Safety Considerations for Carbon Dioxide Systems

Transcription

1 Safety Considerations for Carbon Dioxide Systems Andy Pearson & Angus Gillies Star Refrigeration Ltd., U.K.

2 Safety issues with CO 2 Physiological effects Pressure and temperature Safe maintenance Materials selection Component installation Chemical reactions ammonia Chemical reactions - water

3 Physiological Effects

4 Refrigerants & asphyxiation

5 Effect of reduced oxygen 21% - normal atmosphere 16% - breathing becomes laboured 14% - moving becomes an effort 12% - muddled thinking 10% - nausea, vomiting, collapse 8% - loss of consciousness 6% - respiratory failure

6 Air displacement 100% 80% 60% 40% 20% Asphyxiant Nitrogen Argon CO2 Oxygen 0%

7 3 classes of refrigerant Those which smell strongly and have an acute toxic effect Ammonia, sulphur dioxide, methyl chloride Those which have no particular smell or taste, and no short-term toxic effect Fluorocarbons, hydrocarbons, nitrogen Those which are directly involved in the respiration process Carbon dioxide

8 Air displacement 100% 80% 60% 40% 20% Asphyxiant Nitrogen Argon CO2 Oxygen 0%

9 Asphyxiation by nitrogen 100% 80% 60% 40% 20% Asphyxiant Nitrogen Argon CO2 Oxygen 0%

10 Asphyxiation by CO 2 100% 80% 60% 40% 20% Nitrogen Argon CO2 Oxygen 0%

11 CO 2 and respiration O 2 releases energy by burning carbohydrates in cells CO 2 transferred from cells to lungs in carboxyhaemaglobin CO 2 used for ph control to ensure brain function Breathing rate is controlled by blood levels of CO 2

12 CO 2 and respiration Excess CO 2 causes increased breathing rate ( hyperventilation ) Further CO 2 causes numbness and dizziness ( narcosis ) At higher CO 2 levels the breathing reflex is disrupted ( hypoventilation ) This is followed by coma and death

13 CO 2 and respiration Return to fresh air (300ppm CO 2 ) Effects are dependent on rate of increase and on physical condition Typically 3% (30,000ppm) causes hyperventilation, 5% causes narcosis 10% causes coma

14 Asphyxiation by CO 2 100% 80% 60% 40% 20% Nitrogen Argon CO2 Oxygen 0%

15 Conclusions CO 2 is different to all other gases. but it is not more or less safe! CO 2 effects are felt at lower levels than R134a or N 2 but the warning signs are as clear as NH 3 to those trained to spot them

16 Gas detectors for CO 2 Infra red technology has proved to be: accurate robust reliable and doesn t give false alarms!

17 Gas detectors for CO2

18 COSHH Assessment CO 2 Health hazards: Increased respiration and headache Asphyxiation Freeze burns and frost bite Explosive release of pressure

19 COSHH Assessment CO 2 Control measures: Ensure good ventilation Do not trap refrigerant Ensure all portable tools are suitable for high pressures Design staff training Commissioning and maintenance staff training

20 COSHH Assessment CO 2 Risk No and phrase: RAs Asphyxiant in high concentrations. RFb May cause frostbite

21 COSHH Assessment CO 2 Safety No and phrase: S9: Keep container in well ventilated place. S23: Do not breathe the gas. S36A: Use suitable protective equipment

22 Pressure and Temperature

23 Physical properties

24 P-h chart for CO2

25 P-h chart for CO2

26 Physical properties

27 pressure issues with CO 2 At 50 o C the pressure is 6 Bar(G) At 20 o C the pressure is 19 Bar(G) At 0 o C the pressure is 34 Bar(G) At 20 o C the pressure is 57 Bar(G) It is the pressure/temperature relationship that is important!

28 pressure issues with CO 2 HT cascade Ps = 40 Bar(G) LT cascade Ps = 40 Bar(G) Defrost system Ps = 52 Bar(G) Transcritical Ps = 100 Bar(G) Ambient standstill Ps 80 Bar(G) Fade out vessel 90 Bar(G)

29 Key temperatures for CO o C o C critical temperature 0 o C -25 o C -50 o C -75 o C -100 o C

30 Key temperatures for CO o C o C critical temperature 0 o C -25 o C -50 o C o C triple point temperature -75 o C -100 o C

31 Key temperatures for CO o C o C critical temperature 0 o C -25 o C -50 o C -75 o C -100 o C o C triple point temperature o C atmospheric temperature

