Bulletin 71.2:CRONOSMarch 2002

Type Cronos Pressure Reducing Regulator

E0838

Figure 1. Type Cronos Regulator

Features! Control Accuracy—Keeps constant inlet pressures to downstream equipment by accurately controlling system pressures at widely verying flow rates andsupply pressures for maximum efficiency and best
operation.

! Easy Startup—Equal inlet and outlet pressure
ratings allows easier startup and shutdown.

! Versatility—A wide variety of applications such as
medium to high pressure natural gas transmission
distribution systems, power plants, monitors, and sourgas.

! High Capacity—Available in 1, 2, or 3-inch (DN25, 50, or 100) body sizes, the body design and flow
passage allows exceptionally high capacities with amaximum Cg of 5100.

! Easy Adjustment—Easy adjustment of the outletpressure by simple adjustment of the pilot.

! Quiet Operation—Optional noise abatement trim
is available for up to 20 to 30 dB(A) reduction.

! Tight Shutoff—A combination of soft seat andmetal plug with a knife edge provides a long lasting
reliable shutoff in normal and erosive service.

! In-Service Travel Indicator—The travel indicatorwith protective cover, responds to the precise movementof the diaphragm and plug assembly and shows theactual valve position.
D103071X012! No Atmospheric Bleed—Main regulator loadingpressure from pilot bleeds downstream through pilot
bleed.

www.FISHERregulators.com

Bulletin 71.2:CRONOSSpecifications
Available Configuration
Type Cronos: Pilot-operated pressure reducing
regulator for low to high outlet pressure.
Body Sizes
1, 2, and 3-inch (DN 25, 50,and 100)
With Type SRS Silencer (inlet x outlet):1 x 4, 2 x 6, and 3 x 10-inch (DN 25 x 100, 50 x
150, and 80 x 250)
End Connection StyleANSI 150, 300 or 600 RF Flanges

Pilot Flow Coefficients
Cg: 10.5; Cv: .36; C1: 29
Pressure Registration
External
Maximum Temperature Capabilities(1)–20° to 150°F (–29° to 66°C)
Approximate Weight (Including Pilot)
See figure 5
Construction Materials

Main Body Pressure Ratings(1)ANSI Class 150 RF: 290 psig (20,0 bar)
ANSI Class 300 and 600 RF: 1450 psig (100 bar)
Maximum Inlet Pressure(1)ANSI Class 150 RF: 290 psig (20,0 bar)
ANSI Class 300 and 600 RF: 1450 psig (100 bar)
Maximum Outlet (Casing) Pressure(1)1450 psig (100 bar)
Maximum Emergency (Design Pressure)1450 psig (100 bar)Miminum Operating Differential Pressure
ANSI Class 150 RF: 2.9 psid (0,2 bar d)
ANSI Class 300 and 600 RF: 7.3 psid (0,5 bar d)Outlet Pressure Ranges
See table 1
Flow Coefficients
See table 2

Cronos Series Main Valve
Main Body: Steel, ASTM A352 LCB
Flanges and Diaphragm Casings: Steel, ASTM
A350 LF2
Diaphragm Plates: Sheet steel
Diaphragm: Nitrile (NBR) with PVC coating
O-ring and Seat: Nitrile (NBR) or Fluoroelastomer(FKM)
Type PRX Pilot
Body: Steel, ASTM 105
Trim: Stainless Steel
Diaphragm: Nitrile (NBR) with PVC coating
O-ring and Seat: Nitrile (NBR) or Fluoroelastomer(FKM)
Type SA/2 Supply Pressure Regulator
Body: Steel
Diaphragm: Nitrile (NBR) with PVC coating
O-ring and Seat: Nitrile (NBR) or Fluoroelastomer(FKM)

1. The pressure/temperature limits in this bulletin or any applicable standard limitation should not be exceeded.

Table 2. Flow Coefficients
Table 1. Outlet Pressure Ranges
TYPE

PRX/120
PRX/125

PRX/120-AP
PRX/125-AP

2

OUTLET PRESSURE RANGE,
PSIG (bar)SPRING
COLOR

7.3 to 16
14.5 to 26
23 to 44
41 to 80
73 to 123

(0,5 to 1,1)
(1 to 1,8)
(1,6 to 3)(2,8 to 5,5)
(5 to 8,5)

