Is basis for minimum flow in pumps volumetric? 1

[MarkkraM]

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)

 

[25362]

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 Q_min, m³/h by:

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

 

[MarkkraM]

Good explanation 25362,

Thankyou