Suction Recirculation

Suction recirculation is caused by large inlet areas coupled with flow incidence angles that are significantly dissimilar to the inlet vane geometry.  The stalled area on the pressure side of the inlet vane produces a hydraulic instability resulting in the formation of fluid swirl.

suction recirculation

When severe, the fluid circulates out of the impeller eye, interfering with normal suction flow.

This can cause localized pressure drops that can reduce the total head generated and cause the suction pressure to fall below the fluid vapor pressure.  The latter is known as separation/cavitation. The result is a separation of the fluid from the vane that has enough room (via the large inlet area) to recirculate.

The video clip below demonstrates the violent effects of suction recirculation experienced at various operating ranges.

 

The design emphases to significantly reduce or eliminate these issues are:

  • Reduce the impeller eye diameter (and hence eye area) to the extent allowed by suction (NPSHa) conditions
  • Operate as close to its best efficiency point as possible to minimize separation effects (obtain coincidence of the flow incidence and vane angles)
  • Provide state-of-the-art air foil shapes the the impeller inlet vanes to be more permissive to off-peak operation

The pump suction specific speed, a dimensionless parameter, defines the suction characteristics of the unit.  The suction specific speed is based on the net positive suction head required (NPSHr), speed, and flow per eye.

suction specific speed

This value is always determined at the pump’s best efficiency point and the maximum impeller wheel diameter.

The higher the Ss, the larger the impeller eye area.  To detune the pump to inlet separation, it is desirable to limit the SS to 7,500 (for water operations) based on an NPSHR @ 0% DH (9,375 based on an NPSHR @ 3% DH).  If inlet separation/cavitation exists and is reducing the effective life of the pump or causing material damage to the first stage, then the following actions should be taken:

  1. Calculate the actual NPSHA for both steady state and transient conditions
  2. Redesign the 1st stage inlet to provide a smaller eye area if there is sufficient NPSHA to NPSHR margin at the pump’s maximum flow condition
  3. Obtain accurate performance (head, flow, efficiency) data
  4. Verify that the pump is operating at, or extremely close (within 5%) to its best efficiency point for all plant loads that result in the majority of annual hour usage
  5. If not, trim or extend the impeller outer diameter as necessary (via the affinity laws) to allow the pump to operate at or near its best efficiency point

The affinity laws state that the flow is proportional to the change in speed or diameter, and that the total developed head is proportional to the square of the change in speed or diameter.   If the pump is operating away from its BEP flow for sustained amounts of time, the affinity laws can be used to modify impeller output to operate more closely to the BEP.  This can be accomplished by changing the impeller diameter or by upgrading to a variable frequency drive.

affinity laws
Affinity Laws

Changes in impeller diameter do not precisely follow the affinity laws. Adjustments are required in calculating new diameters using the affinity laws since exit velocity triangles are changed.  The chart above provides the percentage to be applied to the calculated diameter to obtain the actual diameter change.

calculated diameter cut

Changing the pump speed is a more costly design change; however, this allows the pump to operate at its BEP for its whole range of operating requirements.  Changing the impeller diameter only allows the pump to operate at its BEP for a single operating mode.

It may not be possible to always run the pump near BEP if it is subjected to variable operating conditions.  For these situations, the impeller inlet blade geometry can be redesigned as an air foil to further reduce separation effects at off-peak operation.  This modification is typically referred to as a bias-wedge impeller.

bias wedge impeller

 

With this modification, divergent flow angles follow the more rounded air foil blade shape more closely, reducing separation effects.

The modification requires careful consideration of the nature, location, and extent of damage.  Vane thickness, inlet vane angle, vane shape, material, and flow velocities are important parameters in developing the bias wedge/anti-stall hump design. The alteration to the inlet blade is only provided at the entrance of the blade so that there is no choking of the required flow.

 

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