Severe surge incidents at Process Air Compressor, their after effects

and problem resolution A 37 years old centrifugal Process air compressor suddenly starts surging at its normal operating

point, after overhaul in 2004. Detailed investigation reveals a design issue in HP case of the machine which was hidden since 1966. Surges on the machine results in damage of downstream secondary re-former catalyst and fouling of waste heat boilers. This article explains the investigation details, root

cause and how to avoid such pitfalls in design/relocation of any centrifugal compressor.

Muhammad Majid Latif Engro Chemical Pakistan Ltd.

Introduction

ngro Chemical Pakistan limited is the second largest manufacturer of Urea N46 in Pakistan. The manufacturing site is located in a small rural town of Pakistan known as Daharki which is 600

km north of famous port city of Karachi. The site was setup in 1965 after discovery of vast resources of natural gas in vicinity and was named as Esso Chemical Paki-stan, which was later named as Exxon Chemical Paki-stan. In 1991 Exxon decided to divest its equity from global fertilizer manufacturing. At that time Pakistan based employees of Exxon along with reputable interna-tional investors decided to purchase major share hold-ing. Exxon’s patent of brand name “Engro” was also a part of the deal. The Company's original plant was commissioned on December 4, 1968, and since then, except for short maintenance shutdowns, the plant has been in continu-ous operation. The initial urea capacity was increased from 173,000 to 268,000 tonnes per annum till 1993. In 1993, Engro relocated Bechtel design Ammonia and a TOYO Total Recycle Urea plants from Pascagoula, USA and Bellingham UK respectively and increased its

urea capacity to 600,000 tonnes per year. In 1995, the urea capacity was expanded to 750,000 tonnes per year. In 1998 urea production capacity was increased from 750,000 to 850,000 tonnes along with an overall energy efficiency of 7%. Further to this other projects were im-plemented which have increased energy efficiency by another 5%. Daharki site is currently operating one 1660 tonnes per day steam reforming ammonia plant and two Toyo De-sign Urea plants of combined capacity of 2830 tonnes per day.

Background

In original design of plant (when commissioned at Pascagoula), process air compressor was designed to deliver 2050+ KSCFH (Kilo Standard Cubic Feet) of air to meet H2/N2 ratio of 3.0 for ammonia production. However after relocation to Daharki, Pakistan the air demand reduced considerably (to around 1700 KSCFH) the main reason was that unlike Pascagoula, at Daharki the feed Natural Gas contains about 18% Nitrogen.

E

Compressor Details

Process air compressor was originally designed and manufactured by “Clark Brothers” in 1966, the company owner ship and name changed sev-eral times after that and now it has became Dresser Rand. The machine consists of four stages, which are contained in two casings (i-e LP casing and HP casing). Compressor Driver consists of one 17000 HP GE frame 5 single shaft gas turbine which is-

coupled with a steam helper turbine. (For more details please refer to figure 1 ).

Machine is connected with Secondary reformer of Ammonia plant, through primary reformer convection section air preheat coils. A simpli-fied flow diagram of the circuit is given as fig-ure 2. Original antisurge scheme on this machine con-isisted of single antisurge blow-off valve (FR-117) at the final stage of the machine.

1st StageGasTurbine

HelperSteamTurbine

3rd Stage 4th StageGBGB

NH3

Suc

tion

filte

r

Suction filter

2nd Stage

FR119

FR117

FRC-117

PRC-335

FR116

FR

118

ATM

70 K

SCFH

Air

1700

KSC

FH

60 KSCFH

E-2502 D-2507 E-2504 D-2509

E-2503 D-2508

Simplified Diagram of Process Air compressor, K-2501 Train.

Anti SurgeValve

ExtractionAir

Inst

t. A

irto

UTY

-1Pr

oces

s A

irto

Sec

Ref

.

LP Case HP case

(Figure 1)

Overhaul History

HP case of the process air compressor, K-2501 was first time overhauled in 1993 (at the time of its commissioning at Daharki site). After 1993 al-though small reduction in machine efficiency was noted, however only LP case of the machine was overhauled in 1995 and HP case was never opened.

It was decided to overhaul the machine during annual shutdown of plant in April 2004. Both LP and HP cases of the machine were opened up and completely overhauled. While only erosion was noted in LP case of the machine, consider-

able damage was observed in HP case of the machine. It was found that the IGVs (Inlet Guide Vanes ) and rotor of the 3rd stage of the machine has been damaged and eroded (Please refer to damage photos given at end ).

Damaged rotor and IGVs were replaced and complete overhaul of HP casing was carried out.

