I've been attempting to fully understand the valve sizing procedures that Fisher provides for their valves. Where I'm running into some issues is determining the Fp coefficient for scenarios other than where either the valve matches the linesize, or the valve is installed between identical reducers, in a larger pipe size. Historically, we've usually ignored Fp when sizing valves as we don't have full piping details that early in the process.
In the particular scenario I'm looking at, I'm coming from a larger header (4" XS), into a control valve I've originally estimated at 2", and then into some 1-1/2" (0.120" w.t.) tubing. The application is screw compressor oil injection. Because my piping design will have the valve connected directly to a reducing tee from the main line, I've treated this as a standard reducer at 4"x2". Then on the downstream side, I've considered the piping to tubing transition to be equivalent to a 2"x1-1/2" reducer. I was running into issues finding a 2" valve with a large enough Cv, so I was looking at larger valves, but the way the Fp formula works, I've actually getting reduced flow at larger sizes, which doesn't make any sense.
Are there limits on Fp? I'm getting values less than 0.5, which seems quite low, I wouldn't expect that the piping geometry could cut the total flow in half. I'm also wondering how far from the valve I should be considering fittings for affecting the valve sizing. Can I eliminate or reduce the effect of Fp by having some straight pipe at the valve size immediately upstream and downstream of the valve? If so, how much is needed? Or have I made incorrect assumptions by assuming that a reducing flange and tube fittings immediately downstream of the control valve would have a similar effect to a reducer fitting? In that case, how do I determine the effect on valve sizing?
In the particular scenario I'm looking at, I'm coming from a larger header (4" XS), into a control valve I've originally estimated at 2", and then into some 1-1/2" (0.120" w.t.) tubing. The application is screw compressor oil injection. Because my piping design will have the valve connected directly to a reducing tee from the main line, I've treated this as a standard reducer at 4"x2". Then on the downstream side, I've considered the piping to tubing transition to be equivalent to a 2"x1-1/2" reducer. I was running into issues finding a 2" valve with a large enough Cv, so I was looking at larger valves, but the way the Fp formula works, I've actually getting reduced flow at larger sizes, which doesn't make any sense.
Are there limits on Fp? I'm getting values less than 0.5, which seems quite low, I wouldn't expect that the piping geometry could cut the total flow in half. I'm also wondering how far from the valve I should be considering fittings for affecting the valve sizing. Can I eliminate or reduce the effect of Fp by having some straight pipe at the valve size immediately upstream and downstream of the valve? If so, how much is needed? Or have I made incorrect assumptions by assuming that a reducing flange and tube fittings immediately downstream of the control valve would have a similar effect to a reducer fitting? In that case, how do I determine the effect on valve sizing?
