A Guide to Choosing the Right Control Valve for Combustion Processes

Combustion engineers understand, better than most, that a process is only as reliable as its weakest link. And in a combustion system, few components carry more weight than the control valve.

Select the right valve and the system responds the way you need it to. But select the wrong one, and you’ll spend the next several months chasing instability, fighting inefficiency, or worse, troubleshooting a safety issue.

The frustrating part is that many control valve problems don't announce themselves at startup. They show up gradually, like a burner that hunts at low fire, an air-to-fuel ratio that drifts under load, or a valve that cycles constantly near its seat because the sizing was based on peak demand rather than actual operating range.

By the time the problem is visible, the root cause is often weeks or months in the past.

Let’s walk through the key decisions you need to make in control valve selection for combustion applications. In doing so, we’ll look at this from the perspective of how your system actually behaves in the field. This can help you keep operations running smoothly and avoid expensive downtime.

Why Combustion Applications Demand More from Control Valves

Most industrial processes ask a control valve to do one thing well. But combustion asks a valve to do several things well, and simultaneously, across a wide range of conditions.

Consider what a fuel gas control valve actually faces in service. It needs to:

• Modulate cleanly across a broad firing range, from low-fire trim to full load, without hunting or causing pressure disturbances that affect burner stability
  
  
• Respond quickly enough to follow demand changes (but not so aggressively that it introduces oscillation into the control loop)
  
  
• Hold tight shutoff when the burner trips

And it needs to do all of this reliably, at elevated temperatures, with process fluids that may vary in composition.

That's a demanding set of requirements. The good news is that matching a valve to those requirements is a learnable process, and it starts with understanding what the valve is actually being asked to control.

Valve Types: What Each One Is Good At

There is no single valve type that performs best in every combustion application. The choice depends on what you're controlling, at what scale, and under what conditions.

Globe valves are the most common choice for fuel gas modulation, and for good reason. Their inherent flow characteristics make them well-suited to throttling service, and they offer good controllability across a wide range of operating points. When the application requires precise, stable fuel flow regulation, the globe valve is often the starting point.

Butterfly valves make more sense when the task involves larger line sizes or high-volume airflow control. Combustion air handling and forced draft systems are natural fits. The tradeoff is that butterfly valves typically offer less precise control at low flow conditions, which matters in applications where turndown is a priority.

Ball valves show up in combustion applications primarily in on/off or limited-throttling service. Their fast actuation makes them useful in burner management sequences, where timed shutoff or rapid cycling is required. They are generally not the right choice for continuous modulation across a wide firing range.

Rotary and eccentric plug valves occupy a useful middle ground. They can deliver tight shutoff alongside reasonable rangeability, which makes them worth considering in higher-pressure fuel applications where both shutoff quality and modulation range matter.

The right starting question when evaluating valve type isn't which valve is best in general. It's which valve will fit the specific service conditions, flow characteristics, and control requirements of the application in front of you.

Sizing and Selection: The Decisions That Matter Most

Getting the valve type right is only part of the job. Sizing and specifying the valve correctly is where the real technical work happens.

Flow Coefficient and Rangeability

The flow coefficient, or Cv, describes how much fluid a valve can pass at a given pressure drop. Oversizing is one of the most common control valve problems in combustion applications.

An oversized valve operates near its closed position under normal conditions, which means small movements of the actuator create large changes in flow. Control becomes coarse and unstable, and the valve is subjected to erosion and wear in a narrow zone of its travel.

Proper sizing matches the valve's Cv to the actual operating range of the system, not just the maximum capacity. Rangeability matters here. A valve with a high rangeability can maintain acceptable control accuracy from low fire to high fire without losing precision at either end. For combustion applications with significant turndown requirements, this is worth prioritizing.

Flow Characteristics

A valve's flow characteristic describes the relationship between valve travel and flow rate. Equal percentage characteristics, where flow increases by a consistent percentage for each increment of valve travel, tend to suit fuel gas applications well. The response is more linear in terms of process effect when system pressure drop varies with flow, which is common in combustion systems.

