Effective commissioning for variable-volume systems

Commissioning FCU
Embracing change in commissioning variable-flow systems — Martin Lowe.
Is proportional balancing still the best approach to commissioning variable volume systems? Martin Lowe, technical manager at Marflow Hydronics, suggests a faster and more practical method.Variable-volume fan-coil and chilled-beam systems continue to increase in popularity, but, unfortunately, many of these systems are still commissioned using methods that were designed for constant-volume systems. A single-station balancing method that takes full advantage of the new valve technologies used in variable volume systems has the potential to balance the system more accurately and troubleshoot the installation — as well as saving a great deal of time. Many variable-volume systems combine 2-port valves on the fan-coils with a balancing valve and differential-pressure control valve (DPCV) serving each zone of fan coils. Although this method is effective, it incurs high capital costs and carries a heavy commissioning burden. An increasingly popular alternative is the pressure-independent (PI), dynamic flow-control valve. PI valves combine a DPCV, a 2-port valve and a flow-limiting valve in one assembly, reducing installation work and, most importantly, giving a valve authority of almost one. However, to take full advantage of PI valves we also need to take a fresh look at commissioning them if we are to ensure they will perform efficiently in day-to-day operation, with ports opening and closing continuously. In addition, the system employed should help to speed up the entire commissioning process. The combination of a manifold-based system and the single-station balancing (SSB) technique described below delivers all of these benefits. Manifolds and SSB The practice of using a manifold system rather than installing individual valve assemblies at each terminal unit is now well established and offers a number of widely recognised benefits. Using prefabricated valve assemblies offers a cost-effective quality assured solution, whilst providing fewer points to be visited during the process of commissioning. Even greater advantages are achieved when a manifold system fitted with PI valves is combined with the SSB technique that we are proposing as best practice for the commissioning of variable-volume systems.
EXTRA PICTURE

The benefits of commissioning groups of terminal units such as fan-coil units from a manifold are further enhanced when a manifold fitting with pressure-independent dynamic flow-control valves are combined with single-station balancing.

Faced with conventional proportional balancing, the commissioning engineer needs to balance the entire system before problem circuits can be identified. Once those problems have been resolved, the whole system needs to be balanced again, leading to a lengthy process that can impact on the whole construction schedule. The SSB method, developed by our in-house engineers and used successfully in a number of projects, is different because it uses a ‘subtraction’ technique to identify problem valves. This technique is based on knowing the design flow rates for each valve and, therefore, the total flow rate for that fan-coil group. Assuming all the valves are functioning correctly and bypasses are closed, isolating a group of valves will have a predictable effect on the total flow rate for the remaining valves. Where an unexpected result is observed, it is a simple matter of elimination to quickly identify the valve that causes this — enabling the commissioning engineer to home in on the problem area. Crucially, as well as greatly reducing commissioning time, the SSB method is far more appropriate to the dynamic nature of a variable-volume system as it replicates the day-to-day process of opening and closing valves in a variable-volume system. Bearing witness Clearly, any such system will need to undergo the usual flow-witnessing procedures. When dealing with variable-volume systems we would strongly encourage engineers to adopt a more flexible approach, for the following reasons. To comply with the -0, +10% tolerances stated in CIBSE Code W (which was written before PI valves were developed), it is common practice to play safe and design the system to run at 10% higher than should be required. However, most PI valves have tolerances of ±10%, so there is a slight chance that the system could end up running at 120% of flow rate if all the valves were at the +10% end of the tolerance band. In all likelihood, the pump would not cope efficiently with such oversizing. This is clearly a worst-case scenario, and the chances of all ports being fully open at the same time in a variable-volume system (except during conventional commissioning) are remote. Consequently, we would suggest a more flexible solution for variable-volume systems. For instance, if a system were designed at 105% flow rate, these potential problems would be eliminated. However, in the highly unlikely event of all of the manufacturers’ tolerances being -10% on every valve in the system, flow rates could fall to 95% of design flow rate. Such a small shortfall in flow rate would not make any appreciable difference to the performance of the system — and there is hardly any chance of it happening in real life situations. So, while there are very good reasons for the increasing popularity of variable-volume systems, their full benefits will only be realised with a more open-minded approach. This requires a combination of innovative valves and commissioning methods — backed by specialist expertise to put it into practice. Martin Lowe is technical manager with Marflow Hydronics.
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