03 Jun What are Swirl Breaks?
Swirl breaks are designed into high energy centrifugal pumps (especially modern Boiler Feed Pumps) to dampen vibrations especially at higher speeds caused by rotor dynamic forces. These forces occur at the impeller annuluses (between bushings) due mostly to non-concentric run-outs between stationery and rotary wear rings parts. Hence why we concentrate heavily on ensuring we have low run outs on our rotating elements prior to final assembly.
The general unspoken rule of thumb with OEM’s is that pumps operating up to 3000rpm 0.097mm run out is acceptable as the maximum for the rotary components, and up to 6000rpm the maximum is 0.05mm. Now most ‘pump people’ think these tolerances are purely to make assembly and setting of the radial running position easier and also for dynamic balancing purposes, but in fact these run outs have a larger affect than this – without uniform pressure distribution around the circumference of the sealing gaps (between bushings) we create asymmetric pressure distributions, coupled with pre-rotation of the fluid as it enters the seal area you create “cross coupled stiffness”. This has a number of effects but to keep it simple, swirl breaks can be used to reduce this pre-rotation and dampen these forces which will reduce stresses and vibrations on the rotating element.
This is typically why with any multi-stage Boiler feed pumps you can see increased vibration at low flows when for example testing at full speed through the low level leak off (LLLO) lines for commissioning, these vibrations through the shaft cause flexing and given the design of these pumps having a balance disc device there is a certain amount of force placed on the balance device faces as it tries to remain ‘running true’ to counteract the shaft run-outs since it is effectively also working as a thrust bearing; this is why axial vibrations at low flows may also be high and show a 3X running speed frequency typically attributed to angular misalignment. Swirl breaks at the balance device will dampen the affect of all of the above, but it will also allow a straighter more consistent flow with far less pre-rotation (With the use of Swirl Breaks, you will reduce the pre-rotation by around 60-70% at the entrance to the balance device) to enter the balance device which in-turn will help to maintain the gap at the balance device faces with less fluctuations in pressure.
When we redesigned the balance device on a clients pump it was noted during testing that more thrust load is applied to the balance device towards the outboard direction after introducing swirl breaks. The additional force from the straightened flow can only help, combined with the other modifications we have recommended we hope that we can see more reliable pumps, although we may will still have the operational influences that can cause these failures we may be able to at least make the pumps last longer. If we can gain more flow through the (LLLO) line from the changes that have been proposed then things should certainly be looking much better going forward.