The machine is running and the vacuum can be pulled to more than -0.08 MPa, yet only half as much oil comes out as before. This is not something that is solved by turning up the pressure, because the flow is the result of the whole oil path: how much is drawn in from the tank, how much resistance the filter sections add, and how much the pump itself can deliver. If any one of the three gets smaller, the output falls with it. The search should follow the oil path rather than start with the pump.
First find out whether the oil cannot get in or cannot get out
The first step is to find out where the flow is lost. Look at how the machine takes in oil: if the suction line shows bubbles or makes a broken sucking sound, the oil is not being drawn in continuously and the fault is on the suction side. If the suction is smooth and the pump is turning normally but the output is still down, the fault is in the filter sections or in the output of the pump. This step cuts the field of search in half without opening anything.
First step: look for air on the suction side
The most common cause of air on the suction side is a suction pipe with the wrong fall or a pipe that holds air. If a gas lock forms in the line, the pump loses the continuity of the liquid column for short periods and the flow drops in and out, which shows as an output that varies rather than one that is steadily lower. The suction end of the pump is normally installed below the liquid level of the oil tank, and the end of the pipe that runs into the pump has to be higher than the other end; if the difference in height is too small, or if the pipe has a bend that dips in the middle, the air collects at the top of the pipe and the pump draws that in first.
Checking it is not complicated: lift the section of suction pipe near the pump and look for air at the bottom of the pipe, or fit a short length of transparent pipe at the pump inlet and watch whether the oil stream is continuous. While you are there, look at the suction strainer and check whether it is blinded with debris; the two look alike, but the way of dealing with them is completely different.
Second step: check the differential pressure of the filter sections
The resistance of the filter sections rises with the amount of dirt they hold. The fine filter stage of the ZYD-100 is 5 elements of 150-500-5 in parallel, the filtration accuracy of the machine is ≤1 μm and the cleanliness is NAS 1638 class 5. The more dirt there is on the surface of the element, the higher the differential pressure and the smaller the flow that can be pushed through. The coarse elements 150-250-50 and 110-250-50 keep the large particles back and the second stage 110-400-20 does one more layer; their job is to stop large particles reaching the fine section first, and their condition decides the life of the fine section. Reading the differential pressure gauge is far quicker than opening the machine to look at the elements.
Third step: check the pump and the pipework
If the first two places are sound and the flow is still low, look at the pump. Wear in the screw oil pump ZH, a loose coupling and a valve that sticks in the inlet or the outlet all reduce the output. The way to tell is to short the outlet back to the inlet and run a short circulation: if the short circulation still cannot reach the expected flow, the fault is in the pump or the coupling, and if the flow is normal in the short circulation but drops as soon as the filter section is connected, the fault is in the filter section. One short circulation splits the field of search in half. The short connection has to be kept short in time, and it should be confirmed that the oil in the system is clean, because the machine should not be run for long in that state.
| What you see on site | The most likely cause | How to confirm it | Does it need a shutdown |
|---|---|---|---|
| The output varies and the pump makes a broken sucking sound | Air in the suction pipe or insufficient fall | Lift the pipe section and look at the bottom, or fit transparent pipe at the inlet | The pump has to be stopped |
| The output is steadily lower and the differential pressure reading has risen | The filter sections are close to their dirt limit | Read the differential pressure and compare with the record from before the element change | It can be watched under pressure |
| The differential pressure is normal and the flow is still low | The output of the pump has dropped or the coupling is loose | Run one short circulation with the outlet shorted back | The pump has to be stopped and shorted |
| The flow falls and at the same time the vacuum will not come up | Air on the suction side has already reached the vacuum side | Check the suction pipe and the seals first, then the vacuum line | The pump has to be stopped |
- Record a comparable output figure under the same conditions on every run; without a baseline there is no such thing as a drop in flow.
- Do not go straight back to full rate after changing a filter element; watch the differential pressure and the flow for a while first.
- Confirm the fall of the suction pipe every time it is connected again; the check takes less than two minutes.
- Keep the short circulation test within a set time and record the oil temperature and the vacuum level at the time.
When the flow drops, walk the oil path once: what goes in, what passes the filters, what the pump delivers. Check the three sections and the answer is there.
