The olefin plant, one of those most commonly found in the chemical industry, produces such end-products as ethylene or propylene. These end-products are regarded as basic building blocks in the petrochemical industry because they are used as intermediates for many other end products, such as, polyethylene and polypropylene. The most usual method of obtaining olefins is by steam cracking raw materials such as ethane, naphta or condensate. Figure 1 shows a simplified diagram for the production of olefins by steam cracking.
Linear valves have traditionally been used as control valves in the petrochemical industry, although rotary control valves are seen more and more often nowadays in the industry, for example in olefin plants. Some major petrochemical companies have even taken the decision that they prefer rotary valves for control applications.
The ever-growing emphasis on environmental issues has meant the industry is paying increased attention to the emissions from control valves.
This trend has made rotary valves more attractive, because typically they have lower gland packing emissions than globe valves. In addition to producing low emissions, rotary control valves have also proved that they can handle the vast majority of petrochemical industry applications.
Versatility
Rotary control valves typically use metal or soft polymer seats. Metalseat technology has been shown to provide a reasonably tight shut-off (ANSI V), long-lasting performance and a relatively low torque requirement. Reliable metal-seat technology is capable of handling temperatures from cryogenic conditions up to +600°C.
Coating of the trim and/or the seat is normally needed to reduce friction and wear in metal-seated rotary valves.
The rotary control valves that are most commonly seen in petrochemical plants are eccentric plug valves and segment ball valves. Trunnion mounted ball valves are typically utilised in very demanding applications, and butterfly valves are used only where large sizes are required, because of the obvious economic savings. However, way too often butterfly valves are forgotten when it comes to smaller-size control applications. Contrary to general opinion, modern high performance butterfly valves with triple offset designs are excellent products for numerous control applications, but in many instances they have suffered from poor valve sizing and valve selection. By using a simple perforated plate, which is capable of balancing flow forces, butterfly valve pressure drop capability can be improved and the effect of dynamic torque decreased.
In contrast to standard linear valves, rotary valves typically provide low volatile-organic compound (VOC) emissions through the gland packing over long periods and without external maintenance.
Another important advantage of rotary valves, and especially of eccentric plug valves, is that it has been proven that they are not vulnerable to clogging by impurities.
Rotary control valves also have high inherent capacity and rangeability, which means that it is possible in many applications to select rotary valves that are smaller than the equivalent globe valves. However, the valve outlet velocity should not be too high, especially if the fluid contains impurities that may cause erosion. Due to their compact, smaller design, rotary valves typically also offer weight and size advantages over globe valves.
Anti-cavitation and noise reduction trims are also available for rotary valves. One such example, shown in Figure 2, provides high capacity combined with good cavitation resistance and noise reduction in a way that no sliding stem valve can match. In contrast to multistage globe valves, these rotary valves can handle very dirty fluids because the trim rotates with the closure member. The same trim can usually be used for liquid, gas or steam applications.
Petrochemical applications - olefin plant
Eccentric plug and segment ball valves are economical control valves that can handle the vast majority of applications throughout the petrochemical complex. Because eccentric plug valves typically have wider temperature and pressure ranges, they are typically used in more demanding applications than segment ball valves. Trunnion mounted ball valves are commonly used in particularly demanding applications. High performance butterfly valves provide an economical solution to various low pressure, hot/low temperature and large size applications.
Typically, the use of exotic materials is not required in olefin plant valves; standard carbon steel and stainless steel are suitable in the vast majority of applications. Unless fire safe conditions are not needed for the control valves, graphite packings are needed only for control valves operating above PTFE temperature limits. There may be some exceptions - for example in butadiene and ethylene or propylene oxide applications, which might not be suitable for standard PTFE packings. NACE requirements may need to be met for sour hydrocarbon applications in order to prevent sulphide stress-corrosion cracking from causing failure in a valve. The pressure class for most control valves is ANSI 600 or less.
Olefin plants and steam crackers are complex facilities where technology and conditions may vary. However, three main zones can be found in a typical olefin plant: the hot zone containing the furnace and quenching section, the compression zone comprising the compression, washing and drying of cracked gas, and the cold zone, which includes cryogenic distillation and/or the fractionation of final end products.
The hot zone
As mentioned above, steam crackers can accept a variety of feedstocks. Typically, segment ball valves or eccentric rotary plug valves are used for feedstock. Heavier condensates, which may occur at higher temperatures, may need eccentric plug valves with graphite packings.
The heart of an olefin plant is the furnace in the hot zone, which is used to crack the feedstock. Typically, the furnace operates at a high temperature – about
800°C. Valves in the furnace area may handle fluid containing hard impurites, such as coke particles, and may need to withstand high temperatures. For isolation purposes, seat-supported or trunnion mounted ball valves or butterfly valves are suitable solutions. High temperatures may need high temperature extensions. Fuel used for burners in the furnace may range from fuel gas to oil. These are typical applications for segment ball or eccentric plug valves. Steam supply valves for the furnace may be segment ball, eccentric plug, trunnion mounted ball or even butterfly valves, depending on the pressure level, temperature and size involved.