White
Yellow
Green
Blue
Black

116 to 210
203 to 334
319 to 435
421 to 609(8 to 14,5)(14 to 23)(22 to 30)(29 to 42)

SilverGold
AluminumRed

435 to 1160 (30 to 80)

Clear

BODY
SIZE,
FLOW
INCHES COEFFICIENT(DN)

REGULATING AND WIDE OPEN FOR
RELIEF SIZING
S TD

WITH
TYPE SR

WITH
TYPE
SR S

WITH
WIDE
OUTLET

Cg
Cv
C1

550
18.3
30

540
18
30

500
15.2
33

580
19.3
30

2 (50)

Cg
Cv
C1

2250
77.6
29

2200
75.9
29

1900
59.4
32

2300
79.3
29

3 (80)

Cg
Cv
C1

5100
176
29

5000
172
29

4200
131
32

5200
179
29

1 (25)

Bulletin 71.2:CRONOSIntroduction
Type Cronos regulators are accurate pilot-operated,pressure balanced, soft seated regulators designed forhigh pressure transmission/city gate, large capacity
distribution systems and power plant feeds. They provide
smooth, quiet operation, tight shutoff and long life.
The regulator utilizes a main valve actuator, a Type PRX
pressure reducing pilot, and a Type SA/2 pilot supply
pressure regulator. The Type PRX pilot uses inlet
pressure reduced by the Type SA/2 supply pressure
regulator, as loading pressure to operate the main valve
actuator. The outlet pressure is sensed through acontrol line on the main valve actuator and also on theType PRX pilot diaphragm.
The superior performance of this regulator is due to theamplifying effect of the pilot and the two-path control
system. Changes in outlet pressure act quickly on theactuator diaphragm to provide fast response to systemchange. Then the pilot amplifies any small systemchanges to position the main valve for precise pressure
control.

Pilot Type Descriptions
A pressure reducing pilot that has the ability to handle
a wide range of set points from 7.3 to 1160 psig (0,5 to
80 bar) in two versions:Type PRX/120: A pressure reducing pilot with an outletpressure range of 7.3 to 609 psig (0,5 to 42 bar). The
Type PRX/120 can be used as the pilot on single stage
pressure reducing regulators or as the monitor pilot oras the working pilot in wide-open monitor systems.
Type PRX/120-AP: A pressure reducing pilot with anoutlet pressure range of 435 to 1160 psig (30 to 80 bar).
The Type PRX/120-AP can be used as the pilot on
single stage pressure reducing regulators or as themonitor pilot or as the working pilot in wide-open
monitor systems.
Type PRX/125: This pilot is identical to the Type PRX/
120 except that the restriction screw is removed. The
Type PRX/125 can only be used as the monitor override
pilot on working monitor applications. The pilot has anoutlet pressure range of 7.3 to 609 psig (0,5 to 42 bar).
Type PRX/125-AP: This pilot is identical to the Type
PRX/120-AP except that the restriction screw isremoved. The Type PRX/125-AP can only be used as
the monitor override pilot on working monitor applications. The pilot has an outlet pressure range of 435 to1160 psig (30 to 80 bar).

Type SR
E0822

Type SRS

Figure 2. Noise Abatement Constructions

Pilot Supply Regulator
The Type SA/2 pilot supply filter regulator, provides aconstant supply pressure to the Type PRX pilot that is
45 psig (3 bar) over set pressure. The Type SA/2 has a
integral 5 micron filter.

Noise Abatement
Type SR: The Type SR (figure 2) high velocity (up to
262 ft/s at the outlet flange) noise abatement device isincorporated into the regulator on the downstream side
and consists of plated stainless steel wires containing nosound deadening materials. Depending on flows andpressure drop, the silencer can reduce noise levels asmuch as 20 dB (A) with an approximate 3% Cg reduction.
Type SRS: A second noise reduction device may be
added to the Type Cronos regulators. The Type SRS(figure 2) consists of an Type SR plus a widened outlet
flange in which a second silencer is fitted. Noise
reduction is based on the principle of stream splitting
and gradual expansion in several stages. By using awider outlet flange the silencer can be mounted integral
to the regulator body which allows installation of theregulator directly into the downstream piping without an
expansion joint. Noise reduction up to 25 dB (A) isreached without limiting the velocity in the outlet flange.
However, there is a Cg reduction of about 15%.

