DISCHARGE RECIRCULATION D018
This term is used to describe two separate conditions:
- The first describes a bypass line from the discharge to the pump stuffing box to provide circulation through the stuffing box and prevent the build up of unwanted heat.
- The second describes the condition when the pump is operated 50% below its best efficiency point and a portion of the discharge flow is re-directed back to the impeller.
Let’s start with the first definition:
The high-pressure discharge fluid is circulated through the stuffing box to the back of the impeller and eventually to the pump discharge. This technique presents several problems for maintenance people:
- If the fluid contains solids (and most of them do) the centrifugal action of the impeller will concentrate the solids on the inside diameter of the pump volute and it is this dirty fluid that is being circulated to the stuffing box. Needless to say this will not be good for the mechanical seal because the solid particles will act as a “sand blaster” cutting into the lapped seal faces and clogging the sliding seal components.
- The pump wear rings, critical tolerances and close fitting bushings will experience rapid wear as the solids pass through the narrow clearances.
The only legitimate use of this technique is to pressurize the stuffing box to prevent a liquid from vaporizing.
Be careful if you use this method in hot water applications especially if a heat exchanger is installed in the recirculation line. A high temperature water or steam leak in any of the fittings could be dangerous for any personnel in the area, and the solids can clog up the heat exchanger.
When this line is used to pressurize the stuffing box you should keep several additional things in mind:
- Install a close fitting bushing in the bottom of the stuffing box. The clearance varies with the bushing material but it should be about 0.002 inches/ inch (0,002 mm/mm) of shaft diameter.
- Be sure to direct the discharge recirculation line away from the lapped seal faces and the thin metal plates if you use a metal bellows seal.
- If you are using properly installed, balanced O-ring seals (and you should be), The sealed product will not flash between the faces as long as the stuffing box pressure is a least one atmosphere higher than the liquid vapor pressure. The discharge recirculation line should guarantee you would have this pressure difference.
Now to the second definition:
- When the pump discharge is throttled, the flow has to go somewhere and the direct route is back to the impeller.
- The returning liquid will heat the pump cavity, possibly causing cavitation problems.
- Vortices will form on either side of the impeller causing vibration problems.
- The moving impeller will cause high thrust forces on the thrust bearing
- The radial bearing is often retained with a simple snap ring that can be damaged or broken by the axial thrusting.
- The impeller can contact the cutwater damaging both parts.
- An oscillating impeller can damage the impeller shrouds.
- Closed impeller pumps will have their wear rings damaged, and the clearances increased, lowering pump efficiency.
- This recirculating flow is unstable and causes erratic vortices, resulting in cavitation and consequent pitting of the pressure side of the impeller vane at the discharge end.
- The recirculating flow quickly alternates from one side of the impeller to the other, causing an oscillating axial pressure unbalance on the impeller that imposes oscillating axial movement of the impeller and high thrust forces on the thrust bearing.
- Cracking or failure of the impeller shrouds near the discharge may occur, as well as damage to the casing volute tongue.
What can you do to prevent these problems?
- One solution is to install a bypass line from the discharge back to the supply tank, or a low-pressure part of the system.
- If you tap into the pump suction you will heat the incoming liquid and that could cause cavitation problems.
- You might be able to get away with a different specific speed impeller, rated for lower flow
- Your best solution is probably to use a variable frequency drive. Use shaft speed control rather than valving to control flow.
- As the pump speed is reduced, the flow related to discharge recirculation is also reduced, resulting in a lower flow before the problem begins.
- The pump also operates at a lower energy level, which helps reduce damage.
- When dealing with open or semi-open impeller designs, be sure the recommended impeller to volute or back plate clearance is within manufacturer’s recommendations.
See; Suction recirculation S104