Q & A > Thermal Technology
Date  Replies
20/04/2017 Q: What is the relation between the amount of overflash used in a vacuum distillation tower and the stripping steam injected at the bottom of this tower? (3)
20/04/2017 Q: For a cooling top pumparound for a vacuum tower, which is more useful: to use a larger flow rate (140 m3/h) at higher temperature (78C) or use smaller flow rate (100 m3/h) at lower temperature (70C) (2)
30/03/2017 Q: How can I determine the amount of stripping steam for a side stripper? (3)
30/03/2017 Q: What is the typical composition of VGO (Vacuum gasoil) and what is its cracking temperature? (3)
22/02/2017 Q: Does anyone know about the economic feasibility of Organic Rankine Cycle in Crude Distillation Unit? We are having sub-cooled condenser duty of 20-24 GKcal/hr with overhead temperature in range of 120-130°C. (1)
27/09/2016 Q: What is the importance of minimum CCR and ashpalatens in Coker feed. Do we really need to maintain certain minimum limits for these feed properties? (3)
29/06/2016 Q: I am working on performance review of sulphur recovery unit (SRU). Recently we conducted a performance test in SRU. In this regard I wish to know the correct method of measurement of sulphur recovery. Some people believe that sulphur recovery should be merely based on measurement of sulphur in feed minus that in incinerator stack whereas I strongly believe that sulphur balance must be carried out which includes sulphur in feed and stack plus the product sulphur recovered in pit/storage. Material balance is a key to any performance test of the plant. Hence I believe that product sulphur measurement in pit is essential for estimating the % sulphur recovery. Kindly confirm if my understanding is correct. Alternatively please advise other methods of sulphur recovery measurement and the most accurate method to be followed. (1)
29/06/2016 Q: Can someone advise the method of sulphur recovery in a Performance test of sulphur recovery unit (SRU) in the sense whether % sulphur recovery can be measured by
Sulphur in feed X 100/product sulphur received in sulphur storage? There are some people who believe that sulphur recovery can be estimated by measuring the sulphur in feed (A) and sulphur species in the incinerator stack (B) and simple subtraction of (A)-(B). However I strongly believe that the sulphur balance of the whole plant should be done which includes quantity of sulphur received in storage pit. Please advise. I would be grateful if somebody advises various methods with pros and cons of each and finally the best method practiced in the industry.
27/06/2016 Q: In case of heavy residue upgrading, we are encountered with vacuum residue as feed. The main features of this feed especially about contaminants and problematic materials are as below:
Total sulfur>4.5 wt%
Conradson Carbon >25 wt%
Ni+V >500 ppmwt
Nitrogen ~ 1 wt%
We have two cases for VR upgrading project, One is RCD+RFCC and another is HOIL(Hydrocracking)+FCC. Both of these cases use huge amount of fresh catalysts because of high possibility of catalyst deactivation and poisoning. So the operating cost should be high.
Is this rational to charge such a feed to the catalytic system directly or is it better to use the process to somehow get rid of metals at least? If we need to use the solvent deasphalting system at the upstream of two before-mentioned cases and draw off about 20% of feed as pitch, we will succeed to lower the operating cost and increase the reliability of catalytic system because of the elimination of the major part of the metals. But in the opposite side, we have missed 20% of primary feed as pitch that it is a low value product. So the profit margin of the residue upgrading cases will decrease. However, as a second question, can we miss 20% of feed charge at the expense of increment of catalyst life cycle?
15/05/2016 Q: Crude heater is twin cabin type which is operated at 367degC for heating crude from preheat-2 to crude distillation column. There are total 8 passes, 4 passes in each cabin.Pass 1-4 in one cabin and rest 5-8 in another cabin.From past 8 months, To minimize the deviation in all passes coil outlet temperature,when flow is increased in pass 5 & 8,its coil outlet temperature increased rather than decreasing and pass 6 & 7 coil outlet temperature reduced when its flow reduced.Also, this effect is observed in skin temperature.
However, it is confirmed by checking temperature of radiation outlet with temperature gun and ensured the deviation.
When flow in passes 5-8 is made equal, deviation in its coil outlet temperature reduced for the same heater box temperature.
However no such issue is found in rest passes i.e. pass 1-4. What can be the possible reason?
13/05/2016 Q: What are the ways to reduce cutter stock % in visbreaker unit without affecting fuel oil quality? What are the factors one should consider while selecting particular stream as a cutter stock? And, what is typical % ? (2)
10/05/2016 Q: Our crude vacuum distillation column overflash pump suction strainer gets fequently full of coke.
Overflash pump suction temperature is 374℃~385℃, flash zone pressure 21mmhg, top pressure 5mmhg.
Metal contents of overflash are 154.7ppmw(Ni 36ppm, vanadium 118.7ppm) and Metal contents of Vacuum Residue are 223.8ppmw(Ni 49.2ppm, Vanadium 174.6ppm). How can we prevent this?
