“A new way of commissioning?”
Constant volume vs variable volume
Constant volume systems dominated hydronic fluid distribution for years. Their main characteristic was based on keeping the water mass flow rate constant during the lifetime of the system using diverting three port valves and by-passes limiting the flow to terminal unit coils when sensing devices, like thermostats, reached the temperature set limit.
Since a continuous flow of water requires significant use of electrical energy, the industry slowly introduced the concept of variable volume systems, where a variable speed pump can reduce the rotation of the impeller according to system pressure demands. Varying the system pressure saves large amounts of pumping energy over time and the adoption of variable volume systems almost totally replaced traditional constant volume designs.
This also meant that consulting engineers were faced with new challenges, as variable differential pressures across different points of the distribution pipe network would create imbalances during commissioning, therefore hindering measurements and correct settings of design flow rates.
Differential Pressure Control valves
The concept of pressure controllers or pressure equalisers was already well known in the Danish industry after the oil crisis of the Seventies, when new technical solutions were needed for the early Danish district heatingnetworks, which had been designed with large differential pressures feeding entire towns and city districts.
A new range of valves was therefore designed -with the Danish company Danfoss in the front line the Differential Pressure Control Valves. DPCVs are as simple as they are ingenious.
A diaphragm senses any change in the upstream differential pressure and transmits the resultant force to a spring, carefully calibrated to the valve orifice. The spring oscillation assures a constant flow of downstream water, independent of pressure oscillations, allowing precise commissioning operations in a variable volume branch.
Nowadays probably more than 95% of hydronic systems in the UK are based on variable speed pumping and their design and characteristics are largely understood by the industry. DPCV valves are still the most accurate means of controlling variable differential pressure and energy and carbon emissions savings gain more importance every day.
Design and commissioning criteria have changed and engineers and technicians are continuously working to improve system performance during and after commissioning.
Branches and circuits in the pipeline are virtually kept pressure-independent by the modulating action of DPCVs and their set pressure is calculated by design, according to the total resistance of the branches they are installed.
Good quality DPCVs can modulate the downstream differential pressure quite precisely and the design flow rate is being kept constant for commissioning purposes. Because of the characteristic of variable volume systems though, the commissioning operations – and consequently the measured mass flow rates – might change slightly , as the system pressure is not constant.
We can assume an almost continuous variable pressure in the system and the DPCV will react to pressure variations through its spring oscillation. A spring is defined as a basic mechanical system where the oscillation from its rest position is called amplitude or the distance the spring can oscillate when under the influence of a force. In hydronic systems this spring amplitude is conventionally called p-band or proportional band. On rising differential pressure, the spring will tend to be compressed by the diaphragm and will oscillate around its centre. On decreasing pressure, the spring will tend to position itself back to its original configuration.
DPCV springs are calibrated to equalise pressure in kPa and p-band values can be found in the range of 10-40kPa (-5, +5kPa, -20, +20kPa), where the lower the value, the more precise is the valve modulation.
Good performing DPCVs have a proportional band no higher than 10kPa, usually less, about 2-8kPa.
From the definition of p-band we can observe that mass flow rates will change according to the oscillation of the spring and particularly if the valve spring is compressed or released to equilibrium.
What is important is that the average mass flow rate will be kept constant through the dynamic oscillations of the spring. Therefore the terminal branch return flow temperature will also be kept constant, delivering high energy savings.
This aspect of dealing with variable systems and pressure controllers is still not very well known and commissioning operations should be performed with the distinction between increasing pressure and lowering pressure. This double side commissioning can lead to different results and this is a relatively new aspect of variable systems.
Traditional commissioning methods and tolerances might not completely apply to devices designed to deal with variable volume because instead of a constant flow we have a dynamic system with fluctuating differential pressure, and the mass flow rate we might set on a balancing valve will be slightly different when the system characteristics will change.
In these images we can see the measurement of pressure fluctuation or “proportional band”.
Danfoss DPCV valves are the core of our FloCon Commissioning Modules. We have more than 8,000 modules installed in the UK alone.