137â148. pn-en-1050. Zasady oceny ryzyka, 1999. pn-ieC 60300-3-9. Analiza ryzyka w systemach technicznych, 1999. Rak J., Babiarz B., Tchórzewska-CieÅlak ...
Safety and Reliability: Methodology and Applications – Nowakowski et al. (Eds) © 2015 Taylor & Francis Group, London, ISBN 978-1-138-02681-0
Risk assessment in heat supply system B. Babiarz
Rzeszow University of Technology, Rzeszow, Poland
ABSTRACT: The task of a heat supply system is to provide the consumer with the required quantity of the energy in the form of the “usable energy”- heat. Both risk and uncertainty are connected with the functioning of communal systems involving heat supply systems. The issue of risk assessment in heat supply systems requires considering their specific character. Heat supply systems characterize changeable heat load during the year. The delivery of heat to consumers depends on changeable external conditions. These conditions affect energy security, security of energy supply and safety of people as users or operators of system. The paper presents an approach to risk assessment in the heat supply system, as a system consisting of three sub-systems, taking into account the location of the risk, identification, hazard classification, estimation of risk and risk responses. The proposed approach to risk analysis has been summarized with the requirements of the current legislation concerning heat engineering. Method of risk assessment using an event tree method, possible to application in heat supply system considering changeable atmospheric conditions have been presented in this work. 1 InTroduction The task of a Heat Supply System (HSS) under the relevant legislation is a current, continuous and reliable heat supply to the needs of local communities through the provision of services, which are commonly available. Ensuring continuity and reliability of HSS must meet the requirements to ensure: • energy security—as a condition of the economy which enables full coverage of the customers’ ongoing and prospective demand for fuels and energy in a technically and economically justified manner, with the observance of the environment protection requirements; • security of energy supply, forming an ability of the system to ensure the security of the operating network and balancing energy supply with demanding for this energy; • safety of people, as users or operators of a system. The measure of the loss of security is the risk, resulting from the likely sequence of events affecting the level or from the possible consequences of the decisions taken. Risk is characterized by the quality of the fulfillment of the system task. Taking into consideration, that risk is inherent in the businesses of heating, and the quality of services provided is a measure of competitiveness, we can conclude, that these companies should strive to eliminate or minimize the hazards, which may affect the level of risk. It is possible to achieve through improving operating process of the subsystems, objects or
elements that are components of the entire heat supply system by introducing into traditional management system methods aimed at identifying hazards, eliminating or reducing their potential effects in order to estimate the risk and to take action with the aim of responding to risk. The results of the risk assessment process should serve as an important basis to determine the strategy of actions to meet the safety requirements in the area of economically reasonable as well as draft a plan for responding to risk. 2 the TERM and kinds of risk Risk is the basic category of security issues. It is derived from the theory of reliability and safety of systems. This is connected with situations of slight, uncertain or problematic effect of exposing to danger, loss or harm—with the likelihood that something will bring negative results. In this perspective, the risk is identified with the loss of safety (Babiarz & Rak 2001). The risk is at least two-dimensional, complex concept involving the possibility of an adverse outcome or uncertainty about disadvantageous events, which are the occurring time, kind of event, its duration, the size of an adverse outcome. In addition to the term risk is used term hazard, which is more than the risk associated with a sense of danger. The concept of risk, in turn, is more associated with the estimate of uncertainty and possible losses related to this.
