Pressure-Gradient Diagrams

A pressure-gradient diagram is a graphical presentation of the static pressure throughout a fluid flow system

The pressure-gradient diagrams includes only the static and physical pressures in the systems and the diagrams are suitable for checking energy transformation throughout the flow systems. Velocity heads are dynamic heads and not included as done in the Energy and Hydraulic Grade Line Diagram.

The vertical dimension of the pressure-gradient diagram is in scale of the pressure or head unit used. The horizontal dimension is not to scale and is used to separate the specific head and pressure losses throughout the system.

A pressure-gradient diagram for a basic circulation pump system can be illustrated as:

The horizontal falling lines in the diagram visualizes the major head or pressure loss due to friction in the pipes. The vertical lines visualizes the minor head or pressure loss due to valves and components. The vertical lines are important for the sizing of reduction valves needed for balancing the system.

The diagram above shows a static steady state stabilized flow where the flow rate and the minor and major head or pressure losses are constant. In reality there is more common with modulating systems where the flow rate changes due to the external conditions - as heating systems when it's get colder or warmer, or a domestic water supply systems where the demand of water changes over the day.

The pressure-gradient diagram should be drawn for

• the normal condition with normal flow rate
• the extreme condition where the pump deliver a maximum flow rate at the lowest pump head
• the extreme condition where the pump deliver a minimum flow rate at the highest pump head

Analyzing the minimum and maximum conditions are important for correct design of the reduction valves needed for balancing the system. This is also important for optimized design of modulating control valves in the system.

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