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Improved Safety and Reliability by Improved Trip Interlock Systems

It is challenging and demanding to achieve both Safety and Plant Reliability simultaneously when the trip interlocks o/the plant is active. Plant personnel's aim is (0 keep trip interlocks of Ammonia Plant in line without/acing plant shut down due to false or spurious trips. Gaps in redundancy,

switch ma/functioning, control system card/ailuTes, impulse line choke, instrument isolation valve failures and solenoid valve failures are discussed. Case studies for solving these problems are

successfully explained in this paper. This paper provides insight into how Nagarjuna Fertilizers and Chemicals Limited (NFCL), India has aclrieved both Safety and Reliability by implementing these

modifications.

KUMARKRR Nagarjuna Ferti lizers and Chemicals Limited

RAGHAVANR Nagarjuna Fertilizers and Chemicals Limited

Introduction

T he following types of failures with respect to trip interlock system can lead to plant shut down and/or potential accidents to

the operating personnel, damage to the catalyst and equipment and environmental incidents

1. A single trip switch system malfunctioning leading to plant shut down.

2. A failure of trip interlocks when demanded by the process.

3. A Manual Override Switch (MOS) initiated for maintenance of instruments.

4. Replacement of failed instrument demands plant shut down.

5. The trip interlock system partly equipped with reliability or redundancy

The reliability of a trip interlock system depends upon the philosophy and design of the interlock system. A good design of the 2 out of 3 voting logic system shall allow preventive maintenance of individual trip switches while the plant is in

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operation. In an ammonia plant. non-availability of a relevant interlock feature can even cause incidents like furnace explosions and lead to severe production and/or energy loss. Ensuring plant availability by avoiding spurious trips due to instrument malfunctioning is vital for smooth operation. A 1000 MTPD (1102 ST) capacity Ammonia Plant Front End false trip (Short Shut down) can cause a loss of - 500 MTPD (SSt ST) Ammonia and --4000 G. Calories (15873 MM BTU) of energy and also be a concern for the environment. Hence, various ideas and methodologies applied to trip interlock system to overcome the difficulties are explained in this paper and will be useful for the Ammonia Industry at large.

Background

Nagarjuna Fertilizers and Chemicals Limited, under the flagship of Nagarjuna Group, operates a large, modem, integrated Ammonia-Urea complex with an annual capacity of 1.56 Million MT (1.71 Million ST) of urea, laid out in two

AMMONIA TECHNICAL MANUAL

streams, located at Kakinada on the East-Coast of India.

Brief Process Diagram efrlgerotio ".~

HTShin Ccnvertor

LTShifl r COrT/ertor _ ...... C02 Absorber I III Etil analor ...-.,_ ...

~yn. ,,, Comp" . .... "

Ammonl ~ Convertor .... "'...,i11i.' ,.,;1 t~

,.,.-... ~J!r:If

C" l~ CDR ~-+ C02 0 (1$ 3lripper f orfiing POP Unit Iic------'

Figure J. Typical ammonia process

This fac tory is surrounded by an excellent green belt of more than 283 hectares surviving with flora and fauna, which is 70% of total plant area. The ammonia plants are based on the Haldor Topsoe's (HT AS) Steam Refonning process and the urea plants are based on Snamprogetti ' s ammonia self-stripping process. The figure above gives a brief typical ammonia process. The interlock system is heart of the plant's intc:nt to c:nsurc: both safety and uninterrupted running of the plant.

Interlock Modifications

The fo llowing important modi fi cations were implemented at NFCL for improving reliability and safety of trip interlock system.

I. Modification of Trip Interlocks in Reforming section of Ammonia Plant-I:

Steam to Carbon ratio and Natural Gas to Air ratio are the main controll ing factors in any ammonia plant. Low natural gas flow, Low steam flow, Low air flow tri ps are also given as protection for any typical ammonia Plant. In case of lower SIC ratio there is a risk of carbon lay down on the catalyst and in case of lower Gas/Air ratio the secondary refonner catalyst,

AMMONIA TECHNICAL MANUAL 28

Refonned Gas (RG) Boiler inlet ferrules and reformer gas boiler can get damaged.

