Balance line typically goes from a high pressure tapping to the low pressure end of the compressor to help "balance" out the thrust forces.
As it involves moving parts and seals there is some leakage. This is in essence simply a flow from high to low pressure around the compressor and hence if it becomes significant then the compressor is doing the same work, but the overall flow is reduced as some is going back to the low pressure end instead of flowing out of the compressor.
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I would add to the excellent reply by LittleInch. Excessive flow through the balance line is a concern because it represents unintended recirculation which is a waste of energy and a reduction in available capcity. But, it is also an indication that the clearance in the balance device (disk, drum, piston) is increased. The balance device is necessary to balance out the thrust forces. If this device fails, excessive thrust forces could result in a thrust bearing failure. A thrust bearing failure in a centrifugal compressor can be an extremely destructive and dangerous failure. If the machine is not instrumented with thrust position monitoring, armed to automatically trip the machine, I would be very concerned continuing to run with excessive balance line flow.
to add to LittleInch explanation, balance line leakages can also be internal AND external on back to back configurations.
Physically the internal leakage is through the inter/stage balance piston for back to back. The external leakage for inline and back to back types, is flowing via a pipe physically coming out of the machine from discharge side and going in again on suction side (that is to say it is `end to end`).
The leakage rate depends on several construction and performance factors, some that can be mentioned are
- Operation on the performance map, which typically relates to speed, flow and compression ratio
- Type of seals being used on the balance drum or piston (teeth, stepped, honeycomb, etc.) as well as the geometry (number of teeth, geometry etc.). Increasing the number of teeth will lead to a longer span, well very slightly but noticeable in some cases.
As already pointed out, the `price to pay` for balancing the thrust forces across a machine is performance-efficiency penalty (like LittleInch indicated, leakage accounts as internal recirculation and thus impacts the overall compression work). It also induces increased destabilizing effects in terms of lateral stability (so called cross coupling stiffness) but that is beyond the original question scope.
Not to mention that recirculation on high pressure machine is often causing a non negligible increase of temperature at the suction considering hot gas from discharge side is being mixed with the incoming main suction stream. For instance on some production - upstream applications, the leakages can amount for up to 10 percent of the nominal flow of the machine, and that is by design, one has to be careful.
