Seal chamber pressure with closed impeller and no back wear ring

[tcoffin014]

I was hoping to get some help on calculating seal chamber pressure on a single stage overhung Union HHS8. It has a closed impeller with no balance holes and no back wear ring. I have found estimations for closed impellers, but I think they are assuming there is a back wear ring: stuffing box press = suction press + 10% of differential press

I know it will raise the seal chamber pressure, but by how much? Will it see full discharge pressure?

There is also a throat bushing on the seal drawing that specifies a clearance of 0.019".

My reason for asking is that the seal has a plan 11 discharge flush, and they are worried that since there is no back wear ring, there will be no flow to the seal. This must have been overlooked years ago when designing the flush system.

Thanks for any help.

 

[JJPellin]

All of our Union HHS pumps have back wear rings and balance holes. I assume that the impeller has pump-out vanes on the backside. If so, this will draw the seal chamber pressure down below discharge pressure by some amount. The amount will depend on the design of the pump-out vanes and the clearance between these vanes and the head.

The best option is to contact the manufacturer and get them to give you and estimated seal chamber pressure based on their design. If that is not possible or you don’t have time, you can measure the seal chamber pressure and change the seal flush orifice accordingly.

I would install the plan 11 flush with a block valve and pressure gauge. I would assume seal chamber pressure was 75% of the difference between suction and discharge and size the orifice accordingly. I would start up and get measurements of seal chamber pressure with the flush blocked in. I would use this to recalculate the correct orifice size and then change the orifice to that size. Maybe you will get lucky and the original orifice will work.

As an alternative, I would change to a Plan 13 flush and use the same exercise to measure the seal chamber pressure and resize the orifice in the flush line.


Johnny Pellin

 

[tcoffin014]

This pump was sent out for repair and from the pictures looks like it didn't have a back wear ring even back then in 2006. They just had the pump apart in the shop and there is definitely no back wear ring and no balancing holes, pump out vanes, or anything. It's a completely closed impeller with no back wear rings.

I will try to find out how the pump was ordered and see if they have the spec sheet or something. I will see if they can hook up a gauge so we can see the actual pressure, but it is going to be hard to get them to do the extra work.

I know they aren't going to go for redoing the piping to add a block valve and gauge for the plan 11. I know this would be the best way to do it though. That's why I was hoping someone could give me an estimate on how to calculate stuffing box pressure with a closed impeller and no back wear ring so I can resize the orifice accordingly.

Again, for the plan 13, it is hard to get them to agree to change anything. Especially since it seems like this pump fails for other reasons than seal failures.

In a situation like this, would the close clearance throat bushing be enough to keep chamber pressure low enough to still have flow from discharge?

 

[Artisi]

As not having any reliable data, I would be assuming and working on the fact that the stuffing box would see something like full discharge pressure.
Of course the leading question is - what actual problem do you have, if any?

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[tcoffin014]

We aren't really having a problem, but someone noticed there were no wear rings and started questioning if the plan 11 would work since he assumed the same thing of the stuffing box seeing full discharge pressure.

 

[JJPellin]

As you describe the pump, the seal chamber pressure would be equal to discharge pressure. The Plan 11 flush would produce no flow. A close clearance throat bushing would do nothing to reduce the seal chamber pressure. They need to convert to Plan 13 or accept poor seal reliability.

Johnny Pellin

 

[tcoffin014]

That's what I needed to know. Thanks for all the help