Outlet Pipe Sizing After Steam Trap

[learner08]

Hi All,

Want to know if for example we have X kg/hr condensate at Y barG pressure to be discharged to a return of Z barG pressure, how do you calculate the the size of pipe required at the outlet?

There is going to be 2 phase at the outlet with these conditions. I am not looking for any charts but rather logical calculations.

Another question is if we work out that the outlet size to be like 50mm, would it be OK if the steam trap when sized correctly is a 25mm trap of certain model, we install a reducer/expansion at the outlet to 50mm.

Thanks for any help given.

 

[katmar]

I agree with your attitude of wanting to understand the reasons behind the calculations rather than just using the charts published by the trap manufacturers. But don't totally disregard those charts. Once you have done your calculations compare the results with the charts and make sure you understand the reasons for any differences.

Strictly speaking, the degree of flashing at any point in the condensate line depends on the pressure drop from that point to the line's discharge point. But the pressure drop depends on the degree of flashing. This means that you have to do an iterative calculation until the calculated pressures converge.

A simplification I use, is to assume for the purposes of the flash calculation that the condensate immediately after the trap has already flashed to the line's discharge pressure (Z barg in your example). You can now do a two-phase pressure drop calculation with a fixed gas to liquid mass ratio, making the calculation much simpler. This is a conservative assumption and you will have to use your judgement on where it is reasonable. As I said above, compare the results with the Spirax or Armstrong charts.

There is nothing wrong with putting the reducer immediately after the trap. It is quite likely with high pressure steam systems discharging into long, low pressure condensate lines that you would require a pipe size larger than the trap connection.

Harvey

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[25362]

Whatever the diameter selected on friction drop considerations, don't forget to do a check on possible hammering due to a differential (velocity) shock.

The actual design of the pipe should preclude steam entrapment, which, also could, by itself, generate hammering by sudden condensation and imploding (i.e., thermal shock).

Following an old ROT, to minimize erosion caused by flash steam carrying water, the flashing steam (taken alone) velocity is limited to a maximum of 5 m/s/inch of pipe internal diameter for trap dicharge lines, keeping a limit of 25 m/s for the design of return lines.

See also:

http://www.valdeseng.com/pdfs/water_hammer.pdf