Standard Tests, Common Problems, and Troubleshooting

Bearings are key components of any rotating equipment. Any forces generated on rotating parts are a source of vibrations. Such forces could be created by misalignment, friction, missing stiffness, electric problems or imbalance coming from parts or growing layers on the pump rotors or particle intake.

Vibrations are measured in three perpendicular axis:

• X-Horizontal direction
• Y-Vertical direction
• Z-Axial direction

Vibrations are measured with a Vibrometer. Vibrometer consists of a piezoelectric sensor and magnetic base to give velocity of vibrations in the root mean square (RMS in m/sec). Sensors should be placed close to the bearing zone. Avoid painted surfaces, unloaded bearing zones, split housing and structural gaps. The maximum permissible vibration level for Everest Dry Screw Vacuum pump is 2.8 m/sec. Vibration test is also important in order to prevent the following scenarios [1]

• Sporadic increases in vibration and noise level can indicate that the vacuum pump system needs to be serviced before a more catastrophic failure occurs.
• Intermittent increase in vibration results from low pockets in inlet piping, causing the flow to be irregular through the pump.
• Loose piping separators are also commonly a cause for pump vibrations.
• Finally, if the vacuum pump is only vibrating sometimes, it may indicate that parts are beginning to fail within the motor itself and it’s time to service the system.

In a dry screw vacuum pump, optimization of temperature plays a vital role. Sudden and huge variations will result in the stalling of the vacuum pump. Temperature is measured at the following points in a dry screw vacuum pump

• Vacuum Dome Temperature
• Body Temperature
• Gear Oil Temperature
• Suction Oil Temperature
• Water Jacket temperature
• Discharge gas temperature

A large percentage of applications require a T3 rating.

Another important factor for the smooth operation of a dry screw vacuum pump is the amount of current drawn by the motor. Testing is done in order to ensure the motor rating specified and the current drawn are in agreement with each other. If the pump is producing too much horsepower, it can cause additional problems with the entire system if it’s not built to withstand the excess power or if the material being pumped needs to be transported at a lower speed.

The intensity of sound is measured in sound pressure levels (SPL) and the common unit of measurement is decibel, dB (A).This scale resembles the audible response of the human ear. Sound test provides an idea about the condition of the pump. Abnormal sound can be due to reasons like presence of debris or pieces, worn out bearings etc. Noise level for Dry Screw Vacuum Pump should be less than 85dB(A).

This is a very important parameter for the integrity of the pump. If unchecked, this can create long term issues like loss in suction power which will be difficult to handle in the long run. A vacuum surge is detrimental not only to the pump but the entire associated system. The seals need to be properly checked because any air leakage will make it difficult for the pump to create a consistent vacuum. Further, the life of the valves, gaskets and flanges need to be checked thoroughly before certifying the pump to be ready for shipment.

Some of the most commonly observed problems in dry screw vacuum pumps along with suitable trouble-shooting guidelines are presented below [2]:.

Insufficient Pumping Capacity

Probable causes:

• Suction filter or strainer is clogged – Clean or change it
• Too much Clearance – Check clearance

Overload on Electric Motor

Probable causes:

• Foreign matter could be caught in the pump – Adjust or replace the screw and casing
• Pressure loss in piping increases – Check the pressure difference between inlet and outlet
• Interference between screws – Adjust side clearance, clearance between screw and casing larger
• Discharge port or line is clogged & back pressure is increased – Drain and clean discharge port and line

Overheat

Overheating of the pump could be due to:

• Excessive lubricant in front end cover – check oil level
• High temperature at vacuum pump inlet – Reduce inlet temperature
• High compression ratio – Check suction and discharge pressure
• Interference between screw and casing – Search for the cause of interference
• Problem with cooling water flow – Clean cooling water line
• Discharge port or line is clogged and back pressure is increased – Drain and clean discharge port and line

Knocking

Probable causes:

• Incorrect positioning between timing gear and screw – Reposition the system.
• Improper assembly – Reassemble
• Damage on gear due to overload or improper lubrication – Replace timing gear

Damaged Bearing or Gear

Probable causes:

• Improper Lubricant – Change Lubricant
• Lubricant level low – Refill lubricant