Substantially Improving Downtime, Heat Transfer and CO2 emissions
With low emissions, minimal environmental impact and high energy efficiency the Texas Tower (also known as a CCR Platformer, reformer or vertical combined feed exchanger, VCFE) is an environmentally friendly piece of heat transfer equipment found on refining and petrochemical plants throughout the world. The function of the Texas Tower process is to efficiently convert paraffins and naphthenese to aromatics with as little ring opening or cracking as possible.
Like many other pieces of heat transfer equipment the Texas Tower is an unsung hero of many industrial processes and as such tends to be taken for granted. Texas Towers are supposed to be one of the ‘cleanest’ exchangers and are often only cleaned once every 7 – 10 years or more!. They provide duty for so long that when they start to drop in efficiency it is usually a gradual process which goes largely unnoticed – until their performance has deteriorated sufficiently to be a problem, and then it really is a problem.
Texas Towers are not supposed to foul and are therefore cleaned so infrequently that the knowledge and experience needed to clean them is often lacking, both by the operators and by many traditional cleaning contractors. Texas Towers are huge structures, the cleaning of which presents significant safety issues.
Traditional Texas Tower Cleaning Techniques
The height of Texas Towers ranges between 18 – 25 m and they are often very difficult to access. Typical access can be via an open tube face or an 18 i.n manway, however sometimes there is no manway at all.
In the case of restricted or no manway access is required from below. Drilling and cleaning 25m vertically, upwards has its own problems when using traditional water jetting or other mechanical means so instead requires a highly specialised approach.
The length of the tubes within the exchanger and the difficulty accessing the tubes has led many operators to delay cleaning their Texas Towers until the drop in efficiency starts to severely impact the performance of their refinery. Often clients with no knowledge of specialist techniques in existence find the only way is to replace them.
The use of traditional high pressure water jetting on platformer and reformer Texas Tower tubes has proved difficult if not impossible on many occasions around the world. There are typically 3000+ tubes within an exchanger, the tubes on average have a 12mm internal bore and the tube face often has a width of 1.5 m. Problems using traditional high pressure water jetting include access and operational difficulties. The vast length of tubes (i.e. 18-25 m) creates a dangerous risk of hydraulic action when feeding high pressure hoses down the small tubes from above. This occurs within small tubes when both water and deposit tries to exit between the gap that exists between the wall of the tube and the outer casing of the jetting lance. At such high flow, typically 40 to 60 litres per minute at 15,000 psi (1000 bar) there is often no space and not time for the mix of scale and water to escape thereby creating a temporary hydraulic ram effect which the traditional manual operator cannot escape from. Operators have reported 20 to 50 x 12 hour shifts to clean Texas Towers using high pressure water jet but still with limited success.
Due to these access difficulties operators often have to calculate the costs associated with pulling these huge structures so they can be cleaned in a cleaning bay – costs for cranes ($200k is not unheard of), replacement bolts plus the associated potential damage risks make pulling Texas Towers an unattractive option.
Chemical cleaning of Texas Towers has also shown limited success. Fouling within Texas Towers is varied and can include iron sulphide, iron chloride, ammonium chloride, ammonium salts, hydrocarbon coke scale, gummy poly cyclic aromatics and ferrous oxide scale distributed unevenly within the tubes. With so many different deposits and build materials to deal with, chemical cleaning often compounds the fouling problems as no chemical exists to remove inert coke, especially when tubes are blocked or the if the shell side is bridged between tubes. Chemicals are simply not designed to remove such a wide variety of deposits in contact with differing metals via such small inlets and outlet apertures.
The shell side of Texas Towers, according to their manufacturers, should rarely become fouled but when they do the fouling usually results from PNA (polynuclear aromatics) and/or ammonium salt deposition. PNA are very carcinogenic meaning industrial cleaning contractors must take additional safety precautions. As with most shell side cleaning techniques (after pulling) traditional high pressure water jetting removes very little fouling but more often than not just ends up pushing the fouling further into the shell as traditional techniques cannot reach the centre of the shell nest. In-situ chemical cleaning methods require careful analysis of the Texas Towers structural support to ensure it can tolerate the weight of the additional liquid.
If these exchangers are exposed to the atmosphere via a poor water jet clean where water sits within the tubes and soaks the blockages, or via a chemical flush shell or tube side, it is quite common for the deposits to become even harder and thus harder to remove next time. The performance might rise initially after going on line but often drops off significantly several months afterwards.
