Improvements to the Diaphragm Valve Type for Chemically Aggressive Slurry Applications

Transcription

1 Improvements to the Diaphragm Valve Type for Chemically Aggressive Slurry Applications Presenters: Massimo Amato Business Manager IDV CRANE ChemPharma Flow Solutions, Saunders Ian James Polymer Technologist, CRANE ChemPharma Flow Solutions, Saunders

2 ABSTRACT For years, the diaphragm valve has been one of the most commonly used valves for handling chemically aggressive applications where abrasion and corrosion are combined challenges. Over the last year, new technologies have been carefully formulated and developed to specifically target demanding applications, such as mining and chemically aggressive slurries. These exciting new developments are uniquely formulated for industrial applications and build on the foundation set by the leaders in diaphragm technology. Diaphragm valves are used in a number of different industry sectors including, as mentioned above, chemically aggressive applications. For instance, the slurry services found within mining applications demand a valve that can withstand harsh and abrasive media and can also handle high solids content. Certain valve types are more susceptible to abrasion and will wear quickly. Some valve types are unable to achieve complete closure when handling fluids with a high percentage of solids. Similarly, valves operating within Phosphatic Fertilizer facilities are exposed to a range of aggressive and demanding media. The Sulphuric Acid, Phosphoric Acid and Fertilizer production plants within these facilities handle acids (Sulphuric Acid and Phosphoric Acid), slurries, and gases that render valves susceptible to abrasion, corrosion, scaling, and erosion. Damaged valves can result in down-stream leakage, external emissions, and frequent maintenance repairs and replacements. Consideration of these key process issues is imperative to successful valve selection. This paper will present and explain a new diaphragm valve design for industrial industries that will incorporate all the acclaimed and demonstrated benefits of the previous design approach, while implementing new approaches that alleviate prior design constraints. Page 2 of 9

3 OVERVIEW Diaphragm valves are used in a number of different industry sectors, including chemically The Diaphragm Valve aggressive applications. For instance, the slurry services found within the mining applications demand a valve that can withstand harsh and abrasive media and can also handle high solids content. Certain valve types are more susceptible to abrasion and will wear quickly. Some valve types are unable to achieve complete closure when handling fluids with a high percentage of solids. Similarly, valves operating within Phosphatic Fertilizer facilities are exposed to a range of aggressive and demanding media. The Sulphuric Acid, Phosphoric Acid and Fertilizer production plants within these facilities handle acids (Sulphuric Acid and Phosphoric Acid), The original diaphragm valve was invented in 1928 by PK Saunders who slurries, and gases that render valves susceptible also founded the Saunders Valve Co. to abrasion, corrosion, scaling and erosion. Damaged valves can result in down-stream leakage, external emissions, and frequent maintenance repairs and replacements. Consideration of these key process issues is imperative to successful valve selection. For years, the diaphragm valve has been one of the most commonly used valves for handling these applications where abrasion and corrosion are combined challenges. New technologies have been carefully formulated and developed to specifically target demanding applications, such as mining and chemically aggressive slurries. This exciting new development is uniquely formulated for these industrial applications and builds on the foundation set by the leaders in diaphragm technology. DIAPHRAGM TECHNOLOGY An EP-based polymer is still the preferred choice of polymer for aggressive chemical slurry applications. EPs are derived from the common thermoplastic polymer Polyethylene, abbreviated as EP and commonly known as Polythene (although this is not recognised scientifically). Polyethylene is one of the most common polymer materials used globally and is widely used because of its ability to successfully Page 3 of 9

