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Process Safety > Reliability and Trip Systems > Flashcards

Reliability and Trip Systems Flashcards

1
Q

What is reliability?

A

The probability that an item will perform a characteristic function under stated conditions for a stated operating period, t.

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2
Q

What is the one limitation for t in the reliability?

A

t must be similar to the expected life of the item.

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3
Q

What are two useful measures of the reliability?

A
  • failure rate

- mean time between failures

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4
Q

What is the assumption of the failure rate?

A

It is assumed to be random.

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5
Q

What is the relationship between the failure rate and the mean time between failures?

A

The mean time is the inverse of the failure rate

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6
Q

Describe the bathtub curve for the failure rate?

A

There is high failure initially due to burn in. There is then a horizontal line for steady random failure and then high failure rate towards the end of the lifetime due to wear out.

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7
Q

How is the burn-in period controlled?

A

This should be controlled through the selection of the equipment and good installation.

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8
Q

How is the wear out period controlled?

A

Equipment should be replaced before the wear out period.

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9
Q

How is it ensured that a pump will work reliably?

A

It wouldn’t be bought from a cheap source unless there was quality assurance. Good installation, inspection and training can assure that when the pump is started it works reliably.

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10
Q

What is the relationship between R, lambda and t in the constant failure rate period?

A

R = exp (- lambda * t)

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11
Q

What is the probability that an item will fail, F, given by?

A

F = 1 - R

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12
Q

Is this equation used for systems with high or low failure rates?

A

Low failure rates

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13
Q

What is the reliability for N items running in series?

A

Rs = R1 x R2 x R3 x … x Rn

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14
Q

What is the failure for N items running in parallel?

A

Fs = F1 x F2 x F3 x … x Fn = 1 - Rs

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15
Q

So what can be said about the failure rate, if the reliability is high?

A

The failure rate is low

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16
Q

Why is it significant in series for the reliabilities to be multiplied?

A

Series systems fail much more frequently. If one component fails then the whole system shuts down so you multiply reliabilities. No matter if the reliability is high then the reliability goes down once the product has been taken.

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17
Q

Why is it significant in parallel for the failures to be multiplied?

A

The system fails only if all the components have failed. The reliability is high.

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18
Q

Is the reliability higher for series or parallel systems?

A

Parallel systems

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19
Q

What are the two types of device for trip systems?

A
  • Active devices

- Passive devices

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20
Q

What is an active device?

A

They are doing something observable/ active.

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21
Q

What is a passive device?

A

Devices only operating as required. You don’t know that they work until they are tested.

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22
Q

What is the definition of a failure?

A

Something is unable to perform its normal function

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23
Q

What are the two types of failure?

A
  • Active failures

- Passive failures

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24
Q

What is an active failure?

A

Active failures are revealed. Something was functioning and stops.

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25
Q

What is a passive failure?

A

Passive failures are unrevealed. The item will not perform its function when it is required sometime after the failure. You don’t know about the failure until the device fails to respond to a demand.

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26
Q

What is the definition of the fractional dead time?

A

It is the proportion of all time for which the device will not respond correctly to a demand. This includes the time that is needed for repair as the device is not operating.

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27
Q

What are trip systems?

A

Trips are control devices which shut down all or part of the plant when control limits are passed.

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28
Q

What is the objective of trip systems?

A

Trips return the plant to a safe state but not necessarily one from which it is easy to restart.

29
Q

Are trip systems part of the normal control system?

A

No. Their only function is to respond to make the plant safe. They are a last resort and they are specific safety devices. They are constantly monitoring, if a value is too high or too low then it shuts down the plant or returns the plant to safe operation values.

30
Q

If there is a setpoint where operations run then why is it necessary for there to be trip systems in place?

A

There are always some fluctuations around the normal operating conditions.

31
Q

What can be installed to show that a value is straying before a trip is kicked in?

A

An alarm

32
Q

Why is it useful to install an alarm?

A

Because the operator can step in if the temperature is outwith normal operating limits.

33
Q

What are the two types of alarms?

A
  • High and high high above

- Low and low low below

34
Q

What is the purpose of the high alarm?

A

It will sound or alarm flashes and the operator is able to check what went wrong and takes actions to correct it.

35
Q

What is the purpose of the high high alarm?

A

An automatic response which means the operator is not relied on e.g. pressure relief valve kicks in. This occurs when the operator tries everything but yet can’t fix the problem. If it keeps increasing then the trip activates and the process is shut down.

