The first law is that all liquids flow downhill. This rule may seem obvious, but it's fundamentally important to a plumber who deals with gravity. For instance, sewage lines must have a slight slope in order for flow to occur. Most sewer pipe lines have a ¼-inch drop for every foot of run. Suppose a sewer pipe run is 30 feet long. At the inlet, suppose the pipe is 10 feet off the ground. Thirty feet times ¼-inch drop per foot is a 7½ -foot drop. Therefore, the end of the pipe run is 2½ feet off the ground. For multistory buildings with pipes running overhead, this pipe must be hidden in a wall, otherwise a tenant will have a sewer pipe running through his living room.
Many times, a piping system has to flow against gravity, or uphill. This poses unique problems, since gravity becomes a foe to contend with. A common example of this is a well supplying water to a house. Water has to be brought up from the depths, much like a drink through a straw. Specially designed well pumps generate suction to accomplish this task, according to the Alabama Cooperative Extension System.
Piping systems in many installations are extremely complicated. For example, pipelines in modern milk processors or large steam power plants may have hundred of nodes. Nodes are points where three or more pipes are joined together. Flow analysis through nodes is complex, requiring the use of computer flow programs. The University of Illinois at Urbana requires engineering students in some courses to develop programs that analyze complex flow.
Flow sometimes has to come out of one large pipe but feed many smaller branch pipes. To accomplish this, a device called a manifold is used. Manifold is a combination of two words, "many" and "fold." A manifold has one large opening, but many small openings. Engineers specify the number of small openings. Manifolds are commonly used in irrigation systems. A large water supply pipe has a manifold at its end. Many small pipes attach to the manifold and feed water to spray nozzles placed between the rows of crops.