Low Natural Gas (NG) flow trip is provided as a protection for primary reformer as tube skin temperatures will shoot up due to low reforming reaction. If natural gas feed is totally stopped and steam alone continues for longer time, then in absence of recycle hydrogen it may lead to other problems such as catalyst oxidat ion and hot spots on the reformer tubes. Similarly, Low steam flow is provided to prevent carbon lay down on the catalyst. If steam is totally stopped and NG alone continues for even short time, it may lead to Carbon lay down of the catalyst, hot spots on the reformer tubes and appearance of Zebra bands etc. Low process air flow can cause a fl ame failure, over heating of process air coil , cata lyst damage and possible down stream explosion. Hence, protections with interlock system are provided to avoid process or equipment trouble.

The original plant was provided with only single trip switch for the steam to carbon ratio and Gas to air ratios. These ratio trips were modified to 2/3 voting logic system to further improve reliability soon after commissioning. However, modifying 2/3 voting logic system does not alone improve the reliability. The configuration before modification is explained below.

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Before modification: Natural gas feed line, Steam line and Process Air line each had a single set of orifice flanges, from which all low flow trips, ratio trips and control valve transmitters were installed. Before modification Natural Gas scheme alone has been explained below. Original Natural Gas line flange was having single set of orifice tappings connecting to a common branch of HP and LP impulse lines on which all 7 transmitters were installed as detailed below:

• First Set: Natural Gas, very low flow trip system having 2/3 voting logic system with three transmitters.

• Second Set: Three transmitters for ratio control

• Third Set: NG feed flow control valve transmitter - one transmitter

The arrangement of ratio transmitters in the network described below is how the system looked like, before modification. Each transmitter gives input to steam to carbon ratio relay, FFY/(S/C) and as well as Gas to Air ratio relay, FFY/(G/A). Each ratio relay gives digital output to very low Steam/Carbon ratio low low switches and also Gas to Air ratio 's low low switches. These ratio switches are having 2/3 voting logic system, which will activate in case of process conditions reaching the set points i.e. in case of abnonnal conditions.

NG Feed Lint Insh,unent Tappb.g cOlU.ecIiOIlS: 1krOl~ l\'lodlllcalioll

FFSXL : Low Low Switch 213 voting logic system Steam/Cuban R atio _

FFSXL : Low Low Switch 2/3 voting logic system GurAif'Ratio -

Low FlowTransmitters with 2/3 voting logic system

Instrument Im.pulse lines

<I : ec. e .

:'"IIi!." ? ?

6'2)n;;) ~-~-~

Ratio tnnsmitters with 213 voting logic fry'Stem

Natural Gas Feed line to PrimaJY Reformer

Figure 2a. Common impulse lines network/or all instruments

Problems anticipated before modification: The above arrangement is having less reliability as mentioned due to incomplete redundancy (or single point of failure). The problems like occurrence of instrument root IN (isolation

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va lve) near the orifice, gland leak may lead to a plant shut down. Visualize the scenarios of such problems: In case of HP root IN gland failure or leak may lead to activation of Very Low flow of NG and


also Gas to Air ratios. This incident will lead to plant shut down. Similarly. LP root IN gland failure or leak may activate the steam carbon ratio low switch activation and leads to plant shut down for which production and energy loss will be significant.

