Guidelines for predictive maintenance of mechanical seals

For optimum mechanical seal MTBFs, execute the following mechanical seal monitoring best practices.

• Confirm that the pump is operating in its Equipment Reliability Operating Envelope (EROE). Also called the ‘heart of the curve,’ it assures maximum centrifugal pump MTBF by avoiding all operating areas of hydraulic disturbances.

• Check for a plugged flush line orifice by taking temperature reading across it.
• Confirm seal chamber pressure is at least 345 kPa (50 psi) above the fluid’s vapor pressure.
• If the flush system contains a cyclone separator or strainer, check for plugged components by taking temperature readings.
• If the flush fluid is cooled, confirm the proper function of the cooler by checking the temperature drop across the cooler (should be approx. 10 to 38^oC [50 to 100^oF] normally) and the temperature rise of the cooling medium
• Check the temperature difference on seal reservoirs (pots) between buffer/barrier in and out lines to ensure circulation to and from the outboard seal. If there is no temperature difference, the circulation through the buffer/barrier circuit has halted.
• Always vent seal pot systems at the highest point to ensure pumping ring circulation.
• For dual pressurized seal applications install a permanent differential pressure gauge to ensure that the seal pot filling pressure is not excessive, which will force open the inner seal.

We recommend that the above predictive maintenance (PDM) guidelines be followed for all centrifugal pumps in your facility at the following times:

• At pump commissioning
• After the pump operating conditions are normalized
• Whenever the plant condition monitoring results indicate a change (in vibration, temperature, seal leakage, noise, etc.)
• Immediately after seal replacement

Failure to monitor centrifugal pump performance and the effect of the process conditions on pump flow rate are the major contributors to centrifugal pump mechanical component failure (seals, bearings, wear rings and impeller).

Most plant condition monitoring programs do not integrate centrifugal pump performance (operating point and produced head) with mechanical condition (vibration and temperature). Neglecting pump performance, in FAI experience, neglects consideration of approximately 80 percent of the potential root causes for mechanical seal failure.

This best practice has been used since the mid-1990s to recommend plant PDM practices for pumps with mechanical seals to optimize plant centrifugal pump safety and mechanical seal reliability (MTBFs above 48 and as high as 80 months).