Linear characteristics can be appropriate where pressure drop is relatively constant across the operating range, but this is less typical in practice.

Selecting the wrong characteristic for the application produces a control loop that behaves differently at different operating points, which makes tuning difficult and often leads to cycling or sluggish response in parts of the firing range.

Temperature, Materials, and Sealing

Combustion environments are hard on components. Body materials need to match the fluid and temperature envelope of the application. Carbon steel is adequate for many fuel gas services, but stainless steel construction becomes important when the process involves corrosive gases, high temperatures, or fuel oil service (where elevated heat can degrade standard sealing materials).

Seat and seal materials deserve the same attention. High-temperature sealing requirements in fuel service are not the place to make a cost-driven tradeoff. A seal that degrades under service conditions compromises shutoff quality, and in a burner management context, shutoff quality is a safety function.

Actuation and Fail-Safe Positioning

The actuator is part of the control system, not an accessory. Pneumatic actuators remain the most common choice in combustion applications, offering reliability and well-understood behavior in control loops. Electric actuators have become more capable in recent years and are worth considering where instrument air supply is limited or where precise positioning feedback is a priority.

More important than actuator type, in most cases, is fail-safe positioning. When power or signal is lost, the valve needs to move to a position that protects the process. For fuel gas control valves, fail-closed is typically the correct fail-safe position. This should be specified explicitly and verified, not assumed.

Shutoff Classification

Control valves in fuel service are also safety devices. ANSI/FCI 70-2 defines leakage classes for control valves, from Class I through Class VI, with Class VI representing the tightest shutoff. For fuel shutoff applications within a burner management system, the leakage class should be treated as a safety specification, not a performance preference. Verify that the valve you select meets the shutoff requirements your system demands.

Common Selection Mistakes Worth Avoiding

A few patterns come up repeatedly when control valve problems trace back to selection decisions:

• Oversizing, as already noted, is the most common. The instinct to size for worst-case conditions produces valves that work poorly under normal conditions, which is most of the time.
  
  
• Ignoring actuator response time relative to burner management system requirements is another. If the BMS expects the fuel valve to close within a defined time window on a safety trip, the actuator needs to be specified to meet that requirement. A valve that closes slowly in a safety event is a liability.
  
  
• Selecting materials based on cost alone, without fully accounting for the operating temperature and fluid composition, tends to produce early failures and maintenance headaches that cost more in the long run than a better initial specification would have.
  
  
• Treating shutoff class as a secondary consideration, rather than a fundamental requirement in fuel service, understates the role the valve plays in system safety.

Working with a Distributor: What Good Support Looks Like

Combustion engineers often have established manufacturer relationships, and those relationships have real value. But even experienced engineers benefit from working with a distributor who understands the application deeply, not just the product line.

A knowledgeable distributor can help evaluate options across manufacturers, identify configurations that match the actual operating conditions, and reduce the friction that comes with specifying and sourcing components across multiple vendors. When the application has unusual requirements, such as high-temperature fuel oil service or a tight-shutoff requirement in a complex burner management system, that application knowledge matters as much as product availability.

At ACI Controls, we’ve spent more than 30 years working with combustion engineers across the Northeast and Mid-Atlantic on exactly these kinds of specifications. We carry products from Parker and other leading manufacturers and bring hands-on application experience to every conversation.

When the selection decision is straightforward, we can help confirm the right choice quickly. When it's complicated, we work through it with you.

Getting the Selection Right, the First Time

Control valve selection in combustion applications rewards careful thinking. The right valve for the job is the one matched to the actual service conditions, sized for the real operating range, specified with the correct materials and shutoff class, and paired with an actuator that supports the safety requirements of the system.

Get those things right, and the valve becomes a non-event. It does its job, the process runs well, and you're focused on things that actually need your attention.

If you're specifying control valves for a combustion application and want a second set of eyes on the selection, we'd be glad to help. Reach out to our team or browse our control valve offerings to get started.