RO sizing in 1" warmup line centrifugal pump 1

[jimty1]

Hi Folks!

I'm looking for guidelines/calculations for sizing RO's in 1" warmup lines.
We're speaking of centrifugal pumps, both horizontal and vertical.


Why?: when an operator opens the valve in the bypass line over the checkvalve too far, it causes a flow large enough to spin the impeller backwards, causing the mechanical seal to break.

I've seen this thread: thread407-133249
Here JJPellin states he uses 1/4" for pumps under 200 bhp and 3/8" for pumps 200+ bhp. I'd like to know if there is a source that could confirm or explain this.

I hope I've given enough information

Jim,

 

[EmmanuelTop]

I would contact the manufacturer and ask for advice. Some designs utilize 1" globe valve on the recirculation (discharge NRV bypass) line, with purpose to adjust the circulating flow rates depending on the actual heating requirements.

See also attached article - it discusses some of the key points:

http://turbolab.tamu.edu/proc/pumpproc/P5/P5109-115.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[jimty1]

In the current situation we do have these globe valves installed, the point is that the people operating them have little knowledge of the effects caused by a backwards rotating N2 Seal in the pump. The operators tend to open the valve too far just to make the warm up process quicker. The RO is there to limit the pressure in the recirculation line. (training the employees is not considered a good long term solution, as these results degrade over time)

Because we will be creating a limitation to the warm up process I'd like to find the point where we can get the best warm up rates without breaking the N2 seal.

 

[EmmanuelTop]

I couldn't imagine a calculation procedure that would provide you with such an exact result, given that there are so many variables in the play: fluid temperature, fluid viscosity (and fluid-specific viscosity-temperature relation), ambient temperature, insulation, impeller size, and other characteristics immanent to the pump. API RP 686 Section 6 provides general guidance on warm-up piping design, but there is no calculation formula for the minimum backflow.

In absence of feedback from OEM (if the case), I'd go for what JJPellin recommended in the referenced thread - start with a 1/4" hole and see where does it get you. If insufficient, you can always increase the hole diameter.

Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

Put a check valve on each of the recirc lines if there is a risk of backflow from other streams running through this recirc line.