Valve Selection for Process Heating
Understanding what kind of valve to use where as well as the applicable safety standards for your process equipment will help you select a combustion control system that will operate reliably and safely.
Figure 1. Solenoid-operated butterfly valves for air are designed for high and low control of air and are used in pulse-fired or frequency-fired combustion systems.
All combustion systems -- from the simplest to the most complex -- have six basic components in common:
- Air source.
- Fuel source.
- Airflow controls.
- Fuel flow controls.
- Fuel and air mixer.
This article defines air and fuel sources and discusses selection issues related to airflow and fuel flow controls, generally called valves.
Air Source. The air source normally is a combustion blower (turbo blower) that supplies pressurized air to the system at high enough pressure to overcome pressure drops caused by the piping, control equipment and burners.
Fuel Source. Gaseous fuels normally are delivered under pressure by pipelines or from storage tanks. As a rule, utility- or facility-supplied gas pressure is high enough to overcome pressure drops in systems. If pressure is not high enough, a gas booster can be used to increase the gas pressure, or the size of gas control components (valves, regulators, filters, etc.) can be increased to minimize the gas pressure loss that occurs as the gas travels through them. If the supplied gas pressure is too high, use a gas pressure regulator to reduce and control pressure. It is important to have steady gas and air pressures to ensure accurate fuel and air ratios at all times.
Figure 2. Normally closed for on-and-off control of gas and air flow, solenoid-operated safety shutoff valves are used as gas safety shutoff valves.
Airflow Control Valves
The most common air control components are butterfly valves, adjustable port valves and rotary-plug, pulse-air valves (either butterfly or solenoid styles). A typical ratio-controlled combustion installation includes one air butterfly valve in each temperature control zone. The butterfly valve is opened and closed by a control motor to provide more or less heat to the control zone.
Solenoid-Operated Butterfly Valves for air are designed for high and low control of air and are used in pulse-fired or frequency-fired combustion systems (figure 1). The high capacity butterfly valves have adjustable high and low flow stops to simplify system setup. These valves also are available with damping units that provide a smoother transition from low to high fire. This can be important in applications where maintaining an accurate fuel/air ratio is important. (The slower opening time of the dampened solenoid makes it easier for a ratio regulator to maintain the appropriate gas flow.) The dampened units typically are set to open in about 2 sec while the fast opening valves open in less than 0.5 sec.
The continuous duty cycle of the solenoid operated valve makes them well suited for the continuous cycling of frequency firing control systems. Choose a valve that has a high cycle-life rating (2 million cycles) and that has a minimum in-rush current.
FFigure 3. Choose a butterfly valve with gear motor for systems using constant-air, throttled-fuel or in metered ratio control combustion systems.
Gaseous Fuel Control Valves
Ratio Regulators are the most commonly used control valves for gas. Ratio regulators are similar to pressure regulators except that the downstream pressure is set by the combustion air pressure instead of a spring. Temperature in a control zone is controlled by adjusting the airflow and using the ratio regulator to match the gas flow to the air pressure. To achieve this, an impulse line from the air piping downstream of the air control valve is cross-connected to the spring side of the ratio regulator diaphragm. Because this regulator is built upside down compared to most regulators, the spring is used to balance the weight of the regulator internals and to make fine adjustments to the outlet pressure.
As the air pressure changes, the changes are transmitted to the regulator through the impulse line. If the air pressure increases, the regulator opens until the outlet pressure of the regulator is the same as the impulse pressure. An internal impulse tube supplies the reference signal to the opposite side of the diaphragm. When forces on both sides of the diaphragm are equal, the regulator stops and holds steady. If the downstream pressure goes down, the regulator closes until the two pressures are again equal. Ratio regulators are used mostly with nozzle-mix burners.
Figure 4. Suitable for much higher inlet pressures than solenoid-operated valves, motorized valves work by controlling and regulating the gas and air flow to burners and devices.
Zero Governor Regulators commonly are used for premix burner systems. The zero governor works on the suction principal: Air flowing through a venturi mixer creates suction in the gas line that opens the zero regulator, allowing gas to flow to the burner. In this system, the outlet pressure of the regulator always equals atmospheric pressure, which is why it is also called an atmospheric regulator.
Control valves ensure good combustion, but safety shutoff valves (SSOVs) at the fuel source are required for any combustion system. These normally are solenoid-operated gas safety shutoff valves that are closed when de-energized.
Solenoid-Operated Safety Shutoff Valves are normally closed valves used for on-and-off control of gas and air flow (figure 2). They are widely used as gas safety shutoff valves for industrial applications. Depending on the application, local or national codes such as NFPA 86 may require use of visual indication or proof-of-closure for safety shutoff valves. Normally, each main and pilot burner needs to be equipped with two safety shutoff valves.