10/03/2016 Q: Does anyone have any experience in using fuel oil for heaters/boilers firing?
What is the typical size of bucket filter (i.e. mesh size) at pump discharge that supplies acceptable particle size to the burners?
What would be the typical minimum filtration size, anything lower than < 10 micron with bucket filter?
06/01/2016 Q: Is necessary to remove all coke in a soaker drum of vibreaking before starting? What the risks?
The coke covers 20% of the flow area.

22/04/2015 Q: There are several different options to increase liquid production and reduce coke yield in residue coker units. One of the most widely used is try to operate coker drum at low pressure.
But what are the possible options to reduce further the operating pressure in the coker drum?
Could the minimum pressure limit (alarm) at the wet gas compressor inlet be reduced? We currently operate at 0,4 kg/cm2. Has anyone experience operating at lower pressure at this point?
Has anyone experience installing an on-line water wash in top of main fractionator or condenser to remove salts? In that case, which is the maximum DP reduced with this option?
Has anyone experience implementing any other modifications (increase vapor lines diameter, etc)?
25/03/2015 Q: I would like to know if when we design a transfer line of CDU or VDU heater then do we consider erosional velocity as a constraint? The mixed phase velocities in transfer line are frequently higher than calculated erosional velocity (from API-14E). (4)
21/02/2015 Q: In a Sulfur recovery unit, reaction furnace minimum skin temperature should be how much to avoid corrosion due to acidic condensation?  
02/02/2015 Q: During Crude distillation unit start up activities, water travels from crude storage tank to crude tower when furnace outlet temperature was 172C. It caused crude tower trays to dislodge. What if level of crude tower remains high then flash zone, does level of crude tower have significant effect on tray dislodge? Our system is furnace operated crude tower. (2)
14/01/2015 Q: What should be the design pressure for wash water system in air cooled exchanger in atmospheric distillation unit? Is it mandatory to apply wash water in spray form? It will be helpful to if anyone provide reference about the spray nozzle for this application. (4)
25/12/2014 Q: To increase the energy efficiency of Steam turbine, we take it in extraction mode from total condensing mode. When we tried to increase the extraction steam flow, the turbine exhaust temperature increased. Why does the turbine exhaust temperature increase at these conditions? (2)
21/11/2014 Q: We have different fired heaters with fuel oil and gas burners in our refinery. It is seen that the metal tube surface is coated with fine layer of powder (slight yellow colour) externally. This is loosely held to its surface (as seen through inspection door) and reduces the heat transfer leading to high skin, arch and stack temperature. My suspicion on the Fuel oil quality became stronger. So I found that Clarified oil from FCC is not going to the Fuel oil tank. Upon testing the IFO for ash content, it came out to be 0.075%. Is this okay? Any other testing is required?
What probably might be the reason for this powder coating and measures to solve the problem?
01/11/2014 Q: We would like to reduce the net minimum flow of our Visbreaker unit in order to increase our flexibility. Our concern is that this will lead to shorter runs due to coking in the furnace. Do you have experience regarding the various available options: recycling VSB gasoil or VSB residue? Co-processing LCO or slurry from FCC? Increase the injected BFW/steam to the VSB furnace passes? What would you advise? (3)
13/08/2014 Q: What is the welding procedure of T-joints in 8 mm bottom plate of tank? Can T-joints be welded before completing short and long joints?  
13/04/2014 Q: We have a Steam methane reformer having side fired self respiratory burners. To attain the correct O2 in flue gas of primary reformer, burner dampers are being adjusted. What is the correct sequence for throttling the burners? Should the bottom most burners should be throttled more than the top ones or vice versa?  
11/04/2014 Q: In case of side fired self respiratory burners in reformers what is the correct sequence of adjusting the air?
From bottom row burners to top row burners in increasing trends:
in 1st row 40%, 2nd row 40%, 3rd row 40%, 4th row 30%, 5th row 30% & 6th row 30%
in 1st row 30%, 2nd row 30%, 3rd row 30%, 4th row 40%, 5th row 40% & 6th row 40%.
This flue gas is going to convection section for heat recovery.
13/01/2014 Q: We have steam methane reformer. The outer surface of the tube is having deposits and leading to high fuel consumption as well as high temperature in flue gas side in waste heat section. During turn around we want to clean the outer surface of reformer catalyst tube so that we can reduce the fuel consumption and reduction in waste heat section temperatures. Is there any standard method available to clean the outer surface of the tubes? (5)
25/01/2013 Q: In case of non-contact temperature measurement of the skin temperature of furnace tube, which instrument is better: infrared thermometer or laser pyrometer? What is the allowable temperature difference between thermocouple or thermowell temperature measurement to non-contact temperature measurement?  
