Saunders® on/off industrial diaphragm valve
Saunders® control diaphragm valve utilizing a PMV EP5 Electro-pneumatic positioner
An important regulating element of any control loop within a processing plant, the control valve works with other parts of the loop to ensure the programmed parameters of the process variable (e.g. pressure, flow, temperature, or fluid level) remain within a required operating range.Control valve and control loop assembly specification is typically based on performance characteristics such as speed, precision, and stability. However, the selection of the most appropriate control valve for a specific location is also governed by the nature of the process fluid. Traditional control valve technology is often restricted to use on clean fluids, and is not suitable for services which include the presence of slurries or abrasives. In addition, incorrectly tuned or over specified control valves can lead to operational wastes and inefficiencies.The selection of the most appropriate control valve for a specific plant location is governed by basic process parameters, e.g. the nature of the fluid, temperature and pressure of the system, the flow rate, and the pressure drop. Safety, reliability, and maintenance are also important aspects which subsequently influence the engineers choice of valve for a particular process.The globe control valve, or sliding stem valve, is the most commonly used control valve. Offering the greatest range of control, it moves/pushes an element of the valve into a closed position. Some of the benefits and factors behind the widespread use of this valve type include: A high pressure and temperature rating Variable trim geometry to suit the required valve characteristics Relatively simple actuation and actuation control.However, use of globe control valves in highly corrosive services requires expensive valve materials and such technology is not typically suitable for abrasive services. In addition, globe valves are not necessarily designed to provide 100% shut-off, rather various degrees of leak-tightness are available, and often an inline companion shut-off valve is needed to provide complete closure when required.Quarter-turn, or rotary valves, on the other hand, feature a rotating closure element (i.e. ball, disc or plug), that when rotated 90 degrees, completely closes or opens the valve. They have become a popular alternative to sliding stem valves in control applications, despite not offering the same level of control. Generally quarter turn valves are smaller in size and weight than linear valves, and they also address some of the limitations of globe valve technology: Handling of corrosive fluids Tight shut-off capabilities The ability to handle high flows and lower pressure drops.PK Saunders invented the diaphragm valve in consisting of three main components: the body, the diaphragm and the operating mechanism. The diaphragm is the dynamic component which restricts or isolates the media flow, and the original valve design included a weir in the valve body over which the media flows and against which the diaphragm seals. A straight-through (or full bore) design was subsequently introduced as a valve for handling solids, and both designs can utilize a wide range of different diaphragm and lining materials.Simplicity of design, coupled with more than 85 years of innovation, has resulted in the diaphragm valves ability to handle a wider range of fluids than any other valve type. However, the diaphragm valves success as an on/off valve has meant that its ability to operate as a modulating control valve is often over-looked. There are a number of performance features that make diaphragm valves a viable alternative to traditional control valve technology, particularly in 'tough' abrasive or corrosive applications: The weir type valves inherent linear flow characteristics. Throttling and control characteristics are enhanced by a streamlined flow path that is cavity free and provides excellent flow control capabilities. The straight-through design also demonstrates a linear flow characteristic, albeit within a more limited range of travel. Under both positive pressure and vacuum services (from 1 x 10-5 torr to 16 bar, valve size dependent), diaphragm valves deliver 100% leak-tight shut off in accordance with standard BS EN -1. This performance is maintained even after thousands of operations and with solids present in the line. The lack of stem packing or glands minimizes potential leak paths. The diaphragm forms a natural seal between the operating mechanism and process fluid, isolating the valves working parts from the line media. A wide range of valve body, valve lining, and diaphragm material options ensure suitability for use on abrasive and corrosive media. These material options provide an effective and economical solution by eliminating the need for the exotic alloys used in alternative valve technology. In a similar fashion to the rotary valve, diaphragm valves demonstrate a high flow capacity, particularly when compared against globe valve technology. A wide range of ancillaries can be attached to diaphragm control valves to meet the plant requirements and maximise operational efficiencies. Pneumatic, electro-pneumatic, intrinsically- safe, explosion-proof and smart positioners are available with features including characteristic cams which are able to alter the valve performance to produce the desired flow profile. The relatively low purchase price and maintenance costs of the diaphragm valve ensure value. During servicing the valve can be disassembled without having to remove the body from the pipeline.Industry & Location: Chlor-alkali, India Plant(s): Cell House, HCl Plant, Chlorine Liquefaction, Brine Preparation Media: Chlorine Water, Sodium Hypochlorite, Hydrochloric Acid, Brine, Demineralised Water Valve specifications: Weir type DN20 DN80 Plastic (PVDF or ETFE) and hard natural rubber linings with two-piece PTFE faced diaphragms. For the most severe services, the three-piece PTFE diaphragm, including PVDF interlayer, was utilised Fail-to-close and fail-to-open diaphragm operated actuators with either explosion proof or weather proof Electro- Pneumatic positionersControl Purpose(s): Flow Control, Level ControlA key industry/application for diaphragm valves, the