Pipe, fittings, and valves must all fit together to form a piping system. To ensure uniform sizing and rating, a system of describing sizes and ratings has been developed.
Piping Sizes and Rating To understand piping installation, you must first know how piping is measured and rated.
Piping is manufactured and sold by two independent designations:
- Nominal size
- Schedule
When fully describing a pipe, both designations must be used. In describing a pipe in the field, usually only the nominal size is used. The schedule of a pipe in service cannot be determined visually without reading the marking.
Nominal Size Nominal size is approximately the inside diameter of the pipe or fitting, expressed in inches, or fractions of an inch. The smallest nominal pipe size is 1/8 inch. In larger nominal sizes, 48 inches is not uncommon. Nominal sizes increase by standard steps in size, such as 1/8, 1/4, 1/2, 3/4, 1, 1-1/2, 2, 3, 4, 6, 8 inch, and so-on.
A standard length or joint of any size pipe is 20 feet long.
Schedule
Pipe schedule is a number that expresses the ability of the pipe to resist pressure. The higher the schedule number, the greater the strength of the pipe. Pipe schedules are rated in standard units. Common schedules for piping are 20, 40, 80, 120, 160, and so-on. For the common nominal sizes up to 8 inches, standard-service pipe is rated schedule 40: below schedule 40 is light-duty piping and above schedule 40 is heavy duty piping. This designation is not true for the larger nominal sizes.
Pipe schedule is a number that expresses the ability of the pipe to resist pressure. The higher the schedule number, the greater the strength of the pipe. Pipe schedules are rated in standard units. Common schedules for piping are 20, 40, 80, 120, 160, and so-on. For the common nominal sizes up to 8 inches, standard-service pipe is rated schedule 40: below schedule 40 is light-duty piping and above schedule 40 is heavy duty piping. This designation is not true for the larger nominal sizes.
The schedule rating for piping is raised by increasing the wall thickness. The outside diameter for any nominal size pipe must remain fixed so the fittings will always fit. Therefore, to increase the schedule of a pipe, the inside diameter must be reduced to increase the wall thickness. A higher schedule number has less capacity to carry fluid than a lower schedule for any given nominal size.
Pipe Construction
Pipe is constructed by two common methods:
Pipe is constructed by two common methods:
- Welded seam
- Seamless
Both types of construction have advantages and disadvantages.
Both types of construction have advantages and disadvantages.
Welded Seam Pipe
Welded seam pipe is rolled from a flat plate, and the seam is welded where the edges of the roll butt together. Welded seam pipe has uniform wall thickness but may have flaws in the weld. Welded seam pipe should not be used for corrosive liquids that could attack the weld flaws.
Welded seam pipe is rolled from a flat plate, and the seam is welded where the edges of the roll butt together. Welded seam pipe has uniform wall thickness but may have flaws in the weld. Welded seam pipe should not be used for corrosive liquids that could attack the weld flaws.
Seamless Pipe
Seamless pipe is extruded or forged from a solid billet of steel. It has the advantage of having no weld seam that can be attacked by corrosives. A disadvantage is the inside diameter is often oval-shaped, so the wall is not uniform in thickness. This could cause the pipe to be downgraded slightly in rating because of a thinner wall section.
Seamless pipe is extruded or forged from a solid billet of steel. It has the advantage of having no weld seam that can be attacked by corrosives. A disadvantage is the inside diameter is often oval-shaped, so the wall is not uniform in thickness. This could cause the pipe to be downgraded slightly in rating because of a thinner wall section.
Pipe Fittings
Pipe fittings are the connections used to join lengths of pipe, to change the direction of a pipe run, to reduce pipe size, or to branch a pipe run.
Pipe fittings are the connections used to join lengths of pipe, to change the direction of a pipe run, to reduce pipe size, or to branch a pipe run.
Some common pipe fittings are:
- Caps
- Couplings
- Ells
- Ells
- Flanges
- Metering flanges
- Reducers
- Tees
- Tees
Size and Rating of Pipe Fittings
Pipe fittings are sized by nominal size, the same method used for pipe.
