At sites in north-east China, north-west China or at high altitude in winter, the most common fault with an oil purifier is not that it cannot process the oil but that it will not start at all. Energising a cold machine straight into the oil feed, the load on the circulation pump at the moment it starts is far above its rating, and the result is either that the thermal protection operates or that the pump and the seals are damaged by the hard start. The instinct on site is that a low oil temperature means heating it for longer, but what really decides the outcome is the order of the heating, not the length of it.
What the rise in viscosity means
The viscosity of a liquid rises as the temperature falls; that is the most basic property of a material. At ten-odd degrees below zero the viscosity of hydraulic oil and gear oil can be several times that at ambient temperature, and the starting torque of the pump rises by the same proportion. A motor starting in a low-viscosity medium is lightly loaded; starting it in a high-viscosity medium is close to a stall, and a slightly longer duration will trigger the thermal protection. And the pump in that condition is carrying an impact load, while the contact face of the mechanical seal is also unstable at the moment of start-up, which is where the oil leaks seen on many low-temperature sites begin.
The vacuum pump is affected in the same way. The viscosity of the vacuum pump oil rises at low temperature, the pump takes longer to build up its speed, the pumping speed falls, and the equipment shows as a vacuum level that will not come up. If this is judged as a seal leak and the machine is repeatedly leak-tested, the direction is completely wrong. The way to tell them apart is to look at the pressure holding result after shutdown: a leak shows up in the rate at which the pressure recovers, a fault in the pump does not.
Why the start-up order has to be reversed
The intuitive order is: start up, heat, feed the oil, start processing. On a low-temperature site it has to be reversed: preheat first, then feed the oil, and establish the circulation last. This looks like nothing more than moving the steps about, but in reality it changes the mechanical state of the whole machine at low temperature, so that the pump and the seals are already in working condition before the low-viscosity oil enters. The steps cannot be compressed into one another, and in particular the preheating period before the oil is admitted.
| Step | What is done | How to tell whether it is there |
|---|---|---|
| One | Do not load the oil, energise the heating section first and let the tank and the pipework come back to temperature as a whole | The oil temperature display starts to rise steadily and the condensation in the cabinet stops getting worse |
| Two | Let the oil circulate slowly through the system and carry the heat to the section furthest from the pump | The end of the pipework no longer feels cold to the touch |
| Three | Bring the oil temperature up to near the lower limit of the working range | The temperature reading is steady and no longer falls back quickly |
| Four | Only then feed the oil and start the vacuum degassing and filtration | The vacuum level builds up steadily |
How far the preheating has to go
The answer is not a particular temperature figure but whether the pump can be brought under load without trouble. Taking the TYA series as an example, the temperature control range is set at 45–80 °C, with the upper limit brought down to 70 °C where the water content is high, so in the cold season the point is to hold the temperature steadily near the lower limit rather than running straight up to the upper one. Too high a temperature brings other problems: the pump oil volatilises faster, and once the vacuum pump oil is hot the pumping speed of the pump falls as well.
So in very cold conditions the more reasonable strategy is a long preheating at a low temperature and a short period of processing, rather than a short preheating at a high temperature. Building up the heat takes time, and that cannot be shortened by raising the set point; it can only be reduced by distributing the heat more evenly over the whole oil circuit. Schedule the preheating as an independent step that takes time in the process, rather than as something done on the side before processing.
The bill for forcing a cold machine to start
Set out the consequences of forcing the start and there are three items in it. The first is the motor and the thermal protection: repeated stalled starts keep the motor windings in an overcurrent state for long periods. The second is the pump and the shaft seal: the momentary wear on the seal face under an abnormal torque is often irreversible. The third is time: after the machine trips, it has to be preheated again, all the waiting so far is wasted, and the working window on site is shorter by that amount.
- Before start-up confirm that the temperature control set point is in the normal working range, and only then close the switch; do not close the switch and then adjust the temperature slowly.
- Allow the pump a separate period of no-load preheating so that the medium enters the pump chamber first.
- Where the machine is remote or unattended, write the preheating logic into the operating procedure rather than relying on the experience of the site staff.
- In the cold season schedule the start-up of the vacuum pump separately; do not start it together with the circulation pump.
In low temperatures the first thing to be assured is not the treatment result but that the pump and the seals come smoothly into their working condition.