The pointer on the pressure gauge climbs steadily and finally stops in the alarm range, or triggers the audible and visual over-pressure alarm and the machine shuts down. This is one of the most common faults on an oil purifier site. Most people reach for the filter element first, and that judgement is right in many cases, but not always. A rising pressure has at least two completely different origins, and the two call for opposite treatment.
Two routes to a rising pressure
One is increased resistance: the element is blocked by particles, debris has built up inside the housing, or a section of the oil circuit is obstructed by a foreign object. The same flow then needs a greater driving force, and the pressure rises with it.
The other is reduced driving force: the pump itself delivers less, the coupling slips, the motor load is abnormal, or a downstream valve has been closed by mistake and the bypass is not open. The oil is then held up in front, and that also shows as a rising pressure.
The two routes are handled differently. The first has to be opened up and cleaned, the second has to have the flow restored, and there is a third case in between: a falling oil level lets the pump lose suction, the pump draws no oil, the outlet flow falls and the pressure at the front climbs as well.
Three readings separate the two directions
There is no need to open the machine. Three quantities can be read at the same time: the pressure gauge reading, the running current of the motor, and the actual outlet flow. The way they combine separates the direction directly.
| Pressure | Current | Outlet flow | The more likely direction |
|---|---|---|---|
| High | High | Falling | Resistance in the filtration section has increased and the pump is working against it |
| High | Low | Falling | The pump delivers less, or a downstream valve has been closed by mistake |
| High | Low | Close to zero | The pump has lost suction because the level is low or air has entered the suction line |
| High | Normal | Normal | A local obstruction; check the pipework and the valves after the pressure tapping point |
| High, then falls back on its own | Fluctuating | Fluctuating | Pulsation or a gas and liquid two-phase condition; check the pump and the routing of the pipework |
The relationship between the three is what the judgement rests on. When the resistance rises, the pump draws more load to hold the flow and the current goes up with it; when the pump delivers less, the pressure rises while the current is actually low, because the pump itself is not doing more work.
Where to look when the filtration section is blocked
- Start with the coarse filtration section. The oil inlet and the coarse element are the first things the incoming oil meets, and if the large particles are stopped there the fine elements downstream are still clean.
- Then look at the differential pressure of the element itself. The alarm value of the 150-500-5 fine element is 0.35 MPa, and a differential pressure that climbs step by step points somewhere different from one that jumps.
- Confirm that the element is not fitted the wrong way or not seated. Fitted the wrong way or without seating, the flow path changes, and the differential pressure behaves differently from a blockage.
- Check the parallel branches. With five elements in parallel the total differential pressure is set by the tightest of them, and looking at the total alone tends to hide a problem on a single branch.
How to confirm a fault on the pump side
The pump side speaks up more: it is noisier and easier to hear, and it leaves a trace on the current as well. A motor overload trip, noise from the pump body and heat at the coupling all point to the driving side. A pump that draws no oil and air trapped in the line are the same class of problem: the oil is not coming in, or what is coming in is air rather than oil, and the pressure then rises together with a collapse in the flow and a change in the sound.
One downstream factor is easily overlooked: the valves. A valve that does not close properly or has been closed by mistake makes the pump push against a blocked path, and the pressure climbs to the top while the load on the pump may still look normal. Checking the valve positions needs no dismantling, only a comparison against the flow diagram.
Work through a fixed order
The mistake most easily made on site is to start dismantling several places at once and then not to know which of them mattered. A fixed order saves time: read the pressure, the current and the outlet flow and decide the direction; then look at the oil inlet and the coarse filtration section; then check the valve positions and the state of the element installation; and only once the filtration section is clear should the machine be stopped to examine the pump and the suction line.
The value of this order is that every step can be verified against a reading instead of a guess. If the direction is judged wrongly, most of the dismantling is wasted, and if it is judged rightly the search is much shorter.
A rising pressure is a result, not a cause. Sorting out increased resistance from reduced driving force with the readings first saves most of the downtime.
