Control valves for feedstock, dilution steam and fuel to furnaces
This is Part 2 of our series of three posts on valve selection for ethylene cracking. An ethylene steam cracker can accept a variety of feedstock. Naphtha, ethane, propane, gas oil, and other similar feedstock can be cracked to produce ethylene. The choice of feedstock depends on the cracker furnace construction, the availability of the particular feed, its current market price and the desired end products. The amount of the particular feedstock required depends on the desired product yield.
The process system in an ethylene cracker must supply the correct amount of raw materials to produce the desired end product. The feedstock supply valve must be able to control this as accurately as possible, taking into account that in some cases the feedstock can be liquid and sometimes gas. This makes it one of the most important control valves. As the flow rate difference between low flow and full capacity can be huge, some ethylene producers use separate valves for each case. Fire-safe design is also often a requirement as the feed can be volatile. Depending on the feedstock, cavitation or high noise may also be an issue. Typically, the temperature is 120-150 °C (250-300 °F), the upstream pressure 6-10 barG (85-150 psig) and the downstream pressure 2 barG (30 psig).
Dilution steam is injected into the furnace tubes to protect the pipes from fouling or coke formation. It works like a protection layer inside the coil. The HC-to-steam ratio should be optimized to get the optimal flow condition. The flow rate of the steam depends on the severity of the feed stock. Steam can also be used for decoking the furnace, so rangeability of this valve can also be required. Poor performance of the steam supply valve can lead to excessive fouling of the cracker. Steam also often has the tendency to cause high pressure drops and noise, which has to be taken into account when choosing an appropriate valve. Typically the temperature is 180–200 °C (350–400 °F), the pressure 10 barG (150 psig) and the pressure drop 2–7 barG (30–100 psiG).
The burners create the heat required to crack the feedstock. They are also used during decoking, though the temperature requirement is lower in this case, and therefore, not all burners are in operation. Steam crackers can use a variety of fuels to feed the burners, depending on the most economical or practical fuel available at the time, and can range from natural gas to crude oil. Especially modern crackers are capable of taking advantage of various fuels to feed the burners. The different heat generation properties of the fuels require a valve that can regulate the flow accordingly. In addition, the relatively low temperature required during the decoking of the furnace demands that the valve is able to control small flows of fuel, as not all the burners are operating. To ensure a more reliable operation, a fast reaction to signal changes is required to quickly adjust the temperature to account for fouling and switching to decoking. Noise reduction capabilities may also be necessary, especially if fuel gas is being used. Typically, the temperature is 40–200 °C (100–400 °F) and the pressure 2–10 barG (30–150 psig).
The valve selection depends on the type of feed and piping layout. Especially in the case of a gas feed, Metso’s Neles GB or GU-series globe offers an excellent option for these applications, ensuring accurate control with a variety of trims available, including noise and cavitation reduction. Leaks are minimized as the rugged one-piece body structure eliminates potential leak paths and ensures that volatile fuel does not leave the piping. Valves are fugitive emission certified according to ISO 15848. Different inherently characterized trims are available as equal percentage, linear and quick open. Interchangeable trim parts make it possible to easily change the flow characteristics.
Metso’s Neles segment valve is well suited to both liquid and gas feeds. It provides an economical high performance solution for the application. The single valve solution, thanks to its wide rangeability, eliminates the need for split range control, and its compact size makes it easier to allocate space for the valve in the piping design. Cavitation and noise can be reduced with the patented Q-Trim design. The best possible rangeability ensures that the same valve can be used for various types of fuel, and during start-up, full capacity and decoking conditions. There are no potential leak paths,even if subjected to pipe bending forces, as the valve features a one-piece body construction. Fugitive emissions are reduced by design,as the valve utilizes rotary operation, which is inherently less prone to leaks. Low torque requirements reduce wear and minimize actuator size.
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