Severe Service Valves means different things to different people. Until today, defining SSVs had little if any global agreement or recognition. That is about to change as the Manufacturers Standardization Society (MSS) has accepted an application to produce a Standard Practice so defining them.
This paper provides information about the selection of SSVs in all industries but focusses on metallurgical processes and applications and offers examples to illustrate the successful and unsuccessful use of these valves. All of this with the purpose of raising the awareness of the industry on all sides, from the suppliers and manufacturers, specifiers and to the users and owners of them. It also supplies tools to understand where and why to separate SSVs from general purpose or commodity valves (GPVs).
SSVs are often identified by applications, and these applications are challenging to the valve’s ability to survive. Within these applications the elements that make the service severe are being analyzed, quantified and qualified. From this, we expect to offer objective and repeatable definitions and guidance to improve the experience of SSVs, reduce unnecessary costs, provide longer service life and process runs, improve safety, and reduce environmental issues.
SSVs can be found throughout all industries, however some industries harbour many more challenges than others. For example, municipal water treatment will have fewer SSV opportunities or needs than mining or chemical industries. In general, valves have two basic uses; they either control a process variable like pH, or they isolate the process. No matter what type of valve – from ball, butterfly, check or globe – all fit somewhere into the basic role of control or isolation.
The dilemma is for the valves to remain in service providing one of those singular functions and performing at a basic level required or demanded by the process. Severe services challenge that performance and life expectancy. SSVs must deliver a minimum performance over a minimum period of time. The valve industry does have a better understanding and agreement of what defines a Severe Service Control Valve (SSCV). Table 1 provides some reasonable thresholds that can be applied to any control valve situation and be used to make a reasonable determination that the application is severe and therefore requires a SSV.
An example would be where the fluid will likely be near or at a cavitating state. ISA RP75.23 provides basic formulas that can be used to determine whether the fluid will or will not cavitate in service. At this state, fluids are accelerated and their vapour pressure is reduced in a proportional relationship. Should the pressure reduce below the media’s vapour pressure, the fluid will separate into two or more phases during a condition known as flashing. Flashing by itself can and will be erosive, but if the downstream pressure recovery is such that the fluid is above its vapour pressure, the resultant collapse of the flashed gas creates the damaging condition of cavitation.
For isolation valves we have far less agreement or easy to accept definitions of SSVs. Table 2 provides some reasonable thresholds, although it is admitted, we are still debating and discussing these, and it appears we may need a combination of thresholds to occur in order to qualify as severe. For example, if we use the temperature threshold of 260 °C – the upper useable limit of fluorocarbons – this eliminates one of the world’s best seat material options and we are forced into metal seats which are far more challenging to use and produce “tight” shut-off isolation performance. “Tight” itself needs further definition and objective measurements and is in fact being defined by MSS.
One significant lack in most isolation valve datasheets is a clear expectation of isolation performance. For many in the industry, we blindly use FCI 70.2 as the performance level. You will see Class V or Class VI listed frequently. These classes do offer a measurement for “allowable” leak rate. The anomaly is that the title of the Standard is Allowable Leak Rate for Control Valves. Control valves can leak because they should not be used as isolation valves, and SSIVs should not.
The valve industry is only just now catching up with the demands of highest performance valves and providing us with better tools that have been available in the past. For instance, we currently don’t have an industry Standard to source an isolation valve performance that doesn’t allow some passing (seat leakage). One can reference a valve test Standard such as API 598 or ISO 5208 and add a required performance statement like “valve seat testing to API 598 resilient seat”, but that eliminates metal seated valves from selection even though some metal seated valves are capable of the tightest isolation. For now, the most common isolation valve performance Standard in North America is FCI 70.2, yet it has no category for zero seat leakage.
An example of a Severe Service Isolation Valve (SSIV) could appear as a process isolation requirement to provide better than FCI 70.2 Class V shut-off for two years continuous service. A typical application for this example could be a high pressure steam boiler drain. This is because a drain valve contains the energy inside the boiler until it is time to empty it and while isolating the steam, any passing (leakage) will lead to loss of efficiency, wasted resources (fuel, demineralized water) and that leakage will lead to increased seat wear and even more leakage.
The focus on SSVs has uncovered a lack of data that has been responsible for making the proper selection of SSVs more challenging and therefore more prone to failures. When one compares the datasheet of a control valve with one for an isolation valve, you will commonly discover that a fundamentally important element is missing from the isolation valve datasheet. You will be provided the static conditions in the datasheet; maximum design temperature, pressure, pipe size, media, Class, material of construction and often flowrates from minimum to maximum.
