Striking The Perfect Balance
System balancing - Separating Fact from Fiction.
A well-balanced system starts with good system design. The designer calculates the mass of water required at each terminal unit, whether heating or cooling. Balancing valves create additional resistance in each branch to direct water in the required proportion to each terminal unit.
Design considerations will take into account: the size of space to be conditioned, space usage, occupancy, construction materials and their thermal properties, heat losses and gains and general climatic conditions associated with the building.
Achieving optimum flow rate is essential - too high a flow rate may cause noise, whilst too low a flow rate may not dislodge entrapped air. Flow velocities will generally be in the range 0.75 to 1.15m/s for pipe sizes 1/2" to 2" and 0.75 to 3m/s for size DN65 to DN300 with pipe pressure drop per metre length generally calculated at 140 to 280 kPa.
Fixed orifice regulating valves (FORV) or variable orifice regulating valves (VORV) are an option to consider and system specification will dictate which type is to be used. Fixed orifice will nearly always be specified in the UK, with VORV more common in continental Europe and North America.
It is generally accepted that fixed orifice will provide a more accurate flow rate, usually to a tolerance of plus or minus 5% - compared to plus or minus 10% (or greater) of design flow rate in variable orifice systems.
Separate Metering Stations
Some systems will specify metering stations to be installed separately to the regulating valve. Generally, they are available threaded or wafer style for fitting between flanges and are suitable for chilled water, LTHW and MTHW.
Combined Metering and Regulating
These products are easy and quick to install and commission, and are suitable for chilled water, LTHW and MTHW.
A combined valve and metering station benefits from having a single piece body with no joint between body and metering station. This reduces potential leak points as well as being compact and a lower cost option than a 2-piece commissioning set.
To ensure accurate flow measurement, guidelines for the installation of the valve or metering station will specify the length of uninterrupted pipe required on either side of the valve. This is to avoid turbulence caused by other fittings within the system or change of flow direction, which otherwise may be close to the valve, that could affect the accuracy of the reading.
In this example, 5 pipe diameters of straight pipe upstream and 2 pipe diameters of straight pipe downstream are required, with the 5 diameters upstream being the most important.
General Installation Considerations
Because valves are often installed in ceiling voids it is important that hand-wheels/levers/test points and drains point down for ease of access and commissioning.
Pipework and valves in CHW systems should be insulated and provided with effective vapour seals to minimise condensation and thus prevent damage to ceiling voids.
It is important to ensure that all air is removed from the system as this can seriously affect commissioning results and remember - commissioning should only be carried out when the system is cold.
Albion Valves UK
Tel: 01226 729900
Published in Valve User Magazine Issue 23
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