4 withstand a wide range of different media. It is from manipulation of the polymerisation process that an elastomeric version of this versatile material is created. At room temperature, polyethylene is a crystalline plastomer, but on heating, it passes through an elastomeric phase. By interfering with crystallisation of polyethylene, that is, by incorporating in the polymer chain elements, which inhibit crystallisation, the melting temperature and therefore the elastomeric phase can be reduced considerably to below room temperature. Such amorphous and curable materials can be considered rubbers, and can be obtained by copolymerising ethylene propylene with certain catalysts of the Ziegler-Natta type. The resulting socalled EPM s are amorphous and rubbery, but since these do not contain unsaturation, they can only be cross-linked with peroxides. If during the copolymerisation of ethylene and propylene, a third monomer, a diene, is added, the resulting rubber will have unsaturation. These rubbers are so-called EPDM s. [1] TECHNOLOGICAL DESIGN REALIZATION AND IMPLEMENTATION Intentionally, the new diaphragm was created to utilise this third diene monomer. The unsaturation provided by the third diene monomer provides more flexibility to modify cross-link structure of the compound, enabling the enhancement of the inherent good properties of EPDM. All of EPs inherent good properties are derived from their chemical structure; since in EPDM the active sites of unsaturation reside in the side groups, the polymer chain is completely saturated and stable. The saturated backbone gives the polymer a natural resistance to ageing and weathering (i.e. Oxygen EPDM is noted for its oxidation resistance, which results in excellent heat aging characteristics. No special compounding is required for continuous exposures up to 125 C. Service temperatures to 150 C and beyond can be achieved by proper compounding techniques and polymer selection. Ozone Resistance Because of its saturated polymer backbone, EPDM is inherently resistant to attack by ozone. Compounds are suitable for use in high ozone environments without the need for antiozonants, waxes, or other additives). [2] Resistance to Oxygen, Ozone, Water and elevated temperatures, make EPDM vulcanisates very durable. This combined Page 4 of 9

5 with the polarity of EPDM means that any compound formulation based on EPDM will have an excellent resistance to most acids and alkalis commonly found in the mining sector. EPDM articles have good resistance to many chemical agents such as alcohols, esters, acids, alkalis, ketones, hydraulic fluids and anti-freeze fluids. [2] For this reason EPDMs are deemed as the perfect building block to meet the needs of the industrial abrasive and corrosive media. With a foundation in place, further improvements in compound design are made through the careful choice of additives and the ratio at which these are used to bestow the optimum balance of properties required for the application. The diaphragm has been formulated to have balanced physical properties; a high molecular weight EPDM has been chosen to make the compound intrinsically strong and tough. As molecular weight increases so does the tensile strength and tear strength. As ethylene content increases so does cold green strength, tensile strength. As diene content increases, so does compression set and modulus. [3] All these factors have been considered to ensure the best possible base material has been chosen. The cure system has been selected to compliment the choice of polymer, ensuring that while increasing the overall strength properties of the compound (i.e. Tensile Strength, Elongation at Break and Tear Resistance). The polymer retains the mobility to flex repeatedly, all of which are required to enable the diaphragm to remain intact and fully functional when subjected to the type of abrasive solids experienced in heavy duty applications. The preferred cure system chosen for the new diaphragm has purposely remained the same as previous offerings, utilising the benefits provided by a peroxide cure within an EPDM base polymer; the principal vulcanisation site is at or near the side chain double-bond on the diene. Reactivity rates are significantly greater than for EPM copolymers due to the presence of allylic hydrogen on the diene. Peroxides are used with EPDM when the ultimate in heat and compression set resistance is required. [4] The carbon-carbon bonds formed during peroxide cure process have greater bond strength than those generated by the more common Sulphur cure process. Improved bond strength, again, enhance the good chemical / water resistance and enable excellent high temperature stability and good elastic recovery over a wide range of temperatures. Average temperatures for processing in the mining sector are approximately 80 C; the benefits of the peroxide cure is that it supplies additional security that the diaphragm can withstand excursions to elevated Page 5 of 9

6 temperatures caused by spikes in the process, or improve longevity when installed on more demanding applications where elevated temperatures are experienced. The high temperature stability and good chemical and weathering resistance mentioned previously is further complimented by the addition of anti-ageing additives. The compound used in the new diaphragm technology has been finely tuned to extend the normal service life of the diaphragm in such demanding applications. The Elastic Recovery of a compound has a direct effect on improving a rubber diaphragm s ability to retain a seal to atmosphere. After installation and under the load of the fastenings, the cross-link structure will be prone to stress relaxation as time elapses, effectively meaning the cross-link structure eventually conforms to its newly deformed position. This ultimately results in a loss of sealing force acting between the body and bonnet of the diaphragm valve. Additional effects of temperature can accelerate this effect. Considering the harmful, corrosive environment the diaphragm will have to work in, it is of paramount importance that the diaphragm retains its sealing force at all times. The base polymer was chosen for best compression set properties, avoiding high crystallinity grades that reduce elastic recovery. Peroxide cures are employed, to establish carbon-carbon crosslinks which again help prevent permanent set. The ratio of peroxide is increased to improve cross-link density within the compound, helping to reduce the measured compression set. The ratio of reinforcing fillers and oil are reduced to improve resistance to compression set and not hinder the compound s elastic properties. [3] Improvement in the compound s elastic recovery gives the end user greater security against potential leakage to atmosphere, offering both safety and environmental benefits. PRACTICAL RESULTS Abrasion resistance is a key element of the new diaphragm. Abrasion is clearly not only an important factor in determining failure of particular products but it can also be used to gauge the limit of useful life, as for example in shoe soles and rubber flooring. The mechanisms by which wear of rubber occurs when it is in moving contact with another material are complex, but the principle factors involved are cutting and fatigue. It is possible to categorise wear mechanisms in various ways and common distinction is made between abrasive wear, fatigue wear, and adhesive wear. Additionally, wear by roll formation is sometimes considered as a separate mechanism. There can also be corrosive wear due to direct chemical attack on the surface and the term erosive wear is sometimes used for the action of particles in a Page 6 of 9