09/12/2012 Q: Is it a myth or reality that in a refinery fired heater for the same throughput, same coil outlet temperature and everything else being the same, a fuel oil fired furnace will give a lower skin temperature in the convection section than a natural gas fired one? (5)
17/09/2012 Q: What is the expected life of fin tube of overhead air cooler of Atmospheric distillation unit? (1)
03/05/2012 Q: We have four pass vertical tube furnace at our hydro-cracker unit. The feed of furnace is catalytically cracked VGO. After turnaround some tubes of all passes at inlet sides turn their color to black. Why this happened?  
15/04/2012 Q: We need to build very small vacuum distillation unit . We cannot find out how many of oil will crack and we cannot evaluate how many m3 of gases will be generated . So our questions:
What should be a capacity of vacuum pump in m3 per 1t/h ?
How many gases are usually released ?
or give examples from your plants.

08/03/2012 Q: What is 'HSS mode' on a heater? (2)
15/01/2012 Q: My question relaters to the maximum temperature that can be reached in the feed preheater furnace in FCC unit. We operate one of our FCC units in maximum distillates mode and we want to decrease cat/oil to minimum. Currently, we have the following design limits in the feed preheater furnace: 360C (680 F) in the process size and 419C (786F) in the skin points of the furnace tubes. According to a study by our engineering department, temperature in the skin points could be increased to 467C (873F). But our main concern is that an increase in temperature in furnace tubes could cause coking of the feed. Although the feed to the unit is Mild Hydrocracker residue, that has low tendency to coking.
Has anyone experience running FCC units at feed preheat temperatures higher that 360C (680F) in process / 419C (786F) in skin point?
11/11/2011 Q: We find the crude heater tubes started slightly bowing towards the burner inside the radiation zone. The investigation drives my mind over the below written questions...
1. What can be the maximum height of the Fired heater's radiation zone (or) the maximum tube height allowed inside the radiation zone (vertical coil type) as per standard?
2. What is the efficient ratio which can be achieved between the radiation:convectional zone heat transfer(in percentage)? Its a balanced type heater and we could heat the combustion air up to 275 C max?
3. We use P9 material tubes inside the furnace (cylindrical-twin zone). We are puzzled as to why the bowing is towards the burner side? Why not towards the side and backwards?
4. What is the maximum pressure drop across the burners allowed? As we go increasing the throughput in varying the Fuel oil and Fuel gas burning, the skin temperature response in all the section of the heater is not uniform. So the heat flux variance is also expected. I would like to know the methods available to find the heat flux variance inside the radiation zone.
5. The burners (Low NOx/SOx) used are stretching over the design sometimes due to the lower inlet temperatures. Flue gas recirculation is also included in the design. What can be the problem when a burner is running over the design limits? We have oxygen, CO, NOx/SOx analysers but they don't seem to be reliable most of the time.
09/08/2011 Q: What are current views on twisted tube heat exchanger configurations in refineries, particularly in comparison with conventional shell and tube configurations? (7)
22/06/2011 Q: Recently it was observed that the some of the radiation tubes of our atmospheric distillation heater were deformed. The tubes have been in operation for almost 30 years. Some of the tubes (specially at the middle section) deformed to the center of the furnace. Some deformed laterally to the adjacent tube. I want to know the possible reasons behind the phenomenon. Also please advise me what is the standard of replacement of the tubes in this mentioned condition.

29/05/2011 Q: We are facing frequent fouling in our falling film evaporators because of tar content in our feed. Currently we down our plant once in 3 months for cleaning these evaporators. Can anyone suggest the best method in reducing this fouling?? how about tube inserts ?? or do we have any advance technology in heat transfer for reducing fouling??
I know twisted tubes can reduce fouling but can we use for falling film evaporator??
26/10/2010 Q: We are fixing for choosing a new compressor for VRU; vapors recovery unit; the light hydrocarbon gas mixture has Mw about 42, K{Cp/Cv} =1.12 , Rc {Pd/Ps} =3.2 , Z factor is about 0.97, Inlet temperature is 43 C and flow rate Q is 600 CFM, discharge Pressure, Pd= 72 psia...
I'd be more than grateful for any tips, Equations, etc to use...
