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:
The Valvescan Type VSD Controller is an Features and Benefits integrated valve information device for Emergency Shutdown (ESD) valves. Combining valve position monitoring and partial stroke test (PST) functionality, the VSD unit is an information hub for the ESD valve, enabling plant operators to verify the capabilities of the most critical valves in their installations without having to significantly modify existing operating methodologies.
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When control valve professionals talk about “control valve sizing,” they really mean the entire process of selecting the valve that will do the best job of controlling the process. Selecting the right size valve is an important part of the process, but there are other equally important considerations as well.
The control valve’s size should be selected so that it will operate somewhere between 60 and 80% open at the maximum required flow rate and whenever possible, not much less than 20% open at the minimum required flow rate. The idea is to use as much of the valve’s control range as possible while maintaining a reasonable, but not excessive, safety factor.
If the valve is too small, it will be obvious immediately, as it will not be able to pass the required flow. In actual practice, under sized valves are fairly uncommon.
Commonly, the valve is too large. An oversized control valve will cost more than is necessary, but more importantly, an oversized valve will be very sensitive.
Small changes in valve position will cause large changes in flow. This will make it difficult or even impossible for it to adjust exactly to the required flow. Any stickiness caused by friction will be amplified by the overly sensitive oversized valve, reducing the precision to which the flow can be controlled.
Rules of Thumb for Control Valve Sizing
In the illustration above, assuming that both valves are capable of positioning within 1%, the properly sized 3 inch valve will be able to control flow within 8 gpm, while the oversized 6 inch valve will only be capable of controlling flow to within 20 gpm.
Liquid applications must always be evaluated for cavitation. Not only does cavitation cause high noise and vibration levels, it can result in very rapid damage to the valve’s internals and/or the downstream piping. Especially with rotary valves, the prediction of damaging levels of cavitation is more complex than simply calculating the choked flow pressure drop.
As a result of flow separation and the formation of eddies within the valve, localised areas of pressure reduction and recovery can cause damaging cavitation at pressure drops well below that which results in fully choked flow. One proven method for predicting cavitation damage in rotary control valves is based on a correlation between calculated sound pressure level and the potential for damage.
In addition to the fact that a noisy valve in liquid service will most likely suffer unacceptable rates of cavitation damage, high noise levels usually cause vibration that can damage piping, instruments and other equipment. Control valves in steam and gas service can generate noise levels well in excess of plant standards, even at moderate pressure drops, especially in sizes above 3 or 4 inches. As a result, the valve sizing and selection process must always include noise calculations.
Installed Flow Characteristic
In nearly all applications, a control valve should have a linear installed flow characteristic (the relationship between controller output and flow in the system). The control valve’s inherent (published) flow characteristic interacts with the system’s flow vs. pressure loss characteristic to yield the installed flow characteristic.
If the installed characteristic deviates significantly from linear, it will be difficult or impossible to tune the loop for both accurate and stable control throughout the entire flow range. Acomputerised analysis of the installed characteristic should be part of the control valve sizing and selection process.
Sizing actuators for on-off service is fairly straight forward, requiring only that an actuator be selected with a torque output slightly higher than the seating and unseating torque of the valve. The situation is more complex with control valves. The torque output of most rotary actuators changes with the angle of opening. At the same time, the valve’s torque requirement depends both on the opening angle and the throttling pressure drop at that particular angle. To ensure adequate spare torque to guarantee smooth, accurate control, a computerised analysis is recommended.
Selecting Control Valve Style
The choice of control valve style (globe, ball, butterfly, etc.) is often based on tradition or plant preference. For example, a majority of the control valves in pulp and paper mills are usually ball or segmented ball valves. Petroleum refineries, oil and gas traditionally use a high percentage of globe valves, although the concern for fugitive emissions has caused users to look to rotary valves because it is often easier to obtain a long lasting stem seal with rotary valves.
Globe valves offer the widest range of options for flow characteristic, pressure, temperature, and noise and cavitation reduction.
Globe valves also tend to be the most expensive. Segment ball valves tend to have a higher rangeability, and size for size, nearly twice the flow capacity of globe valves, while they are typically less expensive than globe valves. On the other hand, segment ball valves are limited in availability for extremes of temperature and pressure and are more prone to noise and cavitation problems than globe valves.
High performance butterfly valves are even less expensive than ball valves, especially in larger sizes (eight inches and larger). They also have less rangeability than the ball valves and are more prone to cavitation.
