Tags: Gas Valves

Combination Gas Valves

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A combination gas valve (or combination gas control) combines in a single unit all manual and automatic control functions required for the operation of gas-fired heating equipment. In other words, a single valve replaces the various individual pilot line and main line gas controls. A gas-pressure regulator is usually optional.

Many manufacturers of gas controls offer a complete line of combination gas valves; each valve is designed for a different kind of installation or application. Usually these valves will differ on the basis of the controller voltage or voltage source, valve application or function, required Btu capacity for the installation, and type of gas used.

Honeywell manufactures a line of standardized and interchangeable gas control components. A complete preassembled combination gas control can be ordered from the factory or one can be assembled in the field from a variety of different standardized components in order to meet the needs of a particular installation. This add-on feature also allows field replacement of a defective component without removing the complete valve from the installation. A number of possible combinations are illustrated in Figures 5-45 and 5-46.

gas regulator Combination Gas Valves

standard gas regulators Combination Gas Valves

Solenoid Gas Valves

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The solenoid gas valve is commonly used on gas-fired heating equipment to provide on-off control of the flow of gas.

The primary function of a solenoid valve is to provide direct valve operation. The power to operate the valve is obtained from the magnetic flux developed in a solenoid coil. A valve disc in the valve body is connected by a rod to the core of an electromagnet. When the room thermostat or power switch directs an electrical current to the solenoid, it pulls the rod (plunger) to the top of the plunger tube and lifts the attached valve disc. Gas then flows through the main valve port until the electrical circuit is interrupted by the controller. This action releases the rod, which falls and shuts off the valve. The weight of the rod and seat assembly and the gas pressure on top of the valve seat ensure a tight shutoff. The ITT General Controls K3 Series gas valve shown in Figure 5-8 is an example of a direct-operated solenoid gas valve.

direct solenoid gas valve Solenoid Gas Valves

Some solenoid valves use a balanced diaphragm to control the flow of gas (see Figure 5-9). When the solenoid coil is energized, it lifts the rod or plunger just enough to open a bleed valve (or socalled pilot valve). Gas then bleeds from the area above the the pressure above the diaphragm being the same as the pressure below the seat disc. This is referred to as a balanced or unloaded condition. The solenoid coil lifts the complete interior assembly to full open position. When the solenoid is deenergized, the pressure recovers above the diaphragm. The weight of the interior assembly and the gas pressure across the seat disc are sufficient to hold the valve closed. In this type of valve, the pressure of the gas is used to control its operation.

balanced diaphragm solenoid gas valve Solenoid Gas Valves

A third type of solenoid valve consists of a solenoid-operated (i.e., magnetically operated) puff bleed three-way valve and a diaphragm valve in a single unit (see Figure 5-10). The combined unit provides on-off control of the gas to the gas-fired heating equipment.

electric diaphragm solenoid gas valve Solenoid Gas Valves

The three-way valve (also referred to as a pilot valve), responding to electrical signals from the limit or safety controls, opens or closes the gas valve by controlling the gas pressure bleed-off above the diaphragm in the main valve body. In the normally closed position, inlet gas pressure above the diaphragm prevents the valve from opening. In the open (energized) position, the three-way or pilot valve closes off the inlet gas pressure and allows the gas pressure above the diaphragm to bleed off so that gas pressure below the diaphragm forces the diaphragm up to open the valve.

Dual-solenoid valves are designed for three-stage control (highlow-off) of the flow of gas (see Figure 5-11). Both a high-fire solenoid and a low-fire solenoid are used to accomplish this purpose. Low-fire adjustments can be made by turning the adjustment screw clockwise (to decrease low fire) or counterclockwise (to increase it).

dual solenoid valve Solenoid Gas Valves

Figure 5-12 shows a schematic wiring diagram of a two-stage control containing a dual-solenoid valve. If both solenoids are to be energized at one time, the circuit requires a 40-volt transformer.

dual solenoid valve circuit Solenoid Gas Valves

Gas Valves

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The valves used to control the flow of gas through a gas-fired furnace, boiler, or water heater can be divided into two basic categories: (1) manually operated valves and (2) power-operated valves.

The two manually operated valves (gas cocks) used on gas-fired heating equipment provide a backup safety function in case the automatic gas valves fail to operate. One of these valves is located in either the main gas supply riser or the manifold. The other one is located on the pilot gas line.

The manual gas valve installed on the main gas supply line (riser) or manifold is variously referred to as the main gas shutoff valve, manual shutoff plug cock, or simply the gas cock (see Figure 5-7). This valve provides manual control of the gas flow to the main gas burners. It is not used to control the gas supply to the pilot burner, the latter being provided with its own separate shutoff valves.

The manual valve located on the pilot gas line is called the pilot shutoff cock or the pilot gas cock. It is usually the first controlling device on the pilot line (see Figure 5-7). It provides complete gas shutoff whenever it is necessary to remove and service other controls on the pilot line, such as the pilot gas regulator or the pilot solenoid valve.

gas cocks Gas Valves

Power-operated or automatic valves are actuated by some form of auxiliary power such as hydraulic pressure, pneumatic pressure, electricity, or a combination of these sources. The following are the principal types of power-operated valves used on gas-fired heating equipment:

• Solenoid valves
• Direct-acting heat motor valves
• Diaphragm valves