SYSTEM OPERATING PRESSURE
The gas pressure in the piping system downstream of the meter is usually 5 to 14 inches (125 to 356 mil-limeters) of water column, but in some cases it can be as much as 2 to 5 pounds per square inch (psi) (13.8 to 34.5 kilopascals [kPa]). Under certain conditions, engineering practice will limit the pressure losses due to friction in the piping system to a range between 0.2 and 0.5 inch (5 and 13 millimeters) of water column, but all model codes allow the engineer the opportunity to take a greater pressure drop. However, local codes may vary, and the engineer should consult the local AHJ prior to the design of any system.
Most appliances typically require approximately 3.5 inches (89 millimeters) or water column, but certain appliances such as water heaters and boilers may require higher gas pressures to operate properly.
Where appliances require higher operating pressures and/or where long distribution lines are encountered, it may be necessary to select a higher pressure at the meter outlet to satisfy the appliance requirements or allow for greater pressure losses in the piping system, thereby allowing economy of pipe size.
Systems often are designed with meter outlet pres-sures of 2 to 5 psi (13.8 to 34.5 kPa), combined with pressure regulators to reduce the pressure for appli-ances as required. In most cases, the utility company will reduce the incoming pressure to a figure that is requested by the design engineer at the start of the project or to conform to local code requirements. In some areas of the country, gas pressures on both sides of the meter are in inches of water column.
The maximum allowable operating pressure for natural gas piping systems inside a building is based on NFPA 54: National Fuel Gas Code, except when approved by the AHJ or when insurance carriers have more stringent requirements. Natural gas system pressures generally are not permitted to exceed 5 psig (34.5 kPa) unless all the following are met:
The AHJ will allow a higher pressure.
1.
The distribution piping is welded. (Note: Some 2.
jurisdictions also may require welded joints to be x-rayed to verify continuity.)
Pipe runs are enclosed for protection and lo-3.
cated in a ventilated place that will not allow gas to accumulate.
The pipe is installed within areas used spe-4.
cifically for industrial processes, research, warehousing, or mechanical equipment rooms.
A maximum liquefied petroleum gas pressure of 20 psig (138 kPa) generally is allowed, provided the building is used specifically for research or industrial purposes and is constructed in accordance with NFPA 58: Liquefied Petroleum Gas Code, Chapter 7.
EFFICIENCY
The difference between the input and the output of any equipment is the heat lost in the combustion process (burner), heat exchanger, and flue gases.
Water-heating and space-heating equipment is usually 75 to 85 percent efficient (although high-efficiency equipment in the upper 90 percent range is also avail-able), and ratings are given for both input and output.
Cooking and laundry equipment is typically 75 to 85 percent efficient, with input ratings that take into consideration internal losses. When only the output required for an appliance is known, it will be neces-sary to increase the volume of gas to account for the loss of its listed efficiency.
CODES AND STANDARDS
At the beginning of any given system design, the plumbing engineer shall determine the primary model code that will be referenced, along with the specific year edition of that code. The following model codes should be viewed for provisions regarding natural gas and liquefied petroleum gas regulations, require-ments, and standards:
Uniform Plumbing Code,
• promulgated by the
International Association of Plumbing and Mechanical Officials (IAPMO)
International Fuel Gas Code,
• promulgated by
the International Code Council (ICC) National Standard Plumbing Code,
•
pro-mulgated by the National Association of Plumbing-Heating-Cooling Contractors The primary code may reference NFPA 54. Other codes and standards that may be applicable are ANSI/
NFPA 30: Flammable and Combustible Liquids Code, ANSI/NFPA 58, ANSI Z83.3: Gas Utilization Equipment for Large Boilers, ANSI/UL 144: Pressure-regulating Valves for Liquefied Petroleum Gas, NFPA 88A: Standard for Parking Structures, and American Gas Association standards.
Insurance carriers such as Industrial Risk Insur-ers and FM Global also may impose standards and requirements that may be more stringent than the applicable code.
GAS METERS
Meters are required in all services. To achieve opti-mal accuracy, the pressure into the meter must be regulated. Requirements for various utility suppli-ers differ regarding the placement and arrangement of the meter assembly. The assembly could consist of filters, valves, regulators, and relief valves. It could be placed indoors or on a slab outdoors either aboveground or underground in a vented pit. The plumbing contractor is usually responsible for a pit, slab, telephone outlet, and electrical outlet adjacent
to the meter if required. In most cases, the utility company provides the meter.
