Pressure holding is a system for maintaining a constant static pressure is used in heating and cooling systems to order to ensure uninterrupted operation. This can be achieved in more ways than one.
In smaller boiler-supplied heating systems, constant static pressure can be achieved with an open expansion vessel. This vessel is placed at the highest point in the system. A static pressure corresponding to the physical height is thus created.
In smaller closed systems, constant static pressure can be achieved with a pre-pressurised expansion vessel. Using a pre-pressurised expansion vessel in larger systems would be inconvenient due to large size necessary.
Therefore, pressure holding systems are used in larger systems. These can contain a pressure expansion vessel, with an attached compressor or a pressure holding pump. The compressor or pump automatically ensures that the correct static pressure is maintained in the system by regulating vessel pressure.
A pump-based pressure holding system is comprised of one or more pumps connected in series. The pumps can be fitted with a speed regulator, e.g. frequency converter, to maintain correct pressure within the system. In this case a pressure expansion tank is not necessary because the pumps themselves are able to regulate the pressure.
Pressure holding systems using speed-regulated pumps are superior to systems that employ expansion vessels. In the latter case, a constant-speed pump that maintains a constant flow can give rise to pressure surges when the pump starts and stops. Furthermore, a pressure holding system requires less physical space since there is no expansion vessel.
A pressure expansion tank combined with a pressure holding pump can be used to remove air from process water. A small volume of water is filled into the container, which is not pressurised. When the pressure falls to atmospheric pressure, most of the air is released. A similar volume of water is pumped back into the system by a pressure holding pump to maintain the desired pressure in the system.
The Grundfos circulator pump range, with or without variable speed control, are ideal as pressure holding pumps.