Welcome to the first issue of Spec Check. We initiated this monthly publication to commemorate Masterflow’s 21st year. We hope you find it challenging as well as a useful reminder to review your Master Specification – that is to Spec Check!
Two-Pipe Buffer Tanks (Four-Pipe Hydraulic Decouplers)
This issue is about buffer tanks with two main pipe connections. These are sometimes used as hydraulic decouplers, in which case they will have four main pipe connections.
Increasing system volume and protecting and improving central plant performance
Buffer tanks with two pipes are used in both heating and chilled water circuits specifically to increase the system volume or to provide an additional buffer to protect and improve the performance of the central plant at low load. For example, in chilled water circuits, the chiller manufacturer normally stipulates the minimum system volume should be 3.5L/kw of chiller capacity. This will minimise the chance of cold water causing chiller faults at low load.
Improving thermal performance with connection locations or baffles
The situation is similar for boilers in heating water circuits. The thermal performance of the tank may be improved by considering connection locations (eg top, bottom and opposite sides) or by adding baffles, all of which minimise mixing of temperature layers or streaming.
Increasing volume for low load situations with four pipe connections
The other option is a buffer tank/decoupler with four pipes; the two extra pipe connections can be used to decouple the pumping system to form primary and secondary circuits. The decoupler could have been formed using a header or similar feature but using a buffer tank with four pipe connections has the advantage of increasing the volume for low load situations.
Getting your Spec Correct
Often the main specification has a generic description and a simple symbol is included on the schematic to indicate a variation. It is not until the order is being fulfilled that it is found that additional features should be included, such as baffles or different connection positions. To safeguard against these issues, review your master spec and insert one of more of the following paragraphs.
A [insert volume] Litre Buffer Tank (2 main pipes) is to be installed in the chilled/ heating water circuit. The purpose of this tank is to increase the system volume to protect the plant and improve the performance in low load situations. Ensure there are sufficient internal baffles and configure pipe connections to minimise any streaming/ thermal layering, stratification.
A [insert volume] Litre Buffer Tank/ Decoupler (4 main pipes) is to be installed in the chilled/ heating water circuit. The purpose of this tank is to decouple to primary and secondary circuits and increase the system volume to protect the plant and improve the performance in low load situations.
In addition, each buffer tank should be provided with a drain valve at the bottom and at the top, a pressure relief valve as well as air vents (both a manual and an automatic vent valves).