This article is mainly meant for newcomers and beginners to the vacuum pumping system and provides them with a satisfactory introduction on roughing pumps in vacuum furnaces. The first part of the article provides an overview of the vacuum pumps and the second part of the article is related to the roughing pump.
This article shares the best practices, recommendations and practical tips on how to deal with situations that may compromise the efficient operation of a customer’s furnace, i.e., problems such as the formation of oil sludge, the presence of a leak in the system or any water in the pump and the improper use of lubricated roughing pumps, arranged in parallel.
Furnace Malfunction: Have You Checked the Oil Level?
Generally, lubricated pumps have an indicator to show the level of oil. Besides checking that the oil level is correct, the indicator also helps detect the presence of a leak in the system or any water in the pump, which leads to the formation of foam on the surface.
Both the presence of foam and a reduction in oil level are signs of a malfunction that should not be ignored. The operator should not be misled. The presence of any water in the pump, remaining under the layer of oil, a fluid having less density than water, cannot be evaluated by merely examining the oil level. In case there is a doubt that the pump has traces of water, it would be better to stop the pump and open the tank for an internal inspection. While the furnace can no longer be used, at least more serious problems can definitely be prevented.
When there is a fault in a valve inside the pump, a reduction occurs in oil level. In such situations, it is important to open the tank, replace the faulty valve and restore the oil level. When an appropriate filter with a condensate trap is fitted, the missing oil can be found within the “fumes” (air and vapors) exhaust pipe. Apparently, the initial check is performed by analyzing the condensate trap. The same thing may occur when air infiltrates the system as a result of gasket failure. In such a case, the furnace should be shut down to prevent damage to the thermal chamber and a search for the leak should be performed.
However, a water leak in the furnace can reduce oil level, but it can also cause this oil level to rise. All this depends on the extent of the leak. In this serious situation, a large quantity of water and hence damage, could be detected by simply opening the furnace door. This event usually occurs when there is a leak in the exchanger — the most thermally stressed component in the entire system.
Troubles in Furnace Evacuation Time: Also a Matter of Sludge
Another problem, generally occurring in rotary piston pumps and not in large flow vane pumps, is when sludge forms inside, due to the pumping of vapors, which polymerize the oil itself and alter its chemical and lubrication characteristics.
It is not simple to decide when to carry out an oil change. Since sludge formation can happen soon after the last oil change, checking the inside of the tank is not a waste of time.
Particular attention should be paid when the pump gets cold as the oil sludge turns into a solid crust. The cleaning required to restart the pump is relatively complex, as it involves dismantling the oscillating pins, rotors, valves and other parts.
In addition, the furnace operator should adopt a practice to check the evacuation time required to reach the operating vacuum level. Any delay related to the reference time is a sign of malfunction — due to a fault on pumping system or the vacuum line, or a leak in the furnace or even a load that is specifically contaminated, with wash liquid residues. In any such event, the operator should consider this as an alarm signal. In case the reason is attributed to the pumping system, it is sufficient to carry out a leak rate test in the furnace, excluding the pumping system by closing the pre-vacuum and high-vacuum valves. It must be noted that the leak rate test is a check that might be performed automatically, before each cycle, to ensure that there are no leaks in the system.
Until now, problems arising from the pump oil itself have been examined but malfunction in high-vacuum furnaces may also arise from an inappropriate use of lubricated roughing pumps.
Lubricated Roughing Pumps Arranged in Parallel: Is it a Risk?
Another precaution concerns the use of lubricated roughing pumps, arranged in parallel, in order to provide the required flow. This solution seems to be advantageous as it reduces the cost of the pumping unit, it should however be avoided. There is still the phenomenon where the pump oil tends to rise up and forms a film in the suction pipe. Most of the oil can be transferred from one pump to the other, causing damage to the pumps.
The pre-vacuum line should be dimensioned in order to prevent any constrictions. For example, between a roots and a rotary pump, it is a good idea for the pipe to be of limited length. Currently, the roots pump is directly fixed onto the rotary pump and it is good practice to join the two pumps using a pipe whose diameter is at least equal to the roots pump delivery flange and not that of the rotary pump suction pipe.
Furthermore, the roots pump being drawn by the rotary pump during the preliminary pump start-up phase, when only the rotary pump is active, should be avoided, since this causes a constriction and evacuation takes longer.
Even in the initial phase, the roots pump can be started along with the rotary pump, provided the roots pump has a motor-pump hydrokinetic coupling (hyperbolic characteristic curve, adjusting the revs on the torque) or through a software-controlled inverter specifically designed to prevent excessive back pressure on the roots pump shaft. This brings down the pumping times, with the roots contributing to pumping rather than causing a slowdown.
If these tools are not available, the operator must wait for the pre-vacuum pump to empty the system to the point that actuating the roots pump increases the pressure between input and output that is lower compared to the value specified by the Manufacturer. If not, the roots shaft would not be able to tolerate the strain.
This article has explained the important aspects of the roughing pump operation in a very basic way so that operators can take their first steps in the vacuum pump market. The article also provides a pragmatic and practical solution, thus giving a fair idea of the most commonly occurring problems.
This information has been sourced, reviewed and adapted from materials provided by TAV Vacuum Furnaces.
For more information on this source, please visit TAV Vacuum Furnaces.