Innovative Texas Tower Cleaning Techniques
Case Study (Middle East Refinery)
Flow through two vertical combined feed exchangers (VCFEs/Texas Towers) at a Middle Eastern refinery of one of the world’s largest Arabian oil companies had become severely restricted. Dimensions were 6000 x 21m long x 14mm bore tubes. Just how restricted became apparent when Tube Tech’s cleaning teams started working on them. Previous cleaning attempts using local traditional contractors had taken approximately 25 x 24 hour shifts.
When Tube Tech was given the order, original estimates were that approximately 600 (10%) of the 6,000 tubes were blocked. In reality it turned out to be 3,000 tubes (50%) that were blocked from top to bottom with a combination of very hard coke and a rubbery gum like deposit.
Within an astounding five days and searing heat of between 40 to 50 degrees, Tube Tech successfully cleaned every tube in the two VCFEs to inspection standard, except for three, thought to be fragments of jetting lances and drills left behind by previous contractors. The Tube Tech team developed it’s very own Super High-Pressure Mini Micro Lance drilling system, manufactured for them under license. A mix of ceramic and spinning darTTs™ were used in conjunction with Tube Tech’s Rotaflex “Soft drill”.
Case Study (Caribbean)
A Vertical Combined Feed Exchanger (VCFE/Texas Tower) at a refinery on a Caribbean island was suffering badly from blockages and restrictions in some of its 2,006 x 24 meter tubes. The client estimated 12% of the 2006 tubes were blocked. This estimate was based on the number that local contractors had previously been unable to clean. Once the exchanger was opened up, it was found that the number of blockages was closer to 50%.
A unique ‘pulsed jet” system was developed to locate blockages, quickly calculate back pressure and blockage location. This was excellent for locating and calculating the severity of the restrictions as well as blocked tubes. A unique, heat-treated, multi-lance system was used to deliver high pressure water within the small tubes while providing sufficient flexibility to clear blockages. This system was backed up by a SafetyBoss™ system, which prevents the jetting lance head exiting the tube whilst under pressure. Once the VCFE was opened up, a variety of deposits were discovered: hard coke like scale, gum deposits, ferrous oxide scale, PCA and ammonium salts. With access to the top of the VCFE limited to an 18″/450mm flanged access port, drilling of the 24 meter long blockages had to be carried out from below, which was itself a considerable challenge – and a world first.
Case Study (Shell Side Cleaning)
Safety, speed and “Shell Side” cleanliness were deemed the most important factors by a 230,00bpd (crude) and 17,000bpd (lubricant) French refinery. The two V.C.F.E. (Texas Tower) platformer combine feed/effluent exchangers were last cleaned on the shell and tube side in 2001 when it took 24 shifts to clean each VCFE Texas Tower, using traditional high pressure water jetting guns and lances but these methods were deemed dangerous, slow and very inefficient with little improvement in performance.
Both platformer combined feed exchangers were pulled and suspended vertically in parallel to their respective shells. Manpower alone was not the answer and Tube Tech went to the drawing board to come up with a unique solution. The shell side of both Texas Towers were cleaned with an ingenious combination of ultrasound, mechanical oscillation and super high water pressures in excess of 2700 bar. What the jet did not remove, the mechanical vibration did. The combined effect of this highly unique jetting process was better than anyone had anticipated.
On the tube side Tube Tech safely and quickly drilled ca 7% of blockages from the 3612 tubes contained in both bundles, in preparation for de-scaling with Tube Tech’s unique duplex ceramic darTT™ technologies. Each projectile was handmade to fit the tube ID. Debris from the tubes was collected beneath the two Texas Towers.
The challenges for refineries and petrochemical operators when faced with a fouled Texas Tower or indeed any unusually long exchanger are numerous. Challenges include accessibility, safety, unknown fouling characteristics, fouling location, associated costs and risks with pulling.
Since Texas Towers are cleaned so infrequently it is essential that when they are taken off line for cleaning they are cleaned fast, safely and most importantly effectively with the “right methods”.
“Operators need to challenge their industrial cleaning contractors constantly and particularly when the heat transfer equipment is critical path. Texas Towers are difficult to clean but building mock ups, using 3D animations to demonstrate techniques and arriving on site with a number of back up techniques has led to Tube Tech becoming a worldwide ‘expert’ in the cleaning of these huge structures,” concludes Mike Watson.