Trouble Shooting Compendium-Sample - Copy

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Summary

Unit Category HDTBRefinery DIGBOIUnit Pressure Swing Adsorber (PSA) of Hydrogen Generation

Unit (HGU)P Instrument malfunction(Control Valve)

Solve Method 1. Installation of No-Play coupling in feed, product and dumpvalves to eliminate valve error due to delay inclosing/opening feedback.

2. Calibration of positioner of equalisation valves for enablingquick shutoff to eliminate delay in actual closing andreceipt of closing feedback.

Report No.Year / Month 2012-13Title of thereport

Fluctuation in purge gas generation and hydrogen productpressure.

Problem 1. Frequent generation of valve error alarm and equalisation-I and equalisation-II deviation alarms, due to play between

valve actuator and valve body, frequently causing PSA toswitch in 4 bed operation.

2. In some steps of PSA, excessive purge gas generationtook place generating DUMP DEVIATION alarm followedby low purge pressure in next step. Hydrogen productpressure also kept fluctuating due to which more hydrogenthan required had to be produced to avoid hydrogenstarvation in consumer units.

Solution Analysis of PSA adsorber pressure, Dump, Providing Purge &Repressurisation trends for identification of malfunctioningcontrol valve.

1. Problem Details:Hydrogen rich gas from (High Temperature Shift) HTS reactor is sent to PSA for purificationwhich produces 99.99% pure hydrogen and rejects other gases viz. CO, CO2, CH4, N2 andsome amount of H2 also to purge gas stream. Pure hydrogen is sent to HDTB, MSQUcontinuously and to CRU, WHFU and Hydrogen bullet occasionally. Purge gas is used forfiring in reformer furnace.

Due to heavy fluctuation in purge gas generation, reformer box was upset as purge gasconstitutes a significant portion of mass flow to reformer furnace. There is a trip interlockon reformer box pressure. This was hence a major problem.

Furthermore, due to fluctuation in hydrogen product pressure, a minimum excess o

hydrogen had to be maintained so that hydrogen consuming units get sufficient supply ohydrogen.

2. Problem Duration:2012-13


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3. Observation and analysisAttached here is an image of HGU PSA HMI screen for reference.

Ref picture above, all the 5 beds of PSA have four valves, detailed as under:

A. Feed Valve: These are butterfly control valves, 10_XCV_8111, 10_XCV_812110_XCV_8131, 10_XCV_8141, 10_XCV_8151, also called 1 series valves.

B. Product Valve: These are butterfly On-Off Valves, 10_XCV_8112, 10_XCV_812210_XCV_8132, 10_XCV_8142, 10_XCV_8152, also called 2 series valves.

C. Equalisation Valve: These are globe control valves, 10_XCV_811310_XCV_8133, 10_XCV_8133, 10_XCV_8143, 10_XCV_8153, also called 3 seriesvalves.

D. Dump Valve: These are butterfly control valves, 10_XCV_8114, 10_XCV_8124,10_XCV_8134, 10_XCV_8144, 10_XCV_8154, also called 4 series valves.

Part I: Purge gas fluctuation problem


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1. The pressure of bed 5 increased immediately after closing of valve equalization valve(8153) in boththe steps 3.1(isolation) & as well as 2.1(dump) , which implied that there was some delay inreceiving of closing feedback and actual closing of the valve in the field. This was further confirmedwhen the valve was observed in the field, as closing feedback was received at HMI before the valvewas tightly shutoff. When bed 5 goes in Dump stage, 8153 closes and 8154 opens. The kink in thedump graph is due to 8153 not being fully closed physically in the field before the closing feedbackis received and gas from other beds entered the 5th bed. Ref photograph attached.

2. When the 5th bed went in isolation in step 3.1 & 3.2, the pressure of bed increased although all 4associated valves with the bed were closed. 8153 was the last valve to close before isolation step.

This further indicated that there was some delay in closing of the valve of 8153, allowing gas fromother beds to enter the 5th bed. Ref Photograph attached.


