Valve definition

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A valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways.
Valves are technically fittings, but are usually discussed as a separate category.
In an open valve, fluid flows in a direction from higher pressure to lower pressure.

The word is derived from the Latin valva, the moving part of a door, in turn from volvere, to turn, roll. The simplest, and very ancient, valve is simply a freely hinged flap which drops to obstruct fluid (gas or liquid) flow in one direction, but is pushed open by flow in the opposite direction.

This is called a check valve, as it prevents or "checks" the flow in one direction.

Modern control valves may regulate pressure or flow downstream and operate on sophisticated automation systems. Źródło: https://en.wikipedia.org/wiki/Valve

Employment contracts signed by a plumber

Persons employed as plumbers can sign various contracts of employment.
They work full-time in a variety of plants, hydraulic or sign contracts-orders for certain services.
Some plumbers also run his own business, under which perform minor plumbing services.
Nowadays, self-employed persons often assume hydraulic websites, which include information on the services rendered by them and their price list. They also by posting information about their activities on bulletin boards, and even spread leaflets around the city to encourage use of their services.

So they can get regular customers, who will often benefit from their services.

Boiler and safety

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To define and secure boilers safely, some professional specialized organizations such as the American Society of Mechanical Engineers (ASME) develop standards and regulation codes.

For instance, the ASME Boiler and Pressure Vessel Code is a standard providing a wide range of rules and directives to ensure compliance of the boilers and other pressure vessels with safety, security and design standards.5 Historically, boilers were a source of many serious injuries and property destruction due to poorly understood engineering principles.

Thin and brittle metal shells can rupture, while poorly welded or riveted seams could open up, leading to a violent eruption of the pressurized steam.
When water is converted to steam it expands to over 1,000 times its original volume and travels down steam pipes at over 100 kilometres per hour.

Because of this, steam is a great way of moving energy and heat around a site from a central boiler house to where it is needed, but without the right boiler feed water treatment, a steam-raising plant will suffer from scale formation and corrosion.

At best, this increases energy costs and can lead to poor quality steam, reduced efficiency, shorter plant life and unreliable operation. At worst, it can lead to catastrophic failure and loss of life. Collapsed or dislodged boiler tubes can also spray scalding-hot steam and smoke out of the air intake and firing chute, injuring the firemen who load the coal into the fire chamber.
Extremely large boilers providing hundreds of horsepower to operate factories can potentially demolish entire buildings.
Źródło: https://en.wikipedia.org/wiki/Boiler#Safety.

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