They are rated by their working pressure, not by schedule. Examples of ratings are 150 lb, 300 lb, 400 lb, and 600 lb. The “lb” refers to the working pressure in psi (pounds per square inch). The rated “lbs” is at elevated temperature, as prescribed by ANSI (American National Standards Institute). At ambient temperatrue the strength of fittings is much higher than their rating.
Pipe fittings are sized by nominal size, the same method used for pipe.
They are rated by their working pressure, not by schedule. Examples of ratings are 150 lb, 300 lb, 400 lb, and 600 lb. The “lb” refers to the working pressure in psi (pounds per square inch). The rated “lbs” is at elevated temperature, as prescribed by ANSI (American National Standards Institute). At ambient temperatrue the strength of fittings is much higher than their rating.
Valves
Valves are inserted into piping runs to control and direct flow. Most valves are hand-operated and used to start and stop flows. For automatic process control, special valves with precision flow characteristics are remotely operated by automatic controllers.
Valves are inserted into piping runs to control and direct flow. Most valves are hand-operated and used to start and stop flows. For automatic process control, special valves with precision flow characteristics are remotely operated by automatic controllers.
Valves are sized and rated in the same way as fittings; that is, by nominal pipe size and by pressure rating. Standard service valves are rated at 150 lbs.
Installing Piping
Straight lengths of pipe are joined together end-to-end to make a pipe run. Fittings and valves are added to complete the run. All of this assembly must be sturdily supported, yet allow some limited movement for expansion and contraction. Because of this, piping installation is no simple thing.
Straight lengths of pipe are joined together end-to-end to make a pipe run. Fittings and valves are added to complete the run. All of this assembly must be sturdily supported, yet allow some limited movement for expansion and contraction. Because of this, piping installation is no simple thing.
How Piping, Fittings, and Valves are Joined
Joining Piping and Fittings
Steel piping and fittings are joined by two methods:
- Threaded connections
- Welded connections
Most process piping and fittings are joined by welding, especially if the piping is handling hazardous materials. Threaded connections have a tendency to crack and break at the threads when any stress is applied.
Piping lengths may be purchased with threaded ends or with beveled ends for welding. The same is true for fittings. Threaded fittings may have either male threads or female threads. (That is, they are either threaded on the outside or on the inside of the fitting wall.)
Sealing material is always applied to threaded connections before the connection is made. Sealing material may be a putty-like pipe joint compound or Teflon tape applied by wrapping the threads.
Joining Valves
Valves are seldom joined to piping by welding. Welding on valves can cause warping and leakage. Though weld valves are manufactured, most valves are installed with flanges or threaded connections. Flanges are cast as part of the valve. The valve flange mates with a flange welded to the connected piping and the valve is bolted into place. Before bolting, a gasket is inserted between the two flanges. Flanges will leak if care is not taken when they are assembled.
Pipe Supports Piping needs to be well supported to take the stress off of fittings and valves. Yet, it needs to be flexible to allow for expansion and contraction.
Pipe Racks Piping in a processing plant is laid out in parallel runs on elevated pipe racks or on piers. Pipe racks are beamed structures with posts and cross-beams at regular intervals. Tie beams and cross-bracing make the whole structure rigid.
Figure 1-2 illustrates a pipe rack. A short length of “structural tee” is welded to the bottom of each pipe, where the pipe rests on the pipe rack. These “tee” shoes raise the pipe off the rack support beam so that insulation will not be crushed by the weight of the pipe.
Off-rack Pipe Suspension and Support Piping that descends a tower or traverses open space, away from the pipe rack, requires special support. The weight of the piping must not hang from flanges or connections or be allowed to flex into a bow. Vertical piping descending a tower is stabilized with side guides, and its weight rests on posts at the bottom. Springs and counterbalances are used to allow the pipe to expand. Horizontal runs of piping located away from the pipe rack are supported by pipe hangers, or on individual posts.
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