mechanical seal

Mechanical Seals with Oil as Bufferin High-Pressure Centrifugal Pumps

High-pressure centrifugal pumps offer such operational advantages as mechanical reliability andlow maintenance cost. Buffer fluid used in most of the pumps is water, which can cause freezing andpump warping. The operating experience with tandem mechanical seals using oil as a buffer fluid

shows that it is reliable and efficient.

Arif JamalFauji Fertilizer Company Ltd., Goth Machhi, Sadikabad, District Rahim Yar Khan, Pakistan

Plant

Fauji Fertilizer Company Ltd. (FFC) operates twoAmmonia I urea plants at Goth Machhi, Pakistan. Thefirst plant (Plant I) was commissioned hi early 1982with design capacities of 1,000 metric ton of ammoniaand 1,725 metric ton of urea per day. The plant wassuccessfully revamped to 122% of design capacity in1990. The second plant (Plant II) with design capaci-ties of 1,100 metric ton of ammonia and 1,925 metricton of urea per day was commissioned in March,1993. Both plants employ Haldor Tops0e technologyfor ammonia and the Snamprogetti process for urea.

FFC opted for use of modern technologies in itsplants. High-speed centrifugal ammonia feed pumpswere selected for the urea unit of Plant 2. The pumpmodel selected was Sundstrand Model HMP 5112,similar to high-pressure carbamate pumps which havebeen successfully proven in many urea plants through-out the world. A similar model for carbamate servicehas been operating virtually troublefree at Plant I.

Ammonia Pump

Table 1 contains ammonia pump data. To produceurea, liquid ammonia is required at very high pressure.Liquid ammonia arrives at a urea plant battery limit ofabout 15 bar and 36°C from the ammonia plant and itis stored in a vessel. Downstream of this vessel, twobooster pumps increase the ammonia pressure fromabout 15 bar to 23 bar. This low-pressure liquidammonia is then pumped to high-speed, two-stagecentrifugal pumps driven by 6.3-kV electric motors. Agearbox is provided to increase pump speed. Bothstages operate at the same speed.

Discharge of the second stage at about 223 bar issent toward the urea reactor as feed for the urea syn-thesis reaction.

Composition of liquid ammonia is as follows:

NH3

H20Oil

99.80 wt. %0.20 wt. %5 ppm (max.)

AMMONIA TECHNICAL MANUAL 160 1996

Table 1. Ammonia Pump Data

Fluid handledTemp, min/oper/max.Density at min/oper/max. Temp.Pump speedSuction pressure nor/max.Discharge pressureDifferential pressureCapacity min/nor/ratedN.P.S.H availableEstimated absorbed powerSealing typeFlushed fluid

No. of impellersDia. first stage design/max.Dia. second stage design/max.

°Ckg/m3

rpmbarbarbar

mkW

mmmm

Liquid Ammonia20/36/40610/586/5792056223/2822320467.7/119/127 (152 max.)1501,220Tendern MechanicalNH3 + Buffer

2130.81/136.27158.24/164.5

At any time, one pump shall remain in service whilethe other is on standby. Changeover/startup is onlydone in the field after thorough checking.Motor/pumps are equipped with various alarms andtrips to safeguard one of the most critical machines ofthe complex.

Seal System

The Fauji HMP-5112 ammonia pumps incorporatetandem mechanical seals arrangements, using a sealflush with process fluid and a seal buffer with oil.

The buffer oil is drawn from the reservoir tank andpumped to both stages. Prior to entering the sealchambers of both stages, the pressure is monitored byorifices which also are provided at the downstream ofbuffer seals to prevent ammonia vaporization.

The buffer system has a bypass, pressure differentialcontrol valve PDCV to reduce the buffer oil pressureduring startup and shutdown. The valve position iscontrolled by pressure transmitters which help inresponding to process pressure and buffer oil pressureand in maintaining a differential pressure betweenprocess and buffer oil. The buffer oil is introduced in

seal chambers of both stages through pressure letdownorifices. Low-pressure differential switches monitor-ing the buffer oil flow in seal chambers of first andsecond stages are set to alarm when flow decreases(Figure 1).

The seal flush is drawn off the first-stage discharge,filtered and introduced through a pressure controlvalve to the first stage and directly to the second stage.Low-pressure differential switches monitor the sealflush flow in both stages (Figure 2).

Seals

The process side seals are John Crane Type 8 ABwith hydro pads (Figure 3). The hydro pad design pro-vides additional film thickness for sealing light fluids.These stationary seals are carbon graphite with 78%balance face. A retainer material SS 316 retains theface, and four antirotation lugs in the retainer preventthe face rotation. Four springs keep the stationary androtating faces together. The seal-mating face (rotatingface) is of tungsten carbide encapsulated by a SS 316ring.

Buffer oil seals are similar in construction except for


*tft

HM-

M'

Figure 1. Buffer oil system of Fauji pumps.