After modification:_Hence. configurations of the impulse line routings were modified to

overcome the problems of reliability. Orifice fl anges with 4 pairs of pressure tappings were procured and installed in Natural Gas. Steam and Process Air lines. Refer to the following figure for details of the modified instrument impulse line connections from the orifice to the transmitters:

NG Fu'(l Lbte: Ins.rUlnult T nppiug COIUlf:-dious: Aftf'-r IVIocliOnttioll

• , FF S X L : Low {.&w Switch .

b lo;,'"o!II1oo" - For Ga s I AirR atio: 213 voting logic system ~ . ~

FFSXL : Low ~ S witch.: r...-.,..,,.. 213 votUlg lOgic system:

S t eMnlCMbon R,,-tio

: (~J(;) , "'~iIIoIIiII LowFlow : ___ ~ Control Valve Ii' 9 Transmitter: Flow Transmitter

FFY. R'"QL:~:~ (':J~~~J ~ 1Ji f Wi f Tr.u:..smitter , ,

Low Flo w-

fr--l~ __ "-__ ,---' ____ .L Tr&l.smitter

- Mod lfl ed _____ ~=:::::!~~~~t:::::=~~N~.~t"'~~~Q~.~'~F~":'.~hn~· ' e to Primaq Reformer III

Figure 2b. Improved network system for impulse lines

As each orifice is having 3 sets of tappings, the transmitte r's arrangement was split after the modification as follows:

• First set: One low flow transmitter and one ratio transmitter (Note: Originally, all low flow transmitters were in one set)

• Second set: One low flow transmitter and one ratio transmitter

• Third set: One low flow transmitter, one ratio transmitter and flow transmitter for NG feed control valve


In case of HP root Vv gland failure for any impulse line branch, one NG very low flow transmitter and one ratio (G/A) transmitter only wi ll activate. As 2/3 voting logic system is there, the ammonia plant does not trip and the problem can be attended while the plant is running. The similar modifications were done on Process steam line and Process air line also. Refer the drawing (orifice flanges having multiple tappings) and the photograph of the new replaced flange ofN.G as mentioned.

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Orifice flanges with multiple tappings

VIEW FROU IhU:T SlOE ORIENTATION FOft 4 PAIRS OF PRESSURE TAPPING

(FOR TAG NO. · 02-FE.Q)6)

set of

Figure 3. Natural gas feed line orificejlange. (Similar modifications implemented/or Process Air and Process Steam lines orijicejlanges)

Benefits realized: The above modifications not only improved the reliability of the plant and these als.o enabled us to take preventive maintenance without bypassing all the trips at once.

2. Reliability enhancement of Ammonia Plant- 1I PLC system

This section explains about overcoming the reliability problem existing with shut down system. In Ammonia Plant-2 shut down programmable logic control system (PLC) system was supplied by HIMA (HIMA H-51 HS). There were 3 PLC stations connected with operating station, engineering work station and event recorder as shown in Figure 4.

SHUTDOVVN SYSTEM LAYOUT OF AMMONIA- 2

PLC# l

I I OPERAT OR

STATION (MASTt;R,

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I

EVENT RE C ORDE

PLC#2

NET~O R.K CONNEC TION

I PLC#3

I I OPERATOR

STA T ION (REDUNDANT)

ELOPBUS

PLESYBUS

I E N GG . J

STATIO N

Figure 4. Brief diagram of PLe

31 AMMONIA TECHNICAL MANUAL

Before modification: The entire Ammonia Plant analog and digital signals were configured in these 3 PLC's. Each PLC station can accommodate approximately 4 numbers of sub racks and each sub rack can accommodate approximately 16 numbers of Input and Out put

cards (110) either analog or digital cards. The 110 cards in subrack-2 are back up cards for subrack-I.

1/0 Cabinet arrangement

Module1 In Subrack-1

B <1cl< up for Module1 In k-2

Sub Rack-2

Figure 5. Modules and back-up modules in sub-racks

Problems faced before modification: Originally all 213 voting logic related analog inputs of one particular fi eld measurement were connected in the same card. Because of this configuration, when there was a failure of that particular card (both primary and redundant) it was leading to tripping of related interlock system logic. Prior to the modification we have experienced the following failures:

1. Ammonia Refrigeration compressor, Anti Surge Valve got opened due to an error in PLC-3 digital output card and back up card did not come in line. Hence, synthesis gas compressor partial trip was initiated. (Year 2000)

