Regulator Sizing Selection

• 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.