chapter 17 17.pdf · opposing fluid or thrust distributed over a surface. expressed in force or...

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PRESSURE HAZARDS DEFINED

Pressure is defined as the force exerted against an opposing fluid or thrust distributed over a surface.

Expressed in force or weight per unit of areaExpressed in force or weight per unit of area.

• Such as pounds per square inch (psi).

Critical injury and damage can occur with relatively little pressure.

We perceive pressure in relation to the earth’s atmosphere—at sea level, an average of 14.7 psi.

As altitude above sea level increases, atmospheric pressure decreases, in a nonlinear fashion.

In human physiology studies, the typical unit of measure is millimeters of mercury (mm Hg).

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Boyle’s law states that the product of a given pressure and volume is constant with a constant temperature:

• Air moves in & out of the lungs due to a pressure gradient or difference in pressure.

– When atmospheric pressure is greater than i hi h l i fl f h id

P1V1 = P2V2 — when T is constant

pressure within the lungs, air flows from the outside into the lungs.

– When pressure in the lungs is greater than atmospheric pressure, air moves from the lungs to the outside.


Gas exchange occurs between air in the lung alveoli and gas in solution in blood.

The pressure gradients causing this gas exchange are called partial pressurescalled partial pressures.

• Dalton’s law of partial pressures states that in a mixture of theoretically ideal gases, the pressure exerted by the mixture is the sum of the pressures exerted by each component gas of the mixture:

PA = PO + PN + Pelse

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Air entering the lungs is immediately saturated with water vapor, which, although a gas, does not conform to Dalton’s law.Dalton s law.

Partial pressure of water vapor in a mixture of gases is not dependent on its fractional concentration in that mixture.

• It is dependent on temperature.

There are many sources of pressure hazards, which result from air trapped or expanded in body cavities from air trapped or expanded in body cavities.

When sinus passages are blocked, expansion of the airin these sinuses can lead to problems.

• The same complications can occur with air trapped in the eustachian tube of the middle ear.

SOURCES OF PRESSURE HAZARDS

In rapid ascent from underwater diving or from high-altitude decompression, lungs can rupture.

Nitrogen absorption into body tissues can become excessive Nitrogen absorption into body tissues can become excessive during underwater diving & breathing of nitrogen-enriched air.

If the nitrogen is permeating tissues faster than the person can breathe it out, bubbles of gas may form in the tissues.

Decompression sickness can result from the decompression 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 to 66 feet underwater.

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Factors influencing onset of decompression sickness:

A history of previous decompression sickness, which increases the probability of another attack increases the probability of another attack.

Being over 30 increases the chances of an attack.

People in better condition have reduced chances.

Exercise during the exposure to decompression increases the likelihood and brings on an earlier onset of symptoms.

Low temperature increases the probability of the sickness.

Speed of decompression influences the sickness..

Length of exposure of the person to the pressure is proportionately related to the intensity of symptoms.


A reduction in partial pressure can result from reduced available oxygen and cause hypoxia.

Too much oxygen or oxygen breathed under pressure that Too much oxygen or oxygen, breathed under pressure that is too high, is called hyperoxia.

The partial pressure hazard, nitrogen narcosis results from a higher-than-normal nitrogen pressure.

When breathed under pressure, nitrogen causes a reduction of cerebral and neural activity.

At d th t th f t it i At depths greater than 100 feet, nitrogen narcosis can occur when breathing normal air.

Effects may become pathogenic at depths over 200 feet, with motor skills threatened at depths over 300 feet.

Cognitive processes deteriorate quickly after 325 feet.

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BOILERS AND PRESSURE HAZARDS

Potential safety hazards associated with boilers and other pressurized vessels include:

Design construction or installation errorsDesign, construction, or installation errors.

Poor or insufficient training of operators; Human error.

Mechanical breakdown or failure.

Failure or blockage of control or safety devices.

Insufficient or improper inspections, or preventive maintenance

Improper application of equipment

BOILERS AND PRESSURE HAZARDS

OSHA recommended accident prevention measures:

Daily check - of water to ensure it is at the proper level.

• Vent the furnace thoroughly before starting the fire• Vent the furnace thoroughly before starting the fire.

• Warm up the boiler using a small fire.

