Suction pipe. Hot condensate.

[Deemter]

Hi!

I really need some tips, or comments, of a pump/ piping problem!
Hot/ boiling water (condensated steam) from a vessel will be pumped.
-The level difference between the pump, suction, and vessel is app 5,5 m (18 feet)
-NPSHr for the pump is 3 m (10 feet).
-The suction line is app 40 m (133 feet) long. With some bend and one valve, pressure drop is app 0,7 m (2,3 feet).
-The “lowest” pressure in the suction pipe is 2,3 m (7,63 feet). Before it goes down to the pump level another 3 m (10 feet) down.

Are there any risk for vapour bubbles in the suction pipe when the “lowest” pressure only is 2,3 m (7,63 feet) (water)?
What is the recommended max velocity for hot condensate?
Is NPHSa- NPSHr= 1,6 m (5,3 feet) enough?


Many Thanks Erik (new on Eng tips)

 

[checman]

Erik: The water temperature is key to evaluating your application. Vapor bubbles can form or be released in areas of low pressure even turbulence in the piping may have a low pressure area that can help form vapor bubbles. That is why it is so important to have the piping arranged so it is self venting. As for the NPSH margin that depends on the specific suction speed of the impeller and the type of pump and where on the pump curve you plan on operating.


Regards checman

 

[rmw]

Go to

http://www.graham-mfg.com

and look for a technical paper regarding piping recommendations for saturated fluids.

Your piping sounds like it must have quite a bit of horizontal runs in it, which is a no-no for saturated fluids. There is the risk of vapor pockets in horizontal runs.

For me and for systems that I design for hot condensate, I try to never exceed 2 m/sec. That may be on the conservative side, but it has always served me well.

rmw

 

[Deemter]

Many Thanks!
The water velocity in the suction pipe is 0,75 m/s (2, 48 feet/s). Pipe diameter is 300 mm (11,8 in).
It is correct that there is a long horizontal part. The lowest pressure in this horizontal part is app 2,3 m (water column ). Im most worried for vapour in this part.

Best regards Erik

 

[25362]

http://www.bellgossett.com/Press/NPSH2.html