19-12 Slurry Pumps and Seals
Slurry Seals are designed differently than chemical seals
- Take the springs out of the sealing fluid. They cannot clog if they are not in the slurry.
- Make sure the sliding or flexing components move towards a clean surface as the seal faces wear.
- Take advantage of centrifugal force to throw the solids away from the sliding/flexing components and lapped seal faces.
- Use a non-stick coating like Teflon® to prevent the slurry from sticking to the sliding components.
- Use only balanced seal designs. The additional heat generated at the seal faces can cause many products to solidify, coke, and crystallize creating an additional solids problem.
- Metal bellows designs can be used but they must have extra thick plates to resist excessive wear. Extra convolutions will have to be provided to compensate for the higher spring rate caused by these additional plates. Rotating the abrasive fluid with the bellows can be a big asset. Some commercial designs have this feature.
- Specify suction recirculation as a standard except:
- When using Flowserve or Durco pump designs
- When pumping low specific gravity liquids.
- When solids in the pumpage float
- Double ended pumps
- Cyclone separators are a poor choice for slurry applications
Slurry pumps have some features that make them different than chemical pumps.
- The pumps are more massive
- They have looser tolerances.
- The clearances are more open.
- The parts have blunt rather than tapered edges.
- The metal parts are harder.
- They utilize “through bolt construction” because it is difficult to drill and tap the harder metal.
- Some designs are rubber lined to absorb the impact of abrasive fluids.
- They are less efficient than chemical pumps.
- Many slurries are dilatants. Their viscosity increases with agitation. You may have to convert to a positive displacement design if you have this problem. Kaolin or china clay is a good example of a dilatant. Some sugar syrups fall into this category also.
The speed of slurry pumps is important because of the wear rate of the pump volute, impeller and wear rings. The wear is generally proportional to relative velocity between the slurry and the pump elements to the power of two or three. If you went from 1750 (1450) rpm to 3500 (2900) rpm. you would get four to eight times the wear.
Although we often talk about shaft speed, impeller tip speed is often used as the guide in selecting slurry pumps:
- For dirty water type applications limit the tip speed to 130 feet/sec (40 m/sec). For medium slurries up to 25% solids concentration by weight and solids size of 200 microns, limit the tip speed to 115 feet/sec (35 m/s)
- For slurries with higher concentration and larger solids, limit tip speed to 100 feet/sec (30 m/s)
- Pumps fitted with elastomeric impellers (rubber like) are commonly limited to 85 feet/sec (26 m/s) tip speed.
Here are a few more things to consider when ordering your slurry pump:
- Cast iron and stainless steel generally do not require any special design modifications.
- White irons and similar hard metals are difficult to machine and more brittle. This is the reason we usually clamp rather than bolt the pump pieces together.
- Elastomers are often used as liners that are clamped inside the pump casing and cover. In some cases the impeller is bonded with an elastomer material, although the duplex metals are becoming more popular.
about my CD with over 600 Seal & Pump Subjects
Link to Mc Nally home page www.mcnallyinstitute.com