Weekly check - of low-water automatic shutdown control, recording results of the test on a tag that is clearly visible.

Monthly check - of the safety valve, recording results of the test on a tag that is clearly visible.

Yearly check - low-level automatic shutdown control Yearly check low level automatic shutdown control mechanism should be replaced or completely overhauled.

• Have the vendor or a third-party expert test all combustion safeguards, including fuel pressure switches, limit switches, motor starter interlocks, and shutoff valves.

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HIGH-TEMPERATURE WATER HAZARDS

High-temperature water (HTW) is heated to very high temperature—but not enough to produce steam.

Human contact with HTW can result in extremely serious burns and even deathserious burns, and even death.

The two most prominent sources of hazards with HTW are operator error and improper design.

Mechanical forces such as water hammer, thermal expansion, thermal shock, or faulty materials cause system failures, more than thermodynamic forces.

HAZARDS OF UNFIRED PRESSURE VESSELS

Unfired pressure vessels include compressed air tanks, steam-jacketed kettles, digesters and vulcanizers, and others that create heat internally.others that create heat internally.

By various means rather than by external fire.

Various means of creating internal heat include:

Chemical action within the vessel.

Application of some heating medium (electricity, steam, hot oil, and so on) to the contents of the vessel.

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Potential hazards with unfired pressure:

Hazardous interaction between the material of thevessel and materials that will be processed in it.vessel and materials that will be processed in it.

Inability of the filled vessel to carry the weight of itscontents and the corresponding internal pressure.

Inability of the vessel to withstand the pressure introducedinto it plus pressure caused by chemical reactions that occur during processing.

Inability of the vessel to withstand any vacuum that may Inability of the vessel to withstand any vacuum that may be created accidentally or intentionally.

The most effective preventive measure forovercoming these potential hazards is proper design.


Specs for design/construction of unfired pressure vessels include:

Working pressure & temperature range.

f i l b dType of materials to be processed.

Stress relief, welding or joining measures & radiography.

• Beyond proper design precautions for fired pressure vessels can be used for unfired pressure vessels.

– Continual inspection, proper housekeeping, periodic p , p p p g, ptesting, visual observation, use of appropriate safety devices.

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HAZARDS OF HIGH-PRESSURE SYSTEMS

Common hazards of high-pressure systems;

Leaks, pulsation, vibration.

R l f hi h Release of high-pressure gases.

Whiplash from broken high-pressure pipe, tubing, or hose.

Strategies for reducing these hazards include:

Limiting vibration by use of vibration dampening.

Decreasing leak potential by limiting the number of g p y gjoints in the system.

Use of pressure gauges & shields/barricades.

Remote control/monitoring; Restricted access.

CRACKING HAZARDS IN PRESSURE VESSELS

A most serious hazards in pressure vessels is the potential for cracking, which can lead to leaks, or to complete rupture, consequences of which include:rupture, consequences of which include:

Blast effects due to sudden expansion of vessel contents.

Possible injuries/damage from fragmentation.

Consequences of a leak include:

Suffocation or poisoning of employees depending onthe contents of the vessel.

Explosion and fire.

Chemical/thermal burns from contact with the contentsof the vessel.

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CRACKING HAZARDS IN PRESSURE VESSELS

Pressure vessels are used in many applications to contain many different types of substances. ranging from water to extremely toxic chemicals.

Leakage or rupture may occur in welded seams bolted Leakage or rupture may occur in welded seams, bolted joints, or at nozzles.

Deaerator Vessels

Deaeration is removing non-condensable gases, primarily oxygen, from steam generation water.

Deaerator vessels are used in power generation, pulp, paper & chemical processing and petroleum refiningpaper & chemical processing, and petroleum refining.

• The most common failures in deaerator vessels are:

– Cracks caused by water hammer at welded joints that were not postweld heat treated.

– Cracks caused by corrosion fatigue.

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Amine Vessels

The amine process removes hydrogen sulfide from petroleum gases, such as propane and butane.

Also used for removing carbon dioxide in some Also used for removing carbon dioxide in some processes.

Amine vessels are used in petroleum refineries, gas treatment facilities, and chemical plants.

The most common failures associated with aminevessels are cracks in stressed or unrelieved welds.

