Flow Coefficients and Computations

1 The pipe size in the system ordinarily will determine the valve size. However, to assure accurate throttling or positioning, it is advisable to calculate the valve size. Formulas for liquid and gas are as follows:Liquid Flow Formula* Cv = Qa sg / PQa = Cv P/sg P = sg(Qa/Cv) Gas Flow Formula* Cv = Q/1360 sg(T)/ P 2/(P1+P2)Q = 1360 Cv P/sg(T) (P1+P2)/2 P = P1 P1 (sg T)(Q/[963 Cv]) * Fluid Controls Institute Inc Standard FCI 62-1 ** SCFH (standard cubic foot per hour) of gas is measured at 60 F ( ) and psia. CFH (cubic foot per hour) is measured at any temperature and of CFH to SCFH SCFH = ( ) ( R/Tactual) CFHactualNote: The design of the Straightway Valves are not conducive to good throttling In general, any reduction in outlet pressure below half the absolute inlet pressure will give no further increase in flow.

2 The value of the ratio of pressure at which maximum flow is obtained varies somewhat depending on the actual 71 0 09/15 Dia-Flo Diaphragm ValvesFlow Coefficients and ComputationsWhere: Cv = Flow Coefficient (gpm/ P) sg = Specific Gravity Qa = Actual Flow (gpm) P = Actual Pressure Drop P1 P2 (psi)Where: Q = Volumetric flow (SCFH)** sg = Specific gravity T = Absolute Flowing Temperature ( F+460) P1 = Inlet Pressure (psia) P2 = Outlet pressure (psia) P = Pressure drop (P1 P2) Cv = Valve Coefficient from TablesWhere: SCFH = Standard cubic feet per hour Pactual = Pressure of gas in psia Tactual = Temperature of gas ( F+460)Sec 5: Tech DataDia-Flo Diaphragm Valves 5 72 0 09/15 Example Flow Computations (flow at pressure drop of 1 psi)Weir Valves Problem: Find the rate of flow of water through a 1 " unlined cast iron flanged valve, half open, with a pressure drop of one : From the Flanged End Unlined Weir Cv Table, the corresponding rate of flow is 42 : Find the valve position of 2 " Glass Lined valve table , with a water flow of 170 gpm and a pressure drop of 1.

3 From the Glass Lined valve table the corresponding valve position is 70% : Determine the flow in cubic feet per hour of air through a wide open 2 inch unlined valve. Inlet pressure at 60 psig, outlet pressure at 40 psig, and temperature at 60 = (1360) (70) 20/520 ( + )/2 Q = 150,400 SCFHP roblem: Find the rate of flow of water through a 2 " soft rubber lined valve, half open with a pressure drop of 3 : From Flanged End Unlined Weir Cv TableCv = 85 gpm Qa = 85 3 Qa = 147 gpmStraightway Valves Problem: Find the rate of flow of water through a 1 " unlined flanged valve, half open, with a pressure drop of 1 : From Cv Table, the corresponding rate of flow is 72.

4 Find the valve position of a 2 " rubber lined valve, with a water flow of 285 gpm and a pressure drop of 1 : From Cv Table, the corresponding valve position is 70% : Find valve size and valve position of an unlined valve, with flow at 200 gpm and an actual pressure drop of 5 psi for a liquid with a specific gravity of : Cv =200 = 200 .36 = (200)( ) = 120 From Cv Table: A 2" valve has a Cv of 120 at 40% Determine the flow in cubic feet per hour of air through a wide open 2" unlined valve. Inlet pressure at 60 psig, outlet pressure at 40 psig, and temperature at 60 : From Cv Table: Cv = 275 Q = 1360 (275) 20/([1][520]) ( + )/2 = 374,000.

5 04 = 374,000 Q = 602,140 Standard Cubic Feet per HourProblem: Find the rate of flow of water through a 2 " #5 rubber lined valve, full open, with a pressure drop of 3 : From Cv Table, Cv = 365 Qa = 365 3 or 632 gpmProblem: Find the pressure drop across a 1 " plastic lined valve 100% open with water flow of 63 gpmSolution From Cv Table, Cv = 80 P = (63/80) = psiFlow Coefficientsand ComputationsSec 5: Tech DataFluid velocity is a very important design consideration when selecting diaphragm valves. As mentioned previously velocity should be limited to 15 20 fps for clean fluids and 8 10 fps for slurries.

