Catalog - Tri-State Technical Sales Corp.

SIZING AND SET PRESSURE CONSIDERATIONS FOR BACKPRESSURE
18
Constant Backpr essur e Sizing Factor (K b
)
(Gases & V apors Only)
In general, backpr essur e has no ef fect on the flow
capacity of the FLOW SAFE F7000/8000 Series of pilo t-
operated valves as long as a critical flow condition exists.
The critical flow pr essur e ratio for a particular gas may be
estimated per API RP 520 by the equation below:
P cf
[
2
]
=
P 1
K + 1
k /(k-1)
k = Specific heat ratio for gas, C p /C v
(see page 20)
P 1
= Upstr eam relieving pr essur e (psia)
P cf
= Critical flow nozzle pr essur e (psia)
P B
= Backpr essur e, psi a
P s
= Set pr essur e, psi a
P o
= O verpr essur e, psi
W hen suf ficient back pr essur e exists to cr eate
a subcritical flow condition, the critical sizing equatio n
shown on Page 16 should be used along with the Backpr
essur e Sizing Factor (K b
) shown in the chart below .
This method is described in detail in API RP520.
Set pr essur es ar e not af f ected by backpr essur e when
using the F300 or F500 pilot valve as it is balanced against
such pr essur e, or the F200 as it is normally vented to
atmosphe re . Caution should be used if the F200 vent is
r outed to the main valve discharge or other pr essurized
vessel, as the set pr essur e and blowdown may be
af fected.
Backpr essur e gr eater than the system inlet pr essur e
normally would cause the main valve piston to open.
Because the F200, F300, and F500 pilot valves ar e equipped
w ith backflow pr eventers as standar d featur es, this
cannot occur on the F7000/8000 valves. The
backpr essur e is r outed to the main valve piston
dome, keeping the valve closed and bubble-tight .
Example: See figur e below .
S et Pr essur e = 100 psig
Overpr essur e = 10 psi
Superimposed backpr essur e = 70 psi g
Built-up backpr essur e = 10 ps ig
Per cent absolute
backpressure =
K b
= 0.87(for k=1.4)
70+10+14. 7
100+10+14. 7
x 100 = 76%
F7000/8000 SERIES