Quenching, the rapid cooling of hydrocarbon gas, is a process that involves demanding control valve applications. These valves may encounter application demands such as high temperatures, high pressures and pressure differentials, entrained coke or other particles. Such applications typically require eccentric rotary plug valves or trunnion mounted ball valves.
Metal bearings and metal seat technology provide long-lasting robust solutions for demanding quenching applications. To reduce heavy cavitation and erosion in applications such as handling quench water, a heavy-duty drilled-through anti-cavitation trim may be needed.
Another common application in the hot zone of an olefin plant is handling boiler feedwater, which is often erroneously associated only with globe valves. However, rotary valves can also provide a technically and commercially attractive solution for boiler feedwater applications, especially up to pressure class 600. Some feedwater applications may even require two different valves, but due to their wider rangeability only one rotary valve can handle the whole flow range.
Another issue that should be considered is that such valves typically encounter relatively high pressure drops during start-up, which may sometimes last a long time. Therefore, valves used in this application must be capable of handling high pressure differentials.
Eccentric plug valves and trunnion mounted ball valves are typically used in feedwater applications and they are frequently equipped with anti-cavitation and noise reduction trims. Carbon steel bodies are normally used, but in feedwater applications containing amine some special coatings against erosion-corrosion may be required.
The compression zone
The cracked gas is compressed, washed and dried in the compression zone. Compressors are used, for example, to compress cracked gas prior to fractionation, to compress the purified gases for storage or simply to supply plant air. There is a minimum capacity for each compressor, at every speed, below which the operation becomes unstable. This instability is known as the 'surge limit' for the compressor. To avoid this instability, a by-pass valve is installed between the discharge of the compressor and the inlet. This valve, which is commonly called anti-surge valve, has to meet very demanding speed and control requirements. Depending on the tightness requirement and pressure differential, an eccentric plug valve or trunnion mounted ball valve may used for anti-surge applications.
High performance butterfly valves can also provide an attractive solution for low pressure anti-surge application. Such valves are usually equipped with noise reduction trims and they may also need special instrumentation and coatings to meet the speed requirement. Pneumatic actuators can be utilised, but fail-to-open action is commonly required.
Before hydrocarbon gas enters the final stage of separation into the various desired components, the gas must be dried. Typically this is carried out in a fixed-bed dryer section, which contains at least two dryers (ie, molecular sieves) in parallel or in series. Switching valves are needed to direct feed between the dryer towers. These valves are typically ball or butterfly valves. Very demanding molecular sieving applications might require rugged trunnion mounted ball valves such as Neles X or D series, which can withstand temperature variation together with relatively frequent cycling caused by continuous switching operation.
Molecular sieving application is a very common application, which is also seen in other industries, such as LNG. These switching valves are usually very critical for the process, in which case it is important to check that the valves are operating properly. In order to improve the availability of switching valves, intelligent products such as Neles SwitchGuard can be utilised. Neles SwitchGuard, which provides visibility inside also for automated on/off- valves, is an intelligent valve controller developed for critical and demanding cycling applications (Figure 3).
Cold zone
The cold zone is the last of the three zones in an olefin plant. The temperature of the cracked gas is normally reduced in several stages. A cascade refrigeration system using, for example, propylene or ethylene refrigerant cycles operating at low temperatures, is a typical application for eccentric plug valves. Hydrogen applications are commonly seen in the cold zone, and here the valves, which may require degreasing and drying before shipment, are usually eccentric plug valves or trunnion mounted ball valves. Noise reduction trims may be needed to reduce noise and velocity inside the trim.
All rotary control valves mentioned in this article can be utilised in ethylene or propylene applications. Optimal technical and commercial valve selection depends on the minimum temperature and pressure differentials across the valve. Cryogenic extensions may be needed for some very low temperature applications. NACE requirements are usual for valves in the cold zone.
Flares
Flares are designed for the proper disposal of emergency-released gas, liquids or waste from a plant. They are usually mounted on a tall stack in order to keep the venting well away from the immediate plant areas and processes. A flare system should be able to handle the total capacity of the plant. Flare valves should have wide rangeability and should be able routinely to handle high pressure differentials, even where valve openings are small. Sometimes tight shut-off is required to minimise the waste. Flare valves are typically trunnion mounted ball valves equipped with noise reduction trims. Trunnion mounted ball valves provide smooth operation with wide rangeability together with noise reduction features, which makes them well suited to flare applications.
Attractive solutions for demanding applications
Rotary control valves have proven to be suitable for the vast majority of petrochemical industry applications, for example, in olefin plants. Eccentric plug and segment ball valves can be selected as the primary control valves for a complete petrochemical complex.
One of the major advantages of rotary control valves over globe valves is that they are less vulnerable to impurities and naturally produce lower VOC emissions through the gland packing.
Valves encounter many demands in olefin plants - such as relatively high/low temperatures, high pressure differentials, and the presence of coke or other hard impurities. High temperatures and hard particles are mainly found in the hot zone of an olefin plant. In the compression zone, the compressor anti-surge valve must meet requirements for operational speed. In the cold zone, control valves also encounter low temperatures. Boiler feedwater and flare systems may require control valves that should have wide rangeability and high pressure differentials. Rotary control valves can provide technically effective and commercially attractive solutions to all these demanding applications.
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