27 replies to this topic

[Muhammad farhan]

 DHT PFD.pdf   234.39KB   63 downloads

 

Dear All

 

Good Day 

 

We have commissioned diesel hydrotreating unit in June-2017 with capacity of 150 m3/hr EURO-4 diesel product specification.

 

At start of run we run the unit at design throughput 150 m3/hr.

 

Right now the throughput is dropped to 80 m3/hr and its is decreasing day by day. We have identified the pressure profile in feed pre-heat section is disturbed (hindrance in flow). Feed pre-heat section consist of a flow controller, filter, two heat exchanger (one with two shell and one with 3 shell) in series and at last feed vessel with operating pressure of 3.5 bar. PFD is attached for better understanding. The feed pump is building pressure/head as per its performance curve but flow is restricted in feed circuit. 

 

At start of run feed pump develop pressure of 20 bar and 150 m3/hr flow. Flow controller drop pressure from 20 to 8 bar at 50% opening. Pressure drop across filter is 2 bar and rest of the pressure drop is between exchanger and feed vessel. 

 

Right now pump is producing 23 bar pressure and 80 m3/hr (also matching pump curve), control valve is fully open 100% (its by pass valve is also 100% open). Pressure drop across filter is 2 bar and pressure drop across first exchanger is 1 bar. So the pressure at inlet of second exchanger is 20 bar but we dont have any provision to measure the pressure at outlet of second exchanger. So it is assumed all the pressure drop is now between second exchanger and its outlet piping to feed vessel (20 to 3.5 bar main reason for decrease in flow). We have cleaned all the exchanger but no increase in flow was observed.

 

Kindly suggest the solution for this problem. Also suggest if all the pressure drop is across outlet piping of exchanger to feed vessel the how we can check for blockage in line (Any recommended practice) and also how to clean and purge this line.

 

Regards

 

Muhammad Farhan

Process Engineer

Edited by Muhammad farhan, 14 June 2019 - 01:24 AM.

[PingPong]

It seems highly unlikely to me that a 6 or 8 inch line downstream a heat exchanger with much smaller diameter tubes would be plugged and moreover that that plug would resist a pressure drop of 16.5 bar.

 

My focus would first be on the actual pressure in downstream vessel 306-V03.

You state that its pressure is set at 3.5 bar but how sure are you that the pressure really is that number?

Is there a local pressure gauge on the vessel to verify it?

Can that pressure gauge be replaced by a new one (with a scale up to 20 bar) to verify its reading?

 

What is the operating pressure of the make-up gas supply from CCR/PSA ?

 

What is the set pressure of the relief valve on 306-V03 ?

 

What is the indicated pressure at discharge of pump 306-P02A/B downstream 306-V03 ?

 

 

Instrumentation in a plant is not always reliable, so when troubleshooting whatever problem always keep in mind that one (or more) of the instrument readings could be wrong.

Edited by PingPong, 14 June 2019 - 04:06 AM.

[Arsal]

Most probably, plugging is in the heat exchnagers which causing reduction in feed. Have u checked oxygenate in your feed ? Are your feed tanks cleaned?

 

Pressure drop from exchangner outlet to feed vessel could not be the cause of feed reduction. Any way it would be good to check/clean these piping as well by hydro jetting during shutdown. 

 

Moreover, please confirm, did exchangers cleaned properly in last shutdown? 

[Muhammad farhan]

Dear Arsal

 

Exchangers was specially cleaned for this problem but no significant results obtain. We haven't check oxygenates in feed. The feed tanks are cleaned. If some thing is coming from feed we have filters. Plant is newly built (2 years old).

 

Can we check the line for plugging during operation(Any inspection techniques). The line size is 6'' and it have bends and approximate 20-25 RFT length. Either we to remove complete line or hydro jetting can solve our problems. As the flow is coming from line so we can check whether the line is deplugged or not. Please guide us.

 

Regards

 

Muhammad Farhan   

[Bobby Strain]

Show us your actual pressure measurements, not pressure drop.

 

Bobby

[Muhammad farhan]

Dear Boby 

 

Pump 306-P13A/B discharge pressure is 23 bar (Control valve is fully open, its by pass valve is also open), filter downstream pressure is 21 bar, 306-E04 downstream pressure is 19 bar, 306-E05 downstream pressure (No provision to measure pressure) and 306-V03 vessel pressure is 3.5 bar. 

 

Dear Pingpong

 

We have pressure transmitter and pressure guage at 306-V03 vessel and both are giving same reading 3.5 bar. Pressure make up line is hydrogen fro HPU with supply pressure of 25 bar. Vessel is equipped with PSV of set pressure 25.3 bar.

 

Please help me in getting rid of this problem. We are not able to meet our production target and making significant production loss. Need your expertise.

Edited by Muhammad farhan, 17 June 2019 - 12:10 AM.

[breizh]

Hi ,

Is there a way to use an endoscope to survey the lines/HX  ? Third party should be consulted for this kind of technology and work  .

BTW what is the nature of the dirt collected in the filter , if any ?

Can you use equipment similar to Turbotal to enhance the performance of the HX ( heat transfer and pressure drop) ?

 

Hope this is keeping you in the right direction.

 

Breizh

[gegio1960]

Hi Muhammad.

Following your info, the most realistic situation is an obstruction in the line b/w E05 and V03.

In order to obtain proposals for solution, you should provide the P&ID and also the complete piping sketch of this line.

They should help in answering the following questions.

Are there blinds on the two nozzles? Could you insert them?

Are there vents/drains close to the nozzles? 

Good luck!

Edited by gegio1960, 17 June 2019 - 02:03 AM.

[PingPong]

Once again: What is the indicated pressure at discharge of pump 306-P02A/B downstream 306-V03 ?

