Chemicals and Materials

Lithium and Its Limits

Ian Sutton — Sat, 15 Jan 2022 15:21:19 +0000

Lithium and Its Limits Ian Sutton Sat, 01/15/2022 - 10:21am

Safety Moment #83: The One-Legged Stool

Ian Sutton — Sat, 13 Jun 2020 05:21:49 +0000

Safety Moment #83: The One-Legged Stool Ian Sutton Sat, 06/13/2020 - 1:21am

Reproduced with permission

Further thoughts on this topic are provided at the post The One-Legged Stool.

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Early in the 20^th century a factory in the town of Pitsea in England manufactured the explosives using nitro-glycerine.

Making nitro-glycerine was very dangerous. Concentrated acids were mixed with glycerine in huge vats. If too much glycerine was added too quickly to the mixture, it would become unstable, and a large valve would have to be opened to quickly dump the whole batch into a large vat of water. Failure to do this quickly could have led to a catastrophic explosion.

Mostly, though it was very dull. The operator would sit at the mixing machine for long hours just looking at the dials to make sure the machine was working OK, and there was a good chance they could fall asleep on the job. A one-legged stool made sure they had to perch to stay awake . . . in all the years the factory operated they never once had to dump the Nitro-Glycerine mixture.

In other words, the worker in charge of this process (the rather stout gentleman shown in the picture) was allowed to sit down but only on a one-legged stool. Hence if he dozed off, he would fall and wake up.

Let’s consider this situation using process safety management thinking.

The hazard is “wrong composition”, i.e., too much glycerine.
The consequence is a devastating explosion.
The predicted frequency of the event is very low.

Such a situation is what process safety professionals face all the time.

The difference between then and now lies in the safeguards. In the Pitsea factory the safeguard was a one-legged stool — that’s all. It was cheap, easy to maintain and effective.

Were we to build a process such as this now we would install multiple layers of protection, involving sophisticated instrumentation and backup safety devices. Such systems are expensive, require considerable maintenance and are difficult to understand. Yet they would not necessarily be more effective than the one-legged stool. After all, the process at Pitsea never experienced an explosion.

It might be thought that the time and place of this example is so distant as to be not pertinent to modern industry. But I recall, early in my career, working at two chemical plants, one in south-east Texas and the other in Europe, where the clients made large quantities of ethylene oxide (EO) — a chemical that is both toxic and highly flammable.

EO was stored in a large tanks. The tanks had no instrumentation at all. The only way of measuring the level was with a manual strapping gauge. To modern eyes this situation sounds extraordinarily hazardous, yet it worked — in many years of operation neither facility had a spill or any other type of incident to do with tanks.

The modern process safety expert could not live with either of the above examples. He or she would not accept that the level in both the nitro-glycerine vat and the EO tanks could be monitored without any type of instrumentation. He would insist on conducting elaborate studies that generate recommendations for the installation of expensive level control systems backed up with a high-integrity Safety Instrumentation System. Indeed, an industry regulation or standard may require that such a system be installed.

This new system may or may not make the operation of the tank more safe, but it will most certainly increase capital and maintenance costs substantially. Moreover, a complex system such as this is vulnerable to the Law of Unintended Consequences. If something can break it will. But with the one-legged stool, all that can break is the leg of the stool itself, and that can be fixed in no time flat.

The following Table compares the two approaches to controlling the level in the nitro-glycerine reactor using process safety management terms.

Although the above analysis is written somewhat tongue-in-cheek, it is actually an example of the application of Inherent Safety, specifically the value of Simplicity. The modern, highly instrumented approach, could be considered as an example of, “We’ve found the solution, now where’s the problem?”

Chemicals and Materials

Inherent Safety

Safety Moment #87: Hydrogen Sulfide

Ian Sutton — Tue, 09 Jun 2020 19:12:04 +0000

Safety Moment #87: Hydrogen Sulfide Ian Sutton Tue, 06/09/2020 - 3:12pm

Hydrogen Sulfide

Hydrogen Sulfide is a highly toxic chemical compound that is found in a wide variety of oil processing operations. High concentrations of H₂S may be present in crude oil, molten sulfur, tank and pit-bottom sludge and produced water, all of which may release H₂S when agitated, heated, or depressurized. Typical operational activities where personnel may be exposed to H₂S include drawing samples, handling and testing samples, gauging tanks, and when opening lines and equipment. Typical maintenance activities where personnel may be exposed to H₂S include tank cleaning and repair, vessel or sump clean-outs and repair, and well maintenance. These and other similar activities may place workers at a higher risk of exposure to H₂S.

