Low Pressure Steam Trap Condensate Lift

[BronYrAur]

I really have 3 related questions here, so feel free to answer any or all of them.

1. Is there a rule of thumb for how high an end of main drip trap can lift the condensate on a 15 psig system? I know that in theory 15 psig should give me better than 30' of lift (less friction loss), but what is a safe rule of thumb? How high of lift should I actually try to achieve. Trap is a drip on a main so it will always see full pressure and condensate goes back to a vented tank.

2. How does one determine the friction loss on the trap discharge line since it is carrying a 2-phase flow? Aside from the complicated fluid dynamic equations, is there a reliable method? My design guide sizes the line to maintain a flash steam velocity and not a pipe friction loss.

3. As long as a low pressure condensate return main is pitched toward a vented receiver, it the length of the line really important? I may have as much as a 500' run back to the boiler using only the 15 psig steam discharge from the trap as the motive force. Since the line is pitched and the receiver is vented, will I be OK?

 

[Joesteam]

1. Height
You are right to say you can lift 30', and you can. However you must be aware the two most critical reasons for a drip trap, which are start-up and upsets. During start-up, with the highest loads, you have the lowest pressures. Unless you are willing to manually drain the drip leg during start-up, then do not have much lift after the trap. For upsets (ie: a slug of condensate entering the drip leg), you need to remove this quantity of water ASAP so it will not back into the pipe and cause dangerous water hammer. The lift will reduce the capacity of the trap and it will take longer to drain this slug of water.
My recommendation is to keep the lift to a minimum and install a condensate pump where required and THINK SAFETY FIRST.
2. Pressure
Again you are correct. You MUST size the condensate pipe on the flash steam flow at almost zero pressure (meaning atmospheric steam which has a specific volume of appx 26.8 Cu. Ft. per Lb.). If you do this you will have no worries on the friction loss for the water flow because the pipe will be large enough to provide a very low velocity of the water and almost negligible pressure drop.
3. Pitched Line
Once you get the condensate up to the main line back to the reciever, as long as it is pitched correctly and sized right you are using our good friend gravity to make the water flow. There is no length restriction unless you run out of height and you can no longer pitch the pipe.

 

[willard3]

snip
As long as a low pressure condensate return main is pitched toward a vented receiver, it the length of the line really important?
snip

Yes, the length is important.....if the pipe is not vented to atmosphere you may get steam flashing in the pipe.

 

[mizzoueng]

1. well, yes, with 15psi you can lift to 30'. but to be safe you usually on plan on getting only 20-25'. A condensate pump is always a good idea, b/c you dont want have to answer the question "should the pipe/HX be making that slamming noise?"

2. dont have my steam book handy

3. as lone as the run is straight, you can pretty much do whatever length you want. Its when you start adding bends and dips is when you get limitations. But again, to be safe, i think 200 LF of header is the most you want to do. If you are talking 500 LF or greater, you could look into a pumped condensate return line. IE. dump condensate into a sump pit and pump into a header.

 

[BronYrAur]

On my #3 question,. I actually have closer to 600' of pipe and very little pitch (if any). The run is pretty much horizontal with a drop at the end. At that point, the condenstae enters a "riser pipe" that runs through a y-strainer, then down to the floor and conencts to the bottom of a horizontal floor-mounted receiver tank. The level in the riser pipe will be the same as the tank. I don't know what level triggers the pumps, but worse case, I will still have a 1 foot drop before reaching water level in the riser. The riser is vented, as is the tank itself.

My condensate line starts out at 2" and increases to 3" along the way. The line will be serving multiple end of main drip traps from 2 parallel low pressure steam mains @ 15psig. So I actually have roughly 1,200 feet worth of 8" pipe to consider for startup load.

How do I calculate the condensate load and required pipe size? I am of course hoping that the existing pipe will work. How do I calculate the back pressure of water in a horizontal pipe (assuming no pitch) to know if 15 psig will be enough of a motive force?

 

[quark]

Check this link to an excellent material on condensate recovery and line design.

http://www.armstrong-intl.com/common/allproductscatalog/consguidelines.pdf

 

[katmar]

Quark's suggestion is the right way to go. Spirax Sarco also have very good resources. See

http://www.spiraxsarco.com/

Call the reps from one (or both!) of these companies to come and see you. Certainly here in South Africa the reps are very knowledgeable and helpful. They will be able to help you estimate the loads from your mains under various circumstances. You will have to take your local weather conditions into account, plus the insulation and construction methodology of the pipeline.

They will have tables for sizing the return lines, but because you have such long lengths I would be a bit conservative if it were my plant. I would assume that the 15 psig condensate had fully flashed to 0 psig to work out how much liquid and how much vapor to deal with, and then I would apply standard horizontal 2-phase flow pressure drop calculations to work out the expected pressure drop. You could try to estimate how the flash varied along the length of the pipe, but I would just take the conservative view and assume a constant (worst case) vapor-liquid ratio.

I would not assume that any downward slope in the line would help. The slope would help if you did not have a net flow of vapor, but it would be very difficult to estimate how much the flow of liquid due to the slope would entrain the vapor along with it. Any downward slope would obviously help, but keep this as a safety factor and don't rely on it.

Also check that the vent on your riser at the end of the pipe can deal with the additional load imposed by your new load.

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com