- Regulators
are required to control the downstream pressure in the metering station at one
or more points
- Regulators
are sized for maximum anticipated flow requirement with the minimum inlet
pressure
- Although
many regulators can operate over a wide flow and pressure ranges, often it is
necessary to consider parallel runs to have better control, redundancy and
capacity increase
- Regulators
required to operate nearly closed position over long periods of time will tend
to have more valve and seat damage, than a unit that is sized to have valve
open at least 10%
- A small
regulator can be installed in one line and the larger regulator in the parallel
line to handle larger flows up to the required capacity Adequate working space
should be available for regulator, plug valves maintenance
- Regulators
with an external control line should have sensing point 5 – 10 pipe
diameters. Control line may be.”,1/2” or .” depending upon the types of
regulator and distance from the pressure sensing point to the regulator
- Each regulator
should have a separate sensing tap and control line.
- Sensing tap
should not be installed on the fittings such as expanders, Tees, Elbows etc.
- Continuity
of operation is the most essential consideration. In case of fail to close
regulators, freezing possibilities, it is a good practice to have parallel
regulator runs Pilots require clean and dry operating supply, heating taps,
small filters can be installed on the pilot lines
- For safety
of regulator operation normally, regulators with relief valve or monitoring
regulators are used. For distribution system monitoring operation is preferred
whereas relief valve is used for remote locations in general
- Regulator
by-pass and parallel legs are good for performing routine maintenance
- For fixed
factor applications the droop should be in }1% (accuracy). However, for field tapping up to 10% droop is
available.
Most
control valves are rated with a capacity term called Cv, which is defined as
the number of gallons of water per Minute that will flow through the valve with
1 psi pressure drop across the valve.
Cv = Q / √(ΔP 62.4/6)
Where Q =
quality of water in gpm
ΔP= Pressure drop in psi
Normally Cv
or K values of the regulators are given by the manufacturers and formulae for
calculating the regulator capacity at critical and non-critical flows are
given. The capacity tables can also be used.
If ΔP is less
than 8% of (P1) inlet pressure than use formula:
Qh =
76.99×Cv√(ΔP(P1)) MSCF/Hr … formula - 1
Otherwise
use formula
Qh = 54.5
×Cv √(ΔP(P1+P2)/2) …formula - 2
Capacity
Formula as given by different manufacturers
ROCKWELL
Q = K √(P0(P1-P0))
for P1/P0 < 1.894
Q = K P1/2
for P1/P0 > 1.894
K factor
for various orifices
|
Orifice
|
Single Port
|
Double Port
|
|
1/8”
|
¼”
|
3/8”
|
½”
|
3/4”
|
1”
|
1”
|
1 ½”
|
1 ¾”
|
1/8”
|
3”
|
|
K
|
33
|
132
|
292
|
520
|
850
|
1300
|
2000
|
4270
|
5450
|
8880
|
17740
|
Example:
P1 =
Minimum inlet pressure = 100psia
P0 = outlet
pressure = 60 psia
Capacity =
200,000 SCF/Hr
P1/P0 =
100/6- = 1.66 <1.894 use formula – 1
200,000 = K
√(60(100 – 60))
K = 4081,
so from the above table we can select the orifice size of 1 .” having
K=4270 for monitoring total capacity of both regulators is normally taken as
70% of the capacity of a single regulator. So 2” dia regulator with an orifice size
of 1 .” will be selected
Find Q for
K=4081
Q=4270√(60(100-60))
= 209,230 SCF/Hr
If monitoring
is required,
calculated
K=4081 will become = 5830
Which means
now we need an orifice size of 2 1/8” dia. Which is available in 3” dia as
RW-441-57S regulator. Its MAOP is 175 psig which can handle inlet pressure of
100 psig
FISHER REGULATORS
• Capacities can be calculated / regulator selected from the software
developed by them
• Capacity tables can be consulted
• Formulas
(i) Q = P1(abs)
(Cg) (1.29) when P0/P1 ≤ 0.5
(ii) Q = √(520/GT)
Cg Sin (3417/C1 √Δp/P1) DEG When
P0/P1 >0.5
where
P1 = Inlet
Pressure
P0 = Outlet
Pressure
C1 = Cg / Cv
, Cg = Gas Sizing Co-efficient, See tables from Fisher catalog for 399
regulators
Important
Considerations
A regulator
is usually capable of having more than one orifice size. MAOP of the regulator
defines the maximum operating pressure of the regulator body, but pressure
rating for different orifices may be less than MAOP. So great care should be
taken for the selection of orifice for a particular orifice size, otherwise regulator
would not provide tight lock-up.