3Bulletin 71.2:CRONOS

E0839

INLET PRESSURE
OUTLET PRESSURE
LOADING PRESSURE
PILOT SUPPLY PRESSURE
ATMOSPHERIC PRESSURE

Figure 3. Type Cronos Operational Schematic

Principle of Operation
The pilot-operated Type Cronos (figure 3) uses inlet
pressure as the operating medium, which is reduced
through pilot operation to load the actuator diaphragm.
Outlet or downstream pressure opposes loadingpressure in the actuator and also opposes the pilot
control spring.
When outlet pressure drops below the setting of thepilot control spring, pilot control spring force on the pilot
diaphragm thus opens the pilot valve plug, providing
additional loading pressure to the actuator diaphragm.
This diaphragm loading pressure opens the main valve
plug, supplying the required flow to the downstream
system. Any excess loading pressure on the actuatordiaphragm escapes downstream through the bleed
restriction in the pilot.
When the gas demand in the downstream system has
been satisfied, the outlet pressure increases. The
increased pressure is transmitted through the downstream control line and acts on the pilot diaphragm. Thispressure exceeds the pilot spring setting and moves thediaphragm, closing the orifice. The loading pressure
acting on the main diaphragm bleeds to the downstream
system through a bleed restriction in the pilot.

Adjustment
The adjustment of the regulator is performed by meansof the pilot adjusting screw, which varies the compres-4sion of the control spring. Adjustment is performed
while the regulator is in operation with the aid of a test
pressure gauge of suitable range or of a water column.
The shutoff valve downstream of the regulator must not
be completely closed; it is necessary that a small
quantity of gas flows downstream to allow the outlet
side to vent down, when it is necessary to lower thepressure.
Loosen the locknut and turn the adjusting screw slowly
to adjust outlet pressure. Use a pressure gauge tocheck the outlet pressure until the desired pressure isreached.

Monitoring Systems
Monitoring regulation is overpressure protection bycontainment, therefore, there is no relief valve to vent tothe atmosphere. When the working regulator fails tocontrol the pressure, a monitor regulator installed inseries, which has been sensing the downstream andcontrol pressure, goes into operation to maintain thedownstream pressure at a slightly higher than normalpressure. During an overpressure situation, monitoring
keeps the customer on line. Also, testing is relatively
easy and safe. To perform a periodic test on a monitoring regulator, increase the outlet set pressure of theworking regulator and watch the outlet pressure to
determine if the monitoring regulator takes over at theappropriate outlet pressure.

Bulletin 71.2:CRONOSTYPE PRX/120 OR
PRX/120-AP
TYPE SA/2

A
4xA
6xA

Single Pilot System Installation
TYPE PRX/120 OR
PRX/120-AP

TYPE PRX/120 OR
PRX/120-AP

TYPE SA/2

A
4xA
6xA

Upsteam or Downstream Wide-Open Monitoring System Installation
TYPE PRX/125 OR
PRX/125-AP

TYPE PRX/120 OR
PRX/120-AP

TYPE PRX/120 OR
PRX/120-AP

TYPE SA/2

TYPE SA/2

A
4xA
6xA

E0840

Working Monitoring System Installation

Figure 4. Typical Type Cronos Installation Schematics

5Bulletin 71.2:CRONOS
Wide-Open Monitoring Systems (figure 4)
There are two types of wide-open monitoring systems:
upstream and downstream. The difference between
upstream and downstream monitoring is that thefunctions of the regulators are reversed. Systems canbe changed from upstream to downstream monitoring,and vice-versa, by simply reversing the setpoints of thetwo regulators. The decision to use either an upstream
or downstream monitoring system is largely a matter of
personal preference or company policy.
In normal operation of a wide-open configuration, theworking regulator controls the system’s outlet pressure. With a higher outlet pressure setting, the monitor
regulator senses a pressure lower than its setpoint andtries to increase outlet pressure by going wide-open. If
the working regulator fails, the monitoring regulator
assumes control and holds the outlet pressure at its
outlet pressure setting.

