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Inside Insights Is Bigger Better for Control Valves? Right-sizing combines art, science, experience Steve Tredinnick, PE, Infrastructure Project Engineer/Manager, Affiliated Engineers Inc. Editor’s Note: : “Inside Insights” is a column designed to address ongoing issues of interest to building owners, managers and operating engineers who use district energy services. I , for one, will never complain about a TV that is too large, a stomach that is too flat, a bank account that is too great, a car that is too fast, or having too much vacation time! Yeah, baby! I am not all that different from any other American – we want it all, and we want it big! But having the biggest and ‘baddest’ is not always the best alternative. Just like having too much of a good thing is not always good, having something too big may not be appropriate in all situations. In the fourth quarter 2006 edition of this column, I addressed the size of decouplers in primary/secondary chilled-water pumping systems and how some installation/design deficiencies benefit from ‘supersizing.’ Can this philosophy be applied to other devices in a hydronic system such as control valves? Is bigger better all the time? To explore the answers to these questions, let’s start by defining the main parameters required to correctly determine the size of a hydronic control valve: ● Flow rate (i.e., load) ● Pressure drop (coil, branch piping, valve authority component) ● Pipe size Rangeability (minimum to maximum flow or turn-down) ● Valve-sizing coefficient (C ) v ● Shut-off pressure ● Actuator selection ● Valve body-type configuration (two-way or three-way) ● Valve type (ball, globe, butterfly, etc.) ● Control type – two position or throttling ● Flow characteristics (linear, equal percentage, etc.) There are other valve selection parameters such as allowable noise/cavitation, materials of construction, temperature and pressure ratings, connection types, allowable leakage, normally open/closed, etc., but these do not really affect the performance of the valve. Correct flow and pressure-drop calculations are the two key selection parameters for pressure-dependent valves. ● Evaluating Flow and Pressure Drop The flow rate through the valve and coil are usually sized for the peak-load condition on the hottest or coldest day – an event that occurs less than 1 percent of the total heating/cooling hours in a year (based on ASHRAE climatic data) – so a little oversizing occurs here, since one does not want to undersize the coil. However, grossly overestimating the loads will oversize the control valve. Reprinted from First Quarter 2007 District Energy magazine with permission of IDEA. The pressure drop is comprised of the friction losses of the components within the piping circuit, including the coil, valving, pipe and fittings. Without getting in too much detail, this typically amounts to 30 percent to 50 percent of the total system pressure drop at design conditions. To a certain extent, the temperature control system may correct for errors in estimated pressure losses by intelligently modulating the control valve; however, if the pressure drops are grossly overestimated, the result will be an oversized valve. If a valve is oversized, a small increment of valve movement varies the flow greatly; hence, the valve is always modulating or hunting trying to satisfy a setpoint that is constantly being over- and undershot. It’s just like trying to drink from a fire hose: It is just too much flow to handle, and it’s very hard to control properly. Uncontrolled and unstable flow will often lead to occupant discomfort and system inefficiency... The constant modulation will lead to system control instability. Uncontrolled and unstable flow will often lead to occupant discomfort and system inefficiency (low Delta T). In addition, at low loads, the valve barely lifts off its seat. That creates a high-velocity fluid stream that produces noise from the turbulence and wears a thin channel or groove into the valve seat, reducing its life. The groove becomes a ‘leak’ in the system even when the valve is closed, which results in a wire-drawn valve. Such traditional methods of selecting a valve use the valve-sizing coefficient (Cv). A Cv is a unit-less parameter that is calculated from the flow and differential pressure duty of the valve. (See equation below.) Cv is used in manufacturers’ valve charts to more accurately select a valve. For water at 60 degrees F, Cv = GPM x √1/∆P Not only does control valve oversizing affect system performance, but actuator sizing does as well. The actuator should be sized to close against system pressure greater First Quarter 2007 53 than the distribution system pump head. If the actuator cannot close the valve against the system pressure, then the valve will be lifted of its seat and create a ‘leak’ in the system. System leaks resulting from oversized valves and undersized actuators are probably the largest contributor to low Delta T. Further, excessive modulation can lead to premature actuator failure and valve-seal failure, creating maintenance issues. A Look at Other Valve Types The above discussion pertains specifically to pressure-dependent valves – probably the most prevalent valve type used. The selection process may sound complicated, but it is a mixture of art, science and experience. Other valve types may be easier to select and specify. Pressure-independent control valves (PICVs) are being used increasingly in the HVAC industry. PICVs are selected using a much simpler method based on the maximum flow rate expected in the coil and not on differential pressure or valve-sizing coefficient. A PICV does not allow a change in flow rate when 54 District Energy the pressure differential across the valve changes. Although not a panacea for all applications, the PICV’s use has improved the Delta T performance of many water systems and does not require any balancing. Traditionally, throttling valves are one size smaller than the pipe size serving the branch piping, and a line-sized valve may be oversized depending on its duty. However, just because a valve is line-sized does not mean that it is oversized. Line-sized valves are acceptable in some applications, such as two-position valves (e.g., actuated isolation valves) where lower system pressure drop is required. Line-sized valves are usually not acceptable in throttling applications. Right-Sizing Is Best Since it regulates the system’s flow and provides comfort to building occupants, the control valve is probably the single-most important element in a hydronic system. A valve must be ‘right-sized’ for its specific application: A valve that is too small will not have sufficient capacity, and a valve that is too large will cause instability issues and add cost. A successful control-valve selection procedure that follows acceptable engineering practice and manufacturer’s recommendations will help ensure that occupant comfort and efficient operation are achieved. Right-sizing wins every time! Steve Tredinnick, PE, , is a project engineer/manager for Affiliated Engineers in Madison, Wis., with more than 20 years’ experience related to building HVAC systems. The past 10 years of his work have been focused on district energy systems. Tredinnick is a graduate of Pennsylvania State University with a degree in architectural engineering. He is a member of IDEA and ASHRAE and is currently chair of ASHRAE TC 6.2 District Energy. Tredinnick may be reached at stredinnick@ aeieng.com. Column and previous columns available at www.districtenergy.org/de_magazine.htm Reprinted from First Quarter 2007 District Energy magazine with permission of IDEA.