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Risk characteristics are uncertainty, probability, the dispersion of results (adverse effects). There are two ways to define risk. The first of these combines the risk with possible distribution of results—the risk is a possible variance of the expected results. In this perspective, risk is a function of the variability of possible outcomes. A second look at risk refers to the probability of losses and their sizes. The more probable that an event causing losses occur, the greater risk is. According to this definition, the size of the risk can be defined as the frequency of obtaining undesirable results. Then you can bind the level of the risk with the sizes of possible losses. In this perspective, the greater the loss suffered may be the greater risk occurs. There is also a definition of risk as the product of the probability of occurrence of an adverse event and the size of the losses caused by event. From the mathematical point of view the basic definition is obligatory: r = P ⋅C
(1)
where: P—a measure of the system functioning unreliability, corresponding to the category of probability—frequency; C—a measure of the consequences, corresponding to the category of effects—damage, expressed in units of financial resources. 3 the approach to Risk analysis in HSS Risk and safety analysis in HSS is the basis for the proper conduction of the process of exploitation and prevention the occurrence of serious failures that can lead to economic, social and environmental losses. Heat supply systems characterize changeable heat load in the year (Babiarz 2006). The delivery of heat to consumers depends on changeable external conditions. Risk analysis in HSS requires taking action on its: • location (determining the place of occurrence, the type of subsystem, coverage, risk groups); • identification (identifying the types of risks, factors of uncertainties, kinds of hazards); • designation (estimation of risk measures, using appropriate quantitative or qualitative methods); • respond to risk and management of it (controlling and exploration of activities aiming to reduce it). Identification of risk requires the identification of hazards as factors, that may cause dangerous
Figure 1. The approach to HSS risk assessment.
situations to the users of object, the object itself or its environment. From the point of view of the source of danger in HSS, hazards can be distinguished as: internal, external and intentional (targeted). Internal hazards arise from the nature of the system, its specific character and they are based on the kind of technological processes, which are, depending on the degree of system decomposition, production, supply and distribution of heat to the public via the heating medium with appropriate parameters. In particular, these are: • deviations from the established parameters, required in given operating conditions: temperature, pressure, flow, thermal power; • failures of the heat production subsystem components: boilers, pumps, control and measurement devices, etc.; • failure of heat supply subsystem components: district heating pipes, fittings, alarm devices, etc.; • failure of the heat distribution subsystem elements: heat distribution pipes in heating stations, fittings, internal system components, etc.; • error of human—as an operator or conservator of system—maintenance and inspection mistakes; • unreliability of the system supplying the media needed to execute the HSS tasks, in particular: electricity, water, steam, air, oil, etc.
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External hazards are mainly determined by unfavorable weather conditions, or they may be caused by the proximity of communication tractions, and other objects having influence on the state of the object and the consequences of limitations, or the lack of heat supply to consumers. These can be distinguished as: • natural, extreme climatic events such as floods, earthquakes, landslides, strong winds, lightning; • serious failures in electric power systems, causing interruptions in the power supply, which can cause even blackout failure; • other installations or technical facilities located in the vicinity, which could adversely affect the elements of HSS; • transport routes located nearby, which are the source of stray currents that may cause damage to the network elements. Intentional (targeted) hazards are those connected with the operation of unauthorized persons, terrorist attacks. Moreover, the political situation has also the impact on the security of fuel supplies needed for energy production. Considering the impact of adverse events, hazards can be divided into: direct, arising as a result of failures (the direct impact factor of high temperature and pressure) and indirect hazards, which are results of HSS component failures. Direct hazards in the above sense is any factor’s impact on the surrounding environment, people, equipment and buildings. Steam and hot water under high pressure are particularly dangerous in the event of damage in the heating network located in street lane or in its close proximity. Direct hazards are: • failure