Digital valve controllers, like this Fisher™ FIELDVUE™ DVC7K Digital Valve Controller from Emerson, employ a variety of sensors and electronics to ensure even very large valves reach their commanded position despite variable interferences.

LNG facilities and pipelines rely on valve positioners to provide precise and reliable flow control. My article in the January 2025 issue of LNG Industry on pages 35-38, titled “Wise Valve Selection Improves Performance,” discusses key features of modern digital valve controllers that can be used to improve performance and reduce costs.

The purpose of positioners

Valve positioners perform continual adjustments to ensure that the actual valve position matches the commanded position, despite changing process conditions. Modern digital valve controllers provide even greater positioning precision, as well as advanced diagnostic capabilities.

Stopping the bleed

To facilitate very fast response, many valve controller designs use a mechanical balance technique, also referred to as a force beam, connected to the valve position sensor. When the slightest error is detected, the mechanism will move, varying the signal to a high gain pneumatic relay, which adjusts the actuator air pressure to move the valve to the correct location.

Unfortunately, these designs tend to bleed instrument air, even when the valve is not moving. Low-bleed models reduce this issue, saving substantial amounts of money, especially when natural gas is used as the actuating medium instead of air.

A low bleed digital valve controller reduces air and/or natural gas lost to continuous bleed. In the case of natural gas actuated valves, the savings are considerable.

Other types of positioners

There are several alternatives to mechanical balance beam positioners. Spool relay designs have existed for decades, but they are sensitive to instrument air quality and tend to have a slower response. Piezoelectric designs have limited sensitivity to small changes in valve command signal, but unlike mechanical balance beam designs, they have the advantage of essentially zero bleed in the neutral position.

However, a low amount of bleed can be beneficial because it prevents outside plant air from entering the digital valve controller, and in practice, the difference in emissions between low- and zero-bleed designs is insignificant because most control valves are continually in motion.

Selection criteria

While reducing bleed is important, other design factors can contribute additional savings and should also be considered during positioner selection:

• Performance: Tight valve control often yields improvements in process stability and product quality.
• Sensors and diagnostics: Digital valve controllers can monitor many valve parameters and detect problems before performance is impacted.
• Remote alerts: Digital connectivity enables valve controllers to immediately notify maintenance staff of issues.
• Safety: Advanced diagnostics and partial stroke testing features enhance safety and can extend SIL interlock testing intervals for critical valves.

Data management

Staff at LNG facilities should judiciously collect and use digital valve controller positioner data to maximize savings. Whenever a valve is placed in service, baseline tests should be performed to obtain useful data for future troubleshooting. During turnarounds, comparing current to baseline data can be used to limit unnecessary valve removal and servicing.

Baseline valve performance information, like this valve signature test, should be captured when commissioning a control valve so it can be compared against future stroke performance to spot developing issues and troubleshoot failing components.

Virtually all digital valve controllers benefit from enabling three basic alerts. The travel deviation alert, air supply pressure alert, and drive signal alert are simple to configure and typically provide significant long-term value. Ideally, these alerts should be integrated into broader maintenance management systems.

Selecting the right digital valve controller can dramatically improve plant operations and reduce costs. LNG facilities should look for low-bleed models with embedded diagnostics and digital communication abilities as they will provide peak performance.

Visit the Digital Valve Controllers section on Emerson.com to know how you can meet your process reliability and performance goals by bringing intelligence to your entire valve assembly.