Calculating API Plan 14 Flush Rates in Vertical In-Line Pumps 1

[HoustonMechEngr]

I'm looking for the correct procedure to size orifice plates on both lines of an API Plan 14. My plant has a significant amount of vertical in-line pumps that are currently fitted with single seals using plan 13 flushes. We've calculated seal chamber pressures to be barely above suction pressure and we have plenty of applications where the vapor pressure margin is borderline when using the plan 13, so we want to move forward with a plan 14.

Do you size your orifice plates to create a situation where the flow from the plan 11 equals flow through the plan 13? I would assume you want to first calculate seal chambre pressure and then size your orifice based on tubing size/throat bushing to get adeqquate flush through your plan 11 (pending requirement of seal type based on speed, temperature, pressure, etc.). At that point since the differential is lower on the plan 13 portion, do you simply adjust the prifice size on the plan 13 portion until you get equal flow?

 

[bk19702]

Short answer is yes - size for equal flows accounting for tubing, orifice, and throat bushing sizes. However, in my experience since Plan 13 is typically less efficient than a Plan 11, I usually fatten up the flow rate coming out of the seal chamber. Not knowing any of your conditions, pressures, etc, I've installed plenty of Plan 14's on inline verticals using a larger orifice on the 13 side for a higher flow rate just to be on the safe side. I wouldn't go any smaller than 0.125" in diameter for either side, but especially for the flow rate from the seal chamber back to suction. I would typically try to stay away from a Plan 13 on it's own when the differential between seal cavity and suction pressure is less than 25 PSI or so, so it's good that you're transitioning to a Plan 14 for the more effective cooling / self venting characteristics of it if your seal chamber pressure differential over suction is as marginal as you say.

 

[HoustonMechEngr]

I would think one scenario where you would want to have higher flow rate from the plan 11 portion is when dealing with a vertical multi-stage pump that has an upper-shaft line bushing that requires lubrication?

BTW, Thanks for the response.

 

[JJPellin]

Perhaps I am reading this a little bit different. Even if you don't want to force a flow across the throat bushing to lubricate it, you are going to get flow across that bushing if there is a differential pressure across it. That flow needs to be accounted for or you may not build the pressure margin that you want. These are my assumptions:

1. With no flow in or out, the seal chamber pressure is close to suction. This suggests a closed impeller with balance holes and back wear rings.
2. You want to build some differential pressure in the seal chamber above normal stuffing box pressure to suppress flashing between the seal faces.
3. There is a throat bushing in the bottom of the seal chamber to allow the increase in pressure.

I would work the problem this way: I would select a seal chamber pressure target to achieve the desired vapor pressure margin in the stuffing box. I would size the plan 11 orifice to deliver the target flow rate at the known discharge pressure and target seal chamber pressure. I would use the seal chamber pressure across the known throat bushing geometry to estimate the flow across that bushing. I would take the difference between the incoming flow from the Plan 11 and the bushing flow to use as the Plan 13 flow. I would size the orifice in the Plan 13 line to achieve that flow with the target seal chamber pressure and the known suction pressure.

A typical example in my plant might look like this for Naphtha service:

Suction Pressure – 5 psig
Vapor Pressure – 15 psia
Discharge Pressure – 100 psig
Seal Chamber Pressure w/no flow – 10 psig
Desired Vapor Pressure Margin – 30 psid
Desired Flush Flow Rate – 3 gpm

So, in order to get a 30 psid vapor pressure margin, I need to develop a seal chamber pressure of about 31 psig (15 psia – 14 psig (atmosphere) + 30 psid). The Plan 11 orifice would need to be sized to about 0.118” to provide 3 gpm with a 69 psi differential The leakage across the throat bushing would be about 1.8 gpm for a 2 inch bushing with 0.008” clearance and 21 psi differential. The Plan 13 orifice would need to be sized to about 0.093 to pass the remaining 1.2 gpm. If there is concern with plugging off the plan 13 orifice, then you could work the problem in reverse. With a 26 psi pressure drop across the plan 13 orifice, at a minimum 0.125” diameter, it should pass about 2.1 gpm. The throat bushing flow is still the same at 1.8 gpm. So, the Plan 11 orifice needs to pass 3.9 gpm. This requires increasing the Plan 11 orifice to about 0.136”.

Obviously, I didn’t give you enough details to reproduce my results. But, I hope I got the point across. It can become an iterative process until you get a flow rate, seal chamber pressure and orifice size that you are comfortable with.

If the pump was a vertical turbine with full discharge pressure on the seal chamber, then the flow across the throat bushing would add to the Plan 11 flow rather than subtracting from it. And the outgoing Plan 13 flow would need to be increased to pass this additional amount.

Please let me know if I messed up my assumptions or missed something.

Johnny Pellin

 

[HoustonMechEngr]

If the pump is a verticasl turbine style model, than a plan 13 is really all you need. A plan 11 in that situation would provide zero flow as the respective pressures (discharge and seal chamber) would be equal.