The words continuous operation usually appear in a specification for a simple reason: the oil volume is large, the processing cycle is long and the working window is narrow, so the site would like the machine to keep running through the night with nobody there. Writing the words is easy, keeping them is harder. Below the requirement is broken down into four groups, what each one corresponds to on the machine, and how the clause should be worded so that it does not produce a disagreement at acceptance.
The first group: level and foam monitoring on the feed side
Unattended operation requires that the machine does not empty itself and does not overflow. Level monitoring on the oil inlet side deals with the first, foam detection deals with the second. The consequences of the two failures are quite different, but both happen during a period when nobody is there, so they have to be handled automatically rather than reported afterwards for a person to look at.
Infrared level detection combined with foam detection and automatic defoaming completes this chain. The ZYD-100 is fitted with infrared level and foam detection together with automatic defoaming. If a specification calls for continuous operation, this item belongs in it, and it should state what happens after an abnormal condition is detected: the feed stops, the vacuum keeps running, or the machine stops and raises an alarm.
The second group: vacuum and pressure protection on the running side
During continuous operation the load on the pipework and on the filtration section changes as the run goes on, and the sealing state, the condition of the elements and the output of the pumps all move away from their initial values. Without any protection on the running side, the abnormality builds up until the machine is damaged or the oil is spoiled.
The overpressure alarm with sound and light warning and automatic shutdown is the most basic of these protections. Phase failure and phase sequence protection cover the supply side. Both keep the machine in a state it can be recovered from, rather than letting it keep running in an abnormal condition. The working vacuum of the ZYD-100 is -0.08 ~ -0.0999 MPa and the limit vacuum is ≤5 Pa, and whether the vacuum level holds steady is the most direct observation point for whether continuous operation is actually taking place.
The third group: automatic handling during unattended periods
When a person is on site, many abnormalities can be dealt with by eye and by hand: foam overflow can be stopped by switching off a pump, a high level can be answered by changing a valve, a level alarm can be confirmed by a person. With nobody there these actions have to be carried out by the machine itself, otherwise the alarm is only a message waiting to be handled and not a problem that has been handled.
Automatic defoaming is a typical action of this kind. It does not send an alarm for someone to deal with, it removes the foam and lets the run continue. Whether an abnormality should be allowed to continue running or should stop the machine depends on what the abnormality is, and that rule has to be written into the specification rather than left to a decision taken on the spot.
The fourth group: what happens after an abnormality
Continuous operation also means the machine will stop by itself. What is done then is the part of the specification that is most often left out: does it restart automatically or does a person confirm and reset it manually, where does the alarm go, who decides whether the run can continue, and what are the criteria for the working vacuum and the limit vacuum. When this part is written down, both sides have a common basis for acceptance.
| Group | Function on the machine | What the clause should state |
|---|---|---|
| Feed side monitoring | Infrared level detection, foam detection | Position of the sensors, the action on an abnormality and where the alarm goes |
| Running side protection | Overpressure alarm with shutdown, phase failure and phase sequence protection | The trigger values, what the shutdown stops and how the system is reset |
| Automatic handling | Automatic defoaming | The condition for the automatic action and whether the run may continue |
| Handling after an abnormality | The shutdown and reset procedure | Who decides, on what criteria and how it is recorded |
How to word the clause so that it can be accepted
Writing continuous operation as a condition that can be accepted usually takes four elements: how the duration is counted, the condition of the oil, the monitored quantities and the alarm actions, and the responsibility for handling an abnormality. The duration has to say whether it means calendar hours or effective running time. The oil condition has to name the water content and the particle level, because the stability of continuous operation differs a lot between oil conditions. The monitored quantities and alarm actions have to say which values are watched and what the machine does when a limit is crossed. The handling responsibility has to say who judges the situation and who resets the machine.
Continuous operation in a specification is not a promise about one machine, it is an arrangement of monitoring, protection and handling.
