Redundant Solenoid Valve Architecture Improves Safety and Reliability

Redundant solenoid valve architectures allow facilities to perform critical solenoid valve testing while keeping the safety system online.

By allowing facilities to perform critical solenoid valve testing online, redundant architectures help keep personnel and equipment safe while reducing downtime.

Regular solenoid valve testing prevents stiction-related valve failure that can lead to disaster. Redundant architectures make it possible to perform this critical maintenance with no process interruption.

Although it may not be the number one task on every plant’s maintenance to-do list, exercising the solenoid valve is critical to operational safety.

If it goes long stretches without activity, the solenoid valve can experience stiction. Stiction is a lot like it sounds: It’s the sticky friction that can keep stationary surfaces in contact with one another from moving apart. Periodically exercising valves both prevents stiction and verifies it isn’t there to begin with.

While exercising any valve is necessary, it’s downright critical when it comes to the solenoid. If the solenoid valve fails due to stiction, the emergency shutdown (ESD) block valve will not be able to stop process flow. Because this flow can be flammable or, worse, explosive, the ESD valve’s failure to close can be catastrophic.

So why might this critical maintenance not be performed consistently? 

Quite often, it requires downtime. Luckily, there’s a simple solution that allows users to regularly exercise the solenoid valve without interrupting their process.  

A two- or three-solenoid safety methodology enables online valve testing and maintenance, providing greater protection and improving uptime while complying with functional safety requirements.

Facility safety depends on the reliability of the solenoid valve

In facilities like oil refineries, a safety instrumented system (SIS) is legally required on any machinery that processes hazardous chemicals. While the basic process control system, by way of alarms and user intervention, is the first line of defense against incidents, the SIS adds an extra layer of safety that prevents and alleviates possible hazards (see Figure 1).

An SIS typically consists of sensors, logic solvers or controllers and final elements. Sensors measure process parameters, such as pressure, temperature, flow, level and gas concentration. Logic solvers or controllers translate signals from sensors and complete preprogrammed actions to prevent or alleviate process dangers. Final elements, which include the ESD valve and a pneumatic, electric or hydraulic actuator and solenoid valve, bring the process to a safe state.

During normal operation in many SIS applications, the solenoid valve remains energised in the open position. (This indirectly acknowledges energise to trip applications.) If the system senses overpressure or other hazardous conditions, the valve moves to the closed position, which activates the ESD valve to stop the flow.

Yet, in spite of the critical part it plays in protecting the facility and its people, the solenoid valve is rarely used. As it happens, it can remain in the open position for months, even years in some cases. The longer it remains inactive, the more likely it is that it will experience stiction and fail to close when it’s most needed.

Exercising the solenoid valve prevents stiction-related valve failure

In order to better understand how stiction leads to valve failure, it’s important to outline the forces at work within a typical solenoid valve. In most solenoid valves, the O-rings stay in direct contact with the chamber walls. This constant contact creates a seal while the plunger is in motion. To move, the plunger must first overcome the stiction between the O-rings and walls.

If the solenoid valve remains in the same position for a long time, the stiction increases beyond its standard level until the forces generated by the solenoid coil can no longer overcome it. This prevents the valve from closing. And if water or oil is present, these can cause a sticky residue inside the valve that makes overcoming stiction even harder.

Since the possibility of stiction grows incrementally as time passes, infrequent solenoid valve testing increases the probability of valve failure on demand (PFD). Regular testing reduces stiction and lowers the average PFD.

“Mechanical devices, like the human body, work well when they’re regularly exercised,” said Dr. Angela Summers, president of SIS-TECH Solutions. “When you don’t exercise the valve, you’re increasing the potential that it could stick, which could prevent the valve from closing when it needs to.”

In the worst case, a solenoid valve that doesn’t close when it’s needed can lead to potentially deadly risks — like fires and explosions — and complete plant shutdown. In addition to serious danger to personnel, solenoid valve failure can disrupt production and raise costs.

Redundant solenoid valve architectures allow online solenoid valve testing

To prevent stiction and its possible dangers, testing is as simple as bringing the valve through a single cycle. To cycle the solenoid, fully close the valve by de-energising the solenoid coil, then return the valve to the open position by reenergising the coil.

But as easy as it is, exercising the solenoid valve requires taking the SIS offline, which results in downtime. For many facilities, this downtime is real, while the possible danger from solenoid neglect is hypothetical. The cost to productivity may simply outweigh the threat.

