Before getting into the details of pump sealing, a short review of pumping technology will offer an understanding of sealing problems. Pumps are the second most common piece of industrial equipment, next to electric motors. They are divided into two major categories: kinetic and positive displacement (PD). Kinetic pumps move liquid by imparting energy to the liquid by means of an impeller confined in a housing. Rotational velocity of the impeller is key, and these pumps always operate at motor speeds. Positive-displacement pumps move liquid by taking confined amounts of liquid, determined by pumping element geometry, through the pump. Kinetic pumps are often viewed as pressure generators, whereas positive-displacement pumps are viewed as flow generators.
Centrifugal pumps are the most common kinetic pumps and account for about 70% of all pumps sold. Rotary pumps are the most common PD pumps and account for about 15% of all pump sales. Centrifugal pumps do not work well on liquids with a viscosity above 150 cps and will not pull liquid when there is a suction lift. PD pumps work well on a range of liquid viscosities but are the only choice for thicker liquids. They also have excellent vacuum capability - that is, they can pull a suction lift. PD pumps displace the same amount of liquid with each revolution of the shaft, irrespective of discharge pressure, so they are good choices when a predictable amount of output flow is needed. PD pumps generally run at a much slower speed than centrifugal-type pumps.