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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
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Moments that changed communities, movements, institutions and the nation.
Wilmington, North Carolina once had a thriving Black middle class and an elected interracial government. In 1898 white supremacists used violence to overthrow that government, kill Black residents and drive many others from the city.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
Backflow is a term in plumbing for an unwanted flow of water in the reverse direction.[1] It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.
Backflow occurs for one of two reasons, either back pressure or back siphonage.[1]
Back pressure is the result of a higher pressure in the system than in its supply, i.e. the system pressure has been increased by some means. This may occur in unvented heating systems, where thermal expansion increases the pressure.
Back siphonage is the result of supply pressure being lowered below that of the system. This may occur when a supply is interrupted or drained down.
The precise measures required to prevent backflow depend on the risk of contamination, i.e. the condition of the water in the connected system. In the United Kingdom this is categorized into different risk levels:[2]
Backflow prevention must be automatic, and manually operated valves are not usually acceptable.
Automatic check valves are required to prevent back pressure. Regulations for these check valves specify the design capabilities of the valve used, according to the hazard. Category 2 contamination may be prevented by a single check valve, but category 3 requires a double check valve (these are manufactured as a convenient single unit, or even integrated into tap (faucet) fittings). Category 5 requires an air gap, not merely a valve. A recent introduction to the UK has been the Reduced Pressure Zone (RPZ) valve, a form of double check valve where the intervening zone is drained and normally kept empty.[1] If the downstream valve leaks and permits backflow, this will drain out through the vent rather than building up pressure against the upstream valve. These valves are complex, requiring certified installation and annual checks. They are used for category 4 systems, such as fire sprinklers where the system has an antifreeze additive.
Back siphonage may be prevented by use of a vertical air gap. This may be a small gap, such as provided by a tundish (a combined overflow spout and catch funnel) or a large gap, such as a basin tap being above the maximum level of the water in the basin. Standards for these air gaps group them by the amount of separation they provide and their acceptability for the various risk categories.[3] The size of the acceptable gap also depends on the capacity of the incoming supply, such that a stuck-open flow cannot overfill the cistern and close the gap.
Air gaps may also protect against back pressure, and are generally favoured for this.[3] However most air gaps also limit the system pressure that may be transmitted across them. In most cases, they replace mains pressure with the pressure of that from a raised gravity cistern.
Common examples of an air gap in domestic plumbing are:
Backwater sanitary valves (also known as "check valves" or "backwater valves") are also often referred to as "backflow preventers"[4][5] They are intended to prevent backflow of sewage on the sanitary sewer line during a flood or sewer blockage, and have no connection with potable water.
Also sewage lifting stations provide comprehensive protection against sewer backflow. They pump the water above the backpressure level into the sewer. Even when the sewer is completely full.[6][7]
The American Society of Mechanical Engineers (ASME) publishes the following Standard on:
Backflow and prevention | |
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| Backflow prevention methods | |
Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
Backflow is a term in plumbing for an unwanted flow of water in the reverse direction. It can be a serious health risk for the contamination of potable water supplies with foul water. In the most obvious case, a toilet flush cistern and its water supply must be isolated from the toilet bowl. For this reason, building codes mandate a series of measures and backflow prevention devices to prevent backflow.
A backflow prevention device is used to protect potable water supplies from contamination or pollution due to backflow. In water distribution systems, water is normally maintained at a significant pressure to enable water to flow from the tap, shower, or other fixture. Water pressure may fail or be reduced when a water main bursts, pipes freeze, or there is unexpectedly high demand on the water system (for example, when several fire hydrants are opened). Reduced pressure in the pipe may allow contaminated water from the soil, from storage, or from other sources to be drawn up into the system.
"Eurasian backflow" or "Eurasian back-migrations" refers to several waves of early modern-human migrations before and during the Neolithic, when some groups of anatomically modern humans (contemporaneous Homo sapiens) returned to Africa from parts of Eurasia. Modern humans had begun dispersing into Eurasia from North Africa during the Middle Paleolithic. Today, all humans outside of Africa descend primarily from a single expansion that occurred 70,000–50,000 years ago. Modern humans who dispersed into Eurasia came into contact with some groups of archaic humans, such as Neanderthals and Denisovans, and eventually interbred with them. As a result of this phenomenon, which continued until the Upper Paleolithic, those who returned from Eurasia played a significant role in introducing Neanderthal DNA to the African modern human genome, particularly among North Africans. Thus, in addition to intrinsic diversity within the continent borne by population structure and isolation, these historical influxes of Eurasian populations into Africa are regarded as a critical contributor to the existing genetic diversity.
The heart is a muscular organ found in humans and other animals. This organ pumps blood through the blood vessels. The heart and blood vessels together make up the circulatory system. The pumped blood carries oxygen and nutrients to the tissue, while carrying metabolic waste such as carbon dioxide to the lungs. In humans, the heart is approximately the size of a closed fist and is located between the lungs, in the middle compartment of the chest, called the mediastinum. In humans, the heart is divided into four chambers: upper left and right atria and lower left and right ventricles. Commonly, the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. In a healthy heart, blood flows one way through the heart due to heart valves, which prevent backflow. The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium. The heart pumps blood with a rhythm determined by a group of pacemaker cells in the sinoatrial node. These generate an electric current that causes the heart to contract, traveling through the atrioventricular node and along the conduction system of the heart. In humans, deoxygenated blood enters the heart through the right atrium from the superior and inferior venae cavae and passes to the right ventricle. From here, it is pumped into pulmonary circulation to the lungs, where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta into systemic circulation, traveling through arteries, arterioles, and capillaries—where nutrients and other substances are exchanged between blood vessels and cells, losing oxygen and gaining carbon dioxide—before being returned to the heart through venules and veins. The adult heart beats at a resting rate close to 72 beats per minute. Cardiovascular diseases are the most common cause of death globally; an estimated 19.8 million people died from them in 2022, representing about 32% of all deaths worldwide. Of these, 85% were due to heart attack and stroke. Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others. Cardiovascular diseases do not frequently have symptoms but may cause chest pain or shortness of breath. Diagnosis of heart disease is often done by the taking of a medical history, listening to the heart-sounds with a stethoscope, as well as with ECG, and echocardiogram which uses ultrasound. Specialists who focus on diseases of the heart are called cardiologists, although many specialties of medicine may be involved in treatment.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Shirley Chisholm, elected in 1968.