posing a threat to human life and health: the possibility of burns as a result of the impact of hot water and steam, scouring the streets, foundations of buildings, caused by leaching of soil by flowing a heating medium; • failures causing property damage: damage in material objects (buildings, cars, etc.) caused by leaching the soil, damage to existing utilities (sewerage system, water supply and electricity network). Indirect hazards are caused by HSS element failures and, consequently, by the lack or limitation of heat supply to consumers. Depending on the design of buildings, sealing of windows and doors can cause a significant decrease in temperature in rooms inside buildings. Force of the wind and temperature outside, which prevails at the time of damage are measures of an indirect hazard degree. These threads can be distinguished as: • lowering indoor room temperatures, which in consequence leads to closure of nurser-
ies, schools, kindergartens, offices, security or evacuation of hospitals, which do not have an emergency heat sources; • the necessity to drain the internal installations of the water—lack of these activities might result in the possibility of damages in the installation elements, caused by freezing water in it; • the necessity to stop production and the possibility of occurring serious losses as a result of failure to meet the technological parameters in industrial companies, greenhouses, etc. (Babiarz & Rak 2002). Failures occurring in the HSS cause lowering of qualitative parameters of generated heat. This usually makes a thermal power balance not sustainable. The balance of thermal power in the heat supply system ∆Φ may be expressed by a difference between the thermal power supplied to the system Φs and thermal power required (ordered) by the system Φo. In the case of optimal cooperation between the heat source and the entire system, the balance is sustainable and it can be described as follows: ∆Φ = Φ s − Φ o = 0
(2)
In case of irregularities, the balance equation can take two forms: ∆Φ = Φ s − Φ o < 0
(3)
or ∆Φ = Φ s − Φ o > 0
(4)
The case defined by the relation (3) occurs, when the thermal power output by the source to the district heating network is less, than the demand of consumers. Then the thermal power deficit occurs. On the other hand, a case described by formula (4) takes place when the network parameters control is not correct and returning water from the district heating network has a higher temperature than it should be according to the thermal power control charts (Babiarz B. 2014). This involves an indirect threat, which, in the case of simultaneous occurrence of several factors having influence on the heat load, can cause significant social and economic losses. Identification and analysis of hazards give an overview to the characteristics and operating conditions of the system. They are useful in the estimation of risk. Specialized techniques known as risk assessment are created as part of the developing science of safety. They are based on the qualitative, quantitative, semi-quantitative and simulation methods, using control and data acquisition as well as computer databases, operating system based on GIS. Risk assessment may be qualitative, quantitative or semi-quantitative. Qualitative risk assessment is
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applicable when the risk is small and well known. Simple description of the types of major accidents, their consequences and their likelihood and review of compliance with standards are sufficient. Quantified risk assessment is only appropriate where it is both reasonable and practicable in term of the availability of information and data (Arunraj & Maiti 2007). Among the qualitative risk assessment methods there should be distinguished: • CHL—Check Lists; • FMEA—Failure Mode & Effects Analysis; • HAZOP—Hazard & Operability Studies; • PHA—Preliminary Hazard Analysis; • PSA—Process Safety Analysis; • Relative Ranking; • What-If Analysis. Quantitative methods of risk assessment are: • ETA—Event Tree Analysis; • FTA—Fault Tree Analysis; • CCA—Cause & Consequence Analysis; • HRA—Human Reliability Analysis; • Monte Carlo Methods; • Markov Methods; • Matrix Methods. Semi-quantitative risk assessment methods are: • Domino effect analysis; • LOPA—Layers of protection analysis. The choice of the method depends mainly on the complexity of the system, the type of hazard and the probability of their occurrence. In addition, there should be considered: specific character of system, the type and scope of functions, the kinds of hazards and the likelihood of their occurrence, technology and types of databases, etc. Examples of the application of risk assessment methods are presented in many publications (Werbińska, 2006, Zio 2009, Vališ & Zając 2010, Młyńczak et al. 2011, Aven 2012). The next step in the risk analysis is responding to the risk. Result of the risk assessment process is used to make a decision on acceptance, minimization, avoidance or reducing risk. On this basis cost and benefits analyzes and strategies as well as responding to the risk plans are created. According to current law regulations, heating companies should create action procedures, when hazards occur. Restrictions in the heat supply can be introduced in the case of occurrence: • a threat to the energy security of the country consisting in a long term lack of balance on the fuel and energy market; • a threat to the safety of people;
