Risk analysis of an LPG storage facility in India

Risk analysis

of an LPG storage facility in India

A. A. Khan Indian institute of Chemical

Technology,

Hyderabad

500 007, India

An assignment to carry out a hazard study and risk analysis of a gas processing complex has been described briefly, wherein well known procedures have been used to identify and investigate potential hazards. A method of avoiding unintentional overfilling of LPG storage tanks has been recommended, which utilizes the existing level control instruments. (Keywords: risk; hazard; Lffi)

More than a decade’s experience of probabilistic risk assessment methodologies and a series of serious accidents/disasters have resulted in an extensive understanding of the potential hazards associated with the operation of chemical process plants. This is evident from the analyses of unexpected events that occurred and the views expressed on the effectiveness of a priori hazard studies and risk evaluation’. On one hand the quantitative assessment of risk has been totally discardedz, and on the other the onus of preventing undesirable incidents by adopting appropriate procedures for risk management has been placed squarely on the CP13. The latter point of view has been substantiated by asserting that the technology needed to avoid many process accidents is now available and that CPI should evolve policRs for its effective utilization. Hazard studies and risk assessment in all cases are a customized combination of standard procedures to create a model/representation of a complex system4 through which potential hazards are identified, categorized and quantified. The overall risk management effort is then based on meaningful deployment of decision-making tools based on this assessments. Projects at conceptual level as well as at advanced stages of implementation can be evaluated by devising the most appropriate methodology: the present investigation belongs to the latter category. Received I8 October January 1990 09504230/90/o4o4ogo3 0 1990 Butterworth-Hainemann

406

1989;

revised

19

It is with this background that this study was undertaken. The study involved a systematic investigation of an operating natural gas processing complex. All aspects of gas processing were covered, including desulphurization, separation, and fractionation of gas and condensate to obtain LPG and NGL respectively. Sulphur recovery units and storage and loading/unloading of LPG and NGL also formed part of this facility, and as such were carefully scrutinized. Although the entire investigation proved challenging, the fact that much of the plant and equipment was already installed and was not amenable to any major modification made it more so. Analysis and recommendations with respect to the safety of the LPG storage system presented here form a typical example in which existing controls were re-deployed to reduce the associated risk. Methodology Standard procedures6 for identification and quantification of potential hazards were used initially io work out the areas that required specific attention. Characterization of inventories of potentially hazardous chemicals with respect to their physicochemical properties in normal operation as well as during an unexpected event formed a significant part of this effort. A comprehensive evaluation of process control systems together with a HAZOP study of selected units/equipment provided a means of short-listing the potentially

Ltd

J. Loss Prev. Process Ind., 1990, Vol3, October

more damaging scenarios. The software package EFFECTS and the guidelines developed by TN07, were used extensively in computing release rates, vapour cloud dimensions, damage distances due to radiation and overpressures as a result of fire balls and vapour cloud explosions. These estimates were made by taking into consideration meteorological factors and demographic data. A search of a past accident data base ‘FACTS’ at TN08 and CA data base9 was taken up to establish the credibility or otherwise of the selected scenarios. LPG storage Investigations carried out along these lines and an estimated value of Dow’s Fire and Explosion IndexI” of 220 for the LPG storage area revealed that the storage facility could produce the worst possible consequences in the event of an unforeseen ipcident. Examination of the results adequately substantiates this view. For example, the sample HAZOP work sheet given in Table 1 deals with the flow parameter around a node located on the inlet line of the LPG sphere. In this instance a lower-thannormal flow to one of the LPG spheres could be due to unintentional filling of another sphere, located in the storage area, which in turn can result in a hazardous situation. Similarly estimation of damage distances due to overpressures and radiation effects was carried out, among other cases, for the following situations:

Short Communications Table 1 Hm()p work sheet(process unit, Guide word A

Deviation B

Less

Less flow

Instrument tag no. C ROV XV-A TSV-6

LPG storage, node, inlet line to sphere. process parameter.

Causes cl

Direct consequences ofB E

Less flow from LPG and CF units

Loading takes more time

Improper isolation of sphere

LPG level build up in other tanks

LPG going to other suction lines

Quality of LPG in other tanks may get affected

Other ROVs partially open

LPG in othertanks may get contaminated

Leakages

lceing may occur during less flow which may weaken the pipelines

Obstruction in caustic wash unit

Extended time

TSV failure in loading line

Quality control problems in loading section

nozzle failure in the vapour space of LPG storage followed by release of LPG and vapour cloud explosion a LPG feed line rupture followed by BLEVE of the Horton sphere.

l

In both cases results shown in Table2 and Table3 confirm the serious consequences of these scenarios. Cascading effects of such hypothetical scenarios were evolved by superimposing damage distances on layouts of plant and equipment surrounding the LPG storage facility. This exercise further emphasized the inevitability of estimated consequences in terms of damage to plant machinery and environment, and loss of life.

