Pump Capacity

Published: | Updated: September 4, 2020;

Definition - What does Pump Capacity mean?

Pump capacity is a term used to define the flow rate through a pump at its designed conditions. It describes the volume of liquid that is allowed to travel through the pump in a given time.

In other words, pump capacity is the rate at which the pump can push fluid through its system. Based on this definition, pump capacity is expressed as fluid volume per unit time. Some of the most common units of pump capacity are:

  • Gallons per minute (gpm)

  • Liters/minute (L/min)

  • Cubic meters per hour (m3/h)

Figure 1: Illustration of pump capacity showing a specific volume of fluid passing through a tube in a given time (source)

In some applications, pump capacity may also be expressed as the mass of a fluid that passes through the pump in a given time, also known as the mass flow rate. When expressed this way, the volume flow rate is related to the mass flow rate by the following equation:

ṁ = ρ x Q

Where ṁ = mass flow rate (kg/s) (lb/s)

ρ = density of the fluid (kg/m3, lb/ft3, l/ft3)

Q = volume flow rate (gpm, L/min, m3/h)

Pump capacities are essential in trenchless construction projects, particularly in dewatering applications. This measurement determines what type of pump is needed for a particular site and how well that pump is expected to perform.

Trenchlesspedia explains Pump Capacity

Pump capacity ultimately determines how long it will take the pump to complete the task. The higher the flow rate, the higher volume of liquid that the pump can remove; therefore, the faster it will dewater a site.

Conversely, pumps with lower flow rates will take a longer time to remove water from a specific area. However, pumps with higher flow rates are more expensive; therefore, construction teams also need to consider budgetary constraints when selecting a dewatering pump.

In trenchless drilling applications, where pumps are needed to deliver fluids, rather than remove them, capacities also play a crucial role. For example, in horizontal directional drilling and microtunneling projects, drilling mud must be continuously supplied to the drilling head in accordance with soil conditions. If the supply of fluid is too low, (i.e., the pump does not have enough capacity to deliver the right amount of drilling mud,) the duration of drilling operations can increase significantly.

On the other hand, if the pump delivers too much fluid to the drilling head, the increased fluid pressure may damage the drilling equipment or cause excess fluids to seep into the soil and make its way to the surface.

Types of Pump Capacities

Most pump manufacturers typically provide specifications on the capacities of their products. Two of the most common types of pump capacities found on pumping equipment are maximum capacity and normal or rated capacity.

Maximum Pump Capacity

Maximum pump capacity describes the maximum volume of water that can be pushed by the pump in a given time directly from its discharge without having to travel through a piping network. The maximum pump capacity is not likely to be achieved when in operation.

Normal Pump and Rated Pump Capacity

Normal pump capacity is the capacity that the pump will operate at most of the time when in operation. On the other hand, rated pump capacity is the capacity that is guaranteed by the manufacturer, or, the capacity that the pump is designed for. As such, the normal pump capacity is usually lower than the rated capacity.

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