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Is basis for minimum flow in pumps volumetric?

Is basis for minimum flow in pumps volumetric?

Is basis for minimum flow in pumps volumetric?

(OP)

The pump curve for a centrifugal type pump gives a minimum required flow of 0.9 m3/hr using water as the test fluid. The fluid that I'm pumping has a specific gravity of 0.667, so does that mean my minimum required flow would be 600 kg/hr (0.9 m3/hr x 667 kg/m3) or do I need to maintain the mass rate of 900kg/hr (0.9 m3/hr x 1000 kg/m3)?



There are several reasons for setting a minimum flow.
o Cases of heavy leakages from the casing, seal, and stuffing box
o Deflection and shearing of shafts
o Seizure of pump internals
o Close tolerances erosion
o Separation cavitation
o Excessive hydraulic thrust
o Premature bearing failures

o Limit temperature rise of fluid so that it does not vaporise or degrade (already checked this one is OK)



RE: Is basis for minimum flow in pumps volumetric?

The basis is volumetric.
Temperature rise at low flows results from low hydraulic efficiencies (effy in decimals) as energy is lost in friction. Sulzer's formula for the temperature rise in oC:
 
               (0.00981/c)(Head, m)[(1/effy)-1]

where c is the specific heat of the incompressible fluid expressed in kJ/(kg.K); for organics this value is about half of that for water usually taken at 4.18; while 0.00981 is the acceleration of gravity 9.81 m/s2 divided by 1,000 to convert kJ into J.

As you may see, the "heat up" formula already includes the liquid density in the Head factor (=pressure divided by density).

BTW, I've checked it, and the formula is dimensionally consistent:

      (m/s2)(m)(kg.K)/(J)=(m/s2)(m)(kg.K)/(kg.m2/s2)=K

There is more heat up from throttling the liquid in the clearance of the axial thrust balancing device taken as

           (0.00981/c)(Head, m)(1/effy.)

Both results are taken together when determining minimum flow.

Sulzer lists the criteria for minimum flow without sustaining any damage, as follows, I quote:

-temperature rise due to internal energy loss,
-internal recirculation in the impeller (whit large  
 impeller inlet dia. compared with the outside dia., NPSH
 rises in the part-load range),
-increased vibration due to greater flow separation,
-increased pressure fluctuation at part load,
-increased axial thrust at low flow rates,
-increased radial thrust (especially with single volute
 pumps).

Sulzer continues saying -among other statements for high head and high kW pumps- that for small pumps running at temperatures sufficiently far from that corresponding to the VP, it will suffice to determine the minimum flow Qmin, m3/h by:

(P, kW)*3600(s/h)/[(density, kg/m3)(c, kJ/kgK)(20oC)]

RE: Is basis for minimum flow in pumps volumetric?

(OP)
Good explanation 25362,

Thankyou

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