Partial stroke testing (or PST) is a technique used in a control system to allow the user to test a percentage of the possible failure modes of a shut down valve without the need to physically close the valve. PST is used to assist in determining that the safety function will operate on demand. PST is most often used on emergency shutdown valves (ESDVs) in applications where closing the valve will have a high cost burden yet proving the integrity of the valve is essential to maintaining a safe facility. In addition to ESDVs PST is also used on high integrity pressure protection systems or HIPPS. Partial stroke testing is not a replacement for the need to fully stroke testing FST of valves as proof testing is still a mandatory requirement.
Partial stroke testing is an accepted petroleum industry standard technique and is also quantified in detail by regulatory bodies such as the International Electrotechnical Commission (IEC) and the Instrument Society of Automation (ISA).
The following are the standards appropriate to these bodies.
IEC61508 – Functional safety of electrical/electronic/programmable electronic safety-related systems
IEC61511 – Functional safety – Safety instrumented systems for the process industry sector
ANSI/ISA-84.00.01 – Functional safety: Safety instrumented systems for the process industry sector (an ANSI standard)
These standards define the requirements for safety related systems and describe how to quantify the performance of PST systems.
What are the benefits?
The benefits of using PST are not limited to simply the safety performance but gains can also be made in the production performance of a plant and the capital cost of a plant. These are summarised as follows:
Gains can be made in the following areas by the use of PST.
Reducing the probability of failure on demand (PFD)
There are a number of areas where production efficiency can be improved by the successful implementation of a PST system.
Extension of the time between compulsory plant shutdowns.
Predicting potential valve failures facilitating the pre-ordering of spare parts.
Prioritisation of maintenance tasks.
So it’s a good idea to implement a PST or HIPPS right? So why is it that so many operators/EPCs choose NOT to carry out regular PST then?
The main drawback of all PST systems is the increased probability of causing an accidental activation of the safety system thus causing a spurious trip and plant shutdown, this is the primary concern of initiating PST systems by operators and for this reason many PST systems remain dormant after the expensive CAPEX and installation costs.
Different techniques mitigate for this issue in different manners but all systems have an inherent risk.
In addition in some cases, a PST cannot be performed due to the limitations inherent in the process or the valve being used. Further, as the PST introduces a disturbance into the process or system, it may not be appropriate for some processes or systems that are sensitive to disturbances.
Finally, a PST cannot always differentiate between different faults or failures within the valve and actuator assembly thus limiting the diagnostic capability.
The biggest drawback that Operators have in utilising any PST system is in fact the fear that it will lead to a spurious trip and worse shutdown of the entire plant. This can cost hundreds or even millions to any plant or offshore platform facility. There are now PST systems that can eliveate the dredded spurious trip happening. Let’s face it NO one wants to be the one responsible for pressing the button to find they are responsible for bringing down an entire plant or process costing hundreds even thousands of pounds.
This technology such as TripGuard will make sure that the system (Valve) never goes beyond a certain point usually 15-20% of travel. Thus alleviating the common concerns of spurious trips and shutdowns.
This works because of the way in which certain Actuator’s and (PST) controller as a systems integrates together delivering the TripGuard system.
So there you have it, you can utilise PST on your offshore and on-shore critical ESD valves, but you don’t have to worry about the possibility of spurious trips and the cost associated with this.
Total Valve Solutions have conducted research into systems that can deliver. Please feel free to contact the following for further information: