Multiple valves flow Coefficient Cg
Multiple valves flow Coefficient Cg
(OP)
Hi,
I am trying to depressure a plant piping system where we have multiple valves to depressure. i have divided the plant in various sections based on check valve locations and calculated the depressuring time for each section. now I have been told that if we remove all the check valves and let say the whole system is at the same pressure of 800 psig and we open all the blowdown valves at the same time how long it will take to blowdown the whole piping system. now i am struggling with this calculation. i am trying to calculate an equivalent Coefficient for all the blowdown valves (there are three of them) and then use that coefficient to calculate the blowdown time. the formula i am using is 1/Ce^2 = n (1/Cg1^2 + 1/Cg2^...). anyone knows if this formula is can be used to calculate the so called equivalent flow coefficient for valves? is this approach correct or should i use anyother way to calculate that. it is similar to situation where for example we have multiple holes in a vessel or pipe and how long will it take for the pipe or vessel to reach to a lower pressure.
Thanks,
I am trying to depressure a plant piping system where we have multiple valves to depressure. i have divided the plant in various sections based on check valve locations and calculated the depressuring time for each section. now I have been told that if we remove all the check valves and let say the whole system is at the same pressure of 800 psig and we open all the blowdown valves at the same time how long it will take to blowdown the whole piping system. now i am struggling with this calculation. i am trying to calculate an equivalent Coefficient for all the blowdown valves (there are three of them) and then use that coefficient to calculate the blowdown time. the formula i am using is 1/Ce^2 = n (1/Cg1^2 + 1/Cg2^...). anyone knows if this formula is can be used to calculate the so called equivalent flow coefficient for valves? is this approach correct or should i use anyother way to calculate that. it is similar to situation where for example we have multiple holes in a vessel or pipe and how long will it take for the pipe or vessel to reach to a lower pressure.
Thanks,





RE: Multiple valves flow Coefficient Cg
**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/
RE: Multiple valves flow Coefficient Cg
I'll have to think about this to remember how this was handled many years ago.
RE: Multiple valves flow Coefficient Cg
a) pick an initial pressure of the upstream reservoir, Pr.
b) assume a max wide open flowrate (Wn) thru the multiple valves.
c) working backwards from the exit nozzle to atmosphere, calculate what the pressure must be at the nozzle for the assumed flowrate.
d)Then calculate the pressure drop from the exit of the final valveto this nozzle, to determine the pressure at the exit of the final valve.
e) using that final valve's otulet pressure, Cv, Cd, Xt, etc calculate the minimum pressure upstream of the final valve that would pass the assumed flowrate.
f) likewise calculate the minimum pressure upstream of all preceeding valves.
g)If the pressure upstream of the first valve ( Pu1) does not match the known pressure of the upstream reservoir( item a, above), then correct the next iteration as W,n+1 = Wn * Pr/Pu1.
Once the iterations converge on a correct upstream pressure , then calcualte how much the upstream reservoir pressure would drop in ( 10 seconds) at this known flowrate. Revise the reservoir pressure accodingly and obtain a plot of reserovir pressure vs time and valve system flowarate vs time.