Pump torque is calculated using the following formula: T = (5,250 x HP) / N, where T is Torque, HP is Horsepower, and N is RPM.
To elaborate, understanding how to calculate pump torque is crucial when selecting a motor for a pump system. Here's a more comprehensive breakdown:
Understanding the Formula and its Components
The formula T = (5,250 x HP) / N is an empirical formula that uses horsepower (HP) and revolutions per minute (RPM) to derive torque (T) in foot-pounds (ft-lbs). Let's examine each component:
-
T (Torque): This is the rotational force required to turn the pump shaft. It's typically measured in foot-pounds (ft-lbs) or Newton-meters (Nm). The torque represents the pump's resistance to rotation. Higher torque values mean the pump requires more force to turn.
-
HP (Horsepower): Horsepower represents the rate at which work is done. It's a measure of the pump's power output. You need to know the pump's required horsepower at its operating conditions.
-
N (RPM): RPM stands for revolutions per minute and signifies the rotational speed of the pump shaft.
Considerations for Motor Selection
When choosing a motor for a pump, you must consider more than just the torque at a specific operating point. Here are key torques to evaluate during motor selection:
-
Torque at Zero Flow (Shut-off Head): This is the torque required when the pump is started with the discharge valve closed. This is often the highest torque demand on the motor, as the pump has to overcome static inertia.
-
Torque at Rated Flow: This is the torque required when the pump operates at its designed flow rate and head. This is the normal operating condition and an important consideration for continuous operation.
Steps to Calculate Pump Torque for Motor Selection
-
Determine the Required Horsepower (HP): Calculate or obtain the pump's required horsepower at the desired operating conditions (flow rate and head). This information is often provided in the pump's performance curve or specifications.
-
Determine the Operating Speed (RPM): Identify the rotational speed of the pump shaft, usually expressed in RPM. This is also typically found on the pump's nameplate or specifications.
-
Apply the Formula: Plug the HP and RPM values into the formula:
T = (5,250 x HP) / N. -
Consider Shut-Off Head: Calculate the torque required at shut-off head conditions, which may require estimating the HP required under those conditions. This is critical to ensure the motor can start the pump.
-
Select a Motor with Adequate Torque: Choose a motor that has a rated torque equal to or greater than the calculated torque at both rated flow and shut-off head. It's generally advisable to select a motor with a service factor that provides a safety margin.
Example
Let's say a pump requires 10 HP and operates at 1750 RPM. The torque calculation would be:
T = (5,250 x 10) / 1750
T = 52,500 / 1750
T = 30 ft-lbs
Therefore, you would need a motor that can deliver at least 30 ft-lbs of torque at 1750 RPM.
