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T I P S O F T H E T R A D E Spartan Peripheral Devices • telephone: (450) 424-6

T I P S O F T H E T R A D E Spartan Peripheral Devices • telephone: (450) 424-6067 • fax: (450) 424-6071 • E-mail: info@spartan-pd.com • Website: www.spartan-pd.com PERIPHERAL DEVICES 1 SP-V.S.G.-98/09/23-1/12 CONTROL EQUIPMENT AND PERIPHERAL DEVICES FOR EFFICIENT ENERGY MANAGEMENT SIZING SPARTAN WATER VALVES .............................. 2 Calculating Cv ................................................................ 2 Calculating Differential Pressure .................................... 2 Calculating the Flow Through a Valve ........................... 2 Taking Adequate Pressure Drop Across the Control Valve ................................................ 2 Exceptions to a High Differential Pressure ..................... 3 Equal Percentage Plugs or Logarithmic Plugs ............... 3 SIZING SPARTAN STEAM VALVES ............................... 4 Steam is a Unique Medium to Control for it Follows Unusual Laws ................................................................. 4 To Calculate the Cv of a Valve When Using Steam ....... 4 Supersaturated Steam ................................................... 5 Estimating Control Contracts ......................................... 5 Valve Piping ................................................................... 6 TYPICAL BYPASS PIPING .............................................. 7 Three-way Valve Piping ................................................. 7 TYPICAL THREE-WAY PIPING ....................................... 7 Three-way Mixing Valves .................................................. 7 Three-way Diverting Valves .............................................. 7 Constant Flow Systems .................................................... 7 Spartan Four Port Valves .................................................. 8 VALVE SELECTION GUIDE • CONTENTS • Spartan Proportioning Plug Valves ................................... 9 Turndown Ratio ................................................................. 9 Double-seated Valves ....................................................... 9 SELECTING SPARTAN VALVE TOPS ......................... 10 Sizing Pneumatic Tops ................................................ 10 Normally-open Valves .................................................. 10 Normally-closed Valves ................................................ 10 FIGURES 1. Heat Emission vs Flow Characteristics of Typical Hot Water Heating Coil ................................... 3 2. Percent of Full Flow ..................................................... 3 3. Steam Valve Sizing ..................................................... 5 4. Two-way Valve Piping ................................................. 6 5. N.O. and N.C. Valve Bodies ........................................ 6 6. Three-way Valve Piping .............................................. 7 7. Constant Flow Systems ............................................... 7 8. Common Piping Errors ................................................ 8 9. Four-port Valves .......................................................... 8 10. Typical Single- and Double-seated Valves .................. 9 11. Typical Three-way Mixing and Diverting Valves .......... 9 12. Double Seated Valves - Reverse/ Direct Action .......... 9 13. ASHRAE Valve Characteristics Graph ........................ 9 14. View from Side of a Typical Fan Coil Unit ................. 11 T I P S O F T H E T R A D E Specifications believed correct at time of printing; subject to change without notice. 2 SP-V.S.G.-98/09/23-2/12 PERIPHERAL DEVICES CONTROL VALVE SELECTION GUIDE Valve selection is an important part of the design of a control system, and the proper choice will enhance performance while reducing costs. Often, valves are selected too large, impairing performance, requiring stronger operators, and needlessly escalating costs. This section describes how to properly size Spartan valves for best performance and for best value. SIZING SPARTAN WATER VALVES Obviously, the maximum operating pressure of the valve should be first considered. 125#, 225# or 600# class valves may be needed depending upon the maximum pressure the valve will encounter. When choosing this pressure class, allow for the addition of the circulating pump pressure if this pressure will or could be added to the static pressure. Remember that 1 psi is equal to a head of 2.3' and that a building 230' high will have a water pressure at the bottom greater than 100 psig. This can be used as a guide, but it is always best to check with the consulting engineer for the project. CALCULATING Cv The Cv of a Spartan valve is defined as the amount of water at 60°F (15°C) which will flow through it in the wide open position with a differential pressure of 1 psi. A valve with a Cv of 10 will pass 10 gallons per minute with a 1 psi differential across the valve. But the pressure drop does not increase in direct proportion to the flow, rather it increases as the square of the pressure drop. Thus, if the Cv is 10 and the flow is 20 (double the Cv), the differential pressure will be 2 x 2, or 4 psi. KvS The Cv of Spartan valves is published in U.S. gallons, thus all Cv’s are U.S. Cv’s. To convert U.S. Cv to Imperial Cv or to KvS (metric) divide U.S. Cv by 1.2 example: U.S. Cv of 100 is selected as ideal for the application, then a KvS figure for the same valve would be 100/1.2 = 83.3 KvS (we have ignored the change in density of hot water as inconsequential.) 