2. Ammonia Plant Back end tripped due to Shutdown system PLC-3 Card failure (Year 2003)

3. Secondary Reformer Trip owing to failure of PLC cards including the redundant cards.(Year 2008)


4. Back end tripped due to closure of Synthesis Gas compressor I st stage suction inlet valve due to card fai lure.(Year 20 II )

5. Back end tripped due to PLC-3 card failure. (Year 2011)

See the specific example shown for PLC-l in the following figure. Before modification, all 3 switches of low Feed to Primary Reformer were incorporated in the same I/O card i.e. number-I. Even though this card was having a back up card in sub rack -2, but in case of failure of back up card coming in line during the failure of card in operation, the ammonia plant trip occurred.

After modification: To prevent such kind of failures, all these individual trips of low feed to primary reformer were given to 3 different ca.rds (Card 1,2 and 3). In case of any specific card failure including back up card, the trip does not get actuated. Similar modifications were implemented for all cards in PLC-2 and PLC-3 also.

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Distribution of trips in different cards: Example for modification implemented

:a.fore modific ation After Mod ificatio n Card- l Card_1 CMd-2 Card-3 Subrsck_1 Subrack_1 Subl"ltck_1 Subract-1

,. Feed Flow Pr. Re f . ,. Feed Flow Pr. Rof . ,. Fe .. 1 How PI . Ref . 1 . liS St . Flow Pt . Ref. 1. f eed Flow Pf _ R-ef. ,. HS St. Flow Pr o Ref . , HS St. F low Pt. Re f . , LO'W CA Ptes. To Pr ,Ref ~ Fe 4HI Flo w p, .• Re f . 3. Spare Chwvlel 3 La>N CA pres. to Pr .Re f ,. Feed Flow PI . Ref. 4 . $pete Ch-N"lnel 4 . Spete Ch-N"lnel .. Spare CtwYlei 4 . Low Drl'ltt to Pr-.Re f .

I ,. Feed Flow h . Ref. ,. F_d f low PI . Ref . ,. F_ d Flow P,. Ref . 1 . liS St . Flow Pr. Ref. >. Fee d Flo w PI . lRef. ,. HS St. Flow Pr. Ref . , HS St . Flow Pr. Re f . , LOoN CA Pres. To Pr .Ref ,. Fee d Flow PI'. Re f . 3 Sp$"e ChCllYlei 3 . Low CA pres . to Pr .Ref .. Feed Flow PI . Ref_ .. Spare Ch tvV"lei .. Spt!lf. ChtvV"lei .. Spare CJ"\;tV)O eI .. LO'W Dra ttto Pr .Ref .

B&ekup Backup -' '''' s.c' ", CNd_1 Card_1 Ctwd- 2 Card-3 Subrl!lCk_2 SUbrack-2 Subrack_2 SUbrack-2

Figure 6. Feedjlow to primary reformer distributed in separate cards

Benefits realized: Only simultaneous failure of combination of multiple cards will lead to the false trip. Since the fai lure of multiple cards is of least probability, this modification helped in increasing the system reliability, safety and ease of maintenance drastically.

3. Improving the r eliability of Primary Reformer furnace draft indication:

During the normal operation, the furnace draft optimum range is -6 rnm (-0.24") to -9 mm (-0.35") of water column (WC). The reformer trip is provided at +3.00 mm we (+0. 11 8" WC). The draft in the reformer is controlled by operating the flue gas damper at the suction of ID fan. In case of high pressure in the furnace, three high pressure switches are provided which will initiate the trip of reformer. These switches havc bccn convcrtcd to prcssurc transmittcrs with 2/3 voting logic system to improve the reliabi lity. On line DeS indications were also

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provided for monitoring purpose. In case of malfunction of anyone indication, the same can be attended on line.

