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Recognizing Dangers & Ensuring Safe Processes!

Over the last 10 years, the high risk and hazardous potential of hydraulic pressure testing has been brought to recognition by fatal incidents. These inherent dangers make it essential for facilities to take precautions against potential risks. Implementing testing standards, protocols and high quality equipment can reduce the risk of injury or death.

Use our free eBook to learn more about recognizing potential hazards and maintaining secure procedures in hydraulic pressure testing.

Link to your free copy here

Partial Stroke Testing -what is it?


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. 

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. 

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:

Safety benefits

Gains can be made in the following areas by the use of PST.

Reducing the probability of failure on demand (PFD) 

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.

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: 

Emerson Process

www.emerson.com 

Mokveld 

www.mokveld.com 

PetrolValves 

www.petrolvalves.com

Paladon 

www.paladonsystems.com 

Imtex Controls 

www.imtex-controls.com 

Rotork 

www.rotork.com 

Yokogawa 

www.yokogawa.com 

Cameron 

www.caneronslb.com 

Severn Glocon 

www.severnglocon.com 

Valvescan Type VSD Stainless Steel Controller from IMTEX Controls! 



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.

Check it out here VSD

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“When control valve professionals talk about control valve sizing”

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.

Cavitation

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.

Noise

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.

Actuator Sizing

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.


Ensuring Accuracy



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