For a control valve this is all one really needs because the control valve operates 100% in the dynamic conditions that can be calculated from the data between minimum and maximum flowrates. But for an isolation valve which is typically static for most of its service life, without knowing the number of cycles and normal operating position – normally Open or Closed, one cannot properly consider the effects of the dynamic conditions that occur when transitioning between Open and Closed or vice versa. It is this transitional state that exposes the valve to very different conditions than when it is at rest and when it is most vulnerable.
When isolation valves leak, passing energy in the form of differential pressure can produce a velocity increase and the media can become a destructive agent, removing mass from the seating areas. This makes the leak worse and eventually the isolation valve is incapable of operating in its intended form.
Determining the minimum level of isolation that is required at the end of the service life of an SSIV is critical. That demands understanding what the life cycle of the valve will be and all of the conditions that will be experienced during the valve’s life. Without this full knowledge it is extremely difficult to select the valve type, the sealing system, materials of construction, valve bore, the operator, whether manual or automated, or special options demanded by the application.
A simple explanation or definition of a SSV is a valve that survives in the application for a defined duration performing a basic function (isolation or control) up and until the agreed duration is reached. Those valves that cannot demonstrate this performance level are general purpose valves (GPVs).
As indicated earlier, isolation valves have a more challenging definition than control valves. We have noted that severe services are identified by applications. If the process is such that the temperature, pressure, velocity, abrasiveness, corrosiveness or combination of these, challenges the valve’s ability to maintain a basic performance level then a valve that succeeds in the application is an SSV.
SSVs are important because the consequence of a failure or degradation of performance will have a higher negative impact on the process within which it is operating than GPVs. It may be a surprise to most that not all isolation valves isolate to the same ability or tightness, nor do all types of isolation valves have similar or even close performance abilities. Tightness is relative and often misunderstood; our industry has done a poor job of being transparent and objective.
It is important to understand that not all isolation valves need to be perfect in the duty; the application will always dictate what is actually required and there will be applications where some through leakage is unimportant, while others it will be critical. In this article we will try and shed some light on both ends of the spectrum.
As we have stated, SSVs can be used in nearly every process and industry, but they are essential in a few. The Chemical industry has a large number of them and we shall examine some. For our purposes, while everything is a chemical, we will use examples dealing with chemicals that if their containment is lost, can lead to damages to personal health, property and equipment. A good example is hydrochloric acid.
This chemical is corrosive; it will literally dissolve metals into solution, and if the integrity of the containment of the hydrochloric acid is lost, then the acid may be free to play havoc with unprepared people, equipment and ancillary processes. Corrosion is one of the key elements in the determination of SSVs; an SSV’s resistance to corrosion is often paramount.
While there are many benign chemicals (water is a chemical), many chemical manufacturing processes and the chemicals that are used and produced are dangerous. They can be toxic, explosive, aggressively trying to combine with another chemical, or corrosive. These chemicals need containment and management so that they are not released where their properties can do damage or be lost to the downstream process for which they are chosen.
If we look at containment and the purpose of the valve is isolation, we need to think about resistance to corrosion from the process fluid. Design codes like ASME B16.34 provide operating pressure and temperature limits for each pressure Class at various operating temperatures for categories of materials.
For example, valves of ASME Standard Class 300 for Group 2.2 materials, consisting of several grades of stainless steels including the common forged and wrought 316 and cast CF8M, have a working pressure by Class in psig at a range of temperature. A Class 300 valve of this material operating between -29°C to 38 °C, has a 720-psig pressure limit, the valve’s maximum allowable working pressure (MAWP).
Figure 1. Sulfuric Acid Isocorrosion Chart, Courtesy of Flowserve
In order to meet that working pressure the manufacturer will produce a valve body with a minimum body wall thickness. This wall thickness will be thicker than necessary to add a safety margin. In the factory testing, this extra safety is proved by pressurizing the body 50% beyond the maximum allowable working pressure during its factory hydro test.
If the corrosion begins to reduce the wall thickness either evenly or in discrete pockets or sections, the valve becomes vulnerable to loss of containment. When referring to the Sulphuric Acid Isocorrosion chart below, you will find lines depicting less than or equal to 20 MPY (milli-inch or mils per year) corrosion. Note how alloy CF8M is only suitable for very weak or very strong acid at low temperatures and alloy CD4MCu disappears from suitability while two others that were in the same suitable category remain for a range of higher temperatures.
Respecting the overall piping system’s corrosion allowance, often expressed as x mpy, you generally select valves with a trim (the sealing parts) that have less than 1 mpy corrosion rating while the body would have less than 20 mpy. This practice leads to examining the body wall thickness during the time one examines the piping. Some SSVs applications would demand a lower corrosion allowance, based on the severity of an upset, the time period between maintenance turnarounds or minimum process runs.
In the Pulp and Paper industry, chemicals for bleaching the pulp have evolved from chlorine into less dangerous chlorine dioxide, sodium hypochlorite or hydrogen peroxide. For the Kraft Pulp process, cost efficiencies have encouraged the mills to create the necessary chemicals required on-site in a plant within a plant called Chem Prep. One process within this sub-plant can produce the bleaching agent – chlorine dioxide.
Chlorine dioxide can be made by reducing sodium chlorate in a strong acid (sulphuric or hydrochloric) solution and a reducing agent like methanol, hydrogen peroxide or sulfur dioxide. The basic production route is: chlorate + acid + reducing agent → chlorine dioxide + by-products. A commercially important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Advantages of not using chloride-based processes are two-fold; the formation of elemental chlorine is eliminated, and sodium sulfate, a valuable chemical for the pulp mill, is a side-product.
It was this early focus on a chemical process and exposure to several corrosive chemicals that paved the way for our future specialization in SSVs. We gained the confidence to challenge an even wider range of chemicals and chemical processes, exposing us to a range of materials including titanium, super alloys (Inconels and Hastelloys), and fluorocarbons like PVDF and PTFE.
Our decades of experience working with SSVs in Chemical Industries have taught us many important lessons. Below are three we hope you’ll consider when selecting your next valve.
The photograph below is a testament that you get what you pay for, and if you don’t consider the whole of the application you may suffer greatly. In 1995 a new copper mine was built in Chile. It was designed to use 85 g/l weak sulphuric acid to irrigate a heap of copper oxide ore. The acid was sprayed over a huge pile of crushed ore which dissolved the copper into solution where it was later electro-won out into pure copper cathodes. This type of mine and process plant is known as a Heap Leach-SXEW (solvent extraction, electro-winning).
The original process designers selected titanium as their corrosion resistant material of construction for the valves used to direct and isolate the weak acid. These 24” valves cost upwards of $60,000 each originally. When a small brownfield project later arose, the local project team balked at the price and decided to investigate alternatives. They checked the corrosion charts and saw that a Buna-N resilient seated ductile iron butterfly valve with a PVDF coated ductile iron disc was rated A for the temperature and 85 grams/litre concentration of sulphuric acid. They were under $6,000 each, a far lower cost that was irresistible for the buyer to ignore.
The problem is that they only lasted a few months because they were not used only as full Open or Closed isolation valves; besides being used to isolate the centrifugal pumps for maintenance, they were also used on the discharge of the pumps to assist the pump during start-up to develop head pressure. That required them to be placed in the near closed position. Unfortunately the velocity of the acid that was developed while nearly closed was so high it physically removed the thin PVDF coating covering and protecting the ductile iron disc. This exposed the iron disc and the acid simply dissolved it. Of note, the originally supplied titanium valves are still in service 22 years later. Sometimes buying cheap costs more.
At a recent valve conference in Dusseldorf, some of the global audience heard a presentation from the QA manager of a major global chemical company where he identified a significant number (more than 35%) of the Material Test Reports (MTRs) for valves and fittings they purchased during the year were incorrect, missing, contained obvious errors and in some cases were fraudulent. If this is a normal occurrence, then the confidence we have in the valves we use in severe services is surely mistaken.
This statement hit a personal nerve of mine when in 2006 I was hauled before a distraught client in Australia who had a number of valve issues due to the valves we had supplied leaking sulphuric acid all over his plant.
The resulting investigation brought our attention to the knife gate valves we had provided. The manufacturer used castings to make the valve bodies and wrought plate to make the blade. We had selected the casting as ASTM A890 Gr 5a, a Super-Duplex stainless steel well suited for the 70-80°F temperatures of the 40 weight percentage sulphuric acid in the Counter Current Decantation circuit of this hydrometallurgical facility.
For several years these valves worked flawlessly. The client became very comfortable and decided to try them in an upstream, more challenging process, one where the temperatures reached 200°F. This is where the problem started. As you will note in the Isocorrosion Chart for Sulphuric Acid, this acid creates some interesting variations based upon the wt % and temperature. At the higher temperature, the material was no longer as resistant to corrosion.
The investigation also exposed the potential for issues using the A890 ASTM recipe. The non-destructive testing is a little weaker than the more popular ASTM A995 used today. Metallurgical testing showed that the castings were improperly heat treated. While at the lower temperatures, the acid wasn’t as aggressive at exposing the poor manufacturing, but at the higher temperature, the flaws were exposed and the valve failed to contain the acidic solution.