RETAINER

THRUST PUWE

SPRING ^

SLEEVE

STATIONARY FACE

ATING FACE

0-RINGS

PROCESS SEAL(HYDRO PADS)

ANTI ROTATION LUG

Figure 3. Seal details.

ALn'M

M' N

-M-

Figure 2. Seal flush system of Fauji pumps.

RETAINER

f_ ANTI-ROTATI0H PIN

STATIONARY fACE

AHT1 ROTATION LUG

Figure 4. Modification in buffer oil seals.


DISCHARGE UNE

RECYCLE LINE

SUCTION UNE

XX

Figure 5. Nitrogen pressurizing.

hydraulic balance (65%) and face material (graphitewith 70% silicon carbide). The hydro pads are not pro-vided here, since it is sealing viscous oil.

Operating Experience

During the first year of operation, the buffer sealfailure incidents were unexpectedly high. All the fail-ure incidents were investigated. To address the prob-lem, a modification was also carried out by the manu-facturer in buffer seals to improve the reliability ofseals. Eventually, all the problems were overcome, andseal performance is now satisfactory. Following aredetails of some of the incidents.

Buffer oil seal failures

Initially, the average life of seals was about twomonths. The majority of the failures were of buffer oilseals.

Wearing of Antirotation Lugs. Almost all of thefailed seals exhibited a common problem, which waspremature wearing of the antirotation lugs. Prematurewearing of antirotation lugs was due to various rea-sons.

Pressure Differential Control Valve. The buffer oilseal arrangement is simple and employs flow orificesfor pressure monitor. The pressure differential controlvalve is the only control valve buffer circuit; however,its malfunction has direct effect on seal life. Theresponse of the pressure differential control valveinstalled at the buffer oil circuit had not been up to

expectations until its root problem was rectified.Deterioration in the internal parts of its controller andtransmitters was very frequent. Various attempts weremade to calibrate the controllers and transmitters, butresults were not positive. This valve had to be operat-ed manually on a number of occasions when it startedmalfunctioning. During the investigation it was dis-covered that external vibrations transmitting from pip-ing was causing distress in the control valve's trans-mitters. The transmitter was relocated to a moreremote location where vibration could not be inducedfrom the surroundings. This resolved the problem andallowed the control valve to be put back in automaticoperation.

Recycle Flow Valves. Fauji pumps are furnished withrecycle flow valves which help maintain the pumpoperation within the operating range. The pumpstripped a number of times due to malfunctioning ofthese flow valves. These valves were problematicsince the commissioning of the urea plant. It wasobserved that the buffer oil seal life was shortened fur-ther when pumps encountered more startups and shut-downs. Extensive work was carried out to rectify theabnormal behavior of these valves, but the problemcould not be resolved. To avoid such trips, however,the flow is being controlled using a manual isolatingvalve as a recycle control valve.

Process Fluid Temperature Variation. Variation inammonia temperatures at pump suction causeschanges in the axial thrust of the pump rotor. Therotating seal faces that are mounted on the rotor alsomove axially with rotor axial movement. About 10°Ctemperature variation results in 0.02-0.03 mm of rotoraxial movement. This axial movement aggravated thedeterioration of antirotation lugs of buffer seals.

Blockage of Flow Orifices. The flow orificesinstalled downstream of buffer oil seal chambersplugged a number of times. This caused low flow inthe seals which resulted in seal damage. Such plug-ging occurred mainly due to small pieces that occa-sionally dislodged from buffer seal faces and chokedthe orifices. As a countermeasure, strainers wereinstalled upstream of these orifices. Blockage has notbeen experienced after this modification.

Modification in Buffer Oil Seals. To address the fre-quent wearing problem of antirotation lugs, the buffer


seals were modified by the pump manufacture (Figure4). Instead of providing antirotation lugs, an antirota-tion pin of tungsten carbide was installed. This modifi-cation has been proven as the permanent solution ofthe problem. To improve the reliability, seal O ringswere also changed from neoprene to teflon U cuprings.

Shattering of Rotating Faces. During the initial peri-od of commissioning, process seals failed severaltimes when the pump was shutdown and depressurized(Figure 5). In these incidents the rotating faces ofprocess seals were found shattered. Investigationrevealed that rotating faces were damaged due to ther-mal shock. These faces underwent thermal shockswhen the seal was hot and exposed to low temperatureas a result of pump depressurization. The problem wassuccessfully resolved by depressurizing/pressurizingunder positive nitrogen pressure.

Conclusion

Despite the initial seal-related problems, the pumphas achieved a high service factor. After overcomingthe problems of seal failures, the tandem mechanicalseals using oil as buffer fluid is now regarded as a reli-able arrangement in high-pressure ammonia pumps,which can be recommended for urea plant service.

Acknowledgment

The author wishes to acknowledge his gratitude toM/S Adil Bin Rauf and Imran Ghauri, MachineryEngineers of Fauji Fertilizer Company Ltd., GothMachhi, for providing active support during the prepa-ration of this article.


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