For over 100 years, preventing catastrophic failure in the process control industry has been a continuously evolving area of study. To protect life, the environment, reputations and capital investments from compromise, we deliberate over safety and risk in every aspect of a system. As a positive result, systems of designed failure scenarios are strategically located throughout chemical refineries, power plants, floating vessels and offshore rigs. Understanding how process failure modes are achieved and where certain valves and actuation fit into the scenario is key to our efforts.
FAIL-SAFE AND ACTUATION
Within designed fail-safe systems, industrial process control valves are complexly categorized by the method by which each piece of equipment interacts with a process fluid. The flow of that process can be cut, choked, throttled and pinched. From an actuation and controls perspective, valves are more simply categorized by the method of actuation required to operate that valve: rotary and rising stem.
This article focuses on the latter.
Different rising stem alternatives include gate, globe, choke and pinch valves. Each have unique operating characteristics that lend themselves to preferred applications. For example, globe valves are suitable for throttling applications where flow of a process must be precisely controlled. The primary purpose of gate valves is to permit or prevent flow of processes. Pinch valves are ideal for slurry processes that contain solids such as pellets, powders and granules.
Let’s look closer at failure options for gate valves.
When specifying a gate valve failure scenario, we have three options: fail open to permit flow, fail close to prevent flow and fail in last position. The first two options are active failure positions, whereas the third option is a passive failure position. Bear in mind that valve stems on most gate valves experience linear translation during operation to open and close. An engineer is most likely to specify an active failure position with an intention of controlling the process.
The ability to predict behavior and flow of a process during failure is advantageous in scenarios where pressure relief and pipeline isolation are critical to system integrity. In many cases, availability of electricity, pressurized air and pressurized hydraulic fluid dictates the method of actuation used in achieving a failure position. For purposes of this article, we will assume an ideal scenario where all sources of power are available. Let’s zoom in to look at the actuation and controls required to achieve an active failure position on a gate valve.
The engineer has five options when considering a valve actuator solution:
- Multi-turn electric with auxiliary power source
- Spring-loaded diaphragm
- Spring-loaded piston
- Double-acting diaphragm with power storage tank
- Double-acting piston with power storage tank
These systems are all capable of storing potential energy to aid in achieving an active failure position. Let’s take a moment to understand the four presented methods of storing potential energy and their effects on system reliability.
Coiled springs serve as an integral component of the valve/actuator assembly. Springs are a reliable and resilient method of storing potential energy. They act independently of electricity, pipe/connection leakage, severe temperatures and most other types of non-destructive interaction. Tight shut-off applications require springs with large thrusts, which increases the size needed to overcome initial spring forces. Using a coiled spring to increase system reliability demands an increase in actuator size. The attractive feature of spring-loaded systems is that they can store potential energy for extended periods of time with near zero loss.
Hydraulic accumulators are supplementary devices to the main actuator/valve system. This type of system is susceptible to pressure fluctuations in severe temperature variances because pressurized gases are used in maintaining hydraulic pressure. Also, the potential for loss of stored power exists through piping leakage. Aside from these few vulnerabilities, hydraulic accumulators are a reliable method of storing pressurized hydraulic fluid because of their relatively classic concept and design.
Air receivers also serve as supplementary devices to the main actuator/valve system. In these systems, stored compressed air is subject to fluctuations in pressure (stored power) with large temperature variations, while the potential for loss of stored power exists through piping leakage. Aside from the large system footprint of these systems, they are highly reliable with routine maintenance and particularly advantageous in maintaining the ability to alternate between fail-open and fail-closed modes with simple piping changes.
Lead-acid batteries, supercapacitors and backup generators are auxiliary components of the valve/actuator assembly. Backup generators are a reliable source of power supply open to continuous monitoring and commonly found on large tank farms and applications where space is limited at the valve point. Generators provide a single point of backup power supply that can be centrally located while serving multiple valve actuators.
Let’s consider the application of an emergency shutdown valve/actuator system intended for a low-pressure pipeline crossing a water stream. This system would be designed to isolate fluid supply if a predetermined pressure drop between two points is sensed (pipeline rupture). An automated gate valve would be a critical component in safeguarding the integrity of the surrounding community, wildlife and environment. The demanding failure-mode characteristics shown below should be taken into consideration when selecting an actuator solution for this situation.
- Trip criteria: When pipeline pressure, electricity or pneumatic pressure is lost.
- Active failure position: Closed (preventing flow)
- Sensing reaction speed: 1 second
- Failure position speed: 6 seconds
These constraints are a common system design scenario experienced by engineers at organizations around the world. Although many end users value the peace of mind provided with spring-return actuators, space and weight constraints occasionally make it more beneficial to use lighter and more compact systems with air receivers or backup power supplies. As a result, the engineer is often tasked with specifying an optimized combination of failure scenario, weight, equipment size, safety, materials and system reliability. After many years of research, investigation, testing and design, a universal process control solution refuses to present itself. However, industry continually improves established codes, best-practices, standards and proven technologies. With careful design consideration, any combination of the previously mentioned actuator and power storage solutions can be deemed safe and suitable for use.