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TPL (Mechanical) (OP)
20 Jun 11 0:40
We are commissioning some vertical sump (water) pumps, fitted with single inside cartridge seals configured for API Plan 13. The level of the sump can vary between 2 and 4 meters below the discharge/seal. Startups and runtimes are intermittent and on startup, since the flush is taken from the discharge, the seal runs dry and squeals for a few seconds. (the pump isn't run dry because the impellers are always underwater in the sump).

There is an NRV in the discharge line but this is to prevent back flow, doing nothing to maintain fluid in the seal, which just drain away after shutdown.

3-4 seconds after pressing the start button, the squealing stops once flow is made through the discharge – my concern is that this initial period of dry running following each start will shorten the seal life quite considerably – has anyone encountered this before? Is my concern justified? If so, any suggestions?

thanks in advance
 
bk19702 (Mechanical)
20 Jun 11 8:07
If the flush is taken from discharge, then this is not a Plan 13, it is a Plan 11. Further, if the seal chamber is located on the discharge side of the pump, then the differential pressure between the two areas will be marginal (depending on throat bushing length / clearance). You may want to consider actually hooking up a Plan 13 as intended - route the piping from the seal back to suction pressure.

 
TPL (Mechanical) (OP)
20 Jun 11 9:11
BK19702

Please re-read my post - this is NOT a horizontal pump and it is definitely Plan 13.

This is a vertical turbine type pump and the seal chamber is effectively in the discharge line. After passing the seal, water is discharged to atmosphere (i.e to sump/suction) via an orifice.  
JJPellin (Mechanical)
20 Jun 11 10:19
We have a number of large vertical pumps of the type that you are describing. Ours are in cooling tower service and tend to run continuously. But, when the pumps are shut down, they may drain down and end up with a dry seal chamber for start-up. I have never heard any comment by the operators about the dry-run squealing you describe.

A few options occur to me.  The first one would be to add a combination flush plan.  I think that this is referred to as Plan 14. It involves adding a plan 11 and plan 13 to the same seal.  In your installation, this is only practical if you have a group of pumps with at least one running at all times.  The other possibility would be if you discharge into a system that is always pressurized on the other side of the check valve.  But, in either case, it would involve a plan 11 from discharge originating on the down-stream side of the check valve. When the pump is running, the seal chamber would pressure up to discharge pressure and you would not get no flow through this line. But, when the pump is shut down, this would provide flow from the header or other running pumps to keep the seals flooded and pressurized.  In order to maintain flow during times when the pump is running, you would keep the existing plan 13.  You would have to size the orifices for both the plan 11 and 13 lines so that you get good lubrication through the upper bushing when running.  This can be an iterative calculation with an orifice and bushing in parallel with a second orifice in series with this pair.  

This is the basic arrangement that we use for some vertical turbine pumps in vacuum condenser service. When the pump is not running, the seal would be under hard vacuum and could allow air into the process.  We keep positive pressure on the seal when it is not running using this Plan 14 arrangement.  

The second option would just be a suitable Plan 32 flush with a check valve in the line. We would typically use our utility water or well water for this option. The outside flush would flow when the pump was down.  As soon as the pump came up to full pressure, the check valve would close, blocking off the flow of outside flush.  This would still require consideration of the lubrication flow through the upper bushing when running.  And, the orifice in the plan 13 line would need to restrict the flow enough for the plan 32 to be effective when the pump is down.  

Johnny Pellin

Pumpsonly (Mechanical)
21 Jun 11 2:59
In a VTP with just a Plan 13, the liquid has to "seep" through the upper shaft bushing before entering the seal chamber and may take some time to fill it up as encountered by you even though the water level is only 2-4 meters below the seal.
Adding a Plan 11 will shorten the dry running time faced by the seal if those arrangement suggested by Johnny is not feasible for you.
The connection from pump discharge should be up stream of CV.
If the pump discharge pressure is low compared to the seal pressure rating, a RO is not require so as to allow fast filling of the seal chamber.
Spud6414 (Mechanical)
21 Jun 11 17:23
Question, what is the material of the seal face?

Spud says; Meticulous effort and willingness to learn are honorable traits.

TPL (Mechanical) (OP)
22 Jun 11 1:41
Thanks for the replies.

The vendor has told us that the seal faces are carbon graphite on silicon carbide with a low coefficient of friction which can accommodate these 'dry' starts as long as the pump is allowed to run for a reasonable period of time in order to remove any heat from the seal.

I'm happy to go with this. The primary triggers for turning the pump on and off are the sump levels – I will arrange for the control system to limit the number of starts to a maximum of 3 per hour and, once started, to run for a minimum of 10 minutes. The installation can comfortably deal with these constraints.
 
Spud6414 (Mechanical)
22 Jun 11 22:47
First, I must say I agree with pumps only.  But your seal face material is suitible for a dry run, just as the manufacture states.  I think you will be fine.

Spud says; Meticulous effort and willingness to learn are honorable traits.

JJPellin (Mechanical)
23 Jun 11 7:47
I would be very careful about adding a Plan 11 with no orifice.  You still need to be certain that you will get adequate flow across the bushing while running.  If the seal chamber pressure is allowed to equalize to full discharge pressure while pumping, flow across the bushing will be lost.  And depending on the size of the shaft, the speed and the bushing material, this could lead to premature bushing failure.

Johnny Pellin

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