The pump will not draw oil, the flow rate will not come up, there is an abnormal noise in the pump chamber. On site these symptoms are usually put down to the pump. The pump may indeed be at fault, but ahead of the pump there is a section of pipework, and the way that section is installed is enough to stop a sound pump from working, while tracing it back often takes longer than pulling the pump apart.
Three things decide whether the pump can draw oil: submersion, slope and sealing
With positive displacement pumps such as screw oil pumps and gear pumps, the oil has to fill the pump chamber before pressure can be built up. The pump inlet has to sit below the oil level and the suction line has to stay full of oil over its whole length. That is the most basic condition, and both the mounting height of the pump and the routing of the pipework decide whether it keeps holding.
The second thing is the slope. The suction line should always fall towards the pump, with no high point in between and no section running against the fall. Air collects in a section that runs against the fall, the gas cannot be pushed out of the pipe, and on starting, what the pump draws in first is gas. The pump chamber then cannot build up pressure, and what shows on site is that the pump will not draw oil or that the flow rate is clearly short. After a machine has been transported and the pipework reconnected, a slight rise in the line is quite common.
The third thing is sealing. If any joint, flange or hose sealing face on the suction side leaks air, the gas is drawn into the oil by the pump, and the symptoms are almost the same as those of air trapped in the pipework. A machine such as the ZYD-100 has a total weight of approx. 720 kg and dimensions of 1200×1400×1800 mm. If the position of the machine is adjusted after the pipework has been connected, the routing of the suction line changes with it, so after reconnecting, these points have to be looked at again.
Working back from the symptom is faster than pulling the pump apart
The order of the investigation matters because pulling a pump apart is an action with a cost. The pump itself is simple in structure, and after it has been opened and reassembled the sealing and the alignment can both cause new problems. By comparison, working back from the pump inlet along a few metres of pipe and a handful of joints costs far less to check.
| Symptom | Check first | How to confirm |
|---|---|---|
| The pump turns but no oil comes out, and pressure builds up slowly | Air trapped in the suction section, or the suction side not flooded | Stop the pump and look for air at the highest point of the line, or raise the pump and try a test run |
| The flow rate falls after a period of running, and a gurgling sound moves along the pipework | Air collecting at a high point of the line | Disconnect the line section by section to locate where the air is trapped |
| The pressure gauge reading fluctuates, and the pump chamber screams | Air leakage on the suction side | Carry out a pressure hold check joint by joint on the suction side |
| The resistance across the pump clearly increases, and the pressure keeps rising | Rising resistance in the filtration stage | Decide between cleaning and element replacement under the differential pressure management procedure |
Working back from the pump saves time compared with checking behind it, for a simple reason: a problem behind the pump affects the outlet pressure and the flow rate reading, while a problem ahead of it stops the pump drawing anything in, and the two sets of symptoms are easy to tell apart on site. The section ahead of the pump is only a few metres of pipe and a few joints, yet it carries the whole of the information about installation quality.
The first check after the pipework is connected
- The pump inlet must be below the lowest oil level during operation, and no gas may be exposed inside the pipework once the pump is stopped.
- The whole suction line falls towards the pump, with no section running against the fall and no obvious rise in between.
- Where a high point of the line has a vent point, the air should be bled off before the machine is used.
- All joints are tightened before the line is filled with oil, and checked again joint by joint after filling.
- When the filtration stage is fitted on the suction side of the pump, its additional effect on the suction condition has to be taken into account as well.
A faulty pump can be recognised at a glance, while a fault in the pipework only shows up as a pump that does not perform, so working backwards along the direction of the oil is the most efficient way to look for it.
