Balancing holes resulting in reduced performance? 3

[alwayslearning]

We have a pump (high temp service 350 deg C)- sp gr 0.8; flow: 160m3/hr' diff head: 35 mts; etc.

This pump had a history of bearing failures. A decision was taken (after due consultations with OEM, i guess) to drill balancing holes in the impeller to reduce loads on the bearing.

After installation, the pump was reported to be not delivering the required head (ie "poor performance" according to operators).


Can anyone tell me what could be happening here.

We recently took this pump to workshop and inspected the internals: Wearing ring clearances are about 0.5 ~ 0.6 mm; impeller diameter appx 205 mm. Casing and impeller good condition (internally). Bearing condition was good.

Regards

 

[MikeHalloran]

Perhaps the OD was trimmed a bit to make balancing easier?



Mike Halloran
Pembroke Pines, FL, USA

 

[Artisi]

The balance holes should not make any difference to the discharge head.
If postioned in the eye of the impeller the function is to try and balance the "suction pressure" to be the same on each side of the impeller, this minimises the axial load on the bearings.

The impeller eye is at inlet pressure which could can be either positive or a negative pressure depending on the inlet conditions.

How was the "not delivering the required head" measured - by direct measurement or by "ëye".

 

[alwayslearning]

Very little instrumentation is available in terms of measurement. "Not delivering the required head" is reported by the operators, who switch to the (identical) standby pump.

Regards

 

[Artisi]

Maybe a stupid question but it has to be asked, is the pump running in the right direction?

 

[JJPellin]

We are in the process of converting a number of pumps by adding balance holes. In our case we are doing it to reduce seal chamber pressure because we are overtaxing our nitrogen system that supplies the gas to the dry gas seals. However, this conversion typically involves drilling balance holes and installing wear rings on the back-side of the impeller. If the pump was built with no balance holes, there is no reason to have wear rings on the back-side of the impeller, so they would typically be left off. If you drill balance holes, but do not have back-side wear rings, a massive amount of flow will be diverted from discharge, down the back of the impeller, back to suction and this will definitely rob you of discharge head and flow capability. Check for back side-wear rings.

 

[Artisi]

JJPellin
A very astute observation and important star -award comment re the casing/impeller wear rings. I overlooked the fact assuming that the pump had back-side wear rings or else an impeller with close impeller/back cover (casing) running clearance and pump-out vanes.

 

[JJPellin]

Excellent point. I tend to think only of API pumps. If the pump has pump-out vanes, the back clearance for the pump-out vanes is now very important. With no balance holes, this would affect seal chamber pressure, but that might not be critical in this service. If the impeller has been balanced by the end user, it is not uncommon for a mechanic to grind off parts of the pump-out vanes in order to make balance corrections.

 

[EdStainless]

The balance holes should reduce flow, but not head. Maybe the observation was just that the pump wouldn't reach the same head/flow conditions.
The balance holes can be small. As long as they don't clog they will work.

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[JJPellin]

There is an easy check to make that will help you understand what may be happening. Block in any seal flush flow and note the pressure in the seal chamber. If you see discharge pressure or close to it, then the balance holes are not working. This could mean excessive back clearance for a pump-out vane configuration or a lack of back wear rings for an API configuration. If the seal chamber is at suction pressure or close to it, your balance holes are working and you probably have no problem with pump-out vanes or back wear rings. If the seal chamber is somewhere between suction and discharge your back wear rings may have excessive clearance or your back pump-out vanes may not be working effectively. With the addition of balance holes, the seal chamber pressure should be much lower than it used to be. It is worth checking to make sure your seal flush plan is suitable for the new conditions. If you were using a plan 13, you may be getting too little flush flow. If you were using a plan 11, you may be getting too much flush flow. Excessive seal flush flow could also contribute to an apparent loss of pump flow and pressure. This especially true for high head, low flow pumps.

 

[JJPellin]

Have you found the problem with this pump? I am curious if the impeller had back wear-rings or pump-out vanes. Please update us if you have any more information.