09/10/2010 Q: My company aims at further processing the atm. distillation residue (Mazot); and a hydrocracker unit has been chosen for this task. We need to estimate the cost of the unit and its facilities like the vacuum tower and the vis-breaker. How would you suggest we get a rough initial estimate of the costs involved? (5)
15/07/2010 Q: Is there any relation between Velocity steam and Heater Outlet temperature?. Normally Velocity steam is adjusted for heater tubes residence time. (1)
26/05/2010 Q: In our Once Through Hydrocracker, the Fractionator Feed Furnace has options for both Fuel Oil and Fuel Gas Firing. Currently due to some problem in the electrical heater in the Fuel Oil Circuit we are using only fuel gas. Some days back inspection department reported a much higher skin temperature in the radiation section of the Furnace. The same report was also upheld during various cross-checks by other departments. Could this be due to the reason as we are not using Fuel Oil? If so, then could somebody explain? Another thing to consider, we are running at 70% T'Put and design conversion so in general the burners are supposed to operate at the given Heat Duty. (4)
21/05/2010 Q: What are the likely effects of water carry over from desalter on Crude heater and distillation column? What steps should be taken if this happens? (11)
02/04/2010 Q: Sometimes it is seen that the leaky tube of heat exchanger is used by plugging both sides. I want to know the percentage of tubes that can be used in plugged condition in running condition and also the standard for this plugging. (5)
06/02/2010 Q: Recently we are facing topping unit furnace inlet becomes lower than expected. The normal temperature is 215-220 degree centigrade. But now we are getting only 200-205 degree centigrade. What are the probable reasons behind this? And what measures should be taken to overcome the problem? (6)
08/01/2010 Q: In our refinery we are going to replace our Vacuum Distillation Column. Please suggest some designer/manufacturers' names who work in this field. (1)
08/01/2010 Q: I want to know the temperature profile of post weld heat treatment for alloy steel like P5, P9. We have some procedures that was used from a long time. I want to know the source or reference of the temperature range. Please suggest the maximum temperature, holding time, temperature raising rate, cooling rate. (1)
22/12/2009 Q: Is there any simple equation for estimating furnace efficiency only by excess air (oxygen) and flue gas temperature? (1)
14/12/2009 Q: Our furnace has 4 pass flow. Crude enters the furnace by 4" tube in the convection section. Then it changes its size by 5" X 4" reducer in the radiation section. It again changes its size outside the furnace and now this time by 8" X 5" reducer to a common header of 12" pipe line. This pipe line by a 16" X 12" reducer connected to the 16" pipe line that goes to column. My question is why we are using so many reducers in the process line? (3)
08/12/2009 Q: We have a catpoly hydrotreater that converts olefins to paraffins to produce petrol diesel and jet fuel. I just want to know the reaction/chemistry that should take place in the poly hydrotreater and the kinetics associated? (1)
31/10/2009 Q: Is there a heat transfer fluid that can withstand a temperature of up to 600 deg C plus? (2)
18/10/2009 Q: We are going to install anchors in our furnace. We get all the required spacing of anchors for cylindrical radiation shell, overhead arch, convection breeching (roof) and stack, but we have no proper data relating to anchor spacing of conical part of the furnace. Can anybody help me in this issue?  
28/09/2009 Q: What is meant by a 'double fired box-type heater'? (1)
30/08/2009 Q: we have hot oil system for heat load to all reboiler, this system running smoothly for long as more than 3 years. The problem now is the level reducing continuously and very very slowly from the system, but there is no leak in the reboiler. What could be the changes? How do we tackle the problem?  
07/08/2008 Q: In addressing refinery CO2 management, can you comment on CO2 curtailment from on-purpose hydrogen plants through "minimised" involuntary'steam, internal heat recycle and captive integration?
11/06/2008 Q: Is there a rule of thumb for calculating the fuel requirement for a furnace refractory dry out?  
23/05/2008 Q: We have local crudes which are very waxy in nature. The reduced crude from these crudes has a wax content of 40 pct wax and 1 pct asphaltene. The pour point is very high requiring cutter and depressant.
We were thinking of a thermal process like visbreaking or thermal cracking, but this resid is very light and quite a lot of it vaporises at common visbreaking condition unless pressure is increased substantially.
We are trying some pilot runs using makeshift arrangement. Has anyone tried this for light waxy feed and what were the results and operating condition used?
07/02/2008 Q: Have hydraulic power recovery turbines (HPRTs) been included in any of the most recently completed or planned projects where core hydrocracking and amine regeneration is required? Besides energy savings incurred with the installation of an HPRT, how significant a role with HPRTs play in reducing CO2 emissions?  
24/01/2008 Q: Is there a process to make Carbon Black Feedstock (CBFS) from natural Gas? (2)
05/09/2007 Q: We are trying to add heat to the front end (feed stream) of a vacuum unit (part of a crude unit) and wonder if anyone has done this in recent years by using skid mounted equip of some sort or small "package" units of exchangers/heaters, etc. We only want to do this on a temporary basis, say for 4-6 months (1)
31/07/2007 Q: We have been observing bowings in Radiation coils of a Naphtha Cracker Furnace for ethylene production. What technological improvements in furnace design and coil orientation and coil support system might be available to minimise the bowings? What process and inspection monitoring is needed to minimise bowings in operation? What are the effect of side burners? Generally the Cracker furnaces are not provided with explosion doors? In such case how can box overpressurisation be prevented? Please share best practices in interlocks in Cracker furnace operation. (1)