API Plan 13 2

[RS82]

Hi All,

Quick question..vertical pump does not have back wear rings and balance holes due to high suction pressure..we need full discharge pressure in the seal chamber to maintain the API (30%) vapour pressure margin..I am suggesting API Plan 13 to right choice, however, would you open up the throat bush from 0.010" to 0.025", as we are trying to reduce the pressure drop as much as possible?

One seal vendor stated that the plan 13 flow is not as efficient as the plan 11 as it is directed towards the face etc. He said use a close clearance throat bush and have a plan 14..so we would have a pressure drop across the throat bush going into the seal chamber..there would now be a flow through the plan 11 piping and then the total flow, Plan 11 + Throat bushing flow, would be going back to the suction in the plan 13..does this make any sense? Is this preferable?

 

[1gibson]

Yes.

Use a plan 14, with the 11 connection in the seal chamber. Locate the connection so that it flushes directly across the seal faces.

Small plan 13 orifice, larger plan 11 orifice. Do not change throat bushing clearance, if it is a carbon bearing it can "wash out" or have accelerated wear.

 

[JJPellin]

API Plan 14 is one good option as 1gibson points out. We have also used an alternative. We have cut axial grooves through the bushing to open up area and reduce the pressure drop while still maintaining 90%+ of the area at original clearance for shaft support. With an API Plan 13 and a grooved bushing, you can achieve the same result for flush flow and seal chamber pressure. Of course, the bushing needs to be thick enough to allow for the grooves without compromising the integrity of the bushing.

This has been discussed in other threads in the mechanical seal forum including the one I have attached as a link.


Johnny Pellin

 

[RS82]

JJPellin,

The last comment in the thread you referencing is the following:
"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."

That is my question..how are we going with a 14 when the pressure between the throat bush and the impeller, no back wears or balance holes, is full discharge..isnt 13 the only option?

Sorry..I am novice when it comes to this..please dont get frustrated

 

[RS82]

To add to that last question JJPellin, the example you had worked out was determining the target seal chamber first..30psid between the VP and Seal Chamber..but what if we need to have full discharge in the seal chamber to maximize the VP margin..can we still go with a 14? If so, how would this work as the pressure in the plan 11 and the pressure in between the impeller and throat bush is also discharge pressure..we should essentially have no flow..only flow would be through the 13..right?

 

[JJPellin]

You are correct. If the seal chamber must be at full discharge pressure, then you will have no flow through the Plan 11 line. I am assuming that you would be close to discharge, but could still have some differential, even if it was only a pound or two.

If you must have full discharge pressure, then a Plan 13 is all you need. If the pressure drop across the bushing would be a problem, then groove it or drill in bypass holes to allow flow to bypass the bushing


Johnny Pellin

 

[RS82]

Thanks very much for the info! Appreciate all the help!

 

[RS82]

One more question on this topic..say we have bubbles inside the seal chamber prior to starting up the pump but it is flooded..what issues will arise in the pump if the bubbles are recirculated back to suction? Do those implode after passing back through the impeller eye and damage impeller, casing etc? pump capacity will be affected i assume, but is there any other issues that may arise with having bubbles in the stream prior to the impeller eye?

 

[JJPellin]

It would be best if the high point vent used to vent the pump is on the high point of the Plan 13 flush. This will vent the flush piping and the seal, as well as the pump. If this is not practical, a small amount of trapped air in the seal chamber would probably cause no harm. Air bubbles are generally considered to be less destructive than bubbles of vaporized product. The bubbles would most likely pass through the pump with no noticeable effect and be gone in a few seconds.

I would be much more concerned about how long it takes the vapor pocket at the top of the seal chamber to dissipate. Since the seal faces are probably very close to the high point, the seal faces will be running dry until this area is flooded. Once again, no harm may occur if the bubble is small and is forced out quickly.

It is better to be safe than sorry. Place the vent at the high point and vent the seal liquid full before starting.


Johnny Pellin