existing manual and actuated valve installed base at this end user was successfully extended to include diaphragm valve control valves. The originally installed butterfly valves demonstrated poor flow control, resulting in the plant repeatedly tripping. In addition, with the only solution being direct one to one replacement, the maintenance costs for such valves were significant. As a result of the highly corrosive nature of the fluids to be handled, material selection for both linings and diaphragms was advised on an individual line basis. Sizing of individual valves, to accommodate factors including process conditions and pipe sizes, enabled a complete package of manual and actuated control valves to be installed on site (in ), where they have worked well without issue, to date.There is not one single valve type suitable for use in every service, and this holds particularly true for control applications. Over specification is incredibly wasteful and the performance accuracy of the installed control valves should exactly match the required degree of control. The diaphragm valve has been presented as a viable alternative to traditional control valve technology, where the strengths of the diaphragm valve as an on/off valve can be equally applied in control applications: Suitability for use on corrosive/abrasive fluids (wide range of diaphragm and lining options) Leak tight shut off even with solids present Relatively low cost (both purchase price and ownership costs) In-line maintenance capabilitiesAlbeit a versatile option, it should be highlighted that the diaphragm valve is suitable only for use on services which fall within the natural temperature and pressure windows of operation, as they are heavily influenced by the valve materials and linear valve movement. However, continual innovation from diaphragm valve manufacturers has led to developments and improvements in diaphragm valve materials, actuators and accessories. As a result, the potential window of operation has been expanded to include higher temperature and more severe services (in terms of corrosion and abrasion), and is therefore greater than ever.This article was written and supplied by David McClymont, Saunders® Industrial Diaphragm Valve Application Specialist, CRANE ChemPharma and Energy, a Crane Co. business.: +44(0) Email: : www.cranecpe.com
As far back as the late s, the introduction of weir diaphragm valves in pharmaceutical processes was a vast improvement over disk (butterfly) and ball valves for use in sanitary and sterile processes. But why diaphragm valves in the first place? Although originally developed for industrial slurry applications, the weir design was easily adapted to sanitary applications. The internal shape of the valve creates a weir against which the diaphragm seals. These valves allow draining through the system when installed at the correct angle. Compared to ball, butterfly and plug valves the weir design dramatically reduced the potential for product entrapment and this allowed for reliable cleaning and sterilization. When first introduced, the processes where quite different than today and dead legs and product entrapment were not much of an issue.
Towards the end of the s the radial diaphragm valve arrived on the market and shortly after that an even newer valve design emerged. More recently new valves with upgraded diaphragm performance have entered the market and offers key improvements over the first radial valves. The unique right angle design immediately differentiated itself from the more common weir style diaphragm valves that had dominated the pharmaceutical and bio-pharmaceutical landscape for over the last forty years.
In response to process challenges, one of the many drivers of single use technology now almost 20 years ago was eliminating the need to reliably clean and sterilize stainless steel systems. Even to this day industry user groups rank elastomer performance, the standard Tee and dead legs as key areas of focus for increasing system reliability in stainless steel based systems. Each of these areas of concern is closely related to the application and functionality of valves in sanitary process systems and directly affects Continuous Process Improvement and Verification and Design Qualification initiatives. In the U.S. Food and Drug Administration (FDA) issued guidance on Quality Risk Management guidelines and the European Medicines Agency (EMA) has issued similar guidelines in Annex 15.
With the growth of biopharmaceuticals, system engineering to eliminate hidden piping defects created by valves and TEEs is a key design challenge. Radial diaphragm or Right Angle valves have a key advantage over weir valves in that the natural valve geometry effectively puts the line seal at the outside of the valve. Properly designed systems use the valve for changes of direction and elevation resulting in full drainability. Weir valves by their nature place the sealing point in the middle of the valve. From a process piping perspective this design creates a dead leg in the worst case or an unused portion at best. From valves mounted on vessels to valves installed in piping there are countless examples of weir valves creating pockets in the system. Possibly the best example of this hidden piping defect is the weir valve design that incorporates a condensate drain valve into the main valve. These so called sterile access valves solved one issue (condensate removal) but because of physical limitation in valve geometry created a pocket that extends from the pipe wall of the main valve to the closure point of the condensate valve. By comparison the same function can be attained with radial diaphragm valves without the resulting sump or dead-leg.
Valves suitable for sanitary and aseptic piping systems have some attributes in common. Key is the body material of AISI 316L (1.) or for higher corrosion resistance without resorting to higher alloys, (1. BN2). The valves themselves are either forged with a final machining or machined directly from round bar. The most common diaphragm elastomer materials are EPDM, Silicone and several variations of fluoropolymers including PTFE and PFA. In general, weir valve diaphragms are most commonly EPDM by itself or used as a backing piece for some type of fluoropolymer facing the process.