32 Safe Maintenance

33 Don t trap vapour:

34 Solid formation in vents:

35 Solid CO 2 issues

36 Pumping out liquid CO2

37 Venting liquid CO2

38 Pressure issues with CO 2 Considerations with relief valves: Mount valve at the outside end of a high pressure line Size relief valve inlet for pressure drop <3% of actual relieving pressure (EN 13136:2001) Minimal valve outlet vent line to avoid blockage with solid CO 2

39 Venting a CO 2 system Precautions: Vent to open air Do not vent liquid to atmosphere Always use a gauge to ensure line is vented before opening

40 Pumping out CO 2 liquid Precautions: Never apply heat to a vent blocked with solid CO 2 Always confirm a vent is not blocked with solid CO 2 Beware of frost burn from solid CO 2

41 Charging a CO 2 system Charging from a cylinder: Ensure appropriate quality refrigerant Ensure charging line can withstand circa 90 Bar Charge vapour until above 5 Bar(G)

42 Charging a CO 2 system Charging from a tanker: Agree bulk delivery conditions with supplier well in advance Expect mechanical design review Agree charging method Expect tanker pressure circa 20 Bar Charge vapour until above 5 Bar(G)

43 Safe maintenance of CO 2 Solid forms at 56.6 o C (4.2 Bar(G)), and can block vent valves Never apply heat to blockages find another way to relieve pressure Beware of trapped liquid and trapped gas Only vent to open air

44 Materials Selection and Component Installation

45 Materials selection for CO 2 Pipework and vessels can be carbon steel Can be LT50 (ASTM A333 Gr6) for most European design codes Or consider stainless steel.. It is the pressure/temperature relationship that is important!

46 Valve choice Details for ball valves: Petrochemical industry equipment Floating ball design Cavity relief is essential Avoid graphite reinforcement Glass fibre reinforcement is recommended Specify for dry CO 2

47 Valve choice Details for globe valves: Refrigeration industry equipment Up to 52 Bar(G) Beware of stem seal leakage Reasonable cost

48 Valve choice Details for solenoid valves: Refrigeration industry equipment 40 Bar designs common now 52 Bar designs coming soon High differential pressures can rupture diaphragms

49 Chemical Reaction with Ammonia

50 Refrigerant Shapes: Ammonia Nitrogen Hydrogen NH 3

51 Refrigerant Shapes: Carbon Dioxide Carbon Oxygen CO 2

52 Compound Shapes: Ammonium Carbamate NH 4 + White crystalline NH 2 CO 2 - decomposes at 60 o C 25% soluble at 15 o C NH 4 NH 2 CO 2

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54 When heated in open air. CO 2 + NH 3 CO 2 +NH 3

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58 Conclusions CO 2 /ammonia mixtures form Ammonium carbamate This can easily block ports and valves, and withstand pressure Ventilating with hot air clears the solid to gases (CO 2 and NH 3 ) Ammonium carbamate also easily dissolves in water Increased corrosion rate of steel

59 Lessons learned. Deal with leaks quickly to prevent additional damage Design both sides for the same pressure to prevent overpressure or Include automatic isolation to protect the lower pressure ammonia system For simple systems hot air may be enough,.but keep the system well ventilated For more complex systems wash with water then dry thoroughly.

60 CO 2 /NH 3 issues

61 Chemical Reaction with Water

62 Water in CO 2 systems CO 2 + H 2 O

63 Water in CO 2 systems CO 2 + H 2 O = H 2 CO 3 (Carbonic acid)

64 Water in CO 2 systems H 2 CO 3 + H 2 O

65 Water in CO 2 systems H 2 CO 3 + H 2 O = HCO 3- + H 3 O + (Hydronium ion)

66 Picture courtesy of Danfoss

67 Water in CO 2 systems Iron + oxygen + water = rust Mineral oil + O 2 = organic acid + water Ester oil + H 2 O = organic acid + alcohol Wear additive + H 2 O = phosphoric acid + alkanol

68 Water in CO 2 systems Oxidation of mineral oils leads to formation of short organic acids (eg formic acid, acetic acid) Hydrolysis of polyol ester oils leads to formation of long organic acids (less harmful) but both types are counted in the TAN (total acid number)

69 Water in CO 2 systems Carbonic acid + water = bicarbonate + hydromium ion Organic acid + water = carboxylate + hydronium ion Phosphoric acid + water = phosphate + hydronium ion Any acid needs water to make it harmful!

70 Picture courtesy of Danfoss

71 Picture courtesy of Danfoss

72 Water in CO2

73 Water in CO 2 Ensure the moisture content is such that the water concentration remains below the amount that can dissolve in liquid CO 2. Corrosion is not a risk if there is no free water ie it is all dissolved in CO 2.

74 Safety issues with CO 2 Physiological effects Pressure and temperature Safe maintenance Materials selection Component installation Chemical reactions ammonia Chemical reactions - water

75 * Any questions?