How does viscosity affect the hydraulic performance of air operated diaphragm pumps?


Viscosity correction graphs or formulae for air operated diaphragm pumps should be used only as a guide because, to our knowledge, there is no proven theoretical formula. Different manufacturers supply slightly differing information. However, there are some general trends worth noting.

The diagram shows that as viscosities increase, the capacity of the pump decreases. While the graphs are drawn out to viscosities of 22,000 centipoise (100,000 SSU), inlet losses with small pumps would generally dictate the following limits:

  • For ¼" and ½" pumps, maximum of 1000 cps.
  • For 1" and 1 ½" pumps, maximum of 4000 cps.
  • For 2" pumps, maximum of 10000 cps.
  • For 3" and 4" pumps, maximum of 22000 cps.

Notwithstanding the general limits detailed above, other issues to consider when pumping high viscosities include:

  • The rheological behaviour (viscosity) of liquids may vary significantly with shear and this needs to be considered in any pump selection.
  • The pump should be positioned close to or below the level of the fluid source.
  • Suction lines should be increased in size—up to three times the size of the pump manifold inlet. Dual manifolds may be used when available.
  • Suction lift applications must be very carefully considered.
  • The impact of acceleration losses.
  • Start the pump slowly using a control valve on the airline.
  • Maximum air pressure required is reached when increasing the air pressure does not increase the flow rate.
  • If greater capacity is required, select a larger pump.

N.B. Contact your air operated pump supplier for exact details on your pump.

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