As mentioned previously valve diaphragm reliability is a key issue. Proactive to address the issue, industry user groups have collaborated with valve manufacturers to design testing schemes that address extending diaphragm replacement intervals and the associated risk as well as new EPDM compounds and diaphragm design.
You will get efficient and thoughtful service from LEFLOW.
Properly formulated EPDM compounds easily pass testing requirements like USP Class VI for biological reactivity. It is important to note that USP VI does not address in-use performance. One drawback of EPDM is that when hot, the compound can become sticky. One common way to combat the stickiness is to add a vapor deposited coating of medial grade Parylene. Individual users have to determine if this additive is acceptable to their process. One drawback to EPDM rubbers is the phenomenon referred to as Compression Set. Compression Set can be best described as permanent shape change while under load typically the clamping pressure required to affect a seal. This load coupled with elevated temperatures permanently distorts most EPDM compounds. Eventually the seal force on the valve diaphragm will slack and require re-torqueing of the bonnet nuts or replacing the diaphragm. For valves with thin fluoropolymer faces with a resilient backing the same compression set of the EPDM can occur potentially leading to reduced overall system availability and high maintenance costs as valves need to be constantly checked and re-torqued after steaming.
Silicone is used in some radial diaphragm designs because of its flexibility and the tendency not to stick. These first generation valves utilize a bellows above the valve plug to allow movement of the diaphragm plug. However the porosity of silicone can be problematic in some applications with the formation vesicles. It is hypothesized that this occurs during repeated steaming operations resulting in the potential for product diffusion into these areas or at worst a rupture of the vesicle.
Fluoropolymers have some key advantages over EPDM and Silicone. These materials have a high molecular weight and are naturally hydrophilic and therefor highly resistant to steam and stable at temperatures far above EPDM and Silicone. With proper valve body design and clamping force, Cold Flow is not an issue. In fact, when properly designed, valves with solid machined PTFE diaphragms can be opened and inspected repeatedly for valve diaphragm wear and re-inserted in the valve and reliably re-sealed. Unlike composite Fluoropolymer/EPDM diaphragms that can distort while warm and under vacuum, solid PTFE diaphragms have demonstrated the capability of operating under vacuum while hot. This is a major advantage in systems that are dried and cooled using a sterile Nitrogen purge.
The shape of the diaphragm at the sealing point of the valve body is critical to proper cleaning. For fast and thorough cleaning, asymptotic seals and low flow conditions should be avoided. The right angle design of radial diaphragms valves naturally creates needed turbulence for effective cleaning. The ideal body seal should be static and have an open angle that is readily cleanable. Valves that do not have a defined sealing point or that flex in the sealing area are prone to pumping material into an area that is difficult to clean. Heat during sterilization kills what may be left behind. Unless there is an outright leak in the body seal, this condition is only knowable after disassembling the valve. Common issues with first and second generation radial valves related to diaphragm material reliability remain to this day on par with weir valves.
From a purely aseptic processing point of view there should only be one body seal and for good reason. With two seals, if the first seal fails, product between the seal points can be trapped. First generation radial diaphragm valves are designed with an internal body seal in direct contact with the process. In an often over looked design feature there is also second outer body seal. The two seals are connected by a length of elastomer. The assembly of the valve relies upon compression with a sanitary clamp at the outer body seal and according to literature this is providing the primary seal. These valves are characterized with a single bellows which allows movement of the diaphragm.
As to design options, weir valves have one basic flow path and are installed at the required point in the system. Many times this placement results in a dead leg as well as extra system piping. Weir valves need to be installed at a specific angle in order to drain through the valve. Radial diaphragm valves on the other hand can be ported in many different ways. For example, in a diverting valve situation the common inlet can be adjusted for both vertical and horizontal pipe and eccentric or tangential porting. Process designers can then locate the outlets at a favored location that best suites the process, eliminates dead legs and reduces overall piping.
In North America the radial style valve was readily accepted for tank outlet valves and sample valves. The right angle design allows for effectively eliminating the dead leg to the vessel. On vessels with agitators eliminating the sump associated with other valve styles ensured reliable mixing. Beyond use as bottom outlet and sample valves, radial diaphragm valves can be readily adapted to most any application including the most stringent aseptic operations.
For aseptic process piping the design philosophy using the newest generation radial valve is to eliminate piping dead legs while improving cleaning and sterilization times and efficiency. Utilizing solid PTFE diaphragms translates into the ability to sterilize at higher temperatures safely and reliably while greatly increasing the time between diaphragm changes. The bottom line is reduces costs added to increased plant availability - a profitable combination.
To learn more feel free to contact me.