Chapter 17 Pressure Hazards
Major Topics • Pressure hazards • Boilers and pressure hazards • High temperature water hazards • Nondestructive testing of pressure vessels • Pressure dangers to humans • Decompression procedures • Reduction of pressure hazards
Measurement of Pressure • Pressure is defined as the force exerted against an opposing fluid or thrust distributed over a surface. • It is measured in pounds per square inch (or newtons per square meter). • Pressure is measured in relation to the earth’s atmosphere. Atmospheric pressure is usually measured using a barometer. • As the altitude above sea level increases, atmospheric pressure decreases in a non linear fashion.
Inspiration and expiration • Inspiration: When atmospheric pressure is greater than within the lungs, air flows from outside into the lungs. • Expiration: When pressure in the lungs is greater than the atmospheric pressure, air moves from the lungs to the outside.
Dalton’s Law of Partial Pressures • In a mixture of theoretically ideal gases, the pressure exerted by the mixtures the sum of the pressures exerted by each component gas of the mixture:
Water Vapor and Dalton’s Law • Air entering the lungs immediately becomes saturated with water vapor. • Water vapor, although it is a gas, does not conform to Dalton’s Law. • Water vapor pressure is dependent on its temperature (and is not dependent on its fractional concentration in the mixture).
Decompression Sickness • Decompression sickness can result from the decompression that accompanies a rapid rise from sea level to at least 18,000 feet, or a rapid ascent from around 132 feet to 66 feet underwater.
Length of Exposure (to pressure hazards), Bends, Chokes, Aseptic Necrosis • Bends: Common name for decompression sickness. Early symptoms (painful bubbles of gas) occur in body bends or joints such as elbows, knees and shoulders. • Chokes: Coughing and choking due to bubbles in the respiratory system are called the chokes. • Aseptic necrosis: of bone is a delayed effect of decompression sickness. Blood in capillaries supplying the bone marrow may become blocked with gas bubbles.
Hypoxia and Hyperoxia • Hypoxia: A reduction from partial pressure can result from reduced available oxygen and cause a problem in breathing known as hypoxia. • Hyperoxia: Too much oxygen or oxygen breathed under pressure that is too high is called hyperoxia.
Nitrogen Narcosis • Results from higher than normal level of nitrogen pressure. Breathing nitrogen at great depths underwater can cause a feeling of euphoria and loss of reality.
Altitude Sickness • Altitude sickness is a form of hypoxia associate with high altitude. • Ascent to an altitude of 10,000 feet above sea level can result in a feeling of shortness of breath and fatigue. • A person ascending to 14,000 feet may experience euphoria, along with a reduction in powers of reasoning, judgment and memory. • At 20,000 to 25,000 feet a person may lose consciousness.
Trapped Gas Effects and Dysbarism • Trapped Gas Effects: With a decrease in pressure trapped gasses will increase in volume. Trapped gasses in the body include air pockets in the ears, sinuses, and chest. Jet travel causes the most commonly occurring instance of trapped gas effects, during takeoff and landing, which may lead to discomfort and pain. • Dysbarism: The formation of gas bubbles due to rapid ambient pressure reduction. The sickness may occur with the decompression associated with rapidly moving from sea level to 20,000 feet above sea level. Can result in bubble formation in the skin which causes an itchy, crawling, rashy feeling in the skin.
Destructive and nondestructive testing • To detect pressure leaks and incorrect pressure levels. • Nondestructive testing does not harm the material being tested. May include mixing dye penetrants and magnetic or radioactive particles with the gas and then measuring the flow of the gas. Ultrasonic and X-ray are often used to detect cracks and leakage points. • Destructive testing: destroy the material being checked. Pressure stress to the gas container, typically 1.5 to 1.667 times the maximum expected operating pressure for that container.
Proof Pressures • Proof Pressure tests often call for the pressure to be applied for a specific time and released. Stress and strain tests are then applied to the material.
Vacuums • Vacuums are cause by pressures below atmospheric level. Vacuums may cause collapse of closed containers.
Non destructive testing of pressure vessel • Visual Examination: To detect signs of corrosion, erosion, or hydrogen blistering. Need clean surface and good lighting. • Liquid penetration test: Placing a specially formulated liquid penetrant over an area and letting it seep in. When the penetrant is removed from the surface, some of it remains entrapped in the area of discontinuity. A developing agent is then applied which draws out the penetrant and magnifies the discontinuity. • Magnetic particle test: Disturbances are detected by applying fine particles of ferromagnetic material to the surface of the vessel. Magnetic field is produced by using contact probes (prods) in which electric current is run through an area. Irregularities produce disturbance in magnetic flux lines (only useful for carbon and low alloy steels). • Radiography test (X-ray): Irregularities such as holes, voids or discontinuities produce greater exposure (darker) on the X-ray negative. • Ultrasonic testing (radar): Short signals are induced into the material, Waves that are reflected back from discontinuities are detected by one or more transducers.
Decompression time • For employee who works 4 hours under pressure of 20 psig (gauge): • Fig 17-2 p 377. • Decompression occurs in two stages. • Stage 1 of the decompression will require a reduction in pressure from 20 psig to 4 psig over a period of 3 minutes at the uniform rate of 0.20 minutes per pound. • Stage 2 of the decompression will require a reduction in pressure from 4 psig to 0 psig over a period of 40 minutes at the uniform rate of 10 minutes per pound. • The total time for the decompression procedure is 43 minutes.
Summary • Pressure is the force exerted against an opposing force. • Barometers are used to measure atmospheric pressure. • Decompression sickness can occur from a rapid rise from sea level to 18,000 feet or a rapid ascent from around 132 to 66 feet underwater. • Nondestructive testing of pressure vessels can be accomplished by visual examination, liquid penetration test, or ultrasonic testing.
Home work • Answer questions 1, 5, 9, 11 and 15 on pages 381-382. • 1. Against which reference is pressure measured? • 5. Briefly describe decompression sickness. • 9. Discuss altitude sickness. • 11. What is the difference between destructive and non destructive testing? • 15. What is the total decompression time for an employee who works for 4 hours under pressure of 20 psig?