When using variable speed drives to control sewer pump stations, there are a few programming functions that may help in reducing the “ragging” of the pump impeller. The continuous pump ragging can cause high maintenance of the pump and any additional features in the VSD should be utilized to help reduce the maintenance frequency and cost.
The frequency converter must have a dual ramp/acceleration/deceleration function independently programmable for the start-up and shutdown speeds.
To reduce ragging, it is very important to accelerate the pump as quickly as possible to the point where the non-return valve is opened. Typically ragging occurs when a pump is slowly accelerating to the point where there is flow, during this time the flow is not moving through the pump, however during this time, the impeller is rotating and often winding up stringy type media in the waste around the impeller. Having a fast acceleration to the speed where flow begins significantly reduces the ragging problem. At the speed where flow begins, the acceleration rate is reduced to allow the rising main to control the pressure and flow in the main.
The same potential ragging situation exists when decelerating a pump to stop. During the deceleration to the no flow point in the stopping process, there is pressure on the non-return valve. This pressure can close the valve very quickly and damage the valve causing “valve slamming.” This reduces the valve life. Danfoss have solved this problem with a valve ramp time to slowly close the valve before the pump decelerates to stop.
In addition to the set up above, other measures can be taken to reduce ragging on sewer pumps that are designed to run in both directions. This function is simply called “deragging” and it allows the pump to rotate through a program of forward and reverse actions that often relieves the pump of debris. This function can be programmed to be activated on start up or after stop or when the load of the pump has changed, automatically detecting pump blockages.
This feature is standard in all Danfoss Aqua FC200 series VSD’s and does not require additional external equipment. The pump also does not require external programming which prolongs the life of the pump, reduces downtime and the amount of pump blockage call outs.