Page 106 - Applied Process Design for Chemical and Petrochemical Plants Volume I

Fluid Flow 93

Correction Factors1 Factors to consider in evaluating the control valve pres
Volue of C 16017018Ol9Qlroolllolr2olr3oIno suredrop are:
I
Multiplier to Correct Chart j2.57(1.93/1.50(1.221 .01 .84( .71 I .62I.54
k Allowance for increase in friction drop

Establish the ratio of the maximum anticipated flow rate
for system, QM, to the design basis rate, &D or QM/QD.
When QM is not known, nor can it be anticipated, use:
QMM/QD of 1.1 for flow control and 1.25 for level pressure
and temperature control valves to anticipate the flow rate
transients as the control loop recovers from a distur-
bance [9].
At the maximum flow rate QM, the friction drop will
become:

The increase in pressure drop will be:

FD may not necessarily be very accurate at the design stage
where final drawing dimensions for the system are being
Pressure Loss in Feet of Woter per 100 Feet
estimated. For this reason a 10% increase allowance is
Figure 2-24. Friction loss for flow of water in steel pipes. Note c = suggested to Ah-
pipe roughness factor. See Tables 2-9 and 2-22. Courtesy of Carrier
Corp. B. AIlowance for possible falloff in: overall system pres
sure drop, Ps - P,
New flow rate = 110% (G)
If there is an increase in system flow rate
Friction pressure drop will increase to 121% of F,; Overall system pressure drop = PF (all) = 0.05 PS (2-65)

C. Allowance for control valve (base pressure drop at
1.21 (96) = 116 psi = F,
full-open position [9]
Friction increase = 116 - 96 == 20 psi added for rela- This varies with the type and design of valve and can be
tively constant Ps and P, obtained from the manufacturer. It is identified here as
base pressure drop B for the valve itself. Using average
line velocities and assuming that the control valve will be
Available APc = (168 - 37) - 116
APc = 15 pig through th'e control valve, which one pipe size smaller than the pipe line it is connected to,
means that the valve has to open more and using average B values over a range of sizes, the B values
reduce its sensitivity of response, from its for estimating purposes are [9]:
design APc of 35 psig
___.
____- ____ -__.__
Control Valve Type --___ B, psi
For design purposes, the assumed 35 psi for the control Single Plug 11
valve could be used; however, decreasing the pipe friction Double Plug 7
of 6 psi to perhaps % or % by increasing the line size will Cage (unbalanced) 4
help the control of the valve. It would be better to have Cage (balanced) 4
the available valve pressure drop equal to or greater than Butterfly 0.2
the assumed.