Steam Flashing in pipes 2

[dangnm]

Ok, I need your input on a problem we are facing.
4" schedule 40 pipe is carrying 600 gpm, 350 deg. F and 225 pis high temperature high pressure water. Pipe length is 400 feet and has a couple of elbows, offests and an isolation valve.
Pipe is serving a water-water heat exchanger for domestic hot-water generation. Since its a high-pressure application, high-temperature water is flowing through the tubes of the heat exchanger.
My theory is that due to very high friction loss (about 23 ft/100 ft) part of the water will falsh into steam as it reaches the heat-exchanger and it will eventually, damage the heat exchanger tubes. Am I right / Wrong? Why?

 

[lilliput1]

The HTHW will not flash so long as the pressure is higher than the saturation pressure (120 psig at 350°F). The heated water must also be pressurized to above this pressure so it will not flash. The 23 ft w.g./100 ft of pipe pressure drop corresponds to 9.96 psig/100 ft of pipe. Thus you can tolerate about (225-120) x 100 / 9.96 = 1054 ft. equivalent length of piping pressure drop. It is the heated water you have to worry about. With minimum or n heated water flow, the water in the shell will approach 350°F.

 

[TD2K]

Lilliput1 essentially summed it up. You need to lose close to 90 psi before you approach saturation and start to flash.

You are likely about 1/2 of that assuming sch 40 pipe (even if it's sch 80 it doesn't make a huge difference).

Granted, at 15 ft/sec, you don't have much upside capacity on the line.

 

[rmw]

You don't have any control valves to go through that could take that much pressure drop, do you??

rmw

 

[quark]

Lilliput already proved that flashing will not happen.

You shouldn't really worry about mechanical intergrity of the HX eventhough flashing occurs because the pressure at which the liquid flashes will always be less than the maximum fluid pressure.

But your overall heat transfer coefficient will be affected due to vapor film.

Regards,

 

[briand2]

If this installation was in the UK, you'd have to meet the requirements of a document called PM5, "Automatically controlled steam and hot water boilers", which is published by the Health & Safety Executive. Amongst other things, it explains what you must to do achieve an adequate margin against flashing within the system. I assume there are similar requirements in your own area.

Regards,

Brian

 

[friartuck]

Hi ()
A few comments.....

At 180deg C your MPHW will need to maintain typically a 10deg anti flash margin equating to 190 deg C. At 190 deg C the required minimum pressure is 12.55bar abs.If you go below this, then watch out..flash steam will be noisy and you will not be popular

The pipe length I assume is 400feet flow and 400 feet return i.e. 800 feet plus fittings (allowing 30% for fittings) = 1040feet total run. = 317metres equivalent length.

Your pipeline PD is 7m per 30.47m or 2289Pa/m or 0.0229bar per metre=317x.0229=7.25 bar loss (wow, seems like your pipes need upsizing)


You say that you have 225PSI (15.5bar G) so if you start out at 15.5 then - PD of 7.25Bar = 8.26Bar G 9.26Absolute.(assuming I have taken your initial pressure and the pipe length as being as stated.(if not, then if the total distance IS actually 400feet F&R, take PD of 3.63Bar=15.5-3.63=11.87Bar G or 12.87 Abs) You are just in here, but not a massive margin.

Now, the other thing to consider is, does the pipe RISE anywhere. If it does, then for every 10.20m rise in elevation, you will also lose 1 Bar pressure. Like wise it will increase if you go down.

So if your 400 foot of pipe rises 30 feet, the pressure in the pipe will drop and flashing may occur.

Also, I have ignored any 3-port control valves that you may have which could typically have a large PD and give cause for concern.

It might be prudent if you are unsure to lower the Flow temp by say 10 degrees to avoid problems. You can always put a probe in the system (under cold conditions) and check the actual system pressures before risking having the risk of flash steam.

PS I got the steam table info from the chemicalogic website which is pretty good..(cos they are free)

Friar Tuck of Sherwood

 

[lilliput1]

Don't ignore the last part of my response. It is the water in the shell that will flash if it is at less than 120 psig. When it flash it will be like a huge pot boiling over. Have the pump pump towards the HEX to help making sure pressure at the HEX is > 120 psig + safety margin.

 

[friartuck]

lilliputs right. Dont forget the high limit protection which should be manually reset on over temperature detection.

Friar Tuck of Sherwood

 

[fayazdin]

OK, According to my calculation, water may flash. I repeat....... Please Tell me why I am wrong?

P1=225 PSI
P2=225-44.87 = 180.1 PSI

H_fp1= 366.3 "Btu/lbm" Fluid Enthalpy @ P1
H_fp2= 346.3 "Btu/lbm" Fluid Enthalpy @ P2
H_fgp2 = 851.2 "Btu/lbm" "Heat of vaporization" @ P2

Formula:
Flash Fraction = (H_fp1 - H_fp2)/H_fgp2 = 0.023

If 700 gpm are flowing,
Flash / min = Flash Fraction x gpm = 16.24 gallons / min

 

[TD2K]

Look at your steam tables and the vapor pressure of the water at 180.1 psig.

 

[friartuck]

fayazdin

water at 350F=pressure rerqd of 134psi(abs) note the absolute bit and not gauge which would be 134-14.7=119.3gauge pressure

P1=225psi (gauge i presume as nobody works in absolute except for calcs like this)

225psi=239psi abs = equates to a boiling point of 397F

you show p1 as 225-44psi pressure drop=180psi which equates to 373 F boiling point. You have water at 350F which is less than the 373F boiling point......but in addition, I think you actually have a gauge pressure so you would add 14.7 onto the 225psi figure and consequently the 180psi figure which would in turn make the boiling point even higher...180+14.7=194psi abs=380F which is even more margin. either way you are ok..but clarify if you are working in absolute or gauge pressures...it is important

Friar Tuck of Sherwood

 

[25362]

To fayazdin. You are asking about your probable error. Well, from what I gather, as long as water stays at 350^oF, the enthalpy would be around 322 Btu/lb at all envisaged pressures: 180.1 psia, 194.8 psia, 225 psia, or 239.7 psia.
As a matter of fact the NIST Chemistry Webbook gives
for 350^oF and those pressures: 322.02, 322.04, 322.09, and 322.11 Btu/lb, respectively. Correct me if I'm wrong.