36
Q

Why is it useful to have a trip system?

A

The operator doesn’t always have time to react. Humans can’t react as fast as machines because they have a reaction time. Trips allow safe shutdown in response to rapid excursion from safe operation which would be too sudden for an operator to respond to.

37
Q

What are the three elements of an electronic trip system?

A
  • detector
  • control device
  • actuator
38
Q

Are trips passive or active?

A

Trips are passive. They are inactive unless there is a demand.

39
Q

In which two states could a trip be?

A

On or off

40
Q

What is the downside of the trips being passive?

A

You cannot be sure that a trip will respond to a demand until a demand occurs. They are not something easy to test. If it has failed and there is a demand then an unsafe condition occurs.

41
Q

What are the four different trip states and which conditions are they associated with?

A
  • Normal operation (no trip, no demand)
  • Hazard (no trip, demand)
  • Spurious trip (trip, no demand) i.e. false alarm
  • Genuine trip (trip, demand)
42
Q

What is a demand for a trip?

A

When the sensed variable goes outside acceptable bounds?

43
Q

In which two ways can a trip system fail?

A
  • Functional failure

- Operational failure

44
Q

What is a functional failure of a trip system?

A

The system is unable to respond to a demand leading to a hazardous state if a demand occurs. Functional failures are unrevealed - if these coincide with a demand the plant will be unprotected. Functional failures must be reduced as far as possible.

45
Q

How do we detect functional failures?

A

By testing. The testing regimes controls the reliability of the regime. Trips are tested periodically. If its not working then you don’t know when it broke. On average, it’s not working for half the time you tested.

46
Q

What are operational failures of a trip system?

A

The trip is activated without a demand. Operational failures lead to spurious trips. This is revealed, not dangerous but destrictive and disruptive and should be avoided.

47
Q

What is the fractional dead time, sigma?

A

The fractional dead time is the fraction of time for which the trip will not respond to a demand.

48
Q

What is the formula for the fractional dead time?

A

sigma = lambda x tau p / 2

sigma is dimensionless

49
Q

What is tau p?

A

The proof test interval

50
Q

What is the formula for the plant hazard rate?

A

n = sigma x delta

where delta is the plant demand rate

51
Q

On average for how long is a trip dead when it is tested?

A

Half the time for which it is tested.

52
Q

What is the meaning of ‘dead’?

A

It is not reacting to demand.

53
Q

What is the balance for testing and risk?

A

Testing causes reduced risk because you know that the trip system works but the risk in the testing period that the trip is already activated if something goes wrong increases.

54
Q

What is the relationship between the proof test interval and the hazard rate?

A

Decreasing the proof test interval decreases the hazard rate

55
Q

When does the test time become significant?

A

At high test rates.

56
Q

What is the other possibility of the test which increases risk?

A

That the trip is not reactivated after the test.

57
Q

What are m/n systems (multiple trip channels)?

A

M trips from a total of n must respond to a demand in order to trip the system.

58
Q

What happens if m=n?

A

The fractional dead time increases but spurious trips decrease further

59
Q

What happens if m=1?

A

Spurious trips increases but fractional dead is less than for 1

60
Q

Why is it necessary to use multiple trip channels?

A

Everything breaks so multiple trip channels might prove useful. Also there reduces the risk of a false alarm by having multiple sensors activated before trip system is activated.

61
Q

What is r?

A

The number of channels which must fail for the system to fail.

62
Q

Which multiple trip system is popular?

A

2/3 system. There are 3 alarms and 3 need to be activated for trip to activate.

63
Q

The link between the operational failure rate of the system and the operational failure rate of a single channel?

A

gamma = n * lambda
where gamma is the operation failure rate of the system
lambda is the operational failure rate of a single channel

64
Q

What about the link between gamma and lambda for a 2/3 system?

A

gamma = 6 * lambda ^2 * tau R

where tau R is the repair time.

65
Q

What is the definition of fail-to-safe?

A

This means that the trip has failed, leaving the plant in a safe state

66
Q

What sort of trip and failure correspond to fail-to-safe of the plant?

A

Spurious trips or operational failure

67
Q

What is the definition of fail-to-danger?

A

The trip has failed leaving the plant in a dangerous state

68
Q

What sort of trip and failure correspond to fail-to-danger of the plant?

A

Functional Failure where there is a demand which the trip fails to respond to

Process Safety (10 decks)

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