The high draft trip is given for protecting the furnace from high pressure. Various causes for such pressure fluctuations can be due to variations in FD Fan speed while on steam turb ine drive or malfunctioning of the dampers of ID Fan and FD Fan when on motor drive. In case one ofID Fan or FD Fan trips, the furnace may either go to extreme positive or negative pressure which is unsafe condition. Other possible causes could be failure of motive steam due to malfunctioning and closing of inlet solenoid valve, fa ll ing of Turbine trip valve CITV) latch lever, Lube oil failure , etc. In case of failure of power, problem may be due to fa ilures at MCCI Gas Turbine etc. In case of reformer tube rupture or waste heat section tube rupture, this trip protects from furnace box getting pressurized.


Before modification: The original furnace draft measuring system was as follows. The draft measuring transmitter was having two connections; one was connected to Primary Refonner furnace box to measure the draft and the other tapping was kept open to outside without any connections to take as reference of the atmospheric pressure. Refer the Fig. 7.

Problems faced before modification: But this draft indication was not reliable as fluctuation used to be more with the change in ambient air velocity during nonnal plant operation. Whenever there was wind and/or rain, frequent alanns used to appear on shut down system. Hence keeping this trip interlock in line was challenging as it can lead to a plant trip. Many trials were made to modify the arrangement, like free end of transmitter tapping was kept in a bucket filled with stones to reduce the impact of external atmosphere on the draft indication but the same were not successful.

Modification implemented:

The tapping which was connected to atmospheric was connected to a coi l and at the down stream of the coil further connected to semispherical dished heads holding a mesh as shown in the figure. Down stream of this semispherical head, the instrument line was extended downward to nearly 10 meters (32.8 ft) and kept open to atmosphere.

Benefits realized: This arrangement has reduced the fluctuations to a great extent. On October 12th 2014, there had been a cyclone in south India, called as "HUD-HUD". This cyclone had a damaging effect on Visakhapatnam city which is around 160 KM's (nearly 100 Miles) from NFCL site. On this cyclone day, the wind velocity has gone up to -60 KMlHr (37.2 MileslHr) at factory site. During normal running day the typical wind velocity is around 22 KMlHr (13.6 MileslHr) only. Hence, the fluctuations in reformer draft during normal time and cyclone time were compared to evaluate the reliability of indication. During cyclone time also, the fluctuations have not reached the trip va lue

Before Modification After Modification

Anoth er Tapping Open to_ atmosphere "-[?

_Furnace

Primary . R e f or m e r ,-_ Furnoce

Tapping

J

_ Trtlns mltte r

Prima r y . Reformer Furnace

Modific.mio n s h own in BLUE color

Figure 7. Transmitter's HP tapping modification


RcfOI'luc:r dl":'lR I :tJ)Pi.. . ~ . Auuu o lI l:lo PI~.t.1

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+3.00 mm (+0. 11 8") WC. Even during cyclone day the maximum fluctuation was -4.5 mm (-0.18") i.e. from -7.3 mm (-0.28") mm to -11.8 mm (-0.46") of water column (WC). See the below graph for details

Primary reformer draft indication during the cyclone

.;~-------------------------

." Thlelnmln's __

" Figure 80. Variation oJ draft indication when wind velocity is three times normal day (deviation within range)

On a normal day, the refonner draft fluctuations range is 2mm i.e. from around -7 mm (-0.27") to -9 mm (-0.35") are as follows:

Primary reformer draft indication during normal day

·12,1 g ·14.8

1lne In min's --

16:00 1610 16:20 16:30 1640

Figure 8b. Draft indication normal day of plant operation

Thus, this modification had improved the reliabi lity of indication even with significant variations in ambient conditions. This trip interlock was taken In line after the modification.

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4. Improving flame scanners reliability:

Fired heaters like Feed stock pre-heater and steam super heater etc are most important equipments, one should be careful while lighting up or re-lighting up the burners , as t.hat is the time, when most explosions occur. Hence, these fired heaters are provided with necessary trip mechanisms along with flame sensor based trip protections.