• a threat of significant material loss. Restrictions consist in a reduction or an interruption of the heat supply. Supplier of heat introduces restrictions on the provision of heat by: • reduction of the quality and/or quantitative parameters of heating medium to a level enabling maintenance of temperature 10 degrees Celsius in dwellings and 5 degrees Celsius in other facilities heated; • the adjustment of parameters in customers’ systems by changing the controller settings on particular thermal nodes. Supplier can reduce the supply of heat to the limit, which guarantees only maintaining the circulation of the heating medium in the district heating network, which prevent it from freezing in the case of restrictions on sale of coal and electricity consumption. Under the current provisions, in case of default by heating company established customer service quality standards, the recipient is entitled to a decrease of costs. These issues have been regulated in the Ordinance of the Minister of Economy of 17 September 2010 concerning the detailed principles of the establishment and calculation of tariffs and billing principles for heat supply (Journal of Laws No. 194, item. 1291). Recipients are entitled to a decrease of heat costs in case of default by the energy company terms and conditions of sale of heat in terms of start and end dates of heat supply for heating and planned interruptions in the supply of heat in the summer. In accordance with § 40 of the Ordinance, the amount of discounts, referred to above, shall be determined as follows (if the sales contract does not provide differently): • if the start or end of the heat supply for heating were delayed in relation to established quality standards of customers’ service, discount is 1/30 of the monthly fee for the ordered thermal power for plants in which there was a delay—per each day of delay; • if the planned interruption in the supply of heat in the summer was longer than the established quality standards of customers’ service, discount is 1/30 of the monthly fee for the ordered thermal power for plants, in which there was an extension of a break in the heat supply—per each day extension of the break. Furthermore, in accordance with § 44 of the tariff regulation, when due to breach by the energy company quality standards of customers’ service, thermal power has been reduced (if the sales contract does not provide differently), the recipient
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is entitled to a discount, the amount of which is calculated as follows: • if the thermal power limit is up to 40%, the discount is calculated by the formulas: Su 40 = Sum 40 + Suc 40 (5) Sum 40 = 0, 25 ⋅ (Φt − Φ r ) ⋅ 3, 6 ⋅ 24 ⋅ Cn ⋅ hp : 365 (6)
Suc 40 = 0, 4 ⋅ (Φt − Φ r ) ⋅ 3, 6 ⋅ 24 ⋅ hp ⋅ Cc
(7)
• if the thermal power limit is above 40%, the discount is calculated by the formulas: Su 40 u = Sum 40 u + Suc 40 u Sum 40 u = 0, 5 ⋅ (Φt − Φ r ) ⋅ Cn ⋅ hp : 365
(8) (9) (10)
Figure 2. Illustration of event scenario providing greenhouse with heat using event tree.
Su—the total discount for restrictions in heat providing, Sum—crediting (decrease of cost) for limiting of thermal power, Suc—crediting for undelivered heat, Φt—thermal power determined on the basis of calculating flow rate and heat carrier parameters specified in regulatory table [MW], Φr—the actual thermal power, determined on the basis of the flow rate and actual parameters of the heat carrier [MW], 24—multiplier, meaning 24 hours a day [h], hp—the number of days in which there were lrestriction in the supply of heat, due to breach by the energy company service quality recipients’ standards, Cn—the price for the ordered thermal power for the tariff group [PLN/MW], Cc—the price of heat for a given tariff group [PLN/GJ].
adverse events in the scenario and their effects are defined. The essence of the method is illustrated in the following example. A greenhouse is a recipient of technological heat to maintain the proper temperature of the vegetables production process. Alternative heat source is the possibility to use electric power to the provisional, localized heating of the greenhouse capacity. The only security of certainty of heat supply to the greenhouse is reservation of the heating network, supplying heat to heat the greenhouse. The heating medium is supplied to the object by three-wired heating network. One of these three wires is a reserve in the case of failure of any of the two others. Estimation of the risk of incurring identified losses by the owner of the greenhouse is presented is the following example. Figure 2 presents example of event tree representing a risk for greenhouse facilities, to estimate the loss of the owner. Initiating event is a failure to meet the parameters of heat supply or non-delivery of heat. Then the sequence of possible adverse events is considered in case of occurrence or absence of the computational, design conditions. The one, shown in Figure 2, allows calculating the risk of very serious losses rvs, according to the formula:
