The viscosity to which you are correcting must be the viscosity at the operating temperature, not the rating at 100☏. To adjust from one SSU to another, from 100 to 150 SSU for example, take the ratio between the two factors:Ģ9 ÷ 19 = 1.53 increase in flow resistance The Cv may be adjusted by division: New Cv = 5.4 ÷ 29 = 0.186. The flow resistance will be 29 times greater on 150 SSU. Find the corrected Cv for a viscosity of 150 SSU. Factors in the third column may be used as dividers to convert a water Cv rating into a corrected Cv at higher viscosities, or may be used as multipliers to find the increase in flow resistance when using a more viscous fluid.Įxample: A valve has a published Cv of 5.4 on 60☏ water. Table 2 was prepared for conversion from water, which has a viscosity of 1.12 centistokes at 60☏, to fluids of higher viscosity. If valve manufacturer gives the Cv for water flow, fluids with higher viscosity will have higher resistance to flow in proportion to their viscosity, as related to the viscosity of water. TABLE 2įlow resistance is directly proportional to centistoke viscosity. Since both the viscosity and specific gravity of a fluid affect the pressure drop through a valve orifice, corrections must be made for all other fluids. If no definite fluid is specified, it can be assumed that the Cv rating is for water flow. The pressure drop in relation to the GPM flow may then be determined directly from Table 1.
However, if the Cv rating is specifically stated as for a certain fluid and viscosity, this means that adjustments have already been made. If the Cv is stated in terms of water flow, it must be corrected for viscosity and specific gravity of other fluids. The Cv rating published by the valve manufacturer is used for this determination. The most common usage of the Cv flow coefficient is to predict the pressure loss to be expected across a valve while fluid is flowing through it. Certain approximations in the formula may cause the results to vary de to pressure conditions, fluids, or valve configurations.
Information in this data sheet is based on a flow equation published by the Fluid Controls Institute. Find the pressure drop at a flow of 16 GPM at the same Cv = 2.20.įor values of Cv not listed in the table, use this formula for a flow of 1 GPM. Find the pressure drop opposite your corrected Cv factor, then multiply this times the square of the flow of 1 GPM. This table plots Cv factors against pressure drop for a flow of 1 GPM through the valve. ( Multiply table values times the square of the actual flow through the valve)īefore using this chart, take the published Cv of your valve and correct it (if necessary) for viscosity of your fluid. Table 1 - PSI Pressure Drops for Cv Flow Coefficients for a Flow of 1 GPM The published Cv coefficient should then be corrected by the user for specific gravity and viscosity of his fluid. These tests are usually conducted with water. The valve manufacturer must determine the Cv coefficients experimentally, by actual test. Fluids with higher viscosity will have a higher pressure drop than water which has a viscosity of about 35 SSU. The viscosity of the fluid will also affect its flow rate through a valve. For example, heavier fluids will have a greater pressure loss through the same valve passage. Fluids with other gravities will flow at different rates. The definition of Cv is based on water, which has a G (specific gravity) = 1.0. To pass 2 GPM of water at the same pressure drop, the valve orifice would have to have a Cv of 2.0, etc. An orifice or valve passage which has a Cv coefficient of 1.00 will pass 1 GPM of water (specific gravity 1.0) with a pressure drop of 1 PSI. gallons per minute of water that will pass through a given orifice area at a pressure drop of 1 PSI. system of units, the Cv coefficient is the number of U.S. Cv flow coefficient ratings have several advantages: they provide a means of comparing the flow capacities of different brands of valves they simplify the job of selecting an adequately sized valve without wasteful oversizing and they allow the designer to predict with reasonable accuracy just how a newly designed system will perform. Some manufacturers publish Cv coefficients for describing the volume of flow which can be put through their valves without exceeding a certain maximum pressure loss. Cv information for compressed air is in Data Sheet 22. In case you have forgotten now-to-use the Cv flow coefficient (flow factor) for selecting valve size, this data sheet will review the use of Cv coefficients for hydraulic fluids.