Working Monitoring Regulators (figure 4)
In a working monitoring system, the upstream regulator
requires two pilots and it is always the monitoring
regulator. The additional pilot permits the monitoring
regulator to act as a series regulator to control anintermediate pressure during normal operation. In thisway, both units are always operating and can be easily
checked for proper operation.
In normal operation, the working regulator controls theoutlet pressure of the system. The monitoring
regulator’s working pilot controls the intermediatepressure and the monitoring pilot senses the system’s
outlet pressure. If the working regulator fails, themonitoring pilot will sense the increase in outlet pressure and take control.
Note
The working regulator must be rated forthe maximum allowable operatingpressure of the system because this will
be its inlet pressure if the monitoring
regulator fails. Also, the outlet pressure
rating of the monitoring pilot, and anyother components that are exposed tothe intermediate pressure must be rated
for full inlet pressure.
Working monitor installations require a Type Cronos
main valve with a Type PRX/120 or PRX/120-AP working
pilot and a Type PRX/125 or PRX/125-AP monitoring

6

pilot for the upstream regulator and a Type Cronos withthe appropriate Type PRX/120 or PRX/120-AP pilot forthe downstream regulator.

Adjustment
Adjusting the monitor regulator is similar to adjusting
the main regulator. Monitor setpoints are set slightly
higher than the main regulator. However, the value of
this difference cannot be determined in advance, as it
depends on the particular characteristics of eachapplication.

Capacity Information
To find approximate regulating capacities or to findwide-open flow capacities for relief sizing at any inlet
pressure, perform one of the following procedures.
Then, if necessary, convert using the factors provided
below.
For critical pressure drops (absolute outlet pressure
equal to or less than one-half of absolute inlet pressure), use the following formula:Q = (P1)(Cg)(1.29)
For pressure drops lower than critical (absolute outletpressure greater than one-half of absolute inlet pressure).

Q= 3417520
C gP 1SIN 
GT
 C1

∆P 
 DEG
P1 

where,
Q = gas flow rate, scfh
P 1 = absolute inlet pressure, psia (P1 gauge + 14.7)
Cg = regulating or wide-open gas sizing
coefficient from the Specifications table
G = gas specific gravity of the gas
T = absolute temperature of gas at inlet, °Rankine
C1 = flow coefficient∆P = pressure drop across the regulator, psi
To determine equivalent capacities for air, propane,butane, or nitrogen, multiply the capacity by thefollowing appropriate conversion factor: 0.775 for air,
0.628 for propane, 0.548 for butane, or 0.789 fornitrogen. For gases of other specific gravities, multiply
the given capacity by 0.775, and divide by the squareroot of the appropriate specific gravity.
Then, if capacity is desired in normal cubic meters perhour at 0°C and 1,01325 bar, multiply scfh by 0.0268.

Bulletin 71.2:CRONOSInstallation

Ordering Information

The Type Cronos regulators are installed in a horizontal or vertical pipeline. See figure 4 for typical piping
installations.

Carefully review each specification in the Specifications
section, then complete the Specification Worksheet on
page 8. If a pilot setpoint is not requested, the regulator will be set at the approximate midrange.

Dimensions are given in figure 5.

C

C

A

A

B

B
E0841

D

E
Type Cronos with Type SRS Silencer

Type Cronos

DIMENSIONS, INCHES (mm)
BODY SIZE,INCH (DN)

150 RF Flanged
P N 16300 RF or 600 RF Flanged
P N 25 o r 40

WEIGHTS, POUNDS (KG)
300 RF Flanged
P N 25600 RF Flanged
P N 40

STANDARD

WITH TYPE
SR S

A

B

C

D

E

A

B

C

D

E

D

E

1
(25)

8.5
(215)

7.1
(180)

11.2
(285)

7.3
(184)

13.8
(350)

8.7
(220)

7.1
(180)

8.9
(225)

7.8
(197)

13.9
(353)

8.3
(210)

14.2
(360)

79
(36)

108
(49)

2
(50)

9.6
(245)

7.7
(195)

13.2
(335)

10.0
(254)

18.3
(465)

10.2
(260)

7.7
(195)

11.3
(287)

