Published on 2/13/2017
Categories: Insider tips

Roughing pump in high-vacuum furnaces for beginners [2/2]

Roughing pump in high-vacuum furnaces for beginners [2/2]

Welcome back! This is the second part of the article regarding the roughing pump. In the first part, we went through a summary overview of the vacuum pumps available today. Of course, this article is mainly intended for beginners and newcomers to the vacuum pumping system - by providing them with a satisfactory introduction.

Now, I would like to share best practices, practical tips and recommendations on how to cope with situations that might compromise the efficient operation of your furnace. I mean problems such as the presence of any water in the pump or a leak in the system, the formation of oil sludge, and the improper use of lubricated roughing pumps, arranged in parallel.

Furnace malfunction: have you checked the oil level?

Lubricated pumps generally have an indicator to show the oil level. In addition to checking that the level is correct, this indicator also makes it possible to detect the presence of any water in the pump or a leak in the system, in which case a type of foam forms on the surface.

Both a reduction in oil level and the presence of foam are signs that should not be ignored, since they are indicative of a malfunction. The operator should not be misled. The presence of water in the pump, remaining under the layer of oil, a fluid less dense than water, cannot be assessed simply by examining the oil level. If there is any doubt that the pump contains traces of water, the pump should be stopped and the tank opened for an internal inspection. The furnace cannot be used but at least more serious problems will be avoided.

A reduction in oil level occurs when there is a fault in one of the valves inside the pump. In this case the tank has to be opened, the faulty valve replaced and the oil level restored. The missing oil is found inside the “fumes” (vapors and air) exhaust pipe, when a suitable filter with a condensate trap is fitted. Obviously, the first check is conducted by examining the condensate trap. The same thing may happen when air infiltrates the system due to gasket failure. In this case the furnace must be shut down to avoid damage to the thermal chamber, and a search for the leak carried out.

On the other hand, a water leak in the furnace can cause the oil level to drop, but can also cause it to rise. It depends on the extent of the leak. In this extremely serious situation, a significant quantity of water, and hence damage, could be discovered by opening the furnace door. This event is generally caused by a leak in the exchanger, which is 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.

Deciding when to perform an oil change is not simple. A glance inside the tank is not a waste of time, since sludge formation can occur not long after the last oil change.

Pay particular attention when the pump is cold as the oil sludge becomes a solid crust. To be able to restart the pump, the cleaning required is relatively complex, since it implies dismantling the rotors and oscillating pins, as well as the valves and other parts.

One practice that the furnace operator must adopt regards checking the evacuation time needed to reach the operating vacuum level. Any delay compared to the reference time is a sign of malfunction due to a problem on the vacuum line or pumping system, or a leak in the furnace or even a load that is particularly contaminated, and with wash liquid residues. In any event the operator must consider this an alarm signal. If the cause is attributable to the pumping system, a leak rate test in the furnace is sufficient, excluding the pumping system by closing the high vacuum and pre-vacuum valves. It should be noted that this leak rate test is a check that may be carried out automatically, before each cycle, to make sure that the system has no leaks.

So far, however, we have looked into the issues arising from pump oil itself. But malfunction in high-vacuum furnaces may also arise from an improper use of lubricated roughing pumps. Let’s now see what I mean.

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Lubricated roughing pumps arranged in parallel: is it a risk?

Another precaution concerns the use of lubricated roughing pumps, arranged in parallel, so as to give the required flow. This solution, apparently advantageous, since it reduces the cost of the pumping unit, should be avoided. Indeed, there is still the phenomenon where the pump oil has a tendency to rise up, forming a film in the suction pipe. The majority of the oil from one pump can be transferred to the other pump, resulting in damage to pumps.

The pre-vacuum line has to be dimensioned to prevent any constrictions. For example, between a roots and a rotary pump, it is good idea for the pipe to be of limited length. Indeed, nowadays the roots pump is fitted directly onto the rotary pump and, in any event, it is good practice to connect the two pumps using a pipe with diameter at least equal to that of the roots pump delivery flange and not that of the rotary pump suction pipe.

In addition, 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.

The roots pump can be started together with the rotary pump, even in the initial phase, if the roots pump has a motor-pump hydrokinetic coupling (hyperbolic characteristic curve, adjusting the revs on the torque) or by using a software-controlled inverter designed to avoid excessive backpressure on the roots pump shaft. This reduces pumping times, with the roots contributing to pumping instead of causing a slowdown.

If these devices are not present one must wait for the pre-vacuum pump to evacuate the system to the point that actuating the roots pump produces a rise in pressure between input and output that is lower than the value indicated by the manufacturer. Otherwise the roots shaft would not withstand the strain.

The aspects of the roughing pump operation have been described in a very basic way so that you can take your first steps in the market of vacuum pumps - but I also gave you an idea of the most frequently occurring problems by providing you with a practical and pragmatic solution.

If you are not new to high-vacuum furnaces, have you ever tackled a question arising from pumping system? How did you resolve it?

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