Toxicity

Exposures to H₂S at concentrations as low as 600 parts per million (ppm) can cause death in a matter of minutes due to paralysis of the respiratory system. The gas is colorless and flammable. It is also 19% more dense than air. Therefore any H₂S that leaks is likely to accumulate at a low point. H₂S is soluble in many liquids, including hydrocarbons. However, H₂S mixed with natural gas may form a lighter-than-air mixture. The fact that H₂S is "heavier than air" is a statement that should be used with care, particularly when concentrations of the gas are low (say less than 100 ppm).

Table 5.6 summarizes the effects of H₂S at various concentration levels. (Guidance regarding the management of H₂S offshore is provided in API RP 14C.)

Table 5.6
Health Effects of Hydrogen Sulfide (Typical)

Concentration (ppm)	Potential Effect
<10	Not a health concern.
10 to 20	Eye and respiratory irritation.
20 to 100	Inflammation, corneal blistering and opacity of the eye, loss of the sense of smell, headache, cough and nausea.
100 to 300	Respiratory difficulty.
300 to 600	Central and peripheral nervous system effects, i.e., tremors, weakness, numbness of extremities, unconsciousness and convulsions.
600 to 1000	Rapid unconsciousness, death if first aid not promptly administered.
>1000

(1ppm = 1.4 mg/m³).

H₂S oxidizes rapidly in the body; therefore, there are normally no permanent aftereffects from acute exposure if the victim is rescued promptly and resuscitated before experiencing prolonged oxygen deprivation.

H₂S is not a carcinogen.

The gas is approximately 19% more dense than air. Hence it tends to accumulate in low or enclosed places such as pits, trenches, enclosed well bays and cellars, sumps, the tops of floating roof tanks, buildings, shale shakers and portable containers.

Hydrogen sulfide is easily detected by sense of smell up to values of around 100 ppm. (Most texts state that hydrogen sulfide smells like rotten eggs, but, with modern refrigeration, it is probably more apropos to state that rotten eggs smell like hydrogen sulfide.) Above the value of 100 ppm 'olfactory fatigue' can set in, and a person becomes unable to smell the gas. Therefore, the inability to detect H₂S through the sense of smell does not prove that the gas is not present. Moreover, the ability to detect the gas by smell varies widely among individuals. For these reasons portable H₂S detectors are commonly used. Each person at the site carries one of these devices, which is typically set to alarm in the 5-10 ppm range.

In addition to the personal alarms, fixed sensors located around the facility will warn of a release. These sensors should send their signal to the control room.

Flammability

H₂S has a wide flammable range (4.3 - 45.5% by volume in air). When burned, H₂S forms sulfur dioxide (SO₂). In an oxygen-deficient atmosphere, iron and steel will react with H₂S to form iron sulfide deposits on the surface of the metal.

Location of Monitors

API RP 14C provides the following guidance for the location of H₂S monitors for offshore installations.

Atmospheric H₂S concentration is > 50 ppm;
H₂S concentration in piping is > 100 ppm;
Enclosed areas as defined by API RP500 where H₂S could reach >50 ppm;
Poorly ventilated areas;
Sensors should be no greater than 1 meter above the floor/deck with a grid pattern of at least one detector per 400sf (37 m²) of floor space;
Sleeping quarters;
Within 3 meters of applicable equipment:
- Vessels
- Compressors (>50HP/38 KW should have two monitors)
- Pumps
- Headers
- Wellheads

If H₂S is detected both visual and audible alarms should be triggered.

Corrosion

Hydrogen sulfide can cause corrosion of stainless steels such as 316 and 410 stainless in the form of sulfide stress cracking. (Other factors, such as pH, chloride concentration and temperature also affect the potential for steel cracking.) Copper alloys corrode rapidly in H₂S service. An industry value that has been developed is NACE MR-01, 2003 from the National Association of Corrosion Engineers. In the gas phase, a stream is sour if the H₂S partial pressure exceeds 0.05 psia. If a single phase liquid is in equilibrium with a gas phase, where the gas phase H₂S partial pressure exceeds 0.05 psia, then that liquid is also considered to be sour. If the liquid is not in equilibrium with the gas phase, then the liquid is considered sour, if this bubble point gas phase H₂S partial pressure exceeds 0.05 psia. The presence of water is not required for a gas and/or liquid to be considered to be sour, nor is there a minimum pressure to avoid designating a gas or liquid as sour.

In an oxygen-deficient atmosphere, iron and steel will react with H₂S to form iron sulfide deposits on the surface of the metal. Some iron sulfides (known as pyrophoric iron sulfide) are unstable and, when exposed to air, will undergo a rapid chemical reaction creating an ignition source that should be considered during equipment shutdowns.

You are welcome to use this Safety Moment in your workplace. But please read Use of Safety Moments.

Chemicals and Materials

Health

The Chemical Safety Board

Ian Sutton — Tue, 08 May 2018 12:04:59 +0000

The Chemical Safety Board Ian Sutton Tue, 05/08/2018 - 8:04am