1 U.S. Cv = 0.833 Imp. Cv or 0.833 KvS 1 U.S. gallon = 0.833 Imperial gallons 1 Imp GPM = 1.2 U.S. GPM CALCULATING DIFFERENTIAL PRESSURE To find the pressure drop across a valve: PD = (F/Cv)2 Where PD = differential pressure in psi F = flow in U.S. GPM example: The pressure differential (in psi) across a valve with a Cv of 5.5 and a water flow of 16.5 U.S. GPM would be (16.5/ 5.5)2 – 9 psi. CALCULATING THE FLOW THROUGH A VALVE To find the amount of water able to be passed by a valve: F = Cv x √PD example: The flow through a valve with a Cv of 2.5 and a pressure drop of 4 psi would be 2.5 x √4 = 5 U.S. GPM. TAKING ADEQUATE PRESSURE DROP ACROSS THE CONTROL VALVE When selecting the differential pressure (DP) across the control valve in a standard heating system, remember that it is very difficult to undersize the control valve. The ASHRAE manual describes the effect of undersizing the control valve and the resultant reduction in heat transfer. At 50% flow, 90% heating is still effected. The most common error is oversizing the control valve. Refer to ASHRAE guide chapter 30 (see Chart A) Oversizing your Spartan control valve makes it very difficult for the temperature controller to function well unless it represents a goodly proportion of the total DP of the entire water circulating system (the total differential pressure gen- erated by the circulating pump). As soon as the controller asks for a small amount of heating, the valve opens and passes too much water. Then the controller tries to back off the heating slightly, but a small amount of travel fully shuts the oversized valve. The resultant fluctuation, which is sometimes difficult to correct, is referred to as ‘hunting’. For this reason, general wisdom dictates taking 50% of the circulator DP across the open control valves. Many engi- neers mistakenly hesitate to accept this, but the control contractor should at least not accept less than the DP of the heating coil, otherwise stability of control may be difficult or impossible to achieve. Realize that cutting the flow by half only slows the water down in its flow through the coil. If full flow results in a water temperature drop of 20° (180° in and 160° out) then half flow results in about 180° in and 142° out, or a temperature drop of almost double. The coil face temperature has only dropped from a mean of 170 to a mean of 161, so the heat output is still over 90% even though only half flow. T I P S O F T H E T R A D E Spartan Peripheral Devices • telephone: (450) 424-6067 • fax: (450) 424-6071 • E-mail: info@spartan-pd.com • Website: www.spartan-pd.com PERIPHERAL DEVICES 3 SP-V.S.G.-98/09/23-3/12 Where the two temperatures are closer together (the water temperature and the air temperature, as for example in a cooling coil), then cutting the flow has a greater effect at cutting the heat transfer. Study the Figure 2 showing the heat transfer plotted against water flow for a coil working under the following conditions. Air on at 78°F x 70% RH and off at 60°F at 90% RH. Water on at 48°F and off at 56°F FIGURE 2 Percent of Full Flow Now it is clear that the heat transfer vs water flow is a more linear relationship, and it would seem to be less important to select a valve with a logarithmic plug. This same analogy can be used when throttling the flow of water which has been reduced in temperature from an indoor/outdoor type of control- ler. In these applications, a linear plug could perform as well as a logarithmic plug. All good engineering is a compromise, and Spartan has compromised as follows: • All 2-way valves built for stock will be equipped with loga- rithmic plugs. • All 3-way valves up to 2" will also be equipped with logarith- mic plugs, as they are generally being used for heating coils, etc. • All 3-way valves over 2" will be equipped with linear plugs, for they are generally being used on chilled water coils or on mixing applications as in indoor/outdoor control, etc. Exceptions to these requirements will be handled on a build- to-order basis where delivery problems are of little concern. See Figure 1 which plots the heat output of a coil vs flow. FIGURE 1 - Heat emission vs. flow characteristics of typical hot water heating coil EXCEPTIONS TO A HIGH DIFFERENTIAL PRESSURE Exceptions to taking a large DP when selecting a control valve are as follows: • 2-position valves where it does not matter. • Cooling tower control valves where the full circulating pump head is needed to provide adequate pressure for uploads/Litterature/ valve-selection-guide.pdf