When designing fuel trains for an industrial furnace or oven, one of the key parameters you should strive to maximize is flow.
Poor flow in a fuel train can limit the performance of the burner and can also put your entire system at risk of shutting down in the event of low fuel pressure. To avoid these outcomes, each component within the gas train must be optimized to enhance flow performance and the overall efficiency, reliability and cost of your industrial burner system. Let’s dive into what flow is, including its features and benefits, and discuss some strategies to maximize flow within your industrial burner application.
What is flow?
For the purposes of this article, we will discuss flow in terms of volumetric flow, or the volume of fluid flowing through a system over a period of time (e.g., gallons or liters per minute). When discussing fuel-train components like a safety shutoff valve or pressure regulator, flow is often discussed in terms of the flow coefficient (Cv or Kv) – the higher the Cv or Kv, the better the flow.
We can define Cv as the number of gallons per minute of water at 60°F flowing through a system with a pressure difference of 1 PSI between the inlet and outlet. We can define Kv as the number of liters per minute (or cubic meters per hour) of water at a temperature between 5-40°C with a pressure difference of 1 bar between the inlet and outlet. By defining a standard fluid, temperature and pressure differential in this way, fuel-system designers can make apples-to-apples comparisons between components.
Flow is closely linked to another key parameter called pressure drop or head loss. As the fluid moves through a system, each component imparts some resistance or “friction” to the overall flow, resulting in a pressure drop. Pressure drops are additive across the fuel train and must be overcome for the system to operate properly. For example, if a fuel train has a total pressure drop of 5 PSI and the source pressure is 3 PSI, then the burner won’t be able to operate. We can think of both pressure drop and Cv as inversely related – as the Cv of a component increases, then the pressure drop decreases.
The Variables Impacting Flow
When looking at a fuel train, there are a few areas in particular that can impact flow.
- Media. When considering the media, or fluid flowing through the system, density, temperature and viscosity can all impact flow rate. For example, cold oil is more viscous than hot oil and, as a result, will flow more slowly through the fuel train.
- Mechanical components. A mechanical component’s cross-sectional area and geometric complexity can impact the flow rating of the device. In general, larger components with a more direct flow path will have better flow rates. For example, valves with a monoblock design (two valves in one) are designed to have superior flow compared to two single valves piped in series.
- System design. Lastly, we must consider system design. Long piping with many bends and many devices can have an adverse effect on flow. In addition, for incompressible fluids like oil, if the starting elevation of the fuel train is lower than the end elevation, gravitational forces will apply their own pressure loss.
Fuel trains with poor flow performance can have a significant impact on the overall effectiveness and cost of your industrial burner system. Poor flow systems require a significant amount of inlet pressure for the burner to fire. If a system is installed in an area with low or fluctuating fuel pressure, the burner may not light or will periodically shut down.
For example, in densely populated cities with older gas infrastructure, periods of high gas demand (e.g., a cold winter day) can cause gas pressures across the region to drop. As a result, older systems with poor flow may shut down when heat is needed the most. Although this situation can be remedied by adding a booster pump to increase inlet pressure or by selecting larger pipe sizes and components, both solutions can significantly drive up costs.
ASCO Series HOV13B Hydramotor valve (left), Series 266 oil shutoff valve (middle) and Series 377 shutoff valve (right)
The Benefits of a Three-Way Valve Design
Some valves – like ASCO Series HOV13B Hydramotor valves, Series 266 oil shutoff valves and Series 377 shutoff valves – are capable of three-way flow. Thanks to this design, the valve units will recirculate oil back to the heater when closed, keeping the oil from becoming too viscous. These valves are suitable for applications like burners, furnaces, incinerators, ovens and other heating equipment.
ASCO Series 158 gas valve and Series 159 motorized actuator
Comparing the Flow of Two Valves (U.S. Standard Units)
The following example compares the flow of two valves using the following equation:
Where:
Q = Flow in gallons per minute (gpm)
Cv = Flow coefficient
ΔP = Pressure differential between the valve’s inlet and outlet
SG = Specific gravity of the fluid, or the ratio of a fluid’s density compared to water
SG of water = 1
SG of natural gas = about 0.60 to 0.70, with variations due to composition
SG of diesel = about 0.82 to 0.95, with variations due to composition
Valve 1: ASCO Series 158 single body valve
Pipe size = 1.5 inches
Cv = 59
Valve 2: Competitor Valve
Pipe size = 1.5 inches
Cv = 40
Media: Natural gas
SG = 0.65
ΔP = 5 PSI
Results: Valve 1 has 48% more flow compared to valve 2.
Valve 1 Flow (Q) = 164 gpm
Valve 2 Flow (Q) = 111 gpm
Comparing the Flow of Two Valves (Metric Units)
The following example compares the flow of two valves using the following equation:
Where:
Q = Flow in gallons per minute (m3/hour)
Kv = Flow coefficient
ΔP = Pressure differential between the valve’s inlet and outlet
SG = Specific gravity of the fluid, or the ratio of a fluid’s density compared to water
SG of water = 1
SG of natural gas = about 0.60 to 0.70, with variations due to composition
SG of diesel = about 0.82 to 0.95, with variations due to composition
Valve 1: ASCO Series 158 single body valve
Pipe size = DN 40
Kv = 5
Valve 2: Competitor Valve
Pipe size = DN 40
Kv = 35
Media: Natural Gas
SG = 0.65
ΔP = 345 mbar
Results: Valve 1 has 48% more flow compared to valve 2.
Valve 1 Flow (Q) = 37 gpm
Valve 2 Flow (Q) = 25 gpm
Strategies to Maximize Fuel-Train Flow
Selecting the right fuel-train components can overcome these issues without breaking the bank, and there are a few strategies to consider when maximizing a fuel-train system’s performance.
- Focus on high-impact components. In a typical system, up to 70% of pressure drops are caused by the two safety shutoff valves. If the goal is to maximize fuel-train flow, it’s imperative to select a shutoff valve with the highest flow coefficient (Cv or Kv) rating for the given pipe size. Newer safety shutoff valves have been optimized for flow. In some cases, a fuel train can be designed one or two sizes smaller while maintaining performance, reducing the overall cost of the system.
- Minimize the number of components. A typical fuel train contains the following components: a regulator, two safety shutoff valves, pressure switches and a flow control valve. The current trend in the industrial burner industry is to combine these components into a single valve body – typically called a monoblock or double-body valve. This design accomplishes a few things. It improves the flow of the overall system, reduces the footprint of the overall fuel train and reduces installation and maintenance time.
- Preventive maintenance. Regular maintenance is critical to ensure optimal fuel-train performance. Some safety shutoff valves come with an integrated strainer, which protects downstream devices from contaminants in the fuel line. When performing maintenance checks, be sure to clean the strainer and ensure it’s not a source of flow blockage.
Conclusion
Designing a fuel-train system with high-flow components or retrofitting an existing system with new technology can boost the performance of industrial burner systems significantly, unlocking greater reliability, cost savings and energy efficiency.
For more information: Yussef Abou-Ghanem is product marketing manager of Combustion, Americas, at Emerson. He helps customers improve the safety and efficiency of their industrial combustion systems. He can be reached at yussef.aboughanem@emerson.com or 973-966-2578. Contact Emerson at www.emerson.com.
All graphics courtesy of the author except where noted.