SEQUENCE OF EVENTS : During Ammonia plant Startup on 17th April 2004 at 9:00am Process Air compressor Train, K-2501 surged. The machine remained in surge for around 7 minutes. Ammonia plant back end was tripped and machine was offline from the process immediately by fully opening its 4th stage dis-charge vent valve FRC-117. K-2501 surged again on 28th June 2004, due to slight fluctuation in fuel control valves. The ma-

chine had never surged before at similar operating conditions.

Due to similar nature of both events, details of first event only are given below : First Surge Incident :

Ammonia plant was started on 16th April 2004 (after turnaround) and production was achieved at around 2100 hrs. After that ammonia FE load was being increased till the time of the event (at which it was 140%).

April 17th , 2004

FR-1

17

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K-2501

Air I

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Process AirCompressor

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low

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Proc

ess

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Flo

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Waste HeatBoilers, E-2501A/B

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FIGURE 2

Bee Hieve

8:30 am Backend (BE) boardman observed that ammonia loop pressure was going down. This is a clear indication that plant now requires more air addition to maintain same pressure and H2/N2 ra-tio. 8:31 am BE boardman requests Front End (FE) boardman to increase air by increasing speed of process air compressor, K-2501. FE boardman instructs him to close air compressor vent valve FR-117 which was 17% open at that time (this valve remains 100% closed during normal plant operation). 8:35 am With permission of FE boardman, BE boardman starts closing FRC-117 in small steps of 1-2%. Gas turbine was being operated at 4805 rpm at that time. 9:02 am Shift Coordinator was informed by Field Operator that wheezing noises are coming from process air compressor. Alarms started coming on Gas Turbine Speed Control (Mark-V) panel. Tur-bine speed fluctuation from 4400 to 5200 rpm started. Vibration circuit fault alarm (K-2501) ap-peared on Bentely Nevada panel. All vibration in-dications for Process air compressor (especially HP case), K-2501 started fluctuating. 9: 04 am Compressor discharge flow, FRC-117 started varying from zero to max, it was also vary-ing on FRC-116 (air to 2°Reformer), low air flow to 2°reformer security, FSL-313 alarms were also appearing but no action was taken as it was by-passed. FE boardman opened FR-117 from 12 to 15% but compressor surging didn’t stopped. Gas turbine enters temperature control mode (cut back) as its exhaust touches its limit (1000 °F) and alarm appears at Mark V.

9: 05 am It was observed in field that FRC-117 transmitter needle was moving from zero to max along with the wheezing sound and duration for each surge cycle was around 5 sec. FE boardman opens FRC-117 from 15% to 75%. Field operator and Shift Supervisor witnesses in the area that valve moves to 75% opening. Surge didn’t stop.

9:08 am FE Boardman fully opens FRC-117 (veri-fied in field), Ammonia back-end is tripped, air is cut and normal shutdown sequence is followed. 9:09 am Air Compressor comes out of continuous surging Gas turbine rpm and exhaust temperatures normalize. Air compressor vibrations revert back to normal values.

TECHNICAL EVALUATIONS : Detailed evaluations of data captured at Plant DCS (Honey Well, TDC-3000) and at the Emergency Shut Down system (PLC) were carried out and a fault tree was developed. (Please refer to Fault tree analysis given below). Following are the highlights: A single antisurge /blowoff valve, FRC-117 is

installed at 4th stage discharge of the machine. The antisurge system is based on a single point programmed in DCS, TDC-3000. i-e flow point at maximum speed of process air compressor.

On 17th April at the time of surge, the flow at compressor discharge was 1700 KSCFH at gas turbine speed of 4805 rpm. As per compressor map the surge flow value at this speed was 1450 KSCFH .

Compressor i) stage temperature rise profiles ii) Thrust Vibration trends & iii) Noise measure-ments in field all indicate that the surge initi-ated in HP case and the first stage to enter surge was 3rd.. Discharge temperature profiles (captured from TDC-3000) are attached as fig-ure 3.

TDC-3000 DCS has a scan time of >250 mil-liseconds, which is not suitable for detection and control of compressor surge. (< 100 milli-second scan time controllers are required). So it captured only 4-5 surge cycles, which actu-ally were much more than that.

FSL-313T, low process air flow security shut-

down initiated and re-setted (without board-man action) 81 times. This security was by-passed during startup. This indicates that 81 cycles of surge happened. (Note : this security is configured on a much faster PLC system).

Generally in case of centrifugal compressors

if we open the antisurge valve of a compressor in surge, it always comes out of surge. How-ever this was not true in case of Process Air compressor, K-2501. Even though the anti-surge valve was opened from control room and field verified yet the machine remained in surge for around five minutes.

