Thermal - Balance Leak Off Line

The name implies that it prevents excess water from reaching the boiler. You don't don't want more feedwater reaching the boiler than it is able to convert into steam, this would be very bad. That's my educated guess, you may want to double check.

Check this out. Popped up on google.

cr4.globalspec.com/thread/50461

Hi,

Ballance leak off line is fitted to a high capacity BFP to reuse the thrust balancing water from thrust balancing system of a BFP. Generally BFPs are fitted with balance valves or dummy piston arrangement or balancing disc arrangement. The high pr. water comes to one of these balancing system through an annular path in between valve or disc or piston sleeve to balance the thrust towards suction end and after that it has to be discharged through balance leak off line to Deareator for further use as feed water. Balance leak off pr. ranges from 5.5kg/sq.cm to 9.5kg/sq.cm generally. A relief valve is also fitted on balance leak off line so, if leak off isolating valve closed due to any breakdown when the pump is running it ensures flow of leak off water hence saves BFP from seizeing.Hope it will work.

Suranjan

I believe that a good part of its function is to prevent water at high pressure and near the atmospheric boiling point from leaking out at the seals. The water would immediately flash into steam and wire draw the seal faces and the shaft sleeve at the high pressure end. Also one does not want to re-introduce the flow near the suction end or first stage because any flashing at that point could cause cavitation. Since most BF Pumps operate with little excess NPSH it can create major problems so it is often bled off away from the pump itself.

I would agree that it is most often bled off a balancing drum area and I would not rely on the flow to be enough to protect the pump against a dead heading situation should the boiler feed valve close completely. A by-passing arrangement is needed.

Hi Spinco,

Thanks for your comment. In this regards I actually want to mean that the balance leak off water is sent to deareator reservoir for boiler feed pump suction through leak off line and not to put it directly at the pump suction which may cause cavitation as you have pointed out. In all power plant this system is followed. Anyway, I am waiting for other valuable inputs regarding this.

Suranjan