3.2 Pumps: Hydraulics & Operations

Understanding Pump Hydraulics

To operate a water distribution system effectively, an operator must understand the forces working against a pump and the conditions required for a pump to function properly. The most fundamental concept in pump hydraulics is 'head.'

Static vs. Dynamic Head

Head is a measurement of the energy in a body of water, typically expressed in feet. It represents the height to which a pump can raise water.

• Static Head: The vertical distance (in feet) that water must be lifted, or the pressure of water at rest. It is simply the difference in elevation.

  • Static Suction Lift: When the water source is below the centerline of the pump, the vertical distance from the water surface up to the pump is the suction lift. The pump must create a vacuum to draw this water up. Because a pump relies on atmospheric pressure (which is equivalent to about 34 feet of water at sea level) to push water up into a vacuum, it is physically impossible to pump water with a suction lift greater than 34 feet. In practical applications, due to friction and vapor pressure, the maximum suction lift is generally around 20 to 25 feet.
  • Static Suction Head: When the water source is above the centerline of the pump (e.g., a storage tank feeding the pump), the vertical distance from the pump up to the water surface is the suction head. This provides a positive pressure to the pump intake, which is the preferred operational state.
  • Static Discharge Head: The vertical distance from the centerline of the pump to the point of free discharge or the surface of the receiving tank.
  • Total Static Head: The total vertical distance the water is moved (Static Discharge Head + Static Suction Lift, or Static Discharge Head - Static Suction Head).

• Dynamic Head (Friction Loss): As water moves through pipes, valves, and fittings, it rubs against the interior surfaces, creating friction. This friction resists the flow of water and consumes energy. The faster the water moves, or the rougher the pipe, the greater the friction loss.

• Total Dynamic Head (TDH): The total amount of work the pump must do. It is the sum of Total Static Head plus all friction losses in the suction and discharge piping.

Cavitation

Cavitation is a highly destructive phenomenon that occurs when the pressure inside a pump drops below the vapor pressure of the water.

1. Vaporization: When pressure drops low enough (often at the eye of the impeller where suction is highest), the water literally boils at ambient temperature, forming microscopic vapor bubbles. This typically happens when there is excessive suction lift or a clogged suction strainer.
2. Implosion: As these bubbles move outward along the impeller vanes into higher-pressure areas within the volute, they cannot remain as vapor and rapidly collapse (implode) back into liquid.
3. Damage: The implosion of these bubbles creates microscopic, high-velocity shockwaves that strike the metal of the impeller and volute. Over time, these repeated shockwaves literally rip small flakes of metal away, leaving the impeller looking like it was eaten by acid or repeatedly struck with a ball-peen hammer.

Signs of Cavitation:

• A distinct crackling noise, often described as 'pumping gravel' or 'marbles in the pump.'
• Excessive vibration, which can destroy bearings and mechanical seals.
• A sudden drop in pump capacity and efficiency.

Prevention: Cavitation is prevented by ensuring the pump always receives adequate suction pressure. This involves keeping the suction piping clear, minimizing suction lift, ensuring the source water level does not drop too low, and verifying the pump is operating close to its Best Efficiency Point.

Net Positive Suction Head (NPSH)

NPSH is a critical calculation used to ensure a pump will not cavitate. It quantifies the exact pressure available to push water into the eye of the impeller without vaporizing.

• NPSHr (Required): The minimum suction pressure required by the pump manufacturer to prevent cavitation. This value is found on the pump curve and increases as flow increases. The faster you try to move water, the more pressure you need at the inlet to prevent cavitation.
• NPSHa (Available): The actual absolute suction pressure available at the pump inlet in the specific field installation. Calculation: NPSHa = Atmospheric Pressure ± Static Suction Head (or - Suction Lift) - Friction Loss in Suction Piping - Vapor Pressure of Water.

Rule: For safe operation, NPSHa must always be greater than NPSHr. If NPSHa falls below NPSHr, the pump will cavitate.

System Surges and Water Hammer

When operating pumps, operators must be aware of hydraulic surges (water hammer). Starting or stopping a pump rapidly can send high-pressure shockwaves through the distribution system, potentially bursting pipes or blowing out joints. To mitigate this, pumps are often started against a closed discharge valve. The valve is then slowly opened to introduce flow gradually into the system. When shutting down, the valve is slowly closed before the pump motor is turned off.

Operating Multiple Pumps

Water demands fluctuate throughout the day. To meet these changing demands efficiently, systems often use multiple pumps.

• Pumps in Parallel: Two or more pumps discharge into a common header. This configuration is used to increase the total flow rate (capacity) while the head remains roughly the same. This is the most common setup in water distribution pump stations.
• Pumps in Series: The discharge of one pump is piped directly into the suction of the next pump. This configuration is used to increase the total head (pressure) while the flow rate remains the same. Series pumping is common in deep well applications (multistage well pumps) or booster stations moving water up a steep elevation.