It was interesting to note that this was the only

compressor at our site for which we had EIGHT different compressor maps (surge curves) which were obtained from 1966 till 2003. Final evaluations were done by Dresser Rand in 2003 and last set of curves were is-sued in May 2003.

In most of the curve sets, it is clear that 3rd

stage is more susceptible to surge than any other case. However in last set obtained in May 2003, 4th stage has been highlighted as the most susceptible stage.

Surge control point for this machine was set at

1525 KSCFH from commissioning time (1967) till July 2003. Based on new map, the flow point for surge control was revised to a more conservative value of 1625 KSCFH.

Before compressor was brought to Daharki,

Pakistan this machine always operated above 2100 KSCFH flow at Pascagola, USA refinery

(as there was no N2 in their natural gas while gas at Daharki contained around 18% Nitro-gen).

During commissioning time at daharki (1993)

lots of trips happened on this machine During those, very high vibrations were observed at HP case (>5.0 mils) and Gas turbine tripped at high exhaust temperature (over-load). At that time it was attributed to critical speed of HP case etc. and HP case was not opened till 2004. No further investigation of Gas turbine overload was carried out.

When HP case was opened in April 2004

turnaround for overhaul, it was found that the temporary strainer installed at suction of 3rd stage had broken and it had damaged the 3rd stage rotor. As such high vibrations on HP case have not been reported till the time of commissioning in 1993, it is apprehended that 3rd stage got damaged at that time. See photos given below. Another fact which supports this finding is that there is a step change in ef-ficiency of 3rd stage of the machine which re-mained around 69% from 1993 till April 2004 and which changed to around 74.5% after in-stallation of spare rotor.

Unlike standard practice on Centrifugal com-

pressors, no check valve was installed at the discharge of this machine.

CONCLUSION :

Surge at Process Air Compressor occurred : • Compressor surge line moved towards right

side after compressor overhaul. Different compressor maps which are being used since 1967, have became invalid after rotor re-placement of HP case.

• In Pascagola USA, the compressor was al-

ways operated above 2100 KSCFH flow. So it ran much away from surge curve. So it never surged there.

• We damaged 3rd stage of machine in 1993 which changed its surge characteristics, and helped us operate it at low flow of 1500 to 1700 KSCFH for next 10 years. After over-haul of the machine the machine started surg-ing at 1700 KSCFH flow.

PROBLEM RESOLUTION :

The following steps were taken for resolution of the problem.

• A Check valve was installed at the dis-charge of the compressor, in such a way that minimum volume remained between compressor discharge flange, antisurge valve flange and the check valve. (This re-sulted in quick depressurization of ma-chine in case antisurge valve opened).

• All dead legs and extra piping were re-

moved from the process air line. (for quick depressurization )

• Extra flow meter was installed along with

additional pressure transmitters on 3rd stage of the machine.(In order to deter-mine the surge characteristics of 3rd stage).

• Dresser Rand was contacted and on their

proposal, Inlet Guide Vanes, IGVs of HP stage of the machine were changed. This was done to create additional surge mar-gin and improve efficiency.

• Compressor Control Corporation (ccc)

was contacted and their controller was in-stalled on existing 4th stage of the ma-chine. A surge test was carried out in April 2005 using both 4th stage and 3rd stage flow meters and it confirmed that 3rd stage of the machine is surging before 4th stage. However due to change of IGVs, the machine now surged at much lower flow than before (around 1450 KSCFH) at same conditions and around 18% margin was created. Also it was observed that

machine exhibited excellent recovery be-havior, during surge test (which can be at-tributed to decrease in discharge volume by installation of check valve and removal of dead-leg).

• A conservative surge protection curve

based on 3rd stage surge characteristics was programmed in the controller, which actuated 4th stage blow-off valve to pre-vent machine from going into surge.

After the above mentioned changes, the machine is being operated trouble free. To prevent this kind of problem the same design changes can be applied while commissioning a new/old centrifu-gal compressor. AFTER EFFECTS OF THE SURGE INCIDENT : Background : Secondary reformer, D-2504 catalyst was loaded during April 2004 turnaround. Loading was car-ried out using sock loading method. After load-ing, Top and bottom manhole below bee-hive(see picture below) of D-2504 remained open for next two three days. Top nozzle was installed then and bottom man hole closed after that. No damage to bee-hive or fallen bricks or pieces of balls or white powder was observed at that time.

