by Jack Tyler, P.E.,

When pump suction pressure is variable, use the higher value for suction pressure for the stuffing box pressure. If the pump is to be operated against a closed discharge valve at any time, use the pump discharge pressure at shut-off. If the pump is on a suction lift, the stuffing box may be at a pressure lower than atmospheric and air may be drawn through the shaft seal. This is no problem for the LPS since it can run dry indefinitely.


Flexible Sealant

The flexible sealant used in the LPS is also important. There are several types, but the most common is injectable packing. This has been available for decades for use in more traditional stuffing box arrangements in which the injectable packing is pumped into a stuffing box with a ring of packing at each end. In this mode, the injectable packing was intended to maintain a seal for the pumped liquid.

Over-pressurization during installation and the lack of packing pressure maintenance led to irregular and often unsatisfactory performance, such as leaky stuffing boxes. Conversely in the LPS, the injectable packing is held within the system assembly and forms a barrier through which the pumped liquid cannot pass. No over-pressurization occurs since the pressurizing piston assembly maintains a moderate and steady pressure.

The choice of a flexible sealant depends on the requirements of the application. Chemical compatibility, temperature, operating pressure, shaft tangential velocity and FDA certification for food-related products must be considered.

More than 400 variations of flexible sealant are available from injectable packing to high-viscosity greases. Higher stuffing box pressures may be accommodated by a Plan 13 seal piping arrangement.


Typical applications for the LPS include sealing a fluid where flushing the seal faces with water or another barrier fluid is undesirable, applications in which cavitation of the pump is a common occurrence and the cavitation destroys the seal faces in a short time or applications in which abrasives in the sealed fluid lead to premature seal face failure.

Pumping brine solution when dilution of the fluid with seal flush is not wanted can be particularly challenging for conventional mechanical seals. Salt particles in the brine are abrasive and wear traditional seal faces prematurely.

In pulp and paper, sealing black liquor can be problematic because of the need to flush the seal faces with water or another barrier fluid to prevent the liquor from solidifying on the seal faces and causing failure. Some black liquor processes purge the black liquor pumps with live steam while running to clean the system, causing severe pump cavitation. This procedure destroys seal faces in short order. The LPS is able to handle abrasive fluids easily and pump cavitation without failure.


New Developments

The latest development for the LPS is “air flush.” A small amount of compressed air, typically 0.17 SCFM at 8 psig, is fed to the back of the stuffing box via an internal port in the seal. This air forms a bubble between the back of the impeller and the back of the stuffing box. The bubble expands to create a torturous path even more difficult for the fluid pumped to penetrate than the pressurized sealant. The result is leak free

The LPS presents opportunities for handling tough-to-seal pump applications where traditional packing or mechanical seals cannot provide the performance required.


LPS Operating Parameters

LPS without cooling

Shaft tangential velocity: 0.5 meters/second (1.64 feet/second) maximum

Stuffing Box Pressure: 1.5 bar (22 PSIG) maximum

Temperature: 70 degrees C (158 degrees F) maximum

LPS with cooling

Shaft tangential velocity: 9.5 meters/second (31.6 feet/second) maximum

Stuffing Box Pressure: 3 bar (45 psig) maximum

Temperature: 315 degrees C (600 degrees F) maximum

LPS with air flush

Shaft tangential velocity: 9.5 meters/second (31.6 feet/second) maximum

Stuffing Box Pressure: 4 bar (60 psig) maximum

Temperature: 110 degrees C (230 degrees F) maximum


Pumps & Systems, November 2011