The eccentric rotary plug valve combines the features of rotary valves, such as high cycle life stem seals and compact construction, with the rugged construction of globe valves. Unlike the other rotary valves whose flow capacity is approximately double that of globe valves, the flow capacity of eccentric rotary plug valves is on par with globe valves.
While the selection of a valve style is highly subjective, in the absence of a specified valve or plant preference, the following approach can be used to select a control valve style for applications where the valve will be six inches or smaller:
– Considering pressure, pressure differential, temperature, required flow characteristic, cavitation and noise, will a segment ball valve work?
– If a segment ball valve is not suitable, select a globe valve. Keep in mind that cage guided globe valves are not suitable for dirty service.
– For applications where the valve will be 8 inches or larger, first investigate the applicability of a high performance butterfly valve because of the potential for significant savings on cost and weight.
Today control valve sizing calculations are usually performed using a computer program. Most manufacturers of control valves offer control valve sizing software at no cost, though most are specific to that manufacture’s valves only. However some valve manufacturers software, includes a number of generic valves, such as globe valves, ball valves, plug valves and butterfly valves, to choose from.
These generic selections permit the user to investigate the applicability of different valve styles and sizes to a particular application, without showing a preference to a particular valve manufacturer.
Selecting a properly sized control valve is essential to achieving the highest degree of process control for the liquid, gas or multi-phase fluid. To ensure accuracy, use the following information for control valve sizing:
– If a set of loop tuning parameters only works at one end of the control range and not the other, the valve’s flow characteristic is most likely the wrong one.
– If a system has a lot of pipe, use an equal percentage valve.
– If a system has very little pipe, use a linear valve.
– A control valve that is sized to operate around 60% to 80% open at the maximum required flow and not much less than 20% open at the minimum required flow will give the best control.
Properly sized full ball, segment ball and high performance butterfly valves are usually two sizes smaller than the line.*
Properly sized globe valves are usually one size smaller than the line.*
– Most people consider it poor piping practice to use a control valve that is less than ½ the line size or larger than the line size.
– Oversized control valves are very common.
*If you size a valve and it turns out to be different than these, it is a good idea to check your work. You may have made a mistake, or the person who sized the pipe may have made a mistake.
Fisher Valve Specification Software
Published: 08 February 2016 Written by Rich Davis
The API Standard Committee on Piping and Valves has published some standards on fugitive emissions. While other standards do address the issue of fugitive emissions, the API standards have also attempted to address leak rates and other aspects of valve performance.
The standards committee started with API 622 Type Testing of Process Valve Packing for Fugitive Emissions. This standard establishes requirements and parameters for the following tests:
Packing Material Composition and Properties
The fugitive emissions testing includes 1510 mechanical cycles with five thermal cycles—ambient to 500°F. The test methods apply to packing for use in on-off valve rising stem and rotating stem motions.
The latest standard was published in 2011 and currently is undergoing revision. A test chamber for 1/8-inch packing is being added as a number of tests on API 624 have indicated issues with the smaller packing cross-section.
API is also changing the leak test monitoring equipment by opening up the types of equipment allowed to detect fugitive emissions. An important note on this section: The review of the packing manufacturers testing should include an overview of all sections of the testing.
The expectation from the valve buyer some standards on fugitive emission still be an extended warranty and this means that the potential for oxidation and volume loss due to the heat loss of lower temperature materials may cause premature failure, so it is essential to look beyond the fugitive emissions numbers and review all of the results.
The committee hopes the new standard will permit up to 100 ppmv methane leak rate with no allowed adjustments, along with the above-referenced changes.
Following the development of the API 622, the API 624 Type Testing of Rising Stem Valves Equipped with Flexible Graphite Packing for Fugitive Emissions standard was developed and published in 2012.
The API 624 standard specifies the requirements and acceptance criteria (100 ppmv) for fugitive emission type testing of rising and rising-rotating stem valves equipped with packing previously tested in accordance with API Standard 622. The fugitive emissions testing includes 310 mechanical cycles with three thermal cycles—ambient to 500°F.
An optional low temperature test at -2°F (-29°C) may be performed if requested by the purchaser. The elevated test temperature is to be 500°F ± 5% (260°C ± 2%).
The test pressure must be the lower of 600 psig or the maximum allowable pressure at 500°F per ASME B16.34 for the applicable material group and shall be held constant throughout the test. The packing must also be suitable for use at service temperatures 20°F to 1000°F (–29°C to 538°C). Valves larger than NPS 24 or greater than class 1500 are not covered in the scope of the standard.
Both of these current standards identify requirements of requalification, i.e., if the manufacturing location changes or if the valve design changes.
