14 June 2002
Figure 4-9. Calculation of Total Expansion Tank Volume
Isolation Valves. Gate and ball valves are installed to allow components
or sections of the system to be isolated without draining the entire system. Gate valves
are less expensive than ball valves and will be used in locations where only on/off
operation is required. Ball valves are recommended at locations where partial flow may
be required, such as on the outlet side of the collector banks. These valves are
manually operated and may have a key or special tool to prevent unauthorized
tampering. Care should be taken when locating isolation valves to ensure that system
pressure relief cannot be valved off accidentally. Globe-type valves are not
recommended because they can reduce flow (even when fully open), cause excessive
pressure drop, and reduce system efficiency.
Thumb Valves. Thumb valves also function as on/off valves for smaller
sized tubing (typically 1/4 inch (6 mm) or less). They are used to manually open
pressure gauges or flow indicators to local flow and are not meant for constant use.
Drain Valves. Drain valves are required at all system low points.
Specifically, these locations include the low points of the collector banks, the bottom of
the storage tank, and two at the bottom of the collector loop between the expansion
tank and the pump. These latter two drain valves are used for filling and draining and
should be separated by a gate valve. When the system is to be filled, the gate is closed
and a pump is connected to one of the drains. As the propylene glycol solution is
pumped into the system, the other open drain allows air to escape. When filling is
complete, both drains are closed and the gate between them is opened.
Check Valves. A spring-type check valve should be located in the system
between the pump and the collector array, on the supply side. This check valve