Partial stroke testing solution
Richard Harvey examines the strengths and weaknesses of traditional partial stroke testing solutions and looks at where new technology is enabling advancement.
Partial stroke testing (PST) is an important tool for oil & gas operators, providing a method of proving the safety and performance of emergency shutdown valves (ESVs). For many years the technique has helped prove the functionality of final element assemblies by testing a percentage of the failure modes of the final element assembly. However, the two most well used tools for PST do exhibit a number of drawbacks.
PST was originally made possible through the use of simple mechanical systems which could only offer testing. As the need for diagnostic capabilities grew, ‘smart’ systems evolved to take PST to the next level. Today, two main types of system exist – positioners and electronic – both offering different approaches and each having to make compromises to ensure they achieve the original goal of demonstrating safe performance of an ESV.
Positioners are currently the most common form of smart PST system available. They can be a complex solution as although the device can easily be configured to give any desired percentage of partial stroke, other components may need to be added to maintain the desired stroking speed. Derived from control valve position technology, PST positioners were the obvious choice for an immediate replacement for mechanical testing as they already had the built-in capability to reliably move a valve to a given position.
Further improved with the addition of an emergency shutdown (ESD) function, positioners became a fundamental part of the control system for an actuator. This development offered an additional benefit, allowing them to be used to create an effective redundant ESD when used in conjunction with a solenoid valve.
However, positioners have compromises. For example, they do not test the valve at the full designated operating speed, meaning a question mark exists over the results they produce. Their use requires a reconfiguration of the pneumatic control, they can only operate across a limited flow rate, and they are not compatible with all valve types. They also have high safe failure rates, which can result in spurious trips of the safety function. Difficult to commission, install and service, they also give reduced diagnostic coverage which was one of the main misgivings of the mechanical systems they replaced.
As a means of alleviating the shortcomings of positioners as a method of PST, many manufacturers and oil & gas operators turned to electronic systems. Able to perform a partial stroke in a similar way to that which is required in an emergency situation, electronic systems can deliver the required level of diagnostics while also addressing issues associated with positioners. Most of these systems use electronic control boxes which connect the power supply to the solenoid valve and are fitted in close proximity to the actuator itself. They de-energise the solenoid valve in order to perform the partial stroke test, monitoring either the valve position or instrument pressure or both in order to determine the success of the test.
Despite solving some of the issues associated with positioners, electronic systems also have limitations. For example, additional equipment is usually required – such as an electronic control box – which makes the system more costly as well as harder to service. The safety integrity level (SIL) performance can also be compromised through solenoid valve selection while electronic systems can create a spurious trip risk.
While both positioners and electronic systems can offer benefits, neither system was able to offer a holistic, fully reliable solution and given the inherent safety risks of oil & gas operations, there was clearly room for improvement.
In developing a hybrid solution, using the strengths of both the positioner and electronic systems while eliminating the weakness of each, solenoid valves were the starting point for IMI Precision Engineering. It was noted that electronic systems provided simpler operation as no extra pneumatic components were required, while positioners offered the advantage of the diagnostic system being built into one of the control elements. The logical conclusion, therefore, was to integrate the diagnostic system into one of the final element components, and install the diagnostic system into a high integrity solenoid valve. This led to the creation of the first fully integrated PST solution with low safe and dangerous failure rates. Designated the ICO4-PST, the unit combines the company’s high-integrity solenoid valve with an electronic control system to provide high-level diagnostics, simple pass/fail reporting and reduced engineering requirements as well as eliminating the possibility of over-stroke.
Richard Harvey is business development manager at IMI Precision Engineering.>
Source: Control Engineering Europe - All Articles