Pump Off-Design Operation

Pump Off-Design Operation; a Review on Karassik's Paper

The well-known pump consulting engineer, Igor Karassik, has reviewed (in his 3-part paper) the issues encountered when a centrifugal pump is operating at flows other than BEP. Here is a brief review and summary of those.

It should be noted that with modern pump designs the limitation of pumps due to minimum flow is greatly improved but still these guidelines are helpful in understanding the concepts.

1. Motivation

Pump operating point is the intersection of its curve and system curve and can be changed by altering either one or both of these curves:

• Pump (head-capacity) curve is altered by variation of pump speed;
• System (resistance) curve is altered by throttling pump discharge.

Although pump is able to operate at either case as long as it is provided with adequate NPSHA, there are several issues associated with such an operation.

2. Operation at High Flows

This happens in two cases:

• An over-sized pump; pump curve may intersect system curve at a much greater capacity than required system flow; required capacity can be met by throttling back;
• A parallel operation of two pumps when one pump is taken out of service because of a decrease in demand; single operating pump curve may intersect system curve at a much higher capacity compared to required capacity and may reach pump run-out point.

In either case uncontrolled pump operation can lead to cavitation (if NPSHA is inadequate) and power consumption increase (for low specific speed pumps).

3. Operation at Low Flows

This is the case usually encountered when there is a reduction in process flow demand and can have several consequences:

• Operation at lower efficiency;
• Higher radial thrust hence higher load on radial bearing;
• Temperature rise of pumped liquid;
• Internal recirculation (both in suction and discharge);
• Higher power consumption (for high specific speed pumps);
• Pump becoming air-bound.

Pump final minimum flow can be determined considering all above-mentioned effects.

4. Radial Thrust

Operation of pump at flows other than BEP causes non uniform pressure profile around the impeller and this imposes a resultant radial force on shaft and bearing. Generally, radial thrust is a function of total head and impeller width and diameter. Below diagram shows that how changing the single volute design to modified concentric casing or double volute design helps reducing radial thrust at off-design operation.

Radial thrust in volute pumps

5. Temperature Rise

Operation of a pump at reduced flow will be with reduced efficiency. The difference between pump hydraulic power and diver power represents power losses within the pump (neglecting minor losses) which are converted into heat and transferred to the liquid. This causes the liquid temperature to rise and if the power losses are high (at very low flows) the temperature can reach very high values (even in excess of boiling temperature).

Rate of temperature rise can be predicted for shutoff conditions (with engineering estimations) using following formula:

In which "BHP₀" is brake horsepower at shutoff, "W_W" is net weight of liquid in pump [lb] and "C_W" is specific heat of liquid (1.0 for water).

If liquid is flowing through the pump, temperature rise at pump discharge can be calculated from:

In which "e" is pump efficiency. This formula can be used to draw temperature rise curve on pump performance curve. Below diagram presents such an example.

Centrifugal pump performance curve including temperature rise curve

6. Internal Recirculation

Recirculation at suction and discharge areas of impeller occurs at certain flows below BEP and causes a great increase in pressure pulsations. Internal recirculation at the suction is most frequently the cause of pump problems.

6.1. Suction Recirculation

Mechanism: Increasing impeller eye diameter (to reduce NPSHR) leads to lower entrance velocities. In turn, peripheral velocity of impeller at the eye is increased and at some capacity distortion of the velocity triangles causes the flow at the outer eye diameter to reverse and flow back out of the impeller.

Consequences: Very intense vortices are formed as a result of internal recirculation; with high velocities at the core, static pressure is significantly decreased locally. The consequences are intense cavitation, severe pressure pulsations, noise and damage to impeller material.

It should be noted that if NPSHR at the vortex (which is not possible to be calculated mathematically) does not exceed NPSHA, frequently no damage occurs by pump operation in recirculation zone.

Some Notes:

• Larger impeller eye diameter is equivalent to lower NPSHR, higher suction specific speed (N_ss or S) value and higher capacity at which recirculation takes place;

Safe operation zone for normal ("A") and high ("B") suction specific speed

• If damage to the impeller is at pressure side, it is caused by internal recirculation. Otherwise, damage at suction side is due to classic cavitation (caused by inadequate NPSHA);
• Trimming the impeller will move the best efficiency point to a lower flow value but will not reduce the flow at which suction recirculation will occur;
• Minimum flows for pumps handling hydrocarbons need not be selected as conservatively as for cold water pumps;
• Do not specify NPSHR values which result in suction specific speeds of above 9000 (for water) or 11000 (for hydrocarbons) [Again it is emphasized that such a consideration might not be critical with modern pump designs];
• The flow at which suction recirculation starts can be predicted as a function of  suction specific speed and hub-to-eye diameter ratio (h₁/D₁) using below curve:

Suction recirculation flow

6.2. Discharge Recirculation

Mechanism and Consequences: Like suction recirculation it causes hydraulic surge and local cavitation at the impeller tips. Another consequence of discharge recirculation is axial instability which is because of pressure fluctuations outside the impeller shrouds. This can cause a failure of ball thrust bearing (due to lower clearance).

Vane Passing Syndrome: discharge recirculation should not be confused with vane passing syndrome; a phenomenon caused by interaction between moving impeller tips and stationary volute tongues and diffuser vanes. A hydraulic shock occurs as impeller vanes pass the stationary parts. Shock wave magnitude (and the resulted pressure fluctuations) increases with impeller tip velocity and pump size and frequency is a multiple of pump speed and the number of impeller vanes.

Near the central part of the vane (at first stage) shock pressure is momentarily reducing local pressure to less than vapor pressure and causes cavitation erosion. Negative effects of vane passing syndrome can be eliminated by increasing the gap between vane tip and stationary part keeping in mind that too much a gap lowers pump efficiency. The minimum recommended gap is 4% to 6% of the impeller diameter.

6.3. Final Remarks on Internal Recirculation

Following tables present comparisons between classical cavitation and suction recirculation and also vane passing syndrome and discharge recirculation:

 

7. Power Consumption

Power consumption increases for a high specific speed pump as its flow is decreased. If the motor is not selected satisfying lower flow end of curve, it will be overloaded. On the other side, selecting a motor for 0% high specific speed pump flow is not economical and so a minimum allowable flow is dictated.

8. Entrained Air or Gas

Effect of entrained air or gas in the pump liquid on performance and minimum allowable pump flow is presented in below diagram.

 Effect of entrained air on pump performance

9. Minimum Flow By-Pass

If the pump is required to operate below its minimum permissible flow (because of process requirements) then a by-pass line should be installed from discharge line in pump side of check and gate valves. Normally it should not be led directly back to pump suction as there should be some means of heat dissipation.

It should be noted that if the minimum flow is established for reasons other than permissible temperature rise (such as internal recirculation) then it would be normally around 25% to 50% of design flow and temperature rise won't be significant and a major portion of the by-pass may be led back to the suction piping.

Reference: I. Karassik, 1987, "Centrifugal Pump Operation at Off-Design Conditions", Chemical Processing Magazine