attend, operators to whom I am obligated to listen to their gripes, managers to assuage, leaks to fix, and busted equipment to troubleshoot. On top of this, I have exactly one hour to determine the flowrate, exit velocity, and noise SPL from a 300' long, 12" std pipeline flowing 80% saturated steam. Line is a steam generator blowdown header that is charged by several blowdown valves with 850 psig downstream pressure. Line exits to atmosphere. Yes I know my pressure ratio is over the 0.545 critical pressure drop. Beyond that I am stuck and my compressible flow text is 100% useless.

Somebody needs to create a Crane TP410 for compressible flow...

Thanks for letting me vent (ok, bad pun). Pete

 

[chicopee]

Don't you have an idea about the mass flow rate from your make-up feedwater chart?

 

[BigInch]

This looks like it breaks it down into managable segments,

http://www.engsoft.co.kr/download_e/steam_flow_e.htm#1.

Introduction

 

[tr1ntx]

According to Kempe's Handbook:
"the pressure drop of steam flowing along a clean, straight steel pipe can be determined by the Babcock formula:

delta P = 0.4716(1+3.6/d)(LVw^2/d^5)

P in psi
L = length of pipe (ft)
V = volume of steam (cu ft per lb)
w = weight of steam flow (lb per sec)
d = pipe bore (inches)

or by Fritzsche's formula:

delta P = (0.82LVw^1.85)/(d^4.97)
same variables and units.

I don't know if that helps and it's too late I'm sure.

 

[dbecker]

KernOily,

I suggest you get a copy of Crane's manual 410. BUY it NOW. It's the most important reference manual in my opinion for someone like you. It is for the practicing engineer and the theory is not heavy, the application is worth it's weight in gold.

 

[kacarrol]

Ditto dbecker. I've built several spreadsheets based on Crane's manual 410 and every copy I've ever borrowed from senior engineers are falling apart at the spiral binding from their use.

 

[Latexman]

80% sat'd steam? I bet the other 20% is sat'd water, right? If so, can you say "two-phase compressible flow"? I don't think the references given so far are going to be technically correct. I think the only people that can do this problem correctly in 1 hour are the ones that have experience and the tools to do it already sitting on their desktop. So . . . yes . . . throw away your compressible flow text. You may want to farm this one out.

Good luck,
Latexman

 

[CJKruger]

KernOily, download the trail version of Korf Hydraulics from

http://www.korf.co.uk

and use it to solve this problem.

Email me if you are interested in the equations used in the program.

Cilliers (please note I am involved with Korf).

 

[KernOily]

Guys - Thanks for the terrific replies. I appreciate it very much.

I beg your indulgence for a couple dumb questions:

1) Suppose my insulated pipeline of constant size is running in critical flow at the critical pressure ratio (for steam it is Pup/Pdown = 0.545). Now, if I start to ratchet up the upstream pressure, so that the ratio drops below 0.545 (below critical flow ratio), the velocity will still remain at M=1 inside the pipe, BUT the mass rate will start to increase. The velocity will never go supersonic no matter how much I increase the upstream pressure. Theoretically I could infinitely increase the upstream pressure and thereby achieve an infinite increase in mass rate, all at M=1. Correct?

2) No supersonic flow means no shock wave anywhere inside this pipe, not even at the exit. Correct?

Thank you!!!

 

[CJKruger]

KernOily,

1) Partly. If you increase upstream pres (at fixed downstream) then the flow will eventually choke at the pipe outlet.

If you continue to increase upstream pres, the mass flow will increase, but the flow will remain choked at the pipe outlet.

2) Partly. The flow will never go supersonic, but the flow may be choked at the outlet. What this means, is that at the pipe exit (but still inside the pipe) the pres is higher than the atmosphere around the pipe exit. This pressure drop is destoyed as a shock wave.

Cilliers

 

[ione]

KernOily,

1. I suppose you meant Pdown/Pup = 0.545. Please also note that specific heat ratio varies with steam pressure. The “choked” connotation strictly refers to flow rate and not to mass flow rate. Increasing upstream pressure means increasing density and consequently mass flow rate increases as well (another way to get this is to decrease upstream temperature, which again means density increases). The only way you have to pass to supersonic flow (M>1) is to use a convergent-divergent nozzle (De Laval nozzle).
2. If you don’t reach supersonic conditions you won’t have shock waves generation.

 

[sheiko]

For steam issues, go to SPIRAX SARCO website.

"We don't believe things because they are true, things are true because we believe them."

 

[CJKruger]

ione,

2) No, you can (and usually do) have a shock wave by simply going to sonic flow (assuming adiabatic). No need to go supersonic to get a shock wave.

Cilliers

 

[KernOily]

Guys thanks for the replies.

Cilliers - How do I determine if the flow is choked at the outlet of the pipe? I.e. how do I determine if a shock wave is located at the outlet?

 

[CJKruger]

KernOily,

With Excel the easiest way is probably as follows:
- Calculate the mass flow rate using the isothermal compressible equation for a range of pipe outlet pressure (say from Pin to close to zero).
- This is a simple calc that is not iterative (except for f maybe) and depends on the pipe size, length, etc.
- Now plot the mass flow against the outlet pressures, and see where it goes through a maximum.
- If your outlet pres is below the outlet pres at max flow, then it is choked and your flow is simply the max flow rate. If not, you simply read your mass flow rate from this graph at Pout.

Let me know if it is not clear.

Cilliers

 

[ione]

CJKruger,
Thanks for having enlightened me on the shock wave (a star for you).

KernOily,
I have done a quick google search and have found the attached paper (also available here

http://www.aft.com/news/view.php?ID=97),

which revealed to be quite interesting. I suggest you to take a glance at paragraph “Sonic choking”.

 

[sheiko]

A star for ione! This is to my knowledge the best CHE article on the subject, unless someone know another one.

"We don't believe things because they are true, things are true because we believe them."

 

[pipesnpumps]

Perhaps I am mis-reading the thread, but you are dumping into a larger header, right?

It seems to me the flow will choke across the throat of your blowdown valve.. Why then are you concerned about choking in the larger downstream piping?

You have a complex situation that I would get a computer simulation done on if you want a real answer.. Two phase FLASHING flow across the blowdown valve.. Not a simple solution. Upsteam is 80% quality steam, then at some point shortly downstream of the valve, most of the steam is no longer liquid as it as flashed.

Typically in this type of relieving scenario, it chokes in the reducer downstream of the relief valve (as it goes from X NPS to larger Y NPS).

 

[davefitz]

For someone with no time for the fanno calc, then use the asme formuals for choked flow of steam thru relief valve vent lines. See the appropriate relief valve vent piping appendix in b31.1 or section VIII div 1. You will need the piping sum(fL/d) for all bends and straight pipe and inlet + exit loss.

As I recall, the asme curves and formula is limited to relatively small values of fL/d ( maybe fL/d < 4). For larger fL/d you can directly use the fanno equation and iterate on guessed values until you get the apprpriate convergence. Or look for teh original paper that is the basis of teh AME shortcut ( Becghtel's Liao paper circa 1970).

 

[davefitz]

Refer to :" Analysis of Power Plant Safety and Relief Valve Vent Stacks" by G.S. Liao, Bechtel Power Corp., Transactions of the ASME, november 1974