7 liquid stream. [5] In the case of aggressive slurry applications, abrasion is not measured via the expected friction, sand paper type test. The mechanism for abrasion via aggressive slurries is somewhat different and for this reason is measured via Rebound Resilience. Rebound Resilience is used as a direct measure of a rubber compound s ability to resist abrasion from impact of solid particles, perpendicularly; this more accurately replicates actual process conditions. Therefore, increasing the Rebound Resilience will improve the abrasion resistance of the compound and increase the diaphragm s ability to handle erosion by solids and extend diaphragm life on erosive solid duties. The dynamic response of ethylene-propylene rubber compounds is similar to that of natural rubber compounds as indicated in the Yerzley test. Ethylene-propylene rubber, however, is a more popular choice for dynamic parts because its age resistance better preserves initial design characteristics with time and environmental extremes. EPDM is a good first choice when high resiliency is desired. The resilience of ethylene-propylene rubber at room temperature is again slightly less than that of natural rubber and generally equivalent to styrene-butadiene and polychloroprene elastomers. Although the low temperature brittle point of ethylene-propylene rubber is about the same as that of styrene-butadiene rubber, it retains a greater percentage of its resilience at low temperatures. Vulcanisates remain flexible and serviceable down to temperatures as low as -55 C. [4] By design, the promotion of the elastic properties of the diaphragm compound also enhances other critical properties required for this diaphragm to excel in its intended applications. The methodology used to improve Compression Set and Elastic Recovery, also augments Rebound Resilience characteristics. Again the choice of polymer, cure system and reinforcing fillers have all been attentively chosen to optimise this characteristic. For instance, increasing the average molecular weight of the base elastomer used, improves the inherent resilience. Reducing the level of fully reinforcing fillers also aids resilience. High loadings of filler actually increase the degree of hysteresis in a compound during deformation, this occurs from the loss in energy resulting from internal friction or breaking and reforming of the filler/polymer contact points and the filler particle (aggregate) network. Reducing filler loading will reduce hysteresis heat build up. The Carbon Black type has also been chosen to reduce surface area (i.e. increasing carbon black particle size) as this will help to minimise energy losses from hysteresis. [3] As previously discussed the new compound utilises the free unsaturated diene sites within the EPDM structure to Page 7 of 9

8 allow the crosslink density of the compound to be crafted such that it is increased to further aid the resilience of the compound. All these factors have been addressed so that the Rebound Resilience is maximised and able to withstand the demands of aggressive slurry applications. The formulation and production process used to manufacture the new diaphragm have been developed not only to yield this superior performance product but also to produce a diaphragm that is visually superior. CONCLUSIONS In summary, the new diaphragm technology has improved physical properties as detailed in this paper, and numerous benefits for the customer utilising standard diaphragm valves in such harsh environments. These include improved Abrasion resistance, improved Flex and improved Elastic Recovery, all of which will extend the life of the diaphragm, reduce plant downtime, ensure longer intervals between maintenance and limite unexpected failures between maintenance intervals. The new diaphragm exhibits superior sealing performance, minimising any chance of leakage. Along with increased working life in these harsh demanding environments, the new diaphragm technology offers a reduction in maintenance and plant downtime costs. The new diaphragm delivers overall lower cost of ownership for the user. Page 8 of 9

9 References [1] Rubber Technology Handbook - Hofmann [2] Royalene Technical Data [3] How to Improve Rubber Compounds [4] Vistalon [5] Rubber Product Failure Page 9 of 9