CHAPTER 11: PRESSURE REGULATION
TABLE OF CONTENTS
Page
Introduction................................................................................................................................................1
Regulator Classifications...........................................................................................................................1
System Regulators................................................................................................................................1
Service Regulators................................................................................................................................2
Appliance Regulators............................................................................................................................3
Regulator Components..............................................................................................................................4
Adjustable Flow Opening......................................................................................................................5
Energy Source......................................................................................................................................5
Measuring Component..........................................................................................................................5
Control Lines.........................................................................................................................................6
Vents.....................................................................................................................................................7
Regulator Operation..................................................................................................................................8
Direct-Operated Regulators..................................................................................................................8
Pilot-Operated Regulators.....................................................................................................................9
Regulator Design.......................................................................................................................................9
Springs................................................................................................................................................10
Diaphragms.........................................................................................................................................10
Valve Design.......................................................................................................................................10
Droop..................................................................................................................................................10
Pipe Configuration...............................................................................................................................11
Other Design Considerations...................................................................................................................11
Failure Mode.......................................................................................................................................11
Temperature Drop...............................................................................................................................12
Noise and Vibration.............................................................................................................................12
Metering..............................................................................................................................................12
Regulator Sizing......................................................................................................................................12
General Regulator Sizing Guidelines..................................................................................................13
Regulator Installation Requirements........................................................................................................13
Enclosed Locations.............................................................................................................................13
Bypass Lines.......................................................................................................................................14
Over-Pressure and Under-Pressure Protection.......................................................................................14
Relief Valves.......................................................................................................................................15
Pop-type relief valves.....................................................................................................................16
Direct-operated relief valves...........................................................................................................16
Pilot-operated relief valves.............................................................................................................16
Internal relief valves........................................................................................................................16
Monitor Regulators..............................................................................................................................16
Shut Off Devices......................................................................................................................................17
Selecting the Proper Regulator................................................................................................................18
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LIST OF TABLES
Page
Table 11-1: Principal Types of Regulator Installations.............................................................................4
LIST OF FIGURES
Page
Figure 11-1: Pressure Regulator..............................................................................................................1
Figure 11-2: Distribution System Regulator..............................................................................................2
Figure 11-3: Service Regulator.................................................................................................................2
Figure 11-4: Farm Tap..............................................................................................................................3
Figure 11-5: Appliance Regulator.............................................................................................................3
Figure 11-6: Regulator Components........................................................................................................4
Figure 11-7: Energy Source......................................................................................................................5
Figure 11-8: Measuring Component.........................................................................................................6
Figure 11-9: Control Lines........................................................................................................................7
Figure 11-10: Vents...................................................................................................................................7
Figure 11-11: Direct-Operated Regulator..................................................................................................8
Figure 11-12: Pilot Operated Regulator.....................................................................................................9
Figure 11-13: Instrument-Controlled Valve................................................................................................9
Figure 11-14: Pilot Tube.........................................................................................................................11
Figure 11-15: Parallel Regulators...........................................................................................................11
Figure 11-16: Filter.................................................................................................................................13
Figure 11-17: Protected Regulators and Meters.....................................................................................14
Figure 11-18: Bypass Line......................................................................................................................14
Figure 11-19: Over-Pressure (High Flame).............................................................................................15
Figure 11-20: Relief Valve.......................................................................................................................15
Figure 11-21: Large Relief Valve.............................................................................................................15
Figure 11-22: Monitor Regulator..............................................................................................................17
Figure 11-23: Automatic Shut-off Valve...................................................................................................17
CHAPTER 11: PRESSURE REGULATION
Introduction
Pressure regulating equipment controls the amount of gas flow to meet downstream customer demand and to maintain downstream pressure at an established set point.
As downstream load⎯or demand⎯increases, a regulator must increase flow to prevent a shortage of gas and a decrease in downstream pressure. As downstream load decreases, gas flow through the regulator must also decrease to prevent too much gas flow and an increase in downstream pressure. A gas regulator must maintain a constant reduced pressure in spite of varying inlet pressure and flow rates.