Wet Hydrogen Sulfide Vessels

Fluid that contains water & hydrogen sulfide is considered wet hydrogen sulfide, and many vessels used to contain it are made of steel.are made of steel.

Hydrogen is generated when steel is exposed to sucha mixture.

Dissolved hydrogen can cause cracking, blistering, & embrittlement, particularly in high-strength steels.

Low-strength steels are recommended for wet hydrogen sulfide vessels sulfide vessels.

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Ammonia Vessels

Vessels for the containment of ammonia are widely used in commercial refrigeration systems and chemical processes.processes.

Such vessels are typically spheres of carbon steel.

Water & oxygen content in ammonia can cause carbon steel to crack, particularly near welds.

Pulp Digester Vessels

Pulp digestion in the manufacture of paper involves use of a weak water solution of sodium hydroxide and sodium sulfide in a range of 230 - 284 deg F.sulfide in a range of 230 284 deg F.

The most common failure in pulp digester vessels is cracking along welded seams primarily due to caustic stress corrosion.

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NONDESTRUCTIVE PRESSURE VESSEL TESTS

Five widely used nondestructive test methods:

Visual examination.

Liquid penetration test.

Magnetic particle test.

X-ray radiography.

Ultrasonic test.• Visual, liquid penetration & magnetic particle tests can

detect only defects on, or near the surface.

– They are referred to as surface tests– They are referred to as surface tests.

• Radiographic/ultrasonic tests can detect problems within the material.

– They are called volumetric tests.

Visual Examination

A visual examination consists of a thorough look at the vessel to detect signs of corrosion, erosion, or hydrogen blistering.blistering.

It is necessary to have a clean surface and good lighting

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Liquid Penetration Test

A specially formulated liquid penetrant is placed over an area, and allowed to seep in.

When the penetrant is removed from the surface some When the penetrant is removed from the surface, some remains entrapped in the area of discontinuity.

A developing agent draws out entrapped penetrant and magnifies the discontinuity.

The process can be enhanced by fluorescent chemicalsto aid in the detection of problems.

Magnetic Particle Test

Discontinuities in/near the surface of a pressure vessel disturb magnetic flux lines induced in a ferromagnetic material.material.

Disturbances are detected by applying fine particlesof ferromagnetic material to the surface of the vessel.

• Corners and surface irregularities in the vessel materialcan produce the same disturbances as defects.

As this test works only with ferromagnetic material use is As this test works only with ferromagnetic material, use is limited to vessels of carbon & low-alloy steels.

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X-Ray Radiography

An X-ray negative is made of a given portion of the vessel, in the same way as those by physicians and dentists.

Irregularities such as holes voids or discontinuities Irregularities such as holes, voids, or discontinuities produce a greater exposure (darker area) on the X-ray negative.

Ultrasonic Test

Similar to radar &other electromagnetic/acoustic waves for detecting foreign objects.

Short signals are induced into the material and waves Short signals are induced into the material, and waves reflected from discontinuities are detected by one or more transducers.

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PRESSURE DANGERS TO HUMANS

Anoxia refers to the total lack of oxygen.

Hypoxia, when available oxygen is reduced, can occur while ascending to a high altitude or when oxygen in air while ascending to a high altitude, or when oxygen in air has been replaced with another gas.

Which may happen in some industrial situations.

• Altitude sickness is a form of hypoxia.

Hyperoxia, an increased concentration of oxygenin air, is not common.

Hyperbaric chambers or improperly calibrated scuba equipment can lead to convulsions if pure oxygen is breathed for greater than three hours.


Continued exposure to high pressures will result in confusion, convulsion, and eventual death.

Changes in total pressure can induce trapped gas effectseffects.

On a decrease in pressure, trapped gases will increase in volume—according to Boyle’s law.

Including air pockets in ears, sinuses & chest.

Very rapid altitude ascent/descent can lead to lung rupture, caused by a swift return to the surface from diving or decompression during high-altitude flight.

This event is rare and happens only if the person is holding his/her breath during the decompression.

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Evolved gas effects are associated with absorption of nitrogen into body tissues.

When ascending in altitude, nitrogen must be exhaled at a rate equal to or exceeding absorptionrate equal to or exceeding absorption.

