2018-07-11
The ideal pumping system
DHV.600-540, Q=4500 m3/h, H=19m –Proper installation
The fundamental issue that determines the operation of the pump in a given system is how the suction and discharge pipelines are designed. It is very important that the liquid flows into the pump in a uniform manner without turbulence and without the possibility of air pockets. For this purpose, the pipeline connected to the suction port of the pump should be long enough to eliminate turbulence formed behind elbows, tees or fittings. It is assumed that the minimum length of the suction pipeline should be equal to five times the diameter of the pipeline, but in some systems, due to the high inflow velocity (>3m/s), a longer section may be required. Failure to equalize velocity at the pump inlet will result in erratic impeller operation and cause increased vibration and deterioration of the anti-cavitation properties of the impeller resulting in cavitation and faster wear of the impeller, bearings and seals. In principle, there is no retroactive effect of fittings built behind the pump, except for the transmission of vibrations from improperly laid discharge pipelines to the pump.
Apparent cost savings
DVV.200-520 Q=800 m3/h H=100m Evidently bad construction
The drive to reduce investment costs and fit as much as possible into pre-existing pipelines results in a very strong reduction in the life of the unit. Shown above is a drastic case where an elbow has been welded to a shut-off valve. There is an absence of any calming sections, pipeline supports and expansion joints. As a result, fluid flows into the pump very unevenly. In addition, the vibrations and stresses from the system transmitted to the pump are several times higher than permissible, which caused strong vibrations and premature damage to the pump bearings. In order to eliminate the cause, it was necessary to incur the cost of rebuilding the system and, in addition, the cost of replacing the bearings.
The puzzle layout
DHV.200-420, Q=700 m3/h, H=45m - Seemingly correct construction
There are also occurrences of systems seemingly made correctly, in which calming sections and stress compensations are provided, and yet the pump is malfunctioning. Cyclically, every ~1,000 h in the pump in the photo above there was a bearing failure. In addition, the pump was not reaching the set capacity. Measurement of vibrations showed their increase with a change in inflow pressure to the pump, which indicates that the pump is operating in cavitation.
This was particularly strange due to the fact that the anti-cavitation surplus reserve of the NPSHav system was 6 m larger than the required surplus for the NPSHr pump. The problem was solved only on the basis of a simulation of system operation using the CFD numerical fluid mechanics method. 
Based on CFD calculations, it was found that due to the high velocity of the flow through the elbow, the liquid breaks away from the walls and is unable to equalize in the made calming section. As a result, on the inflow to the pump, one half of the inlet flows in at a high velocity and the other half at a low velocity. In addition, the direction of inflow and velocity changes over time. In a two-stream pump, the liquid behind the inlet splits into two streams, which are fed into the mirror-positioned inlets of a two-stream impeller. This arrangement ensures the reduction of axial forces. In the case of strong variation and variation of the inflow field to the pump, different amounts of liquid flowed into each of the impeller inlets at the same time, which causes excessive and time-varying axial forces damaging the bearings. In addition, due to turbulence and high speed, deterioration of the pump's NPSHr and increased vibration were observed. The problem was solved by using a larger-diameter elbow, which made it possible to lower the velocity and not detach the liquid in the elbow.
Dr. Marcin Janczak
Head of Hydro-Vacuum S.A.'s Research and Development Department.
Traps set for pumps
The ideal pumping system
DHV.600-540, Q=4500 m3/h, H=19m –Proper installationThe fundamental issue that determines the operation of the pump in a given system is how the suction and discharge pipelines are designed. It is very important that the liquid flows into the pump in a uniform manner without turbulence and without the possibility of air pockets. For this purpose, the pipeline connected to the suction port of the pump should be long enough to eliminate turbulence formed behind elbows, tees or fittings. It is assumed that the minimum length of the suction pipeline should be equal to five times the diameter of the pipeline, but in some systems, due to the high inflow velocity (>3m/s), a longer section may be required. Failure to equalize velocity at the pump inlet will result in erratic impeller operation and cause increased vibration and deterioration of the anti-cavitation properties of the impeller resulting in cavitation and faster wear of the impeller, bearings and seals. In principle, there is no retroactive effect of fittings built behind the pump, except for the transmission of vibrations from improperly laid discharge pipelines to the pump.
Apparent cost savings
DVV.200-520 Q=800 m3/h H=100m Evidently bad construction
The drive to reduce investment costs and fit as much as possible into pre-existing pipelines results in a very strong reduction in the life of the unit. Shown above is a drastic case where an elbow has been welded to a shut-off valve. There is an absence of any calming sections, pipeline supports and expansion joints. As a result, fluid flows into the pump very unevenly. In addition, the vibrations and stresses from the system transmitted to the pump are several times higher than permissible, which caused strong vibrations and premature damage to the pump bearings. In order to eliminate the cause, it was necessary to incur the cost of rebuilding the system and, in addition, the cost of replacing the bearings.
The puzzle layout
DHV.200-420, Q=700 m3/h, H=45m - Seemingly correct constructionThere are also occurrences of systems seemingly made correctly, in which calming sections and stress compensations are provided, and yet the pump is malfunctioning. Cyclically, every ~1,000 h in the pump in the photo above there was a bearing failure. In addition, the pump was not reaching the set capacity. Measurement of vibrations showed their increase with a change in inflow pressure to the pump, which indicates that the pump is operating in cavitation.
This was particularly strange due to the fact that the anti-cavitation surplus reserve of the NPSHav system was 6 m larger than the required surplus for the NPSHr pump. The problem was solved only on the basis of a simulation of system operation using the CFD numerical fluid mechanics method. Based on CFD calculations, it was found that due to the high velocity of the flow through the elbow, the liquid breaks away from the walls and is unable to equalize in the made calming section. As a result, on the inflow to the pump, one half of the inlet flows in at a high velocity and the other half at a low velocity. In addition, the direction of inflow and velocity changes over time. In a two-stream pump, the liquid behind the inlet splits into two streams, which are fed into the mirror-positioned inlets of a two-stream impeller. This arrangement ensures the reduction of axial forces. In the case of strong variation and variation of the inflow field to the pump, different amounts of liquid flowed into each of the impeller inlets at the same time, which causes excessive and time-varying axial forces damaging the bearings. In addition, due to turbulence and high speed, deterioration of the pump's NPSHr and increased vibration were observed. The problem was solved by using a larger-diameter elbow, which made it possible to lower the velocity and not detach the liquid in the elbow.
Dr. Marcin Janczak
Head of Hydro-Vacuum S.A.'s Research and Development Department.
