All Valve Industries , providers of process valves for various industries and applications, discuss the benefits of using a fixed orifice style double regulating valve.
The Two Types of Double Regulating Valves: The installation of double regulating valves at appropriate points in the system is a basic requirement for effective system regulation. Purpose manufactured double regulating valves should always be specified; gate type isolating valves or full bore ball valves are not suitable for regulation. There are 2 main types of manual balancing valves on the Australian market: fixed and variable orifice valves.
The fixed orifice double regulating valve is a double regulating valve that utilises a separate fixed orifice for flow measurement and designed specifically to overcome the accuracy problems associated with flow measurements across variable orifice valves. The determination of the flow rate is obtained by reference to a single line pressure differential/flow rate chart. The regulating valve is usually an oblique pattern globe valve for smaller sizes or a butterfly type for larger pipe diameters.
Variable orifice double regulating valves differ from fixed orifice double regulating valves where they use the pressure drop across the valve opening for flow measurement. Hence the term variable orifice; for these valves there is a variable opening across which pressure differential is measured. The size of the opening varies depending on the valve setting.
Benefits of Fixed Orifice versus Variable Orifice: For a fixed orifice valve, as implied by the name, the orifice does not change. There is therefore a single kvs value for the flow measurement device and hence a single line graph of flow rate versus pressure drop. Because the orifice is fixed and has no moving parts, its accuracy can be guaranteed at any degree of valve closure. The accuracy of a fixed orifice device can be maintained no matter what the setting of the attached double regulating valve. For a variable orifice valve the valve setting and orifice dimension will vary so that a different kvs value is required for each valve setting. As the valve closes the area open to flow becomes very small and as a result, some variable orifice valves exhibit gradually deteriorating flow measurement accuracy as the valve is closed.
Most heating and cooling systems will contain circulating particles of material which entered the system during installation. For a fixed orifice valve, such material is unlikely to block the circular opening of the orifice. Orifice sizes for the smallest valves are at least 4mm in diameter, which is enough to allow most debris through. For a variable orifice valve that is partially closed, dirt can accumulate around the plug of the valve causing the valve resistance to increase.
Small diameter valves that are partially closed (50% or less) may have openings of less than 0.5mm around the valve plug, which make them very susceptible to blockage. If a valve is partially blocked then a higher than normal pressure drop signal will be indicated across its tappings, suggesting to the user that the flow rate through the valve is much higher than it actually is.
An example of why flow measurement accuracy is important: The importance of flow measurement accuracy is sometimes ignored on the grounds that many types of heating or cooling emitters are relatively insensitive to flow rate variations. However, inaccurate flow measurement has a significant impact on plant sizing and energy consumption, as can easily be demonstrated. Suppose that the minimum acceptable cooling heat transfer for a particular situation was 10 kW and that anything less than this would lead to under-cooling of the room. This kW load at a 6 deg DT gives a flow rate of 0.40 l/s. Hence, any flow rate less than 0.40 l/s could result in under-cooling.
If, when it comes to measuring this flow, the accuracy of measurement is potentially minus 20% (as could be the case using a partially blocked variable orifice valve), then a safety margin must be added to the original load and flow rate to allow for this. It can be calculated by simple arithmetic that one would need to allow a safety margin of plus 25% on flow rate and kW output, in order to compensate for a minus 20% flow measurement accuracy. Hence for the above situation, the installed capacity would need to be 12.5kW with a consequent flow rate of 0.5 l/s.
It can be concluded that in the past, many systems have been designed with significant safety margins that have allowed successful system performance even though flow rates were not accurately measured and set.
However, this has been at the expense of over-sized heating and cooling plant and consequent wastage of energy. In view of the tightening requirements on installed cost and energy consumption, it can no longer be assumed that heating and cooling loads will incorporate healthy safety margins, which can be used to offset the consequences of inaccurate flow measurement. There is now a clear need for flow balancing and measurement devices that give as high accuracy as possible, and for a procedure that ensures that an accurate balance is achieved. In this respect, fixed orifice regulating valves have many advantages over variable orifice regulating valves.