Before modification: Feed stock preheater and Steam Super heater had flame scanners as part of the original installation for both of the Ammonia Plants. These flame scanners originally supplied by M/s.Air Oil - Flare gas India limited (Later on the company became Mfs.FIREYE). The flame scanners were of UV type and were installed in 1992 in Ammonia Plant-l i.e., since inception. There is a trip avai lable for total flame failure of all burners for feed stock preheater and steam super heater. Based flame failure for each burner, individual burner trip is also avai lable. These flame scanners in the field will generate a voltage signa l which is sent to the control module in the control room with the final digital input going to the Emergency shut down system that initiates the respective trip interlock.

Problems faced before modification : Prior to modification, the Ammonia Plant-l tripped in year 2004 and Ammonia Plant-2 tripped in 2009 due to minor explosions in feed stock preheaters. There were many reasons for these incidents. At that time, these flame sensor-based trips were not in service since these scanners were not reliable. Frequently, dust accumulated on these flame scanners. Also, during rainy season, insects used to fa ll on these flame scanners of feed stock preheater (Natural draft fumace) even though Instrument Air purging was used. Due to these reasons fl ame


sensors were not detecting properly. Also, there were no DeS indications available for flame intensity/signal strength at that time. Dependability was based on cleaning frequency only. Also, there were problems of input card failures pertaining to flame scanners. This input cards feed signal to emergency shutdown system. Frequent false alanns used to appear on the shut down panel due to these problems. Hence during that time these trip interlocks were not in service.

After modification: In the year 2009, all these flame scanners were replaced with new DURAG flame scanners of D- LX 100 model and improvements were made as follows:

a) Flames whose UV -radiation is absorbed by dust, water vapor or other materials, and which cannot nonnally be monitored by UV tubes which is the case with the original system was

" .. ~I

relatively overcome by this highly sensitive semiconductor UV -photo elements.

b) DeS on line indication also made available with flame intensity to monitor for issues.

c) These flame monitor sensors can perform self test. This test will be repeated continuously during the operation.

The DeS indication made available for flame intensity of each burner is given below for feed stock pre-heater. These signal strengths are being monitored by panel operator. Whenever this value is coming down to 80% of signal strength, a permit is given to instrument person to clean the flame scanner.

Flame intensity indications on DeS

".'

0.' 0 .•

'0.0

Fig 9. Feed Stock Preheater individual burner flame indications

Even though dead insects and refracto'ry dust are getting accumulated on these scanners, the DeS


indication is now being used to schedule cleaning as needed.

2015

Benefits realized: After implementing these modifications, the flame intensItIes were monitored for reasonable period of time and the total flame failure trip interlock was kept in line for both Ammonia plants.

5. Overcoming the faiJures of Solenoid Operated Valve system:

Solenoid operated valves (SOV) are considered to be the most important controlling element which is supposed to be more reliable as it ensures the safe condition during emergencies.

Before modification: In general the function of a solenoid valve is as follows: a solenoid valve has 3 ports i.e., inlet, outlet and vent or exhaust port. In general, the solenoid valve will be always in energized condition and with help of electro magnetic force due to supply of voltage of - 110 volts. This condition will keep the solenoid valve open, which means that inlet port is connected to outlet port. When trip interlock gets activated, the solenoid valve gets deenergized and subsequently, due to spring force, the outlet port gets connected to exhaust port. On the process side, if the control va lve is air to open type, this air gets vented through this port and the respective control valve gets isolated.

e .g : Feed c o nt.-o l va l v e -D o uble s oleno id v nl v e s lllo difi c nti o n

Problems faced before modification: Depending on the location, the malfunctioning of a solenoid valve such as a coil problem or the power supply failure can lead to the ammonia plant trip. The following failures have occurred in ammonia plants as follows:

I . Secondary Reformer trip activated due to Process Air Compressor (PAC) Anti-surge Valve solenoid malfunctioning. (Ammonia Plant-II in 2009).

