API VS6 Pump Mechanical Seal Flush Plan 32 - Flushing Fluid Pressure 1

[Haroon2020]

I am currently working on a project in which one additional pump shall be added to existing pumps in parallel. OEM for existing and new pump are not same. For new pump two seal flush plans are specified. Flush Plan 31 for startup and flush plan 32 for normal operation. For flush Plan 32, the flushing fluid pressure is 50 to 80 psig for existing pumps. For new pump, vendor is specifying the required flushing fluid required pressure as 198 to 200 psig. We don't have this much pressure flushing fluid available at site and vendor is claiming that required flushing fluid pressure cannot be decreased as it always close to discharge pressure for VS6 pumps. But existing pumps are operating on much lower pressure. My questions are:
1. Is the vendor right in claiming that required flushing fluid pressure cannot be decreased as it is always close to discharge pressure of the pumps.
2. How the existing pump are operating on much lower flushing fluid pressure?

Pump Data (all Pumps):
Psuc = 65 psia
Pdisch = 183 psia
Flow = 5500 GPM
Service = Wet Crude

 

[JJPellin]

The vendor is probably right based on the model of pump that was purchased. A typical vertical turbine pump has the mechanical seal at full discharge pressure and there is almost no way to change it by much. The fact that the existing pumps are a different manufacturer opens up the possibility that they include design elements that can drop the seal chamber pressure. We have VS6 pumps that have a built in feature with multiple bushings below the seal chamber and internal passages that can drop the seal chamber pressure close to suction pressure. One version of this pump is actually convertible by the end user. That is to say, we can remove an internal plug and run the seal at suction pressure or we can install this internal plug and run the seal at discharge pressure. Keep in mind, this is a very unusual design feature.

There is one other possibility. If the existing pumps do not simply have Plan 32, but actually have 32 and 13, this could explain it. The bushing below the seal could be a very close clearance, very long bushing designed to take a large pressure drop. The Plan 13 could be diverting the bushing leakage back to suction and dropping the seal chamber pressure close to suction pressure. But, this would be external piping and clearly evident.

So, you have a couple of options to deal with this. You can modify your Plan 32 system to provide the higher pressure needed. You can beef up the bushing below the seal and add a Plan 13 to drop the seal chamber pressure. Or, you can scrap the new pump you just bought and buy one that is designed to have the seal chamber at suction pressure.

What is the model of the existing pump? Can you provide a cross-section of the head and seal chamber? Are you sure there is no external Plan 13 piping on the existing pumps?


Johnny Pellin

 

[Haroon2020]

Dear Mr. Johnny Pellin,

Thanks for detailed response. I have some details of existing pumps which are attached. Unfortunately I don't have the crossectional drawings. The new pump is still in design process and not manufactured yet. So we still have chance to modify the seals as per low flushing fluid pressure.

 

[Haroon2020]

Snapshot from existing Pump GAD is attached.

 

[JJPellin]

According to the P&ID, you do have a plan 13 on the existing pumps. It is the line back to suction that includes the orifice numbered as RO-3012-A. The size of that orifice must be large enough to drop the seal chamber pressure close to suction pressure. I would probably not place an orifice in that line, at all. The bushing below the seal needs to be non-metallic and very tight clearance. The plan 13 line needs to be sized to pass all of the bushing leakage coming up and all of the Plan 32 (or 31) seal flush. Ideally, there would be a second bushing below the seal to avoid mixing the pumped product into the Plan 32 flush. Otherwise, you are really not getting the full benefit of the outside flush. I will attempt to find a drawing of one of our pumps that use this arrangement.

Johnny Pellin

 

[JJPellin]

I could not find any drawings of our relevant pumps. That return line with the orifice may not be properly called a Plan 13. But, it appears to be an orificed leak off to suction intended to drop the seal chamber pressure. There is probably a double bushing in the bottom of the seal chamber with that leak-off between the two bushings. Basically, it would function like a balance line on a multi-stage, horizontal split case pump. You need to attempt to duplicate that basic arrangement with your new pump.

Johnny Pellin

 

[Haroon2020]

Dear Mr. JJPellin,

Thanks for your interest and response. I am in discussion with vendor and will update this thread as soon as a solution is reached.

Regards,