The utility company typically runs the service on the consumer’s site up to the meter location, termi-nating with a shutoff valve. The utility company also may install wireless radio frequency or encoder-type meter-reading equipment at the meter so that gas consumption data can be obtained remotely by util-ity company personnel. Remote reading is becoming more common and is most often done through an electronic pulse output assembly mounted on the meter. Different styles are available, consisting of
radio frequency (RF) and fixed networks, portable/
handheld devices, mobile systems, Wi-Fi, and direct power line transmission types. Manufacturers have developed pulse or encoder registers that produce electronic outputs for radio transmitters, reading storage devices, and data-logging devices. Pulse meters typically send a digital or analog electronic pulse to a recording device. Encoder registers have an electronic means for an external device to ques-tion the meter register for either the posiques-tion of the odometer wheels or a stored electronic reading.
In most cases, remote meter reading is advanta-geous to the local utility company because it allows utility personnel to obtain meter information without the expense of actually having to travel to the meter location. Nevertheless, the plumbing engineer should always investigate the different options that may be offered by the local utility company for remote meter reading, as a particular facility might specifically desire this option such as where submetering of individual ar-eas or tenants within a larger facility may be necessary or where meter information might need to be obtained from a meter located in a secure area. Another ad-vantage is that customer billing can be based on near real-time consumption rather than on estimates based on previous or predicted consumptions.
Meter Types
The three basic types of meters are diaphragm, ro-tary, and turbine.
Diaphragm meters are positive-displacement devices that have fixed-volume measurement compartments formed by a two-sided convoluted diaphragm. A small pressure drop across the meter causes it to cycle, so the compartments alternately fill with gas at the inlet and then empty at the outlet.
By counting the number of cycles, the meter provides a measure of gas volume.
Rotary meters are also positive-displacement measurement devices. In their case, however, a pair of hourglass-shaped impellers forms the fixed-volume compartments. When downstream demand initiates the flow of gas, the impellers rotate to receive a fixed volume of gas at the inlet and then discharge it at the outlet.
A turbine meter is classified as an inferential meter. It has a rotor in the gas stream in lieu of fixed volume compartments. As gas flows through the meter, the rotor turns at a speed that is proportional to the rate of gas.
Meter Selection
Deciding which type of meter is the best choice for a particular application depends on the following:
The pressure of the gas being metered 1.
The maximum flow rate of the gas being 2.
metered Table 7-3 Approximate Gas Demand for Common Appliancesa
Appliance Input,
Combination broiler and roaster 66,000 (69.6)
Coffee maker, three-burner 18,000 (19)
Coffee maker, four-burner 24,000 (25.3) Deep fat fryer, 45 lbs (20.4 kg) of fat 50,000 (52.8) Deep fat fryer, 75 lbs (34.1 kg) of fat 75,000 (79.1) Doughnut fryer, 200 lbs (90.8 kg) of fat 72,000 (76) Two-deck baking and roasting oven 100,000 (105.5)
Three-deck baking oven 96,000 (103.3)
Revolving oven, four or five trays 210,000 (221.6) Range with hot top and oven 90,000 (95.0)
Range with hot top 45,000 (47.5)
Range with fry top and oven 100,000 (105.5)
Range with fry top 50,000 (52.8)
Coffee urn, single, 5 gal (18.9 L) 28,000 (29.5) Coffee urn, twin, 10 gal (37.9 L) 56,000 (59.1) Coffee urn, twin, 15 gal (56.8 L) 84,000 (88.6) Stackable convection oven, per section of oven 60,000 (63.3) Residential equipment
Clothes dryer (Type I) 35,000 (36.9)
Range 65,000 (68.6)
Stove-top burners (each) 40,000 (42.2)
Oven 25,000 (26.4)
30-gal (113.6-L) water heater 30,000 (31.7) 40- to 50-gal (151.4 to 189.3-L) water heater 50,000 (52.8)
Log lighter 25,000 (26.4)
Barbecue 50,000 (52.8)
Miscellaneous equipment
Commercial log lighter 50,000 (52.8)
Bunsen burner 5,000 (5.3)
Gas engine, per horsepower (745.7 W) 10,000 (10.6) Steam boiler, per horsepower (745.7 W) 50,000 (52.8) Commercial clothes dryer (Type II) See manufacturer’s
data
a The values given in this table should be used only when the manufacturer’s data is not available.