API is currently working on developing a standard to cover quarter-turn type valves. It will be titled API 641 Quarter Turn Valve FE Test.
The standard specifies the requirements for type testing quarter-turn valves for fugitive emissions and applies to all stem seal materials. Conversations have been held about the types of mechanical and thermal cycles, and the committee has settled on 100 ppmv as the maximum allowable leakage.
The wide range of quarter-turn valves complicates the development of the standard. Also, the inclusion of all types of stem seals further complicates the test temperature and pressure requirements.
Finally, the Upstream & Midstream API 6D Valves group has decided to develop a separate standard covering their valves and are currently working with the Association of Wellhead Equipment Manufacturers to make that happen. The group is aiming for completion by the end of 2016.
Many critical applications must be prepared in the event of a catastrophe, such as an explosion, fire, flooding, earthquake, ect. In the past, process engineers often specified remotely operated on-off valves for standby applications. However, today’s emergency shutdown (ESD) standards have been increased so that valves must now be applied using a strict code of testing, identification, auditing, and performance under adverse conditions associated with catastrophic event.
Under common ESD codes, before an ESD valve is applied in a critical application, it must be designed and tested so that the valve goes to the required fail safe position ( fail open, closed or fail in place) and then holds that position for a particular length of time under given parameters of a possible catastrophic event.
Of the three fail safe positions, fail closed is the easiest to achieve since the actuator spring and fluid pressure are both used to move or assist the valve to the required closed safe function.
However, although rare in emergency shutdown applications, the fail in place position often requires an independent power or air supply source to hold the required position. With the use of digital positioners such as DVC 6200 and controllers, throttling control valves used in ESD applications are expected to perform at a higher standard as a critical part of the safety instrumented function (SIF), which calls for a safety loop between distributive control system (DCS) and the ESD final control element.
When a catastrophic event occurs, sensors send a signal to the (SIF) which calls for a safety loop between the DCS and the ESD final control element. When a catastrophic event occurs, sensors send a signal to the SIF, which disconnects the power to the ESD control valve, moving the valve to the correct failure position.
For self-sustaining control valves employing smart technology with an internal safety integrity level (SIL) function, the digital positioners and controller are linked to self-contained sensors, which after sensing the event can employ the required failure mode. For this reason, the digital positioner or controller must be designed with the necessary Intrinsic temperature, and exterior conditional requirements of the ESD valve.
The likelihood of catastrophic event is rare, it’s critical that the ESD valve must be tested and designed for a particular mean time between failure MTBF parameter which is an important part of SIL requirements. The problem with most valves is when they stay in the one position for a very long period of time, they are progressively more prone to stick or require a greater force to move to the fail safe position upon failure. Knowing the MTBF for a particular valves allows maintenance or a failure test to be performed at certain time intervals to ensure proper performance.
ESD valve standards
The most common standards applied to ESD are IEC 61508, 61511. IEC (International Elecrotechnical Commision) is commonly recognised worldwide as the safety standard for ESD valves, and is a requirement for many global insurance companies. This standard provides stringent guidelines for testing and auditing of the functional safety requirements for safety integrity loop, which includes all aspects of the process loop, sensors, positioners or controller, control valve or any exterior fail safe equipment. IEC demands a zero tolerance level of hardware failure (valve, actuator yoke or cylinder, spring ect.) and a minimal tolerance failure rate, diagnostics associated with ESD valve and SIL software requirements.
Partial Stroking Testing
In the past the only way when the testing of the ESD valve was required, was to carry this out during a system shutdown for routine maintenance. This can be an expensive and time consuming process.
However now with the development of smart positioners on ESD valves. Not only does this save time and expense but now gives diagnostic information of the systems performance and strokes the valve partially 10 to 30 percent of stroke, to ensure their is no stiction on the valve stem or shaft. Each test records the ESD valve signature so you can see from one test to the next if any changes occur and look for obvious signs of high friction levels, this would suggest a possible stuck stem or shaft or stiction taking place. Self diagnostic tests can also be conducted to detect leaks to the actuator, or determine spring rate or bench set.
Fisher FIELDVUE™ DVC6200 Digital Valve Controller
FIELDVUE™ DVC6200 digital valve controllers are HART® communicating, microprocessor-based current-to-pneumatic instruments with linkage-less, non-contact travel feedback. Important functions performed: 1 Traditional valve positioning 2 Automatic calibration and configuration 3 Provide instrument and valve diagnostic information.
Linkage-Less Non-Contact Position
Feedback—There are no wearing parts so cycle life is maximised
Encapsulated Electronics–Resist the effects of vibration, temperature, and corrosive atmospheres