Question

The design flow rate for a pumped system is 10 m3/hr. At design stage the pump head was estimated as 19 m water. Based on these figures a FHS 32‐125/11 pump with a 136mm diameter impeller was purchased and installed. When the pump was first run at commissioning stage the volumetric flow rate was found to be 18.5m3/hr. Calculate the pump power (i) If the design volumetric flow is achieved by throttling the flow using commissioning valves or (ii)If the design volumetric flow is achieved by reducing the pump speed.

Answer 1

Pump power when achieving design flow by throttling is calculated with the formula involving water density, gravity, pump head, and flow rate. When reducing pump speed, the power decreases and depends on adjusted pump head and flow rate values. Pump efficiency also affects actual power consumption.

Step-by-step explanation:

Calculating Pump Power Under Different Conditions

To calculate the pump power when achieving the design flow rate by throttling, we use the formula Power = ρ × g × H × Q, where ρ is the density of water (assumed to be 1000 kg/m3), g is acceleration due to gravity (9.81 m/s2), H is the pump head (19 m), and Q is the design flow rate (10 m3/hr). The power required will therefore be higher due to the increased flow resistance when throttling.

When reducing the pump speed to achieve the design flow rate, the power usage typically decreases as well, since it's directly related to both flow rate and pump head. The exact power can be determined from the pump's performance curve provided by the manufacturer, based on the reduced speed. The formula Power = ρ × g × H × Q is still applied, but with adjusted values for H and Q that correspond to the reduced speed.

It should be noted that in practical applications, the actual power consumption will also depend on the efficiency of the pump, which can vary based on operating conditions.