During the surge event, air flow and pressure kept on fluctuating inside Secondary reformer, D-2504. Air flow increased from 1700 KSCFH to beyond 2450 KSCFH , became 200 KSCFH and then the cycle was repeated. As per ESD log of FSL-313,

low flow at secondary reformer occurred 81 times which is consistent with observations in field. This kind of flow fluctuations caused flameout in secondary reformer, which in turn resulted in air escape from the burner. Observations :

• Outlet Temperatures of waste heat boilers, E-2501A/B downstream Secondary Re-former started increasing at a very very high rate (upto 1°F/hr) after restart of Ammonia plant, however after this initial high rate, the outlet temp rise trend slowed down. Detailed investigation was car-ried.out

• It was apprehended that Alumina and re-

fractory material was causing the fouling, however it was not easy to point out the source of this material.

• Ammonia plant was shutdown on 8th May

2004 and inspection + blowing of these exchangers was carried out. Top surface of catalyst and burner was inspected but no abnormality was found. Plant was re-started on 13th May 2004 but the fouling phenomenon continued after the startup.

• Ammonia unit was again stopped on 30th

May 2004 in order to find the source of this refractory material. Catalyst and sup-port material vendor Johnson Matthey rep. was also present at this time.

• Secondary reformer burner was removed

and top surface was examined, No abnor-mality found. No refractory damage in transfer lines was observed. However on opening the bottom manhole of the re-former, water was found filled in side it. Also inspection representative found some pieces of broken Bottom Bee-Hive bricks fallen in water.

• As there seemed to be no other abnormal-ity, it was decided to unload the catalyst in order to find the problem source.

• While unloading, top 1/3 of catalyst was

found in very good shape (no breakage, or dust) however after that, a cylindrical segment of approx 3-4 ft diameter and 4 ft height, just above Bee-Hive was discov-ered in very bad shape, totally crushed and powdered. Except for that portion, all re-maining area contained normal catalyst.

• Going further down, it was discovered that

the alumina balls at the top of the bee-hive have totally crushed and are in powder form (Max quantity : <0.5%). Some of the 2” balls which were 1 ft below 1” balls have moved above them in bee-hive cen-ter. As noted by inspection, all rows of bee-hive (aprox 5 ft height) have broken their edges. This shows that major shakeup of bed has taken place.

• Bee hive roof center and top row of bricks

have been broken, some of the alumina balls were embeded in this structure. Quite a few balls (<0.25% of total) became blue from bright white original color.

Conclusion :

These observations were then analyzed along with Johnson Matthey representative and it was concluded that either localized explosions have accrued inside the bed or the bed has been damaged by 60 to 80 psig pressure ham-mering.

As due to shock wave, gas could not escape

from bee-hive top, it crushed the catalyst and balls present there.

Also due to low pressure zone formed near bee-hive top, the balls moved in reverse direction around bee-hive causing damage to edges of its walls and moving 2” balls above

Problem resolution : Damaged Bee-hive was repaired, additional balls were filled in the bee-hive portion and the catalyst was dedusted and loaded again. Author(s) Profile :

Muhammad Majid Latif is Section Head Process Engineering Am-monia, at Engro Chemical Paki-stan Ltd. Telephone: 92 723 641001, Email : mmlatif @ engro .com. .He is responsible for new

projects, trouble shooting and optimization of Ammonia unit. He has a varied background hav-ing worked at IBM, FFC , CocaCola Export Corp and EngroVoPak terminal Ltd. before joining En-gro Chemicals Daharki. His areas of specializa-tion includes Process design, Process Modeling & Simulation, CFD modeling and troubleshooting. He holds a BE in Chemical Engineering from University of Engineering & Technology, Lahore. Authors’ Acknowledgement I would like to acknowledge the support provided by Dresser Rand and Compressor Controls Corpo-ration (CCC) in the successful completion of this project. I would also like to thank the I&E, opera-tions and maintenance teams of Engro Chemical Pakistan, Plant in implementing the project from concept to completion.

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Flow at which machine will surge, KSCFH(Please note that this includes 10% margin)

Gas

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pm

Very First Batch of catalystjust after unloading start.

Note excellent condition ofcatalyst with no dust.

Very First Batch of catalystin tarpaulin lined trolley.Excellent condition, No

dust.

Start of Damaged balls,just above Bee Hive. Notethat 2” Balls which were 1

ft below 1” balls havecome to top

Small and large refractorypieces found in catalystbed. Two sacks of these

were found mixed incenter of catalyst.

Healthy Bee Hive, Justbefore loading in April

2004

Water filled in bottomportion of secondary

Reformer. Note high levelof debris and Broken Bee-Hive Bricks (indicated byArrows) identified by Mr.

Habib Pervez

Damaged Top portion ofBee- Hive

Damaged Top portion,other side. Please note

how the balls havecrushed into powder andalso Embedded in Bee-

hive top.

Damaged Edges of theBee-hive due to upwardmovement of Aluminaballs during pressure

swings.