When a batch of oil has been processed and the machine is shut down, two actions have to be decided on site: whether the oil in the tank is kept or drained. The choice looks like closing work, but if it is made wrongly it directly affects both the time needed for the next start-up and the condition of the oil. There are only three criteria: how long the machine will stand, what the ambient temperature is during the standstill, and whether the machine still has to process this batch of oil during that time.
The workable conclusion comes first. If the machine will be down for two or three days and is about to be started again on the same batch of oil, keep the oil. If the machine will be down for more than a month, or the machine is to be opened up for maintenance after the overhaul, or the batch has been processed and is ready to go back into the tank, drain it. Where the case falls between the two, look at the ambient temperature and the planned start-up date.
A short standstill: the benefit of keeping the oil and the actions that are required
During operation the processing tank of a vacuum oil purifier is a sealed vessel under negative pressure. The first benefit of keeping the oil is the time saved: the tank already holds oil, the pipework and the filter elements are already wetted, so the machine can go straight back into circulation and heating, which saves the time needed to fill the tank again and to re-establish the negative pressure. The second benefit is less air drawn in. With the tank drained, the tank stands open when the machine is started, and the fresh air carrying water vapour and particles is drawn off in one go; that extra load is effectively created out of nothing. With the oil kept, the air volume inside the tank is much smaller in the first place.
Keeping the oil does not mean walking away as soon as the machine stops; several actions have to be completed:
- Shut down in the correct order: stop the oil feed, switch off the heating, wait for the oil temperature to come down before stopping the pump, and disconnect the power last. The heating circuit has to be switched off first, otherwise the elements keep heating once the oil temperature falls below them.
- The oil level in the tank must not fall below the immersion depth of the heating elements. An element standing above the oil surface runs dry, and that is one of the hardest equipment failures to trace.
- Do not open the sight glasses, the flanges or the vacuum pipework during the standstill; keep the system sealed.
- Do not drain the pump oil of the vacuum pump. The pump oil both lubricates and seals, and on a pump running dry for a long period the bearings are the first thing to burn out.
- Before stopping, read the infrared level and foam detection once to confirm that no alarm is left standing in the system.
A long standstill: draining the tank and what else has to be done
During a long standstill the cost of keeping the oil becomes apparent. The oil stands in the tank, and in oil that contains water the dissolved oxygen and the moisture continue to oxidise slowly; the products settle on the tank bottom and on the heating surfaces, and at the next start-up the heating can stir them up again and carry them with the oil into the whole machine. A second change takes place in the seals: rubber parts that are soaked in oil for a long time swell and harden, and their compression set becomes permanent, so the loss of elasticity is only discovered at the first start-up, by which time the leak points are usually not the only one.
Draining is not a single action but a set of actions, and the order cannot be rearranged:
- First stop the machine, release the pressure, disconnect the power and put up a lock-out tag, then confirm that the pressure in the tank is back to atmospheric before opening the oil drain valve, otherwise the residual pressure will spray the oil out.
- Open the drain slowly and control the rate of discharge, so that a fast-moving volume of oil does not carry the deposits out of the filter elements and the pipework and block the downstream side.
- Have the drip tray and the dedicated container ready first. The treated oil is a product of value; it must not be mixed with waste oil, and it must certainly not be discharged into the sewer.
- Seal the container and label it with the type of oil, the date of treatment and the current condition, so that it can be put back into service or disposed of in accordance with the environmental requirements.
- Oil with a high water content, oil that has already emulsified and oil carrying suspended solids are in a condition that cannot be put back into service directly; hand them to a qualified organisation in accordance with the local environmental regulations, and do not dilute them with water and discharge them.
- After draining, clean the tank walls, the strainer, the drain outlet and the pipework and leave no residual oil; remove the filter elements and store them separately or replace them outright.
The pitfalls of each of the two approaches
| Symptom | Possible cause | How to confirm | Action |
|---|---|---|---|
| The vacuum takes noticeably longer to draw after a restart | The tank is filled with fresh air after draining | Compare with the pump-down time under the same conditions on the previous run | For a short standstill, keep the oil and reduce the exposure to an empty tank |
| After a few weeks of standstill the oil is darker and has an off smell on start-up | The oil has oxidised slowly during a long standstill in the tank | Take a sample and compare the colour and the smell with the sample kept before the standstill | Drain and clean the tank, replace the oil and re-check |
| The pressure gauge pointer flickers and the oil pump makes an abnormal noise after start-up | Settled deposits have been stirred up again | Drain from the oil outlet and look for settled solids | Stop the machine, release the pressure, disconnect the power, then clean the tank bottom and the pipework |
| After start-up the vacuum level does not reach -0.08 MPa | A seal was carried out during draining, or a valve has not been returned to position | Apply soapy liquid point by point to check the tank cover flange and the valve seats | Refit the seals, retighten and carry out a no-load pump-down |
| After a long period out of use the level display does not agree with the actual level | The infrared level probe is affected by an oil film or residual oil | Compare the sight glass and the mechanical level gauge with the displayed value | With the machine stopped and the power disconnected, clean the probe and recalibrate it |
Safety preconditions before starting work
Every action involving draining oil, opening a cover or handling a seal must be preceded by stopping the machine, releasing the pressure and disconnecting the power, and the work is then done in the correct order. Dismantling under pressure sprays oil, and an incomplete release of the pressure brings the residual pressure out the moment the cover is opened; the power is disconnected to prevent the heating circuit and the pump from operating by accident during the maintenance work. The draining work also has to be done with fire precautions in mind: hot oil mixed with air carries a combustion risk, so the area should be ventilated, kept away from naked flames, and the oil cooled before it is discharged.
Whether to keep the oil or drain it depends first on the length of the standstill and the timing of the next start-up. Keeping the oil for a short standstill saves time; draining for a long standstill saves the condition of the oil and the life of the seals.