Risk assessment using 1 to 10 scale

45

46

TABLE III.IV

FMEA OF THE MOST CRITICAL ITEMS INCLUDED IN A HVAC.

FAILURE

MODES

CAUSES OF

FAILURE LOCAL EFFECTS GLOBAL EFFECTS

EFFECTS ON TRAIN

COMPRESSOR

FM1 - Motor does not start on demand

Motor seize up.

Blocked compressor.

Damage winding.

Loss of pumping capacity.

Loss of cooling function.

Loss of cooling capacity.

FM2 – Incorrect signal from thermostat

Overheating of compressor.

Thermostat dirty.

Loss of protection Possible damage of compressor.

Possible loss of cooling capacity.

FM3 – Pump gas leakage

Mechanical failure.

Fretting compressor.

Loss of refrigerant pumping.

Loss of cooling function.

Loss of cooling capacity.

FM4 – Sticking internal valve

Internal failure.

Valve dirty.

Pressure doesn't increase.

Loss of cooling function.

Loss of cooling capacity.

FM5 - Internal overload motor protection

Short circuit.

Electric overload.

Compressor motor protection failure.

Loss of pumping capacity. Short circuit of compressor.

Loss of cooling function.

Loss of cooling capacity.

EVAPORATOR BLOWER

FM6 – Incorrect signal from thermostat

Vibrations over specification.

Aging.

Motor does not work.

No ventilation Low pressure switch will cut off.

Loss of ventilation, cooling and heating capacity.

FM7 – Fails to run

Internal failure.

Coil in short-circuit.

Lifetime of the motor (aging).

Motor does not work.

No ventilation in evaporator coil. The pressure will increase.

Loss of ventilation, cooling and heating capacity.

FM8 -Mechanical crack

Lack of oil.

Bearings deteriorated.

Lifetime of the motor (aging).

Motor does not work.

No ventilation in evaporator coil. The pressure will increase.

Loss of ventilation, cooling and heating capacity.

AIR FLOW DETECTOR

FM9 – It doesn’t detect air flow when there is.

Internal failure.

The heating coil and the compressor are stopped.

Loss of heating or cooling function.

Loss of cooling and heating capacity.

FM10 - It detects air flow when there is no supply air

Internal failure.

There is no detection of air flow.

Possible damage of the system.

No effect

47 TABLE III.V is divided into six sections:

1. Occurrence, Severity and Detection assessment for each failure mode.

2. Classical RPN assessment (“value” column) and its decreasing prioritization ordering of the modes (“rank” column). The highest the rank, the highest the criticality related to this failure mode.

3. IRPN assessment (“value” column) and its decreasing prioritization ordering of the modes (“rank” column).

4. ERPN assessment using base 𝑥𝑥 = 3 and identical weights (set as 1) for the factors (“value” column) and its decreasing prioritization ordering of the modes (“rank” column).

5. URPN assessment using 𝑧𝑧 = 𝑘𝑘 (“value” column) and its decreasing prioritization ordering of the modes (“rank” column).

6. LRPN assessment (“value” column) and its decreasing prioritization ordering of the modes (“rank” column).

The criticality assessment using the classical method leads to RPN values which vary in the first half of the admissible range (i.e. from 1 to 1000). The results are quite distant from each other: this leads to an easy prioritization of the mode from the highest RPN (most critical) to the lowest (least critical).

TABLE III.V

FAILURE MODE OF AN HVAC IN RAILWAY APPLICATION RANKED USING CLASSICAL AND ALTERNATIVE RISK PRIORITY NUMBERS.

O S D RPN IRPN ERPN(3) URPN(e) LRPN VALUE RANK VALUE RANK VALUE RANK VALUE RANK VALUE RANK

FM1 8 6 7 336 3 21 2 9,477 4 22,592 5 2.527 3 FM2 3 5 4 60 10 12 10 351 10 1,780 9 1.778 10 FM3 6 6 7 252 6 19 6 3,645 7 16,944 6 2.401 6 FM4 5 6 5 150 7 16 8 1,215 8 10,085 7 2.176 7 FM5 4 6 5 120 9 15 9 1,053 9 8,068 8 2.079 9 FM6 8 8 4 256 5 20 5 13,203 3 95,390 3 2.408 5 FM7 9 8 6 432 1 23 1 26,973 1 160,971 1 2.636 1 FM8 8 8 5 320 4 21 2 13,365 2 119,238 2 2.505 4 FM9 7 7 7 343 2 21 2 6,561 5 53,735 4 2.535 2 FM10 7 3 7 147 8 17 7 4,401 6 984 10 2.167 8

48

A threshold value which distinguishs the group of the most dangerous modes from the set of the least critical one could be identified comparing the data or by different mathematical approaches (more detail about the determination of the thresholds is reported in the next subsections.

The IRPN scores vary in a limited range, leading to two main problems: item with the same values (e.g. Despite different O, S and D index FM1, FM8 and FM9 have the same IRPN=21) and difficulty in the definition of a RPN threshold rate due to a very compressed scale of admissible results.

Both ERPN(3) and URPN(e) provide outcomes hardly comparable with the classical formulation because of the very wide admissible range. For the same reason the interpretation of the criticality related to these numbers is quite hard.

The LRPN is the only one that maintains the same prioritization ordering of the classical formulation. The results are irrational numbers compressed in a very small range.

Fig. 3.9 illustrates the results obtained from the analysis of TABLE III.V highlighting how the different approaches provide different prioritization orders. The chart shows ten different groups composed by five bars with different colors where the groups stand for the analyzed failure mode, the colors represent the different techniques, and the height of the bars identifies the criticality of the mode. The height of a bar depends on the priority associated to that mode with the specified method: the higher the bar and more critical is the mode, therefore higher is the rank.

Fig. 3.9. Bar plot of the classical and alternatives RPN ranking for each failure mode.

FM1 FM2 FM3 FM4 FM5 FM6 FM7 FM8 FM9 FM10

Failure Modes 10

9 8 7 6 5 4 3 2 1

RPN Rank

RPN IRPN ERPN(3) URPN(e) LRPN

49 What is striking about Fig. 3.9 and TABLE III.V is that the different approaches provide different ordering despite they consider the same O, S and D dataset.

The most evident finding to emerge from the IRPN analysis is the duplicate issue, consequently the IRPN prioritization is meaningless because it is impossible to distinguish the more critical modes in presence of many identical values. The ERPN amplifies the importance of higher values of O, S and D; for instance, FM8 moves from the fourth rank in the classical RPN prioritization to the second position in the ERPN rank, due to the very high value of O and S. In some circumstances, giving high priority to higher O, S and D values could be positive. The disadvantages are evident when only one value of O, S and D is close to 10: in this case the exponential formulation of the higher parameter is dominant making the other one negligible. The URPN amplifies the importance only of the severity, therefore if the severity is very high the mode will definitely be critical. For example, using this method FM6, FM7 and FM8 have the maximum priority because they are characterized by the highest severity of the system (S=8).

In summary, these results show that the most reasonable prioritization order is the one provided by the classical Risk Priority Number.