Figure 3. As Received Microstructure at 200x Figure 4. Microstructure after Proper Heat-Treatment at 200x
It’s other case of you don’t know what you don’t know and exposing us to this issue caused us to re-evaluate our QA systems including purchasing. Key vendors were requested to provide better control of the vendor data so that we could eliminate or at least greatly reduce errors that could lead to future valve issues.
Of interest, the gate material which was made from plate (wrought UNS S32750) was little affected by either temperature. This is not to imply that wrought materials are better than cast ones, only that properly processed materials are what’s really important.
Metals are not the only material of construction used to fight against corrosion. Plastics and elastomers are also effective allies to protect against corrosion from a particular chemical or chemical process. The invention of fluorocarbons like Teflon™ (polytetrafluoroethylene) and variants has given the valve industry a wonderful family of extremely corrosion resistant materials.
Valves for corrosive services are often lined with one of the fluorocarbons, normally Teflon™ PFA, a perfluoroalkoxy copolymer which can fully isolate the valve body from the process medium. These plastic materials don’t corrode like metals, but can and do allow fluids to permeate them. Thus the quality of their manufacture to reduce the permeability, the type of liner, how it is anchored into the body, as well as the thickness of the lining are keys to preventing damage and failure of the less-noble and generally non-corrosion resistant valve bodies which provide the application’s pressure containment and retention.
Very few solid plastic valves have the strength to withstand the needs of even the lowest ASME B16.34 Class 150 pressure retaining requirements and are typically rated to a maximum of 225-psig or lower at ambient temperatures. Lined metal body valves are available fully rated to ASME Class 150 and up to and including Class 300, 720-psig at ambient temperatures. These alternatives to solid metallic valves often offer significantly reduced acquisition costs over high alloy valves and can provide equivalent long lasting service lives if selected and operated correctly. Of course, the maxim “the application dictates the valve” applies and it is incumbent on the valve selectors to consider all aspects of the valve requirements.
A challenge for plastic lined valves has been that there are no industry Standards that gave guidance on what the minimum thickness should be or on any of the other key manufacturing details that made the valve a successful piece of process equipment.
For a Teflon™ PFA lined weir-style diaphragm valve, a typical 3mm thick body liner is the normal and generally sufficient protection for the ductile iron body. This rigid liner is formed over the weir, which serves as the base seat for the moving diaphragm seal which together provides the isolation. Due to the relatively inflexible and non-resilience of Teflon™, the diaphragm consists of a thinner layer cushioned by an elastic resilient material such as EPDM or Viton™.
This combination diaphragm seal must be able to flex multiple times without distortion or damage. Yet there is a line between how thin the corrosion resistant liner should be to allow multiple flexing and how immune it is to the stress caused by the closure of the handwheel and that rotary to linear torque to thrust. Too much torque will cause the PTFE to cold flow and weaken perhaps to a point where the protective liner is compromised and the liner is torn exposing the less resistant elastomer cushion to the process media as well as an uneven sealing surface that allows leakage even while the valve is fully closed.
When it is impractical or impossible to limit the closing torque’s potential for damage, simply changing valve design can be the obvious solution; turn multi-turn linear force into quarter-turn rotary action as shown in the lined ball valve.
Thankfully MSS will shortly publish Plastic-Lined Ferrous Metal Valves as a Standard Practice. The scope covers plastic-lined ferrous metal valves intended primarily for conveying corrosive fluids. This new tool will recommend minimum liner thicknesses, grades and formulations of plastic liners including PFA, PTFE, PVDF, PP and UHMWPe, liner anchoring, inspection and testing.
The valve industry is working to provide better information to ensure the correct selection of SSVs and the proper language to nominate the valve so that it can function in service and meet or exceed our expectations. We have discovered the need for more details on the actual process dynamics including the percentage of time spend during different dynamic conditions as well as upsets, and to consider what those dynamic situations are doing that could damage the function of the valves, whether immediately or over time as the sum of the damage adds up.
There is a place for SSVs just as there is a place for GPVs and knowing where to draw that line is essential in obtaining the best, safest and lowest cost valve solutions. In the coming months, we will see better tools for identifying SSVs, including new Standards of SSV testing, defining SSVs, Standard Practices for corrosion resistant lined valves and deeper knowledge that will assist us all in providing better valves for the most challenging applications.
Our exposure over the last five decades to a wide range of industrial applications in all of the regions of the world has provided us with an appreciation that communication is a key aspect of success and without it, of failure. Industry’s acceptance of terms like high-performance and tight shut-off gave false confidence and assurance that valves will work in difficult applications. We have begun to rid ourselves of subjective terminology and bring objective and measureable performance to the task. This greatly enhances our chances of success.