Suc 40 u = 0, 8 ⋅ (Φt − Φ r ) ⋅ 3, 6 ⋅ 24 ⋅ hp ⋅ Cc where:
4 an EXAMPLE OF DETERMINING THE RISK USING EVENT TREE METHOD The purpose of the risk, hazards and level of safety assessment in HSS is to create a database of relevant, criterial information, necessary in decisionmaking, optimization of processes, exploitation and control of systems, as well as in taking protective measures to prevent the occurrence of adverse effects of events. There are many methods of determining the risk. Event tree method is often used in analyzes of technical systems. The concept of the event tree method is to consider the consequences of adverse effect as the result of adverse events during or erroneous evaluations and decisions (Babiarz & Rak, 2002). Then all possible sequences of events, being a consequence of
rvs = P1⋅ P2⋅ P3⋅ P4
(11)
In turn, the risk of serious loss rs is: rs = P1⋅ P2⋅ P3⋅ (1 - P4) + P1⋅ P2⋅ (1 - P3) ⋅ P4 + P1⋅ (1 - P2)⋅ P3⋅ P4
(12)
5 summary Risk assessment in HSS is connected with the energy security of the country. Current law
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legislations define issues related to energy security, determining the necessity to develop procedures of limitations restrictions in the heat supply by the company in case of emergency. Existing regulations, shaping the issues of decrease of costs, require energy companies to give a discount in the case of occurring certain events failure to meet the quality standards in heat supply service to customers. Very important things in the heat supply management are to assess the risks, inform others of its sizes and initiate actions that must be taken in the face of risk occurring. The approach to risk assessment, presented in the paper, takes into account the decomposition of HSS system in the analysis of the degree of fulfillment of tasks in the exploitation process. The comprehensive approach to risk assessment in HSS requires its location, identification and classification of hazards, estimation of risk and response to risk, including setting priorities for risk management by comparing the level of risk with pre-established targeted levels of risk. Moreover, monitoring of risk and checking of compliance with established procedures are also necessary. The final product of identification, assessment and measurement of risk in HSS may be risk maps, which are a useful tool in the effective management of centralized heat supply to consumers. Assessment of risk using the event tree can be useful in countering threats resulting from damage to system components, taking into account the variability of weather conditions affecting the thermal load. It may be an additional element in the conduction of preventive activities, related to prevention of damage, standardization of repairs, as well as in the development of emergency scenarios. The subject of risk in HSS requires further indepth analysis, based on the available tools and methods, and particularly determining the required level of risk.
REFERENCES Arunraj N.S. & Maiti J. 2007 Risk—based maintenance— Techniques and applications. Journal of Hazardous Materials 142/2007. Aven T. 2012 Foundational Issues in Risk Assessment and Risk Management. Risk Analysis, Vol. 32, No. 10. Babiarz B. 2006. An introduction to the assessment of reliability of the heat supply systems. International Journal of Pressure Vessels and Piping, Volume 83, Number 4/2006, 230–235 Elsevier. Babiarz B. 2014: Heat supply system reliability management. European Safety and Reliability Conference ESREL, Wroclaw 2014. Babiarz B. & Rak J. 2002. The preliminary analysis of risk evaluation in a heat supply system. International Conference of Air Conditioning & District Heating. Szklarska Poręba. Młyńczak M., Nowakowski T. & Vališ D. 2011. How to manage risk? An approach based on standards. Maintenance problems, 2011, vol. 1, no. 80, p. 137–148. PN-EN-1050. Zasady oceny ryzyka, 1999. PN-IEC 60300-3-9. Analiza ryzyka w systemach technicznych, 1999. Rak J., Babiarz B., Tchórzewska-Cieślak B. & Studziński A. 2005: O podstawowych uwarunkowaniach analiz i ocen ryzyka. Gaz Woda i Technika Sanitarna nr 11/2005. Smith D.J. 2001 Reliability, Maintainability and Risk. Practical methods for engineers. Butterwort Heinemann. The Act of 10 April 1997 on the Energy Law. Journal of Laws No. 54, item. 384, as amended. Vališ, D. & Zając M. 2010: Fundamental risk assessment in example of transshipment system. Reliability & risk analysis: theory & applications; Electronic journal of international group on reliability, No. 4. Werbińska, S. 2006: Risk analysis of logistic customer service—case study of Nashuatec products distribution network. Safety and reliability for managing risk. Vol. 2, Eds. C. G. Soares, E. Zio. Leiden: Taylor and Francis, pp. 1127–1135. Zio, E. 2009. Computational Methods for Reliability and Risk Analysis. World Scientific Publishing Co., London.
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