engineering

Partial stroke testing

Partial stroke testing is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Partial stroke testing rather than just read about it. In short: 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.

Key takeaways

Partial stroke testing belongs to engineering; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Partial stroke testing to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Partial stroke testing from memory before moving on to harder problems.

Reference excerpt

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 high integrity 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 valves as proof testing is still a mandatory requirement.

Standards 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

Measuring safety performance IEC61508 adapts a safety life cycle approach to the management of plant safety. During the design phase of this life cycle of a safety system the required safety performance level is determined using techniques such as Markov analysis, FMEA, fault tree analysis and Hazop. These techniques allow the user to determine the potential frequency and consequence of hazardous activities and to quantify the level of risk. A common method for this quantification is the safety integrity level. This is quantified from one to four with level four being the most hazardous. Once the SIL level is determined this specifies the required performance level of the safety systems during the operational phase of the plant. The metric for measuring the performance of a safety function is called the average probability of failure on demand (or PFDavg) and this correlates to the SIL level as follows

One method of calculating the PFDavg for a basic safety function with no redundancy is using the formula

PFDavg = [(1-PTC)×λD×(TIFC/2)] + [PTC×λD×(TIPST/2)] Where:

PTC = Proof test coverage of the partial stroke test. λD = The dangerous failure rate of the safety function. TIFC = The full closure interval, i.e. how often the valve must be full closed for testing. TIPST = The partial stroke test interval. The proof test coverage is a measure of how effective the partial stroke test is and the higher the PTC the greater the effect of the test.

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

Safety benefits Using PST can reduce the probability of failure on demand.

Production benefits 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.

Drawbacks The main drawback of all PST systems is the increased probability of causing an accidental activation of the safety system thus causing a plant shutdown, this is the primary concern of PST systems by operators and for this reason many PST system remain dormant after installation. 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.

Techniques There are a number of different techniques available for partial stroke testing and the selection of the most appropriate technique depends on the main benefits the operator is trying to gain.

Mechanical Jammers Mechanical jammers are devices where a device is inserted into the valve and actuator assembly that physically prevents the valve from moving past a certain point. These are used in cases where accidentally shutting the valve would have severe consequences, or any application where the end user prefers a mechanical device. Typical benefits of this type of device are as follows:

The devices assure metal-to-metal prevention of stroke past the specified set point. Unlike some electronic systems, there is no need to commission and calibrate controls or continually train personnel, resulting in additional significant cost savings. The devices are vibration resistant, making them highly reliable. The risk associated with having an ESD event occur at time of manual mechanical PST may be considered statistically insignificant and allows a rational consideration of the advantages mechanical devices offer. Modular design allows for addition of limit switches, potentiometers, remote control operation, etc. The test is a comprehensive test of the logic solver and all final elements, only the sensing elements of the safety function are not tested. The valve is tested at the designed operating speed as it simulates an ESD event Jammers have a very low probability of causing a spurious trip. However, opinions differ whether these devices are suitable for functional safety systems as the safety function is offline for the duration of the test. Modern mechanical PST devices may be automated. Examples of this kind of device include direct interface products that mount between the valve and the actuator and may use cams fitted to the valve stem. An example of such a mechanical PST system: Other methods include adjustable actuator end stops.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Partial stroke testing

Start with the simplest possible case. Write down what Partial stroke testing claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Partial stroke testing before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Partial stroke testing ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Partial stroke testing

In research: Partial stroke testing appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Partial stroke testing in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Partial stroke testing is common in secondary-school and first-year university syllabi. It links to neighbouring topics Safety engineering, Safety equipment, Valves, so understanding it makes those chapters shorter.
In everyday life: Look for Partial stroke testing outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Partial stroke testing in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Partial stroke testing means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Partial stroke testing out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Partial stroke testing in simple terms?

Why does Partial stroke testing matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Partial stroke testing?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Partial stroke testing.