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Part II: Hydrogen product fluctuation

1. As can be observed in the attached image, there is a strong interaction between bed pressure of 4t

bed (in pink) and 5th bed (in blue). When 4th bed is producing hydrogen and continuous feed gas isbeing fed to it, next bed i.e. 5 th bed is under final pressurization and is also being fed feed gassimultaneously. That is, feed valves of both the beds are simultaneously open. The feed valve of 4 t

bed is completely open and hence could be ruled out as the source of the fluctuation. However feedvalve of 5th bed is in play according to the rate of pressure rise. Hence this was identified as thesource of the problem. As it later turned out, due to play between valve actuator and valve body,controller was not able to maintain a steady rate of pressure rise in 5 th bed. This in turn upset 4t

bed pressure which cause a fluctuation in hydrogen product pressure.

4. Methodology:

The PSA adsorber pressure, Dump, Providing Purge & Repressurisation trends were observed indetail, with a view to identify the deviation in trends from normal operation which gave a generaidea of valves which may be malfunctioning.

The following characteristics of the suspected valves were observed at PSA HMI as well as PSA Skid:

A. Passing of valves in fully closed state was checked.

B. Opening and closing feedback percentages of the valves.

C. Receipt of closing feedback of valve at PSA HMI and actual closing of the same at PSA SkidThe delay between receipt of closing feedback and actual closing of the valve at field wasalso checked.

D. In case of Butterfly Valves, any play between valve actuator and valve body was alsochecked.

E. Opening percentage and bed pressure during dump state of beds were observed.


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5. Corrective Action:

A. No-Play coupling (picture attached below), was installed in the feed, product and dumpvalves of all the 5 beds of PSA by taking PSA into 4 bed operation to eliminate any playbetween valve actuator and valve body. Installation of No-Play couplings totally eliminatedthe Equalisation deviation alarms and valve error alarms. This eliminated the problem offluctuation in hydrogen product pressure; however purge gas generation profile did notimprove.

B. To reduce purge gas profile, PSA was taken into 4 bed operation by isolating the 5th bed andthe Equalisation Valve (10_XCV_8153) was attained. Receipt of closing feedback of valve atPSA HMI and actual closing of the same at PSA Skid was observed and a delay was noticed.

The positioner was re-calibrated for quick closing of the valve. The opening and closingfeedback was then checked and found in permissible limits. The PSA was taken into 5 bedoperation and considerable improvement was observed in the purge pressure trends. Thesame problem up to a little extent was with the Equalisation valve of 2nd bed (10_XCV_8123)which was also attained and calibrated for quick shutoff, thus totally eliminating thefluctuations in purge gas generation profile.

6. Benefits Realized


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1. It can be observed in following image that the pressure trends of all vessels are similaand stable now.

2. Significant improvement observed in purge gas firing pressure can be observed infollowing graph.


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3. Significant improvement in hydrogen product pressure profile can be observed in followingimage.

4. It can be observed that fluctuations in box pressure have significantly reduced infollowing image.

5. When hydrogen is suppiled to HDTU, a minimum excess of 15 Kg/hr of hydrogen ismaintained to avoid hydrogen starvation in HDTU make-up gas pressure. It is to be noted


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that although this excess hydrogen is sent to fuel gas header, it is still a significant loss.Hydrogen production from PSA is an inherently fluctuating process. To maintain aminimum of 15 Kg/hr excess, a higher avergae excess has to be maintained. Earlier due tohigh fluctuation with difference in minima and maxima upto 60 Kg/hr, an average excesshydrogen flow of 45 kg/hr had to be maintained. After installation of no play couplings inall feed, product and purge gas valves, amplitude of fluctuation has come down to 15Kg/hr and an average excess flow of only 22 Kg/hr has to be maintained. Now minimum 10Kg/hr of hydrogen is being saved.

6. Due to excessive purge gas generation, upto 4000 Kg of purge gas had to be flared peday. Now this has been brought down upto 3000 Kg/day and minimum 500kg/day losssaved.

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Trouble Shooting Compendium-Sample - Copy