2. Secondary reformer trip due to Gas/air ratio activation when NG Pressure control valve malfunctioned. Fuse m the Shutdown cabinet failed and lead to power failure to solenoid of NG Pressure control val ve. (Ammonia Plant-I in 2002).

3. Back end tripped due to C02 absorber rich solution outlet solenoid valve fuse failure. (Ammonia Plant-I in 2004).

4. Primary reformer Trip activated due to fuel NG valve closure (solenoid failure) (Ammonia Plant-II in 2000)

After modification: The above problems were solved by adopting the modification in Figure 10.

~---#-# - -.. -~ ---- ~ I : Inlat port: 0 : O uUa t port E : Exhaus t P o rt

~

~ .. .. iSOV1t-----------"; .....

I : 0 ~ - - .... - ~------#- -4'- -# - .... ---~-I--#",

: ..... E : 0 .... ~ . .! Isov2t----------~ .... -# -: .;. : +

: : I '-. - _ . - - .".-_. #- -#- - _ • • . E

Soleno id vn lve connec tio n s

1

Figure 10. Example of solenoid valves modification implemented (Photograph and diagram)

2015 37 AMMONIA TECHNICAL MANUAL

To explain the above modification, primary refonner feed control valve was taken as an example. Another solenoid valve has been added to the existing solenoid valve during the modification.

The various possible scenarios were considered for operation with new arrangement as mentioned below:

1. Normal operation : When both solenoid valves are functioning nonnally, the inlet ports wi ll be connected to outlet port, when control va lve is in open condition and operating properly. When a trip gets activated, the outlet ports will get connected to exhaust ports and the contro I val ve is closed due to depressurizing of diaphragm through the exhaust of vent.

2. Solenoid valve - 1 failure : The vent port of SOY -1 gets connected to outlet port. But as SOV2 is functioning nonnal and SOV2 inlet is connected to SOV2 outlet, the control valves still get the required air pressure through SOY I exhaust and outlet port leading to normal operation. (Path: SOV2 inlet ~ SOV2 outlet ¢ SOY I exhaust ¢

SOV I outlet c:> Control Valve). 3. Solenoid valve - 2 failure : SOV2 vent gets

connected to outlet port. But this condition does affect the operation as SOY I is operating properly, SOVI inlet port is connected to outlet port and Air will continue to supply to the control valve. (Path: SOY I inlet c:> SOY I outlet c:> Control Valve).

4. Both Solenoid valves failure: The control valve goes to fai l safe position leading to contro l valve closure.

This modification has eliminated plant trips due to the failures of solenoid valves. Many important control valves like the following were provided with double solenoid valves:

a) NG feed and its bypass valve


b) Block and bleed valves ofNG fuel and NG feed

c) Process Stearn d) Process Air compressor discharge e) Process air compressor Anti surge valve f) Synthesis gas compressor anti surge valves

Benefits realized: This modification of Solenoid valves arrangement enhanced the plant reliabi lity and avoided unwanted shut downs.

6. DeS and SDS systems upgrades for better reliabiJity in Ammonia Plant II:

Upgrade of the Distributed Control System (DCS) and Shut Down System (SDS) systems have brought drastic improvements in safety and reli ability. It is not a comparison between vendor during the year 1998 and the present vendor. The intention is to highlight latest and additional features available at present, which can be adopted for improving safety and reli ability of the plant.

Des: The original DCS system was UNIX based and suppl ied by Mis.ASS during the year 1998. As the DCS system was 16 years old, the model became obsolete and there was no support for spare parts. Once failure of one the DeS card led to Ammonia Plant shut down in the year 2000. The original DeS system was replaced with new windows based Yokogawa Centum (Vigilant Plant) . After the DCS replacement there were no shut downs due to card failures since June 2014. Further, the liD card redundancy, processor speed, SIL-3 rating with single card and better maintenance features were rea lized with new DeS system.