If nitrogen is absorbed faster than it is exhaled, gas bubbles of gas may form in blood & tissue.

This can cause decompression sickness—the bends—painful, sometimes fatal.


Formation of gas bubbles due to rapid ambient pressure reduction is called dysbarism.

Cause by release of gas from the blood and attempted expansion of trapped gas in body tissuesexpansion of trapped gas in body tissues.

It may occur with decompression associated with rapidly moving from sea level (considered zero) to approximately 20,000 feet above sea level.

• Dysbarism manifests itself in a variety of symptoms.

– The creeps are caused by bubble formation in the skin.

– Bubbles in the respiratory system cause the chokes.

– Bubbles in the brain may cause tingling/numbing, severe headaches, muscle spasticity, blindness and paralysis.

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Aseptic necrosis of bone is a delayed effect of decompression sickness.

Blood in capillaries supplying bone marrow may become blocked which can cause platelets & blood become blocked, which can cause platelets & blood cells to build up in a bone cavity.

• Marrow generation of blood cells can be damaged,as well as the maintenance of healthy bone cells.

DECOMPRESSION PROCEDURES

Employees working under pressure must undergo decompression to return to normal atmosphere.

Based on the amount of pressure to which the employeeis subjected and for how longis subjected and for how long.

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MEASUREMENT OF PRESSURE HAZARDS

Several methods of detecting pressure hazards:

Sounds can be used to signal a pressurized gas leak.

• Gas discharge may be indicated by a whistling noise• Gas discharge may be indicated by a whistling noise.

Workers should not use fingers to probe for gas leaks.

• Cloth streamers may be tied to the gas vessel to indicate leaks.

Soap solutions may be smeared over the vessel surface so that bubbles are formed when gas escapes.

• A stream of bubbles indicates gas release • A stream of bubbles indicates gas release.

Scents may be added to gases that do not naturally have an odor—such as natural gas.

Leak detectors that measure pressure, current flow, or radioactivity may be useful for some types of gases.

Corrosion may be the long-term effect of escaping gases.


Common causes of gas leaks:

Contamination by dirt can prevent the proper closingof gas valves threads gaskets and other closures of gas valves, threads, gaskets, and other closures.

Overpressurization can overstress the gas vessel.

The container closure may distort and separate from gaskets, leading to cracking.

Excessive temperatures applied to dissimilar metalsthat are joined may cause unequal thermal expansion, l i h l l j i loosening the metal-to-metal joint.

Materials may crack because of excessive cold.

Operator errors may lead to hazardous gas release.

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Nondestructive testing methods do not harm the material being tested.

They may include mixing dye penetrants and magnetic or They may include mixing dye penetrants and magnetic or radioactive particles with the gas & measuring the flow.

Ultrasonic and X-ray waves are often used to characterize materials and detect cracks or other leakage points.

Destructive testing destroys material being tested.

Proof pressures generate stresses to the gas container, typically 1 5 to 1 667 times the maximum expected typically 1.5 to 1.667 times the maximum expected operating pressure for that container.

Strain measurements may also be collected to indicate permanent weakening changes to the container material that remain after the pressure is released.

REDUCTION OF PRESSURE HAZARDS

Pressurized vessels should be stored in locations away from cold or heat sources, including the sun.

Cryogenic compounds may boil and burst the container Cryogenic compounds may boil and burst the container when not kept at the proper temperatures.

Hoses should be firmly clamped at the ends when pressurized—whipping action of pressurized flexible hoses can be dangerous.

Gas compression can occur in sealed containers exposed to hheat.

Aerosol cans may explode violently when exposed to heat.

Pressure should be released before working on equipment—check gauges before any work begins.

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Water hammer is a shock effect caused by liquid flow suddenly stopping & produces loud noises.

The momentum of the liquid is conducted back upstream in a shock wave and may damage pipe fittings & valvesin a shock wave, and may damage pipe fittings & valves.

• Reduction of this hazard involves using air chambers in the system and avoiding the use of quick-closing valves.

• Negative pressures or vacuums are caused by pressures below atmospheric level.

– Negative pressures may result from hurricanes and tornadoes.

– Vacuums may cause collapse of closed containers.


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chapter 17 17.pdf · opposing fluid or thrust distributed over a surface. expressed in force or...

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