What is pressurisation unit used in HVAC system of Building system and Offshore platforms

Purpose of the pressurisation unit in building services is to:

1. To keep pressure in pipes above atmospheric at a point where static pressure is at lowest point so that we do not have air infiltration

2. To allow for water expansion due to temperature change

3. To prevent cavitation at valves (this is rarely an issue in chilled water systems)

If we are talking about pressurisation unit for domestic water pumps than another purpose would be to prevent excessive number of pump (motor) starts