SDS: The original (shut down system) was supplied by HlMA H51 PLC. The shut down system was replaced with Yokogawa Prosafe RS PLC System during the recent plant tum around of 2014. Few additional improvements realized, have been mentioned below:

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Improvement obtained Table of comparison

No Original System I It is an

independent system and communicates to DCS through third party interfaces.

2 2 CPU's avai lable and each CPU

with new features -

New System Seamless architecture having direct integration of DCS and SDS -More Safe and Reliable. 2 CPU's available and each CPU

having single having two processor. processors, hence

reliability improved. 3 Error reporting on High diagnostic

I/O card failure coverage factor of was not effective more than 99%. and resulted in Fault detection and Plant trips. Repairing features

also available. 4 Programming is Windows-based

cumbersome and programming. DOS based. Complex Mathematical mathematical calculations cannot be performed. On line download is generally not recommended.

calculations can be performed. Online modifications can be done and downloaded to CPU.

Benefits realized: With above latest features and new system, the Safety and Reliability of SDS have remarkably improved and no down time experienced since June 2014.

7. Enhancing reliability of Critical Trips

As per the original design philosophy, few of the trip inputs were provided with single trip switch only and in case of malfunction, will lead to spurious trips. Hence the following modifications were implemented.

A) De-Aerator Trip: Before modification Deaerator had a single switch for low level.

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Keeping in view of criticality, this trip has been converted to 2/3 voting system in Ammonia Plant-2. There are two stand pipes already avai lable for De-Aerator as shown in the Fig. II. Two more tappings with transmitters were added to the existing stand pipes. One transmitter was connected to the stand pipe of LG and another transmitter was connected to the LT/switcbes stand pipe.

De-Aerator trip interlock modification

Stand pIpe for Switc hes. \ Transmitter

De~Aerator

I

LSXLLOO2A _ Modificat ion

Figure 11. Additional transmitters installed to enhance reliability (Shown in blue color)

The trip interlock system was enacted after this modification, thereby increasing the safety of equipment.

8) Process Air compressor trip on Very Low Lube oU pressure: The nonnal lube oi l header pressure is around 3 kscg (42.6 psig). Before modification this trip was in line with a single switch with a set point of 1.9 kscg (27 psig). Additionally, one transmitter was installed at local gauge location and one more transmitter tapping was taken from downstream of lube oil pressure control valve. With these additional 2 tappings, a 2/3 voting logic system was introduced and trip interlock was taken in line.

Benefits realized: The above two modifications enhanced the reliabi lity of plant by avoiding possible plant trips due to malfunctioning of instruments.


CONCLUSIONS

The fo llowing conclusions are offered:

a) We could achieve the safety and reliability of the plant with in-house capabilities of the plant personnel and utilizing the available latest technology in the industry. With this, we could overcome majority of impending problems faced over a period of time and prevented the poss ible incidents due to safety, loss of production and thereby improving the plant performance.

b) Dedicated driving efforts of management and the plant personnel are required to study, modify and to take all trip interlocks in line without affecting plant reliability.

c) Interaction with other Ammonia industries, technology supplicr's guidance and information from equipment vendors helped for achieving the same.

d) Modifying only the trip interlock systems cannot ensure safety of personnel and


reliability of plant. Elements in Process Safety Management System like standard operating procedure (SOP's), management of change (MOC) procedures and pre-start up safety reviews etc will add to the efforts mentioned above. In particular, the safe working practices are required to prevent accidents. The fo llowing are some of the examples:

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When Ammonia Plant is shut down fuel gas is to be blinded at battery limits and proper purging is to be ensured at the earliest. Pressurizing of fuel Jines is to be done only just before lighting up furnace.

e) Plant operational personnel shall look into instrument configuration reliability along with arrangements like 2/3 voting logic system in the fie ld.

f) New versions of shutdown system and DeS with more safe and reliable features are available in the market. The avai lable systems reliabi lity is to be compared and rep laced if suitable.

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improved safety and reliability by improved trip interlock …/fileser… · chemicals limited...

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