Butterfly Valves with Gear Motor are suitable for regulating gas flow rates for modulating or stage-controlled combustion systems (figure 3). These butterfly valves can be used for a control ratio up to 1:10. The gear motor normally works using a three-step signal or a 4 to 20 mA input. Choose this style of valve for systems using constant-air, throttled-fuel (sometimes called excess-air control) or in metered ratio control combustion systems.
Motorized Valves are utilized for safeguarding as safety shutoff valves (figure 4). They work by controlling and regulating the gas and air flow to burners and components, including two-step operation. Motorized valves are suitable for much higher inlet pressures than solenoid-operated valves. Choose this type of valve for large pipe-diameter, high pressure safety-shutoff applications or where multiple features are desired in a single valve.
Figure 5. A ball valve is used for manual shutoff of gas or air.
Ball Valves. For manual shutoff of gas (or air), a simple ball valve can be used (figure 5). All combustion systems should include a manual shutoff valve for each burner as well as on the inlet to the main fuel train.
Gas Trains. Gas control equipment usually is piped in series in a fuel or gas train (figure 6). These trains can be prepiped to simplify installation on-site and can include the gas regulator, safety shutoff valves and ancillary equipment such as filters and pressure switches.
Modular Gas Trains. While some applications allow for long (10' or more) fuel trains, new modular systems provide the same safety and gas control in as little as 18" (figure 7). Ports on the modular train provide mounting locations for low and high gas pressure switches, a bypass valve, a valve-proving system and a vent valve. Additional options such as a strainers, pressure-reducing regulators and gas ratio regulators provide a complete fuel train. Modular fuel trains are a good choice for pipe sizes up to 3" in diameter and generally are more cost-effective than non-modular systems. The cost savings are even larger when you consider the installation expense of non-modular trains.
Figure 6. Often prepiped to simplify installation, a gas train can include the gas regulator, safety shutoff valves and ancillary equipment such as filters and pressure switches.
Approval and Insurance Requirements
Gas safety shutoff valves normally require third-party agency approvals to comply with application codes. Check with your plant safety officer or local fire marshal to determine which codes apply to your particular situation. Approval agencies include Underwriter's Laboratory (UL), Factory Mutual Research Corp. (FM) and Canadian Standards Association (CSA). Existing standards include:
- NFPA 86: Ovens and Furnaces, 2003 Edition.
- NFPA 85: Boiler and Combustion Systems Hazards Code, 2001 Edition.
- CSD-1: Automatically Fired Burners.
The standards cited above require use of auxiliary equipment on fuel trains to ensure their safe operation. This equipment includes gas-pressure switches as well as proof-of-closure switches or valve-proving systems.
Figure 7. Modular fuel trains are a good choice for pipe sizes up to 3" in diameter. Their modular nature allows them to take up less space.
Low and High Gas-Pressure Switches. To ensure that adequate gas pressure is delivered to your combustion system and that the pressure is within the operating range of the system, high and low gas pressure switches are required. These switches must be part of the safety limit circuit that enables power to flow to the SSOVs. To save time and money on piping, pressure switches can be mounted on modular gas valves directly.
Proof-of-Closure Switches or Valve-Proving Systems. For combustion systems with a capacity greater than 400,000 BTU/hr, a gas safety shutoff valve must have a means of proving valve integrity. Current standards allow use of either a proof-of-closure (POC) switch that indicates the valve successfully closed, or a valve-proving system, which is a safety control used on gas-fired equipment to ensure that a double safety shutoff valve arrangement has no leakage. The advantages of the valve-proving system are that it can eliminate the need for a vent valve in several standards and that it proves the leak rate of the valves is within specified limits rather than simply proving that the valve POC switch is closed.
Whatever its style and function, one of the most important features of a gas valve is serviceability. A breakdown of an industrial gas-fired application can cost thousands of dollars per hour. Being able to quickly and easily replace parts or complete subsystems helps keep operating disruptions to a minimum. Modular gas train construction ensures a short maintenance time because each gas valve can be replaced without piping adjustments.
Following simple selection guidelines will help ensure that you receive the best system for your application:
- Choose products designed, rated and approved for their function.
- Choose products designed for long, reliable life. Savings over the life of the valve will dwarf any up-front capital cost.
- While mix-and-match components can be combined to make a good fuel train, modular systems may provide the best “bang for the buck,” especially when considering installed costs.
Selecting valves for your combustion application requires knowledge of standards, process requirements and the cost tradeoffs associated with the options available.