Selecting the Proper Gas Valves
Previously, the small number of shutoff valve designs provided few gas train options. Today there are many approved valve designs, including multi-functional components, which provide options that may result in savings of thousands of dollars -- per gas train. With the increased competitiveness in the marketplace, an oven or equipment manufacturer must explore these advantages.
The following basic factors should to be evaluated when determining which gas valves are the most appropriate for the application.
Determine the Available Inlet Gas Pressure and Required Pressure at the Burner. Gas valves will have at least one pressure rating -- the maximum operating pressure. This pressure is the maximum pressure against which the valve can open; it has been underwritten by agencies such as Underwriters Laboratories (UL) and Factory Mutual (FM). In addition, many valves have a maximum close-off pressure rating. This rating is the maximum pressure against which the valve will close. An example of this rating's importance is if the upstream regulator were to rupture, the high gas pressure would trip the high gas pressure switch, closing the main gas shutoff valves. If the gas valves do not have a close-off pressure rating higher than the line pressure upstream of the regulator, overpressure protection -- i.e., a pressure relief valve -- must be installed.
Determine the Codes, Approval and Insurance Requirements of the Equipment. Oven and dryer manufacturers need to be aware not only of the insurance requirements of FM and GE Global Asset Protection (GAP) Services (formerly IRI) but also applications but also application codes such as NFPA 86. Local codes such as Minnesota's requirement for gas shutoff valves with a proof-of-closure switch -- including over-travel -- would eliminate the possibility of using most solenoid valves.
Some gas valves offer features that may provide for easier mounting of accessories. Valves are available in single- or dual-bodied designs. Most single- or dual-bodied valves have convenient taps for mounting pressure switches. Some dual-bodied valves offer full port connection for easy install of vent lines (required by IRI code).
Modular gas trains are now typical, taking up less space and allowing manufacturers to meet varied capacity and pressure regulation requirements. Modular gas trains have been particularly beneficial on washer systems and other applications with a small installation area.
In addition to their compact design, modular gas valves also offer cost savings by decreasing the labor time and pipe fittings during assembly. When comparing modular designs, be sure to compare the time required to assemble each modular assembly.
Opening Time of the Gas Valves. Fast-opening solenoid-type valves typically are used in pilot lines, pulse-fired furnaces or low capacity applications. A fast-opening valve may provide for a rough, gas-rich ignition when improperly used on large burners. By contrast, slow-opening valves typically will begin to have gas flow through the valve within 1 sec and reach full-open position within 6 to 14 sec. These are used most commonly on the main gas trains for capacities that exceed 400,000 BTU/hr.
Trial-for-Ignition Period. The time allowed by the flame safeguard to establish flame also must be taken into account. The gas train must be located to allow sufficient time for the gas to reach the burner and ignite reliably.
Process Criteria. When evaluating gas valves, be sure to include any relevant process criteria. This can include the location of the gas train and the type of process application. For example, when selecting gas valves for a paint finishing line, one should ensure that the components and lubricants in the valves are silicone-free. Silicone has been found to cause paint "fish-eyes" in automotive paint finishing applications and is not allowed in many automotive plants.
Serviceability. The serviceability of the gas train components needs to be evaluated. For example, when a gas valve becomes inoperable, what needs to be replaced? Usually, the mechanical valve body will outlast the electrical actuator. Some manufacturers require removal of the entire valve and, consequently, the re-piping of the gas train. Customers and service personnel will appreciate valve components that allow replacement of the gas valve actuator without opening the gas enclosure or unpiping the train.
Take a fresh look at your combustion system. There is no time like the present to evaluate ways to improve your processing business.