Steam Cracker – Take it to the top !

Steam Cracking_ethane cracking_Guichon Valves

“Steam Cracking: the first step to produce the simplest olefin”

Ethylene is one of the simplest of all the olefins and present everywhere around us. It is used as base product for several applications: Plastics, paints, solvents, packaging, cosmetics… The worldwide ethylene demand is now over 140 million tons per year and is still growing by 4-5% every year!

Back to the beginning of the 20th century, Standard Oil’s engineers and scientists patented the first thermal cracking process, called “Burton Process”. It was the first process using destructive distillation of crude oil, under pressure & temperature, through a bubble tower (fractionator). Using this, the fuel production doubled and first cuts of ethylene were produced.

During the 30’s, the first ethylene plant was built. Back to that time, ethylene was separated from coke oven gas. Couple of years after that plant commissioning, the first steam-cracker was built next to Baton-Rouge in the US. Ethylene production started replacing acetylene as raw material for synthesis.

Nowadays, steam cracking is the predominant technology to produce ethylene. The total worlwide production capacity is over 150 million Tons/year.

The main goal: Heating a hydrocarbon mixture (naphtha or gas) with steam inside a furnace to crack hydrocarbon molecules (=decompose molecules in smaller ones).

In case the steam cracker’s feed is Ethane, the chemical reaction will be as follows:

ethane_chemical formula_Guichon Valves

As the feed is never pure gas (ethane) or can be liquid (naphtha), the cracking reaction causes other cracked gas production and coke formation. As a result, the steam cracker must be periodically “cleaned” to ensure an optimal process performance. This step is called “Decoking”.

The coke formation and equipment fouling in the ethylene industry is still a major operation problem, especially regarding pumps and valves. Producers always seek to reduce coke formation, in order to increase the global running time. Design improvements on critical equipment is one of the solution to decrease down time!

“Fighting with coke particles”

The most critical valves on a steam cracker are :

1. The Transfer Line Valves (TLV) – Installed between the furnace header and the main column.

2. The Decoke Valves (DV) – Installed on the specific decoke line. A security small decoke valve (SDV) is sometimes installed to prevent any overpressure.

These valves should meet the following requirements from the process side:

* Operable under High Temperature (400°C or even more, depending on cracker feeds).

* Zero leakage requirement (even after 5 years of operation)

* Double-Block and Bleed feature on the main transfer line (TLV).

* Material have to be resistant to corrosion and pitting caused by low pH condensate.

* Valve body shall be resistant to thermal cycling.

* Transfer Line and decoke valves shall be linked together and synchronized.

ethane process Guichon valves

ethane cracking_Guichon Valves

In both cases, the technical challenge is mainly oriented on coke particles. Effluent is a three-phases flow: gas, quench oil and coke particles.

These particles are transported into the main flow path and erode many components on their way to the main column. Those ones going into every small cavities and can be easily stuck on every metallic components.

During the decoke phase it has been experienced that the coke can drop into the valve body and build up into the body cavities. Whatever the conventional valve technology is: single disc, classic gate valves, butterfly (triple off-set)… It causes problems regarding the full closing position of the valve.

 

guichon valves ethan cracking valve solution

As a result, some coke particles can be stuck on the valve seats. It causes torque peak, leakage and it takes a lot of time to fix that matter every decoking time! In the worst case, it can even destroy the actuating system, resulting in overpressured lines, fire or explosions.

“Avoid valves fouling is the key to performance”

If you want to save time, money and make your process safer, it is highly advised to select the right valve technology for those lines.

As this coke fouling has always been an issue on steam cracker, valve experts found several solutions overtime (single slide valve with metal/metal seating back in the 70’s, then single slide valve with spring loaded soft seats in the 80’s…etc.).

The up-to-date design is the Double Discs Through Conduit Slide Gate Valve (DDTC) equiped/fitted with a Secure Sealing System.

The conventionnal DDTC design is fitted with a ball arrangement between the two discs (wedges between discs) which still generates torque issue and seat erosion. If some coke particles are stuck on the valve seats, the ball arrangement will expend the discs against the seats before the valve reaches its fully closed position.

conventional solution ethane cracking guichon valves

Backed with more than 45 years of experience in steam cracking applications, Guichon Valves experts have been designing a new technology in order to improve valve operation and global process performance.

The main design difference is the “Fixed Wedges”. These wedges are not in the flow path and avoid any blocking of the discs before they reach the correct closing position. The full load/torque from the actuator is used to break any coke particle on the seats and to reach the final end position correctly.

On the following animation, the right hand side represents the conventional design while the left hand side represents the Guichon secure sealing system.

During the first step of the opening movement, the discs are moving back through process pressure in order to decrease the friction between seats and discs over the entire lenght of the stroke. The steam injection cleans the seats and critical surfaces during the stroke. Two guide plates are installed in the body to avoid any particle infiltration into the body cavity.

In the open position, the spring-loaded substitution rings (not represented on the above animation) fit the body bore (like a straight tube) and the fluid goes through the valve without entering into the body. Seat surfaces are totally protected.

The double discs and seats installed on that valve allow to purge between these discs. It constitutes a real Double-Block & Bleed feature and most of the licensors (TECHNIP FMC, CB&I LUMMUS (Mc DERMOTT), LINDE, KBR, TOYO, SINOPEC…) use to advise only one valve on the main transfer line instead of two valves in line.

decoke valves line guichon valves

Guichon Valves designed several tailored valve solutions installed on Steam Cracking Unit :

“Golden Rules – Good practices to keep in mind regarding critical valve selection”

  1. Take a good look on the body rigidity on critical valves. Use valves which front parts of the body are made of thick forged plates, handling distortions due to piping loads.
  2. Use integrated metal/metal seats. Screwed seats generate additionnal leakage path and it is just impossible to remove them after 5 years of cracking conditions. Integrated seats are just more reliable in cracked gas applications.
  3. Ask for a perfect fluid containment. That is one of the most important point. The goal is to prevent the potential coke particles entry into the valve body.
  4. At least, seats, discs and substitution rings should be Stellited.
  5. Always use steam injection with preferred steam path to clean up the valve inside and provide a body positive isolation.
  6. Self-ajusting trim protects the critical surfaces and no settings are required.
  7. Use bidirectional sealing to protect the process in case of back-flow.

Many other features can improve the valve performance and the global process reliability, safety and profitability!

For more information, please ask or check : www.guichon.com

Written by Rudy Repellin