Chapter 7 — Fuel Gas Piping Systems 117
The minimum flow rate of the gas to be me-3.
tered
Depending on the specific model, diaphragm meters have pressure ratings up to 100 psig. Rotary meters can operate up to 285 psig. For applications more than 285 psig, a turbine meter should be selected.
Ratings for these meter types may overlap with respect to capacities and pressures. This overlap al-lows for versatility when selecting the correct meter for certain applications. For example, a project’s maximum flow rate may fit into the “typical” range indicated in a manufacturer’s product data listing for a diaphragm meter, but the pressure of the gas being metered may be more than 100 psig. In this case, a small rotary meter would be selected.
For most projects, the following meter choices are typical:
Diaphragm meters: 10 to 1,000 cubic feet per
•
hour (cfh), 100 psig or less
Diaphragm, rotary, and turbine meters: 1,000
•
cfh to 50,000 cfh, 100 psig or less
Rotary or turbine meters: 50,000 to 325,000
•
cfh, 100 to 285 psig
Turbine meters: 325,000 to 18,000,000 cfh,
•
285 to 1,440 psig
Range ability is another consideration when select-ing a gas meter for a given project. Range ability is the ratio of maximum flow rate to minimum flow rate that can be measured within the specified accuracy of the meter. For example, most diaphragm meters provide an accuracy of ±1 percent of reading with a range ability of more than 100:1. Therefore, a meter with a maximum rating of 250 cfh will provide ±1 percent accuracy for flow rates from 2.5 to 250 cfh.
PRESSURE-REGULATING VALVES
A pressure regulator is a device used to reduce a variable high inlet pressure to a constant lower outlet pressure. The line regulator is used to reduce supply line pressures. If used, this regulator usually is installed outside, upstream of the meter assembly, and it is provided by the utility company. If installed inside a building, a dedicated relief vent pipe will need to be connected at the regulator’s vent connec-tion and routed to the exterior of the building per the local AHJ. An intermediate regulator located downstream of the meter assembly may be used to further reduce pressure from 2 to 5 psig (13 to 35 kPa) or to a pressure suitable for use by terminal equip-ment of approximately 7 inches (178 millimeters) of water column.
An appliance regulator connects the supply to equipment at the point of use and may be provided by the equipment manufacturer, specifically on equip-ment that may include a gas train. Types of appliance
regulators are zero governor, backpressure regulator, and a differential regulator.
When regulators are installed inside a building and require venting, these vents must be routed to the at-mosphere. The vents from individual regulators may not be combined. However, a vent to the outdoors is not required for regulators equipped with and labeled for utilization with an approved vent-limiting device installed in accordance with the manufacturer’s in-structions. Regardless, incorporating vent-limiting devices should be evaluated carefully for each spe-cific project design to ensure that the proper safety measures have been accounted for and that local ju-risdictional requirements have been met. Regulators with vent-limiting devices are in many cases allowed only within ventilated spaces to ensure that any gas escaping from the vent is dissipated safely.
When bottled gas is used, the tank can have as high as 150-psi (1,034.6-kPa) pressure to be reduced to the burner design pressure of 11 inches (279.4 mil-limeters) of water column. The regulator typically is located at the tank for this pressure reduction.
Gas Regulator Relief Vents
Guidelines for the use of relief vents from pressure regulators, also referred to as gas-train vents, can be found in the latest editions of NFPA 54 and FM Global Loss Prevention Data Sheet 6-4: Oil- and Gas-fired Single-burner Boilers, as well as in other publications of industry standards, such as those is-sued by Industrial Risk Insurers and the American Gas Association.
Figure 7-1 Altitude Correction Factor
The Altitude Correction Factor (ACF) should be multiplied by the gas input at sea level to determine the corrected input. Sizing of the equipment is then performed utilizing this corrected input multiplied by the full load efficency.
It should be noted that when pressure regulators discharge (or the diaphragm in the regulator rup-tures), large amounts of fuel gas may be released.
It is not uncommon for a local fire department to be summoned to investigate an odor of gas caused by a gas-train vent discharge. Every attempt should be made to locate the terminal point of the vents above the line of the roof and away from doors, windows, and fresh-air intakes. They should be located on a side of the building that is not protected from the wind.
Refer to NFPA 54, local utility supplier requirements, and local codes for the exact requirements for vent termination locations.
CONTROL VALVES
Excess Flow ValvesAn excess flow valve is a device that shuts off the flow of gas if there is a much larger flow through the pipe or service than that for which it was designed.
