Vacuum pump problems?

Are your vacuum pumps mysteriously breaking impellers, breaking or flogging keys, twisting shafts or prematurely trashing bearings? If so, the culprits could well be 'invisible' slugs of liquid and sludge.

Over the years, service engineers have encountered many badly damaged vacuum pumps. Internally, the pumps appear to have suffered major 'foreign body' impact damage, but usually there's no sign of the 'body'!

Almost always the pumps damaged in this distinctive way are used for evacuating vessels containing liquids or sludges, such as evaporators, condensers, cheese towers and cooking vessels. Almost all of these problems may have arisen from the carryover of fluid from the process, with the most serious form of carryover being a 'slug' of liquid - sometimes only a few litres in volume.

These slugs of liquid can arise from various causes. Sometimes the cause is incorrect valve sequencing in a complex process; other times it's a lack of process control, or someone just 'forgets' to watch fluid levels.

On most occasions, the cause is probably an equipment fault, such as faulty level control, a sticking valve or the failure of a liquid draining pump.

Few plants are adequately protected against such faults. The expectation seems to be that "the vacuum pump will handle it". Sometimes it does - for a while - but repetition of the fault inevitably damages the pump in the end.

Service engineers find that typical signs of vacuum pump damage caused by fluid slugs are:

• Broken impeller vanes;
• Broken or flogged drive key;
• Twisted or broken shaft;
• Leaking pump joints;
• Bearing failure;
• Overloaded motor.

Liquid slugs easily damage vacuum pumps (including liquid ring pumps) because vacuum pumps are designed to pump compressible gases. To provide the required compression ratio, the inlet port is typically much larger than the exhaust port, as shown in Figure 1.

If the inlet port is completely filled with a slug of liquid, the pump is inevitably subjected to very high instantaneous loads because the incompressible liquid cannot exit from the pump as fast as it comes in. Internal pressures rise excessively and the impeller decelerates very rapidly. The shaft, the motor and the bearings are then thrown into overload. Judging by the shearing of shaft keys that have been seen, the instantaneous shaft torques can rise to well over 10 times the design torque.

Protect the pump with an inlet trap

If your process involves a vacuum pump evacuating any form of vessel containing a liquid, or sludge, some form of trap should be provided to protect the pump. We believe the trap should be independent of the normal process control system as there is always a risk that some element of the process system will fail and lead to an overflow. With clean liquids a simple floating ball valve at the exit of the process vessel may suffice.

However, if there is any chance that the ball will stick, a more sophisticated trap is required and it must be easily cleanable. Unless the trap is in an easily visible position, it is recommended that trap cleaning should be automated (but still subject to regular inspection).

An example is the Pumpsaver trap, a mechanically simple and robust trap designed to slam closed almost instantaneously on a slug of liquid or sludge entering the trap.

The partial flow Pumpsaver allows passage of some fluid under high airflow conditions. It has two stainless float balls in the upper chamber of the trap. This version will allow some liquid to pass through the trap as long as it is associated with a high airflow. This may be appropriate for systems generating wet gas streams that are pumped by a liquid ring pump. As long as the liquid amount is small relative to the airflow, the liquid will pass through the trap for disposal in the liquid ring pump's water circuit. However, if a slug of liquid or sludge arrives at the trap, the trap will fill with liquid. The exhaust side float will then rise and block the discharge port.

The high sensitivity Pumpsaver prevents liquid flow under all airflow conditions. The stainless floats are situated in the lower chamber and activate the shut-off valves via pushrods. In this configuration, the floats react more quickly to the accumulation of liquid in the lower chamber. Liquid flow is prevented under all airflow conditions.

Installing the inlet trap

If required purely as a pump protection device, the inlet trap can be installed anywhere in the vacuum line from the process to the pump, as long as there are no other possible liquid sources between it and the pump. Note that possible liquid sources include other liquid ring vacuum pumps feeding the same vacuum line. If the trap is also to serve as a backflow preventer for a liquid ring vacuum pump, the trap should be installed close to the pump.

Installation and maintenance of an inlet trap may provide a simple and economical solution to your vacuum pump problems.