Back Pressure valve on Condensate line 4

[Chemtech2]

Can a Back Pressure valve be designed on a condensate return line to prevent flashing in the line. There is a hotwell downstream on the valve at 0 psig. The condensate is generated by 150# steam @ 360 deg.F .

 

[TBP]

Steam traps are essentially just automatic valves - they open when the see air or condensate, and close when they see steam. The amount of fluid they'll pass is dependent upon the orifice size, and the differential pressure. The orifice size is pretty much fixed. If you increase the downstream pressure, you'll lose differential across the traps, so they won't pass as much air or condensate as they once did. I think you'll have a lot of problems if you install a backpressure valve in your condensate line, especially on any applications with a temperature control valve on the steam supply, such as heat exchangers. If there is concern about the amount of steam venting from the hotwell, make sure that your existing steam traps are in good repair, are the correct style for the application, and are sized properly. A great many traps, like a lot of things in steam systems, are GROSSLY oversized.

 

[Montemayor]

sandeepdalvi:

To answer your question formally: Yes, you can install a back-pressure control valve (or similar device) on your condensate line. However, the obvious result will be just like TBP states: you've going to reduce your condensate capacity (if not halt it altogether) and probably cause condensate flooding upstream of the steam trap - thereby ceasing operation of some of your heat exchangers and coils. In other words, the system will not work as designed and probably cause you all kind of process upsets.

What you're trying to achieve -condensate flashing- is laudable, but not practical. In order to have condensate flow you must (as in all fluid flow) establish and maintain a driving force (in this case, a pressure drop). Without a driving force, you can't have condensate flow - flashing or not. Unfortunately, in this case of steam condensate, you will flash the instant you release the saturated liquid to a lower pressure. That's a price we have to pay if we are not going to subcool the condensate and just deal with it as saturated. In most all cases we don't subcool the condensate because this defeats the economies of maintaining the heat content so the boiler room won't have to consume more fuel.

What makes your proposal even more impractical is that you are going to flash anyway, as soon as the back pressure control valve opens to a lesser pressure down stream! So what have you achieved? Instead of flashing at the steam trap outlet, after you spend money and effort and suffer reduced capacity, you still flash at the control valve outlet. I wouldn't do it because I don't think it will work as you would want it to.

Art Montemayor
Spring, TX

 

[Chemtech2]

TBP/Art,
Thx for your replies. I also think on the same lines and was having a great difficulty proving things on paper. My discussion with a Sr.Process Engineer gave the following scheme which I will design tomm. and check if there is a possibility :

The condensate downstream of the steam trap is att 150 psig/366 deg.F.
The back pressure valve will have a upstream pressure of say 30 psig. so there is some flashing and crresponding cooling of the condensate.
The condensate has a driving force of 150---->30 in a 4" line.
Downstream of the back pressure is open to atmosphere but the steam flashed will be used to heat makeup condensate.
By putting a back pressure valve i want to prevent excessive flashing in line which can lead to hammering/unstable flow.
A manual from Spirax Sarco considers the above problem as 150 psig condensate being letdown to a 30 psig flash drum.
By putting a back pressure valve, I want to save on flash drum /pump since I do not have a user for LP steam.
Your comments/feedback is valuable.Pls. do let me know your views.

 

[byrdo]

Sandeepdalvi,

You might want to look at the driving force for this system again. I agree if you have 150# steam, then ultimately you can have 150# driving force on the condensate. However, for a stable system or unit the true driving force is the saturation pressure corresponding to the steam temperature required to get the Delta T needed to satisfy the required heat flux or Q=U*A*dT. A good example is a C3 column: the steam supply is 230 PSIG and the condensate leaving the reboiler can be as low as 3 to 5 PSIG (depending on column pressure). Also note that a system like this generally requires a condensate pump.

Regards,

Matt

 

[abcmex]

I think that everyone agrees that a condensate system should have a smaller pressure drop (including steam trap, piping pressure losses etc.) than the lowest steam pressure used otherwise the condensate would accumulate (stall, condensate pumping required).

But if the pressure difference between the steam pressure and the condensate header is too big excesive vaporization and other problems would appear. The piping pressure losses will be depending on the flow, at lower flows less pressure drop and more vaporization I suppose.

Yes, somewhere it will flash, or not if you add colder condensate (pumped condensate for example) somewhere, but would not be better to flash at a single point (perhaps with a tank) and reduce the two phase flow?

Depending on the specific system design/conditions a pressure control could be useful.

 

[25362]

Some thoughts for consideration, apart from those raised already:

Primero. At the P/T conditions indicated by Sandeepdalvi, it would be expected that condensate flashing into an atmospheric vented tank would be about 16% by weight, dropping to about 10% if the back-pressure were 30 psig.
Is 10% flashing sufficiently low to avoid water-hammer effects ?

Segundo. If I'm not mistaken, a 4" line would be recommended for a condensate flow of 15,000 lb/h when the back-pressure is 30 psig. The same flow rate when discharging into an atmospheric vessel would require an 8" line.

Any comments ?

 

[Chemtech2]

25362 ,
A pretty good calculation on your part......I did the calculation today. The final size I got was for 4" for a 16000 lb/hr flow.Following things were considered.
1)The point between downstream on Steam trap and before the PCV is like a Horizontal Flash Drum
2)5 cases e.g set points of PCV (considering that it is the pressure of the flash drum) were taken.
3)% Flash was found at the different pressure to get the two-phase flows in the pipe.
4)Now I have 2 phase flow travelling over the distance of the return header.
If the delta P of the two phase flow > the PCV setting for a particular set pressure, the system will work as there is a driving force.Another consideration was to come closer to a velocity of 66 ft/s as recommended by a manual of Spirax Sarco.
On paper it cms that it will work. I will be discussing the design with my Sr.Engineers to come to a conclusion.Do let me know your thought. I do appreciate the help given by all. The reasoning was correct for all.Thx again

 

[25362]

From literature, for flash steam velocitites, u, other than 50 fps, the needed pipe diameter is approximated by:

D[sub]u[/sub]=7.07*D[sub]50[/sub]/(u)[sup]0.5[/sup]​

 

[quark]

Before imposing constant back pressure on the trap it is better to plot the process on the stall chart. You will not have any problems if your system is working for the design maximum load all the time. If the load varies significantly, the back pressure creates problem to the removal of condensate from the system. Otherwise I would go with Byrdo's suggestion of condensate pump.

Regards,


Eng-Tips.com : Solving your problems before you get them.

 

[kirsner]

Sandeepdalvi--isn't this a case where your returning condensate from traps dripping a 150 psig main steam line? If so, is it the case that there is a separate condensate return line for this "high pressure" condensate, or are you trying to put this 360oF condensate in a low pressure return line with cooler condensate from lower pressure applications? If this is the case, is the condensate line hammering?



wayne kirsner (kirsner@kirsner.org)