6 Velocity through a Dia-Flo weir type diaphragm valve can be determined by using the following equation:Area Over the Weir for Standard Weir Valves(Square Inches)NEPO %evlaVSize1020304050607080901001 *. ,11 Over the Weir for DualRange Valves(Square Inches)NEPO %evlaVSize1020304050607080901001"*. ". ". ". ". ". " * Includes all 3/4" flanged valves except solid plasticV = .321 (Q/A)Where: V = Velocity in feet per second Q = Flow in gallons per minute A = Area in square inches at the point of greatest restriction (from table below)5 73 0 09/15 Dia-Flo Diaphragm ValvesFlow Coefficients and ComputationsSec 5: Tech DataDia-Flo Diaphragm Valves 5 74 0 09/15Cv rating Rate of flow depends upon the pressure drop.

7 Cv is the amount of flow in gallons per minute through a valve which results in a 1 psi pressure drop. Throttling characteristics are shown in the same manner with Cv s at various openings. Use formula shown below to compute actual flow at various pressure Flow (gpm) = Cv P/sgWeir Valve Cv Ratings%Flanged End PF A Linedopen1" "2"3 "4"6" End Soft Rubber Linedopen1/23/4 1 11/4& 11/2221 End Unlinedopen1/23/4 111/4& 11/2221 End Plastic Lined (except PF A)open3/4 111/4& 11/2221/23468103510174060105390205152340 7012026560030725376110017040074040831508 2120210505830509366594140245585900601038 6898150265630960701139699916028067010008 0114069100170285680104090103969100175290 68510601001038671001752856901070%Flanged End Hard Rubber Linedopen1/23/4 111/4& 11/2221 End PF A Linedopen1" "2"3 "4"6" End Soft Rubber Linedopen1/23/4 1 11/4& 11/2221 End Unlinedopen1/23/4 111/4& 11/2221 End Plastic Lined (except PF A)

8 Open3/4 111/4& 11/2221/23468103510174060105390205152340 7012026560030725376110017040074040831508 2120210505830509366594140245585900601038 6898150265630960701139699916028067010008 0114069100170285680104090103969100175290 68510601001038671001752856901070%Flanged End Hard Rubber Linedopen1/23/4 111/4& 11/2221 Coefficientsand ComputationsSec 5: Tech Data%Butt Weldopen1/2"3/4"1"11/2"2"21/2"3"4"6*8* Linedopen1/23/4 111/4& 11/2221 End Metalopen1/23/4111/4& 11/2221 Plasticopen1/23/4111/411/223"4" ratings applying to screwed end metal and flanged unlined valves are based on use of cast iron bodies.

9 For socket weld metal bodies, use Buttweld Cv Valve Cv Ratings5 75 0 09/15 Dia-Flo Diaphragm ValvesFlow Coefficients and ComputationsSec 5: Tech DataDia-Flo Diaphragm Valves 5 76 0 09/15 Dualrange Control Valves Cv Ratings%Flanged Unlinedopen3/4 111/2221 Soft Rubber Linedopen3/4 111/2221 Hd Rubber Linedopen3/4 111/2221 Glass Linedopen3/4 111/2221 Plastic Lined (except PF A)open3/4 111/2221 End PF A Linedopen1" "2"3"4"6" Unlinedopen3/4 111/2221 Soft Rubber Linedopen3/4 111/2221 Hd Rubber Linedopen3/4 111/2221 Glass Linedopen3/4 111/2221 Plastic Lined (except PF A)open3/4 111/2221 End PF A Linedopen1" "2"3"4"6" Coefficientsand ComputationsSec 5.

10 Tech DataStraightway Valve Cv Ratings%Flanged End Unlinedopen111/2221/23468101210101530607 5852503504504502019306011513516545070010 5010503026459016018524070010302000200040 3260120205230320950140028002800503872150 2402704001150175033503350604480180265295 4801400205035503550704884210285310560165 0235036503650805287235300335625185027003 9003900905697260350390670205033004300430 0100601152754505257002250425050005000%Fl anged Plastic Linedopen111/223468** #5*open111/22101516222024264030283690403 2481355034591506036641507038661558040691 659041731901004279220%Flanged Rubber Linedopen21/2346810121060659010035055055 0201101251852757001150115030155190255550 1050170017004019023531082514002250225050 2152703509501750280028006023529041510002 1503100310070245315525105025003200320080 2603506451100287533003300902853906851300 3200365036501003654607001800350048504850 %FLG #10*open111 *Flanged #10 = hard natural rubber lining.