In some parts of the country, particularly in areas where earthquakes may occur, excess flow valves are necessary to guard against the possibility of a break during such an event. In other cases, where danger exists for equipment such as large boilers, installation should be considered.
A low-pressure cutoff shall be installed between the meter and the appliance where the operation of a device, such as a gas compressor, appliance, or boiler, could produce a vacuum or dangerous vacuum condi-tion in the piping system.
Appliance Control Valves
An appliance shutoff valve shall be installed at all gas appliances. Valves at flexible hose connections are to be installed prior to the flexible connection that is used to connect the appliance to the building gas supply.
Interlocks and Solenoid Valves
An automatic interlock or gas solenoid valve can be interconnected with the automatic fire extinguish-ing system when required to shut off the gas supply to all equipment in a kitchen when sprinklers dis-charge in the event of a fire. These valves typically are provided by the fire suppression equipment con-tractor and installed by the plumbing or mechanical contractor. (Note: Laboratories may utilize similar automatic shutoff capabilities. Refer to the “Labora-tory Use” section further in this chapter for more information.)
In earthquake-prone areas, a seismic shutoff valve is necessary to shut off the supply of gas if a seismic event is of sufficient magnitude to potentially rupture the gas supply pipe or separate the pipe from equipment.
APPLIANCES
Most manufacturers of gas appliances rate their equipment with gas consumption values in British thermal units per hour (Btuh), which is used in de-termining the maximum gas flow rate to be supplied to the appliance via the piping system. Table 7-3 shows the approximate gas consumption for some common appliances (listed in Btuh). To find the flow rate of the gas required, always use the consumption data listed by the manufacturer and divide it by the Btu per cubic feet content of the gas as provided by the utility supplier. (Note: Natural gas is nominally rated at 1,000 Btu per cubic feet for design purposes.
However, it is very important to verify the actual Btuh content or rating to be used, as well as the altitude/
elevation necessary for deration purposes for a specific project design.)
VENTING
Integral to the design of any natural gas system are the venting and combustion air requirements required for appliances to operate properly and effi-ciently. Appliances are listed by types and categories that shall be used in the design of flue/vent systems.
Along with the requirements for natural gas systems, appliance venting requirements also are stipulated in NFPA 54, the International Fuel Gas Code (IFGC), and any specific requirements dictated by the local gas supplier as applicable. Although not within the scope of this chapter, it is mentioned here for reference purposes due to the fact that the plumbing engineer may be required to provide design input with respect to venting even though not responsible for the actual design of these systems.
It generally is not the plumbing engineer’s respon-sibility to design and specify gas vents. This is done by either the HVAC engineer or the manufacturer.
However, water heater technologies that are currently available, such as non-condensing, high efficiency, and forced combustion, may require specific venting consideration, which in turn should be coordinated with the HVAC engineer to ensure that all aspects of the venting design have been coordinated.
The products of combustion from an appliance must be safely exhausted to the outside. This is accom-plished with a gas vent system in most cases. Where an appliance has a very low rate of gas consumption (e.g., Bunsen burner or countertop coffee maker) or where an appliance has an exhaust system associated with the appliance (e.g., gas clothes dryer or range) and the room size and ventilation are adequate, a sep-arate gas vent system may not be required. Current practice usually dictates the use of factory-fabricated and listed vents for small to medium-size appliances.
Large appliances and equipment may require specially designed venting or exhaust systems.
Chapter 7 — Fuel Gas Piping Systems 119
For reference purposes, a general description of venting categories is as follows:
Category I: Appliance operates with a
•
negative vent static pressure and a flue gas temperature that avoids condensation (nega-tive pressure/non-condensing).
Category II: Appliance operates with a
•
negative vent static pressure and a flue gas temperature that may cause excessive con-densation within the vent (negative pressure/
condensing).
Category III: Appliance operates with a
•
positive vent static pressure and a flue gas temperature that avoids condensation (posi-tive pressure/non-condensing).
Category IV: Appliance operates with a
•
positive vent static pressure and a flue gas temperature that may cause excessive con-densation within the vent (positive pressure/
condensing).
Where the rating of the appliance is not known, it shall comply with the typical demand of the type of appliance as indicated in NFPA 54.
ALLOWABLE GAS PRESSURE
The gas pressure in the piping system downstream of the meter is usually 5 to 14 inches (125 to 356 mil-limeters) of water column, but in some cases it can
The gas pressure in the piping system downstream of the meter is usually 5 to 14 inches (125 to 356 mil-limeters) of water column, but in some cases it can