Figure 11-1: Pressure Regulator
Regulator Classifications
Regulators operate at different locations and have different functions within a piping system. The three common locations for regulators are at the system level, at individual customer locations, and at appliances and equipment.
Regulators can be classified according to their location/functions. The three common regulator classifications are system regulators, service regulators, and appliance regulators.
System Regulators
System regulators move large volumes of gas and regulate pressure at gate stations and at control points within a transmission or distribution system.
Gas supply stations and distribution systems use large regulators and control valves to move large volumes of gas and control pressure. Supply station regulators may reduce gas pressure from transmission lines of 1000+ psig to pressures such as 40 psig. Supply station regulators do not require “snap action” or fast response to changing load conditions. Instead, these regulators handle large flows and are designed for minimum pressure drops, and they do not require as accurate control of pressure as does a service or appliance regulator.
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Figure 11-2: Distribution System Regulator
Source: American Meter Company
The Gas Control section of a company commonly operates major supply system regulators by dispatching or remote control. Gas controllers adjust valves and regulators to maintain either a constant (metered) flow rate or a constant downstream pressure to manage gas supply and demand. Gas regulating equipment at supply stations or on specialized equipment may involve different types of electrical or mechanical controls to operate the regulators.
Distribution system regulators are normally located within a city and reduce gas pressure from high-pressure feeder lines to distribution main pressures. High-pressure feeder lines move large volumes of gas throughout a service territory.
Service Regulators
Service regulators are normally located outside customer building walls and reduce pressure from the service line for metering and delivery to residential, commercial, or industrial customers. Service regulators must deliver a wide range of flows and pressures.
Figure 11-3: Service Regulator
Source: American Meter Company
Two regulators are often installed when a service is tapped from transmission or higher pressure lines. The first regulator reduces pressure at the service connection with the main and is called a farm tap (Figure 11-4).
CHAPTER 11 ⎯ PRESSURE REGULATION 11-3
Figure 11-4: Farm Tap
Source: Fisher Regulators
Appliance Regulators
Appliance regulators are located immediately upstream of customer appliances and large equipment. These regulators reduce the pressure in a customer’s piping to the optimum setting for proper operation of specific equipment.
Appliance regulators are normally smaller in size and have a large opening for high flow rates and low-pressure drops. The use of appliance regulators enables a higher pressure to be delivered at the meter outlet and smaller diameter piping to be installed to feed appliances.
Figure 11-5: Appliance Regulator
The different types of regulator installations are outlined in Table 11-1, which also summarizes the control job to be done in each type of application.
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Table 11-1: Principal Types of Regulator Installations
Pressure Range (approx.)
Type of Installation
Load Served
Usage Rate MCF/hr (approx.)
Inlet
Outlet
Accuracy of Control Required
Appliance
One appliance (range, water heater, furnace)
0.001 to 3.0
4 to 13 in. wc
3 to 8 in. wc
+ 0.5 in. wc
Service
One customer
0.005 to 15
1 to 60 psig
4 to 13 in. wc
+ 1.0 in. wc
Industrial
Industrial equipment
up to 100
1 to 400 psig
¼ to 50 psig
varies widely
District Low Pressure
Mains and services in an LP area
5 to 1000
1 to 250 psig
6 to 12 in. wc
+ 0.5 in. to + 1 in. wc
District High Pressure
Mains and services in an HP area and LP regulator stations.
5 to 5000
5 to 400 psig
2 to 100 psig
+ 1% to + 20% of outlet pressure
City Gate
All load supplied through the same city gate station
200 to 20,000
200 to 1200 psig
5 to 400 psig
+ 1% to + 20% of outlet pressure
Regulator Components
Standard regulating equipment includes three main components. These components consist of an adjustable flow opening, energy source and measuring element.
Figure 11-6: Regulator Components
CHAPTER 11 ⎯ PRESSURE REGULATION 11-5
Adjustable Flow Opening
An adjustable flow opening controls the amount of gas flowing through a regulator and normally involves a valve plug and orifice.