10.5
(267)

18.6
(471)

11.3
(286)

18.9
(480)

137
(62)

192
(87)

3
(80)

13.0
(330)

10.3
(260)

15.7
(400)

11.8
(298)

22.5
(571)

13.8
(350)

10.3
(260)

15.7
(400)

12.5
(317)

23.2
(590)

13.3
(337)

23.6
(600)

282
(128)

419
(196)

Figure 5. Dimensions

7

Bulletin 71.2:CRONOS
Ordering Guide
Body Size (Select One)

Type SA/2 O-Ring and Seat Material" 1-inch (DN 25) Body **" 2-inch (DN 50) Body **" 3-inch (DN 80) Body **" Nitrile (NBR) (standard)** " Fluoroelastomer (FKM)**
Noise Abatement" Type SR** " Type SRS**

Body inlet x outlet with a Type SRS

Pilot Diaphragm (Select One)" 1 x 4-inch (DN 25 x 100) Body **" 2 x 6-inch (DN 50 x 150) Body **" 3 x 10-inch (DN 80 x 250) Body **" Nitrile (NBR) (standard)** " Fluoroelastomer (FKM)**
Pilot Valve Disk Assembly (Select One)End Connection Styles (Select One)" 150 RF**" 300 RF**" None" Nitrile (NBR) (standard)** " Fluoroelastomer (FKM)**" 600 RF**

Pilot Valve Replacement Parts Kit (Optional)

O-Ring Material (Select One)
" Nitrile (NBR) (standard)** " Fluoroelastomer (FKM)**" Yes, send one replacement parts kit to match
this order.

Pilot Type (Select One)" Type PRX-120 **" Type PRX/120-AP **" Type PRX-125 ** (monitor override pilot)" Type PRX/125-AP ** (monitor override pilot)
Outlet Pressure Range (Select One)
PRX120/125

Specification Worksheet" 7.3 to 16 psig (0,5 to 1,1 bar)**" 14.5 to 26 psig (1 to 1,8 bar)**" 23 to 44 psig (1,6 to 3 bar)**" 41 to 80 psig (2,8 to 5,5 bar)**" 73 to 123 psig (5 to 8,5 bar)**" 116 to 210 psig (8 to 14,5 bar)**" 203 to 334 psig (14 to 23 bar)***" 319 to 435 psig (22 to 30 bar)**" 421 to 609 psig (29 to 42 bar)**Application:
Specific UseLine Size
Fluid TypeSpecific Gravity
Temperature
Does the Application Require Overpressure Protection?
" Yes " No

PRX120/125-AP" 435 to 1160 psig (30 to 80 bar)**Pressure:
Maximum Inlet Pressure
Minimum Inlet Pressure
Differential Pressure
Set Pressure
Maximum Flow

Fisher Regulators Quick Order Guide

***
**
*

Accuracy Requirements:
Less Than or Equal To:" 5% " 10% " 20% " 40%Standard - Readily Available for Shipment
Non-Standard - Allow Additional Time for Shipment
Special Order, Constructed from Non-Stocked Parts. Consult
Your Fisher Sales Representative for Availability.

Construction Material Requirements (if known):

Availability of the product being ordered is determined by the component
with the longest shipping time for the requested construction.

Fisher is a mark owned by Fisher Controls International, Inc., a business of Emerson Process Management. The Emerson logo is a trade mark and service mark of Emerson Electric Co. All other marks are theproperty of their respective owners.
The contents of this publication are presented for informational purposes only, and while every effort has been made to ensure their accuracy, they are not to be construed as warranties or guarantees, express
or implied, regarding the products or services described herein or their use or applicability. We reserve the right to modify or improve the designs or specifications of such products at any time without notice.
Fisher does not assume responsibility for the selection, use or maintenance of any product. Responsibility for proper selection, use and maintenance of any Fisher product remains solely with the purchaser.

For information, contact Fisher:
Marshalltown, Iowa 50158 USA
McKinney, Texas 75070 USA
28320 Gallardon, France
40013 Castel Maggiore (BO), Italy
Sao Paulo 05424 Brazil
Singapore 128461©Fisher Controls International, Inc., 2002; All Rights Reserved

www.FISHERregulators.com