Why Liquid Pump Seals Fail

Why Liquid Pump Seals Fail

When operating rotating equipment, some end users do not pay enough attention to transient conditions. Startup, slow-roll and standby pump conditions must be evaluated to ensure proper sealing fluid is being supplied to the seals at all times.

Startup: The pump is charged, but at or near suction pressure. Liquid ethane at the seal faces is slowly leaking and vaporizing. When the pump starts, how long does it take to build the right pressure in the stuffing box and get the pressure above vapor pressure? Additionally, the heat generation between the faces, although not significant, could be enough to increase vapor pressure and vaporize the fluid across the faces. Damage to sealing faces could be a telling sign that this is occurring.

Slow-roll: The same situation as startup but compounded. Without the right speed, the discharge pressure is not generated. The pressure in the stuffing box is not rising quickly enough to ensure the ethane will reach a high enough pressure to overcome the vapor pressure. Also, the heat generation between the contacting faces is increasing, and damage is probably taking place.

Standby: All conditions mentioned above are the same, but the seals are sitting idle for many months without a flush to the seals. During the standby time, evidence has shown that debris has collected at or around the seal faces, which, in turn, adds more complication to the sealing environment.

Inefficient operation: Operating the pump too far outside of the best efficiency range and with the wrong operating parameters results in increased demand for drive power and reduced discharge pressure. Both of these negatively impact the vapor margin in the seal area, which can result in dry running.