Thankfully, facilities no longer have to choose. By adding an additional solenoid valve or two to the ESD valve design, each solenoid can be tested individually and the SIS system can remain online. Operators can even program the controller to automatically run online testing at scheduled intervals, preventing valve stiction, reducing the average PFD and improving facility safety — without any downtime whatsoever.

Installing a second solenoid valve isn’t particularly time-consuming or even complicated. In fact, there is a prepackaged solution that uses redundant solenoid configurations. The redundant control system (RCS) is a proven pilot valve arrangement that provides built-in redundancy and diagnostics.

With no single point of failure, the RCS optimises facility safety and reliability while reducing downtime (see Figure 2). It uses a 2oo2D or 2oo3D architecture and, in addition to redundant solenoid valves, includes pressure switches and a maintenance bypass switch in one easy-to-configure package that helps facilities meet necessary safety requirements. Its high safety availability means that the RCS is SIL 3-certified and meets IEC 61508:2010 requirements for functional safety. And no point of failure means it also greatly reduces nuisance trips.

The RCS’s automated online testing, including solenoid valve and partial stroke tests, allows users to identify 98% of hazardous failure points with no bypassing, while pressure switches provide continuous monitoring and diagnostic feedback. When the bypass is activated, the RCS also makes online maintenance, such as replacing solenoid valves, coils and pressure switches, fast and easy with no process interruption.

With redundant architectures and an all-in-one online testing and diagnostic solution like the RCS, there’s no longer a reason to delay critical solenoid valve testing. Facilities can test valves, even scheduling them to automatically run daily, weekly or monthly, without interrupting processes (see Figure 3). The high level of safety and reliability redundant architectures and the RCS provide helps protect people, equipment, productivity and, ultimately, peace of mind.

New 2oo3D Redundant Solenoid Valve Architecture Enhances Safety and Reliability

Building on the high safety and reliability of its 2oo2 architecture, a redundant control system (RCS) using a 2oo3 architecture is now available. For applications that require increased operational reliability and an extra level of safety, Emerson’s ASCO^TM 2oo3 RCS comes in a basic version as well as 2oo3D.

The 2oo3 Basic RCS and 2oo3D RCS are both comprised of three solenoid valves mounted in 2oo3 configuration, but they have notable features that benefit different applications. In the 2oo3 Basic RCS, valves are mounted inside a 304 or 316 stainless-steel enclosure or on a panel when an enclosure is not required. Optional 316L stainless-steel valves and pressure switches are suitable for use in corrosive environments, like locations exposed to salt, humidity and fluctuating temperatures.

In the 2oo3D RCS, valves are mounted inside a 316 stainless-steel enclosure. Like the 2oo2D RCS, the 2oo3D RCS includes pressure switches that provide diagnostic feedback and online testing, as well as a standard maintenance bypass that allows for online maintenance (see Figure 4).

In both 2oo2 and 2oo3 architectures, the RCS is suitable for use in SIL 3 applications and features the industry’s highest flow rates.

Dr. Angela Summers, president of the consulting and engineering firm SIS-TECH, who designed the technology behind the RCS, said, “Emerson spent a lot of time working to maximise the flow pathways of the manifold to achieve the fastest closure time possible while producing the smallest, lightest box for installation.”

How Different Architectures Affect PFD and STR

In order to provide ever greater safety and process reliability, safety integrated system (SIS) architecture has seen updates over the years. However, each new evolution has drawbacks as well as benefits (see Figure 5):

• One-out-of-one (1oo1). This foundational architectural design only features one element.
• 1oo2. This architecture improves safety by adding redundancy. While it does reduce the average probability of design failure on demand (PFD), it raises the spurious trip rate (STR). If either solenoid valve fails, it will trip the system.
• 2oo2. This design improves process reliability by adding redundancy. While it reduces the STR, it raises the average PFD.
• 2oo3. This architecture improves safety and process availability by adding advanced redundancy. It reduces the STR and average PFD but, because this design integrates more components, adds complexity and necessitates higher input/output (I/O) requirements and increased power consumption.
• 2oo2D. The RCS features 2oo2D, a newer configuration that integrates diagnostics that improve safety and provide high process availability. Its fault-tolerant architecture has no single point of failure and offers a much lower STR than 1oo2 and 2oo3 architectures.
• 2oo3D. The RCS featuring 2oo3D is now available. The newest configuration incorporates the same diagnostics as the 2oo2D, but an additional SOV increases operational reliability and improves safety. It, too, has no single point of failure and offers a much lower STR than 1oo2 and 2oo3 architectures.