[Muhammad farhan]

Dear Sir

 

Pressure at Suction of 306-P02A/B is 4 bar and at discharge is 80 bar. 

[Muhammad farhan]

Dear Gegio

 

P&ID for your understanding Line # is P-1002

 

 P&ID.png   755.6KB   0 downloads

[Bobby Strain]

Can you provide drawings for the last exchanger set? Along with the isometric drawings requested.

[PingPong]

Pressure at Suction of 306-P02A/B is 4 bar

Is that 4 bar a real measurement? 

Usually there is no pressure measurement at pump suction, at least I have never seen one.

 

Or did you estimate it based on the pressure measurement of V03 plus 0.5 bar static head?

[bubbly]

Dear

 

Upload clear and complete copy of P & ID in pdf.

 

Also provide 306-P-01A/B curve and datasheet.

Edited by bubbly, 17 June 2019 - 11:52 PM.

[Muhammad farhan]

Dear Boby

 

Isometric drawing and exchanger drawing is attached   306-P-1002-isometric.pdf   288.28KB   21 downloads   306-E05 drawings.pdf   388.97KB   20 downloads

 

Dear Bubly P&Id is attached   306-P&IDs.pdf   364.05KB   19 downloads

 

Dear PingPong

 

A pressure gauge is installed at suction of 312-P02 A/B. 

 

Please guide us to solve this problem.

Edited by Muhammad farhan, 18 June 2019 - 12:16 AM.

[breizh]

Hi,

I understand that the engineering company is Hualu (China/Pakistan)  , did you approach them with your problem ? What are the comments from the licensor ?

Can you share with us ?

Breizh

Edited by breizh, 18 June 2019 - 11:53 PM.

[Muhammad farhan]

Dear breizh

 

We approach EPCC and licensor but both are clue less. Licensor said you didn't check your 306-E05 outlet pipe line. But we have asked how check for blockage and how to remove blockage again no response. 

[PingPong]

If lines are severely plugged it is usually just after start up (after construction or maintenance) because people accidentally left tools, helmets, clothing, whatever, in equipment such as vessels or columns. It is however hard to imagine that such materials were left inside an exchanger channel and ended up in the downstream pipe.

 

Moreover you stated that after start-up the unit did operate at 150 m³/h so there was no plug at that time, but on the other hand gasoil does not foul so much that it could plug a 6 inch line. That is probably also why EPCC and licensor don't want to get involved in this. After passing the guarantee test run their job is finished. 

 

If there is a gasoil line plugged it is likely caused by a foreign object, possibly caught by the thermowell of TI-342.

 

Anyway, if there is a plug then I suppose that it is removed in the same way as in a sewer drain pipe. Surely those need to be unplugged every now and then. Contact the people in your refinery who normally handle that. I would expect they use some sort of "sewer snake" or whatever it is called in your country. Google that.

 

To remove a possible line plug that part of the HDS unit first needs to be taken out of operation and made free of hydrocarbons. Then remove the channel cover of 306-E05(S1) and inspect whether there is a plugging against the thermowell of TI-342 which is at only 347 mm from the channel. Remove any material and If possible remove the thermowell of TI-342. Then remove the manhole cover of 306-V03. Now you can try to push the snake through the whole line from E05 into V03 but because the line goes upward that might be troublesome and you may need to push the snake through the line from V03 until it exits at E05. Don't overdo the rotation speed of the snake, you don't want to damage the steel walls and bends.

 

After you are absolutely sure the line is clean (use endoscope if available) you can reinstall the thermowell of TI-342. After making sure there is no foreign material left in E05 or V03, close the covers, et cetera.

[breizh]

Hi ,

To me you should consider to remove the thermowell and inspect the line after the SHEX .If you get confirmation that the line is plugged , you should try THP ( very high pressure ) after removing the pump  managing the pot (draining point) .

I still believe you need a contractor equipped with THP (above 1200 bars) , with capability to use endoscope ( before and after the cleaning) .

For SHEX you may need a turn around to investigate or from the investigation with endoscope you may decide to anticipate it.

 

Safety is key before inspection .

 

 

My view

Breizh  

Edited by breizh, 18 June 2019 - 09:20 PM.

[Bobby Strain]

It would be quite unusual to build a deposit in the pipe and not the heat exchanger. I note a 4" line connected to the 6" line. What is it for? Maybe you didn't really clean the exchanger tubes.

 

Bobby

Edited by Bobby Strain, 18 June 2019 - 03:59 PM.

[Muhammad farhan]

Dear bobby 

 

4'' dia line is coming from 306-P02A/B recirculation (pump spill back).

 

Exchanger is clean properly but no action was taken on line.

 

Dear All is there any way to clean without shutting down the plant. 

 

@Dear briezh can you elaborate THP ? 

[breizh]

Hi ,

Sorry THP it's a French acronym for very high pressure .

Breizh

[gegio1960]

https://www.tracerco...2012XM0149C.pdf

[Bobby Strain]

You can use x-ray or a hand held radioactive source and detector, or acoustic measuring device. But I really don't expect that you will find the line is restricted. Is there any inlet device in the separator? My guess is none of us have ever experienced line plugging as you suggest.

 

You need to look at the operation history to see the rate at which the flow has decreased. This might tell you something.

 

Bobby

[Muhammad farhan]

Dear All

 

Thank you very much for your expert opinions. Recently we have shutdown the unit and open exchanger for cleaning. Exchanger 306-E05 found badly plug. Exchanger clean and unit started again, the feed and pressure restored back to its operating conditions.

 

We have found that the previous exchanger cleaning was not good because of time constraint and also no one from process/operation department witness the cleaning of exchanger. We totally rely on contractor and maintenance department but its not good practice.