Don't Let Slurries Ruin Your Seals

Various approaches can protect seals, but only a few are effective

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If the fluid you are sealing is not hot, the cooling jacket will not be necessary. Sometimes, a single charge of clean liquid into an oversized, dead-ended stuffing box is all that is required to seal even severe slurries.

If the solids have a lower specific gravity than the liquid, use a clean flush or one filling of a higher-specific-gravity, compatible liquid.

Good Solution No. 3.

If the solid particles are sub-micron in size, as is the case with kaolin (china clay) and some dyes, two seals with a higher-pressure barrier fluid are required. In some instances, you may want to use two hard faces on the inner seal.

Good Solution No. 4.

Install a large seal chamber on the pump and connect a recirculation line from the bottom of the stuffing box to the suction side of the pump. This will cause liquid to flow from behind the impeller to the stuffing box and then on to the suction of the pump. Fluid entering the stuffing box from behind the impeller has been centrifuged and should be a lot cleaner than the fluid you are pumping.

This solution works well with closed impeller pumps and those open impeller designs that adjust to the front of the pump volute.

However, do not use this technique if:

  • You are pumping close to the vapor point of the fluid. Lowering the pressure could cause the pumping fluid to vaporize in the stuffing box and, in some cases, between the seal faces.
  • You are sealing a pump where the impeller adjusts to the back plate.
  • You are using double-ended pumps where the stuffing boxes are at suction pressure.
  • The solids float on the liquid.

    Imbalance compensation

    The rotating unit will go out of balance, so you should compensate for that. The seal faces have to be vibration-dampened. The elastomer in O-ring-type seals provides a natural vibration damper. Metal bellows seals have to be provided with some other method.

    Shaft runout and vibration can cause the seal rotating components to contact the inside of the stuffing box unless you have installed an oversized sealing chamber.

    Use motion seals if the runout or vibration is excessive. Most of the popular designs can compensate for 1/8 in. in a radial direction and 1/8 in. in an axial direction.

    Move the seal closer to the bearings. Split seal designs are a logical choice because most of them come with a stuffing box extension gland that positions them next to the bearings. A support bushing or sleeve can be installed in the end of the stuffing box to minimize the effects of imbalance, vibration, and shaft whip or wobble.

     

    Pump wear

    The pump will lose its efficiency and experience more shaft movement as erosion takes its toll on close tolerances.

    Open impellers will require frequent adjustment. In this case, a cartridge seal is your best option because impeller adjustments can be made without disturbing the seal face loading. Split seals can compensate for the initial impeller setting; split seals mounted on a split sleeve will easily make up for movement caused by temperature growth or impeller adjustment.

    Closed impeller pumps will have to be disassembled and their wear rings changed when clearances become excessive. If you have adjustable wear rings on your pump, then only an outside adjustment will be needed and the pump will not have to be taken out of service. Cartridge seals can almost always be reused in these applications because the seal faces are not separated when the pump is disassembled.

    Remember that the wear rings in closed impeller pumps will have to be replaced when the normal clearance doubles. A good rule of thumb is that the pump will lose 1% of its capacity for each 0.001 in. of wear-ring wear.

    A few more thoughts

    Sometimes two hard seal faces have to be employed because a carbon seal isn't suitable. Carbon cannot be used with oxidizers or if carbon black could cause a color-contamination problem.

    Use of packing and a split mechanical seal has proved to be an ideal solution in many applications. With the seal installed, there is no pressure differential across the packing and, therefore, the solids do not try to penetrate. Move the packing flushing line to the bottom of the split-seal housing and flush the packing through this connection instead of the lantern ring or seal cage. The flushing is necessary to remove the additional heat being generated by the packing.

    You should be able to cut the flushing fluid volume down to about one-third of the amount you had been using with the seal alone. Because the packing is not being forced to the shaft, only a small amount of cooling is necessary. Caution: It is important that the flushing fluid be kept at a higher pressure than the stuffing box pressure. Loss of this pressure differential could force the packing into the rear of the mechanical seal. A split adapter plate installed between the split seal and the stuffing box face can prevent the packing from blowing out if the flushing pressure is lost.

    If you elect to use a rotating metal bellows, remember that the bellows should rotate the fluid in the sealing chamber. Most bellows designs allow the thin bellows plates to cut through the abrasive slurry and, therefore, the plates suffer severe wear and breakage in a short period of time.

    William McNally has more than 45 years of experience with pumps and seals. He runs the McNally Institute (www.mcnallyinstitute.com), which conducts courses on pumps and seals.

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