Gas flow through the flow opening can be increased until its velocity reaches the speed of sound. At this point (known as the critical pressure drop), a decrease in outlet pressure will no longer increase gas flow through a regulator.
Energy Source
An energy source such as springs, gas pressure, or electricity supplies a force to open or close the regulator flow opening.
Weight-loaded regulators add or delete weight from the diaphragm to regulate pressure. The inertia affect of weights causes the regulator to commonly overshoot, and then re-adjust. These regulators are the oldest design and are normally replaced by spring-loaded regulators.
A spring is the most common energy source used to control regulator operations and eliminates the inertia problems of weight-loaded regulators. The spring, along with atmospheric pressure, applies energy to one side of the regulator diaphragm.
Figure 11-7: Energy Source
A small pilot regulator can also be used as an energy source to increase sensitivity. However, the loading pressure to a pilot regulator must be bled either to the atmosphere or to the downstream piping.
Measuring Component
The measuring component senses downstream pressure to control the energy source and flow. A diaphragm is normally used as the measuring component in regulators.
A regulator diaphragm consists of a round piece of reinforced fabric covered with a rubber-like material. Downstream gas pressure is on one side of the diaphragm. The spring and atmospheric
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pressure are on the other side. The diaphragm seals off the spring and downstream pressure sides and moves up and down in response to changing gas flow conditions. The diaphragm shape is designed so that it compensates for changes in spring force as it moves up and down.
Figure 11-8: Measuring Component
Modern regulator diaphragms are usually made of a sheet of non-elastic, flexible, fabric-reinforced synthetic rubber. The rim of the circular diaphragm is clamped between the two halves of the diaphragm case and the center is supported between two flat metal plates. To permit diaphragm movement, considerable slack is left in the unsupported section between the diaphragm case and plates.
An external device such as a small “pilot regulator” can also be used as a measuring component (and energy source) to improve performance.
Two additional regulator components are control lines and vents.
Control Lines
Control lines are small-diameter pipes that connect a regulator with downstream piping beyond any turbulent flow. Turbulence can occur with large flows and pressure reductions. Control lines are normally used for regulators that handle large gas loads. Each “control line” regulator must have its own separate control line. Control lines must be adequately protected to prevent a regulator from malfunctioning.
In cases of smaller gas loads, downstream pressure travels through the regulator body directly to the diaphragm.
CHAPTER 11 ⎯ PRESSURE REGULATION 11-7
Figure 11-9: Control Lines
Control lines must be at least 1½ inches in diameter and should be increased one pipe size every 20 feet to avoid pressure loss. Control lines should be attached to the main, away from elbows or other fittings at a distance several pipe diameters downstream to avoid turbulence. Control lines should also slope away from regulators and connect to the top of mains to prevent liquid from being trapped.
Vents
A standard regulator must also have a vent that allows the movement of air into and out of the regulator’s spring chamber.
The regulator diaphragm must be able to move up and down freely in response to changes in load. The vent connection allows air to move into and out of the spring compartment so that the diaphragm movement does not compress or decompress air in the spring chamber and affect the pressure set point.
A vent also allows atmospheric pressure to combine with the spring force and increase or decrease pressure on the regulator diaphragm as atmospheric pressure changes.
In order for gas to move through a burner, it must be at a higher pressure than the atmospheric pressure. However, atmospheric pressure can change by several inches (w.c.) due to weather conditions. The regulator vent allows atmospheric pressure to be applied to the diaphragm.
As atmospheric pressure rises, a greater force is required for gas to exit from a burner. The force on the diaphragm changes in proportion to the atmospheric pressure and the regulator set point is maintained. As atmospheric pressure decreases, both the force on the burner and the diaphragm decrease and the set point is maintained.
Figure 11-10: Vents
Source: American Meter Company
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Regulators equipped with internal-relief valves use the vent to expel gas in case of regulator failure to prevent over-pressurization and damage to downstream equipment. The diameter and length of vent lines must be sized according to the manufacturer’s literature to prevent additional pressure buildup from vent stack resistance.
Regulator Operation
Regulators are often grouped into two categories: direct-operated
