## Help me understand Pumps:

The purpose of a pump is to add energy to a fluid. For example a pump may be needed to get the water over a change in elevation (since both friction, gravity, and other minor losses work on slowing down a liquid pumps are often required to keep fluid moving.

In most cases, pumps will add energy to the system in the form of increased pressure. The amount of energy added can be determine by taking the Bernoulli Equation on both sides of the pump and taking the difference.

### What about Turbines?

A turbine is the opposite of a pump, it's purpose is to remove, or siphon energy from the liquid flowing through it. This energy is often used to power machinery and is the basis of hydroelectric energy (think Hoover Dam).

 Pumps Explained

## Power Considerations

The work, measured in horsepower, done by a pump is related to the pump head (hp) through the equation:

HP = \frac{\gamma Q h_p}{550}

where

• HP = work done by the pump (hp)
• γ = specific weight of the fluid (lb/ft3) = 62.4 lb/ft3 for water
• hp = head of pump (ft)
• Q = discharge (ft3/sec)

## Cavitation in Pumps

As a pump's impeller blades move through a fluid, low pressure areas are formed as the fluid accelerates around and moves past the blades. The faster the blades move, the lower the pressure around it can become. As it reaches vapor pressure, the fluid vaporizes and forms small bubble of gas. This is cavitation. When the bubbles collapse later, they typically cause very strong local shockwaves in the fluid, which may be audible and may even damage the blades. Cavitation in pumps may occur in two different forms: suction cavitation and discharge cavitation

### Suction cavitation

Suction cavitation occurs when the pump suction is under a low pressure/high vacuum condition where the liquid turns into a vapor at the eye of the pump impeller. This vapor is carried over to the discharge side of the pump where it no longer sees vacuum and is compressed back into a liquid by the discharge pressure. This imploding action occurs violently and attacks the face of the impeller. An impeller that has been operating under a suction cavitation condition has large chunks of material removed from its face causing premature failure of the pump.

### Discharge cavitation

Discharge cavitation occurs when the pump discharge pressure is extremely high, normally occurring in a pump that is running at less than 10% of its best efficiency point. The high discharge pressure causes the majority of the fluid to circulate inside the pump instead of being allowed to flow out the discharge. As the liquid flows around the impeller it must pass through the small clearance between the impeller and the pump cutwater at extremely high velocity. This velocity causes a vacuum to develop at the cutwater (similar to what occurs in a venturi) which turns the liquid into a vapor. A pump that has been operating under these conditions shows premature wear of the impeller vane tips and the pump cutwater. In addition, due to the high pressure conditions, premature failure of the pump's mechanical seal and bearings can be expected. Under extreme conditions, this can break the impeller shaft.

## Net Positive Suction Head Available (NPSHA)

Cavitation is avoided by ensuring there is sufficient net positive suction head available to keep the fluid pressure above the vapor pressure. The energy equation can be used to calculate NPSHA[1]. Based on conditions at the top of an open fluid source (e.g. tank or reservoir) where there is no kinetic energy:

NPSHA = h_{atm} + h_{z(s)} - h_{f(s)} - h_e - h_{vp}

where

• NPSHA = Net Positive Suction Head Available (ft)
• hatm = atmospheric head (ft)
• hz(s) = static suction head (ft)
• he = entrance losses (ft)
• hf(s) = friction suction head (ft)
• hvp = vapor pressure head (ft)

Atmospheric head can be found using the equation for fluid pressure:

p (psi) = specific wt. (lb/ft3) * hatm (ft) / 144 (in2/ft2)

Static suction head is given by the piping geometry. The friction head is often found by using the Hazen-Williams formula or a Moody diagram. Vapor pressure head is listed in CERM appendix 14.A.

Based on the conditions at the immediate entrance to the pump, there is both potential and kinetic head loss (and so there are terms for static (pressure) head and velocity head at the pump suction). In this second case, the reduced pressure head already accounts for the friction losses and the atmospheric head.

## Resources:

1. F. Spellman; "Handbook of Water and Wastewater Treatment Plant", 2009

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