Understanding Pump Heads in Water Supply Systems

When dealing with pumps used for water supply, it's essential to understand the different types of head components that contribute to the total work done by the pump. These heads represent energy in terms of vertical height of a column of the fluid being pumped. The question specifically asks about a particular vertical distance in relation to the pump's centreline and the discharge point.

Defining Key Pump Head Concepts

Let's break down the common terms related to pump heads:

• Static Head: This is the vertical distance between two fluid levels. It's purely based on elevation difference and doesn't account for flow or friction. Static head is often divided into suction static head and delivery static head.
• Static Suction Head: This is the vertical distance from the pump's centreline to the liquid surface in the source reservoir or tank on the suction side.
• Static Delivery Head: This is the vertical distance from the pump's centreline to the liquid surface in the discharge reservoir or tank, or the point of free discharge.
• Friction Head: This represents the energy loss due to friction as the fluid flows through the pipes, valves, and fittings. It depends on the flow rate, pipe size, pipe roughness, and length.
• Velocity Head: This represents the kinetic energy of the fluid, related to its velocity. It's often significant at the discharge point, especially for free discharge. Mathematically, it's expressed as \( \frac{V^2}{2g} \), where \( V \) is the velocity and \( g \) is the acceleration due to gravity.
• Total Head: This is the total energy added to the fluid by the pump. It's the sum of the static head (total suction static head + total delivery static head), friction losses in both suction and delivery lines, and the velocity head at the discharge.

Analyzing the Question's Specific Distance

The question asks for the vertical distance between the centre line of a pump and the point of free discharge. Let's consider this scenario:

Imagine a pump located at a certain elevation. It draws water from a source (suction side) and pushes it upwards through a pipe to a point where it freely exits into the atmosphere (free discharge). The vertical distance from the pump's centreline upwards to this free discharge point is the static vertical lift on the discharge side of the pump.

Based on our definitions:

• Friction head is about energy loss due to flow, not a static vertical distance. So, option 1 is incorrect.
• Static Suction Head is the vertical distance on the *suction* side, from the pump centreline downwards to the source level (if below) or upwards to the source level (if above). The question specifies the distance to the *discharge* point. So, option 2 is incorrect.
• Static Delivery Head is the vertical distance from the pump's centreline to the level of the discharged water, which in the case of free discharge is the free discharge point itself. This definition perfectly matches the description in the question. So, option 3 is the correct answer.
• Total head includes static head (both suction and delivery), friction head, and velocity head. The question asks only for a specific vertical distance component, not the total energy added. So, option 4 is incorrect.

Therefore, the vertical distance between the centre line of a pump and the point of free discharge is known as Static Delivery Head.

Revision Table: Key Pump Head Terms

Additional Information on Pump Head Calculations

Understanding the different heads is crucial for calculating the required total head for a pump, which in turn helps in selecting the right pump for a specific application like water supply. The total head \( H_t \) can be generally expressed as:

\( H_t = h_{sd} + h_{ss} + h_{fd} + h_{fs} + \frac{V_d^2}{2g} \)

Where:

• \( h_{sd} \) is the static delivery head.
• \( h_{ss} \) is the static suction head.
• \( h_{fd} \) is the friction head loss in the delivery line.
• \( h_{fs} \) is the friction head loss in the suction line.
• \( \frac{V_d^2}{2g} \) is the velocity head at the discharge.

In some contexts, \( h_{ss} \) might be negative if the source level is below the pump centreline (suction lift). The net static head is \( h_{sd} - h_{ss} \).

For a free discharge into the atmosphere, the pressure head at the discharge point is zero (gauge pressure). The energy at the discharge point consists primarily of static delivery head (elevation) and velocity head.