Fault tree analysis While BLEVE of an LPG Horton sphere was taken up along with a cluster of other scenarios for quantifying the

Table 2 Damage distances due to vapour cloud (14 x 200 m) explosion Overpressure

(bar)

0.3 0.1 0.03

Table 3 Radiation LPG Horton sphere

Diameter(m) 140 220 1400

effects

of

Diarter Fire ball dimension 1% lethality distance

1800

BLEVE

(ml

of

Any other consequences F

flow)

Available safety provisions G

Clarification required H

Remarks I Potential hazard associated with overfilling

LSH LAH-A LAH-B

operation

risk involved, a fault tree analysis was carried out simultaneously to assess the probability and sequence of events in overfilling and BLEVB of the LPG sphere, respectively. Analysis of the results showed that human error, associated with the erroneous operation of LPG transfer and recirculation manifolds, apparently enhances the probability of occurrence of the top event in the former case, while the presence of fires or sources of ignition in the vicinity of the Horton sphere along with failure of tire fighting facilities, are the basic features of the latter. Considering the overfilling of the sphere first, it appeared desirable to interlock the valves such that unintentional filling of spheres is totally avoided. Alternatively it seemed feasible, on initial assessment, to control inadvertent overfilling by tripping off the pumps which are used either for transferring or for recirculating LPG . A closer examination of the existing manifolds as well as the types and duties of valves and pumps involved, however, suggested that the system is not amenable to such modifications. Nevertheless it was possible to suitably alter the existing level control instruments to avoid overfilling of any Horton sphere. This could be achieved by arranging for a high level alarm 2-3 h before the LPG in the sphere reached its maximum level followed by isolating the sphere automatically as soon as the maximum level is reached. With this arrangement both primary events responsible for the high frequency of the top event (i.e. inadvertent pumping and wrong setting of

valves), are replaced by the corresponding instrument failure frequencies, thus reducing the probability of the top event from 1O-5 to lo-’ per year.

Level control Details of the modifications required to achieve the above improvement are as A level indicator (LI) is follows. mounted on the sphere for indicating LPG level both locally as well as in the control room. It is shown in Figure I, along with the level transmitter (LT). This instrument was modified to obtain a high level alarm (LAH) from the level transmitter (Figure 2). The set point of the alarm was chosen to allow at least 2-3 h for corrective action before the control system of the high level switch came into action.

m______\, LI

___---

L’

---r-_

8

8,

0 LI

3 LT

L-r

Figure 1 LPG level control in the sphere

J. Loss Prev. Process lnd., 1990, Vol3, Octobw

407

Short Communications To close MOV LPG feed line

on

t_____ _

f-----‘:= ‘.l

To close MOV on LPG recirculation

LXH line p

Figure 2 Location of LSH that activates a trip (U(H) to close the two valves, which isolate the sphere, avoiding inadvertent overfilling

A high level switch (LSH) is provided to actuate a high level alarm (Figure]). The LSH was modified to actuate a trip (LXH) to close the following remote operated motorized

408

valves (Figure 2): main feed/discharge valve of the LPG sphere; and recirculation line valve used for homogenizing the contents. When these valves close automatically, the LPG sphere is totally isolated, thus avoiding any inadvertent overfilling. A push button override is used for resetting the trip. In addition to these modifications of the level control system, other measures to reduce the probability of fires and vapour cloud explosions in the vicinity of LPG storage were also evaluated. For example, a survey of possible sources of ignition was made. Similarly the possibility of accumulation of hydrocarbon vapours, subsequent to a release, due to lack of ventilation and inadequate drainage of spills, was also investigated. Hence recommendations with respect to proper sloping of floors, lifting of the pipe racks, provision for efficient drainage of spills in the vicinity

of the storage area were made, which also helped to reduce the probability of occurrence of the top event. In the case of a fault tree for BLEVE, the substantial reduction in probability of overfilling of LPG storage, as discussed above, reduces the frequency of occurrence of the top event.

J. Loss Prev. Process Ind., 1990, Vol3, October

Acknowledgement The author gratefully acknowledges the assistance rendered by members of the Investigation Team.

References 1 Bendixen, L. M. and O’NeiJJ, J. K. PIant~Op. Prog. 1984,179 ,2 Pilx, V. and Bayer, A. G. Hydrocarbon Process 1980,275 3 Howard, W. B. Chon. Eng. Prog. 1988, 84,25 4 Stevens, F. D., Maher, S. T., Sharp, D. R. and Sloane, B. D. Can. J. Chem. Eng. 1986,64,848 5 Sancaktar, S. PlontjOp. Prog. 1983,176 6 AlChE Manual on ‘Guidelines for Hazard Evaluation Procedures’, 1985 7 ‘Methods for the Calculation of the Physical Effects of the Escape of Dangerous Material’, Ministry of Social Affairs, Netherlands, 1979 8 FACTS, Databank for accidents with hazardous materials, TN0 Department of Industrial Safety, Netherlands, 1988 9 Online information search from data base of Chemical Abstracts 10 ‘Fire and Explosion Index Hazard Classification Guide’, Dow Chemical Company, 5th Edition, 1981