Internal pump minimum flow bypass on Molten Sulfur pump 1

[Raj_2958]

My query is related to flow bypass on Molten Sulfur Pump. I am to review the vendor documents and not aware of the developments in inquiry stage.

The inquiry requirement was as under:

Pump type - VS4 steam jacketed.
Total Rated Flow = 20 + 9 = 29 gpm (where 9 gpm is the assumed minimum continuous stable flow rate)
Net Rated Flow = 20 gpm.


Per the note on the pump data sheet : Rated flow is specified as total flow, including the internal pump minimum flow bypass which stays within the sulfur pit.

The vendor has offered following:
Total Rated Flow = 35 gpm @ 1750 rpm
MCSF = 4.37 gpm (on vendor submitted data sheet)

Questions:
1. Why is the MCSF bypassed in to the pit? Note: I find that bypass flow is not used to flush the line bushing.
2. Due to abnormal viscosity change with temperature (sulfur can only be pumped satisfactorily within the 275-311 °F range) is the pump deliberately selected at higher flow?

 

[JJPellin]

We have three sulfur plants with a total of six of these pumps in service. All of our pumps have the drilled, continuous spillback of the type you describe. All of them are sized for the MCSF of that pump. This works very well and I recommend this arrangement. I do not understand your second question. The offered pump is slightly oversized. This is normal. I see no problem. Two items are more important. A pump at 1750 rpm will be much more reliable than a pump at higher speed. And, you must have a very good supply of low pressure steam for the steam jacket.

The minimum flow is passing through a drilled hole in the discharge elbow. This is neccesary because of the difficult properties of molten sulfur. Piping this flow any distance would create potential problems with plugging or inadequate flow.

Johnny Pellin

 

[Raj_2958]

Thanks Johnny!

I have seen continuous re-circulation on Low Flow OH2 pumps, through an orifice plate where the pump MCF is greater than the process minimum flow mentioned in the data sheet. In such case

Re-circulated flow = Pump MCF - Process Minimum Flow.

This application is unique where MCF on a VS4 pump is being spilled back in to the pit.

 

[JJPellin]

These pumps can tend to have a couple of other challenges. The line shaft bushings are lubricated by a flow of sulfur coming up the column. Above the top bushing, this flow of sulfur is also allowed to spill back into the pit. If this flow of sulfur is allowed to free fall through the vapor space in the pit, it can build up a static charge. When this charge finds a ground, the resulting spark can ignite a sulfur pit fire. We added baffles to the outside of our pumps for this sulfur to flow down rather than free falling. We have fewer pit fires. The second issue is the inherent instability of the line shaft bushings in this style of pump. This is a much more severe problem in 3600 rpm designs. But, even in 1800 rpm designs, if the clearance of the bushings increases beyond a point, the shaft will go into a whirl mode and the vibration will go up dramatically. We have converted to a tri-lobe bearing design to break up this whirl. That probably won't be needed because of your lower speed. As you are aware, if molten sulfur gets above 315 F or so, the viscosity goes up drastically. When this happens in the bushings, the motor may trip off on overload. At these higher temperatures, this sulfur can create a hardened deposit that our process engineers call Suflur-crete. It is hard as a rock and will not re-liquefy at any temperature. Once it is laid down in the bushings, it can only be removed by chipping it off.

Johnny Pellin

 

[Raj_2958]

Appreciate your quick response. I now understand that Client has gone ahead with 3600 rpm pump model. Since, these pumps are replacing an existing smaller pump, going with an 1800 rpm pump leads to bigger foot print. An 1800 rpm pump calls for lot of modification to the existing pit, which is not possible. However, I will keep in mind problems with an 3600 rpm pump during reviews.

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RC