You get up every morning from your alarm clock's warning
Take the 8:15 into the city ...
And if your train's on time, you can get to work by nine
And start your slaving job to get your pay ...

("Takin' Care of Business," by Bachman-Turner Overdrive, 1973)

 

METAL PIPE SIZING AND PIPE SCHEDULES

Definition of: Inside/Inner Pipe Diameter, Nominal Pipe Diameter
and Outside/Outer Pipe Diameter

No pipe manufacturer retools the entire pipe-making foundry to manufacture a "boutique size" of pipe. Commercially available pipe is fabricated in a wide variety of diameters from which a design engineering firm can select to achieve the closest-possible desired PV Flowrate.

Hey! That insight on piping manufacturing seems logical!

Is now a good time to get confused, Fred?

Imagine two Process Operators talking about a "4 inch" process line.

Are the Process Operators referring to the pipe's Inner Diameter or Outer Diameter?

Neither.

These Process Operators are just talkin,' not being too specific.

They both know they mean what is represented by the red diameter line in the nearby graphic, the "Nominal" pipe diameter. The word Nominal literally means "in name only."

These Process Operators may or may not know that ...

Any commercially manufactured metal pipe referred to as "12 inches or less" assumes the Process Operators are talking about the pipe's Inside/Inner Diameter, the green diameter line in the nearby graphic.

Are you awake, Fred because now it gets confusing:

• a "4 inch (Nominal) pipe" might actually have a 3.826 inch Inside Diameter and a 4 1/2-inch Outer Diameter (the gold line in the nearby graphic)!
• not shown but true: a "10 inch (Nominal) pipe" might actually have a 10 3/4-inch Outer Diameter!

 

Wait! Life would not be complete without more confusion:

When the same two Process Operators are talking about commercially made pipe "greater than 12 (Nominal) inches" the assumption is that they are both referring to the pipe's OUTER Diameter!

To recap ...

• When the two Process Operators shown above are referring to a Nominal "14-inch pipe" they are discussing the actual Outer Diameter of the pipe ... give or take a few thousands of an inch due to manufacturing variance.

 

To recap, Fred:

A casual ... ahem, Nominal ... reference to a pipe size below 12 inches assumes the reference is about the pipe's Inner Diameter. The actual Outer Diameter of this pipe size range may be more than an inch greater.

 

A casual, Nominal reference to pipe size greater than 12 inches assumes the reference is about the pipe's Outer Diameter. The actual Outer Diameter of this size range of pipe will differ by a few thousands of an inch.

 

 

Pipe Schedules

Nowadays piping is manufactured according to Schedule ... which doesn't have anything to do with the time of day for an appointment or such.

Identifying a pipe's size by referencing a Pipe Schedule eliminates any confusion regarding if the reference pertains to Inside/Inner Diameter, Nominal Diameter, or Outside/Outer Diameter. The foundry simply manufactures the pipe to the specifications listed for the Pipe Schedule.

The Pipe Schedule specifies the pipe's Outside/Outer Diameter and Wall Thickness.

Pipe manufacturers make pipe from Schedule 10 to Schedule 160.

The greater the Schedule, the thicker the pipe.  For example, Schedule 80 pipe is thicker than Schedule 40.

The most popular Pipe Schedules are Schedule 40 and Schedule 80. Very high PV Pressure service may require Schedule 120 and Schedule 160.

The nearby graphic compares Schedule 40 and Schedule 80 pipe. Tables comparing different Pipe Schedules are easy to find on the internet.

The "Weight per Foot" column helps manage the movement of the pipe; the forklift operator needs to know this information to perform safe transport of a pipe section.

Before the internet existed, the "Flow of Fluids" manual by Crane (simply known as "Crane's") was the go-to reference for understanding PV Flowrate variances through different Pipe Schedules, valves and pipe fittings.

 

THE THREE WAYS
TO JOIN TWO PIPE SEGMENTS TOGETHER

The Piping Network in a processing facility is comprised of metal pipe segments, valves, and fittings that must be joined together.

The three most popular methods that are used to "join" two lengths of industrial pipe together ... and/or insert a valve or pipe fitting ... are:

• Screwed (aka Threaded) Connections.
• Flanged Connections.  
• Welded Connections. 

 

The nearby chart shows the ISA symbols for Pipe Connections/Joints. The Welded Pipe Connection at the top (third column) simply appears as an unmarked process line. The Screwed/Threaded Pipe Connection is indicated as a single vertical hash drawn over the process line. The Flanged Pipe Connection is drawn as two parallel vertical hash lines drawn over the process line. The area between the lines may or may not be left vacant.

Which of the three connection methods is best depends upon:

• The degree of access that is needed for the process line.
• The characteristics of the metal from which the pipe is fabricated (e.g., ferrous or non-ferrous, refer to PTOA #246 and #247).
• The chemical and physical behavior of the flowing fluid and the range of PV Pressures and PV Temperatures the fluid will be exposed to.

 

Flanged Connections and the many special phenomena associated with Flanged Connections are featured in the next PTOA Segment.

 

SCREWED (aka THREADED) PIPE CONNECTIONS 

Fastening the Screwed/Threaded Pipe Connection

Screwed/Threaded Pipe Connections are joined together by meshing the threads on the exterior end of a pipe with the threads that have been cut into the interior pipe wall of a different pipe.

The pipe end that has threads on its exterior surface is referred to as the "male end" of the connection. The mating threads that are cut into the interior surface of the connecting pipe are often referred to as the "female" side of the connection.

Cutting of threads requires tradespeople who possess a passion for precision. Threads must be smooth and properly cut to secure a tight fit. Threads can be tapered or parallel as shown in the nearby graphic.

The connection action requires twisting ... or screwing ... the threads together. The twisting action typically begins by hand and often continues with use of a wrench. Careful meshing of the threads is required to achieve optimal fastening.  Cross threading prevents a successful connection.

Brilliant PTOA Readers have already completed the PTOA Tribology Focus Study (PTOA Segments #177 through #180) and are additionally knowed-up regarding the role of Bearing Seals (PTOA Segment #182). Thus, these brilliant PTOA Readers and Students immediately identify the potential for leakage and heat-generated friction which will develop with any metal-to-metal contact.

To prevent leakage between the metal-metal connection of mated threads, special thread-sealant tape (aka Teflon Tape) is carefully wound onto the "male end" pipe threads prior to twisting the "male end" of a pipe into the "female end" of a second pipe. Sometimes the threads are sealed with "pipe dope" instead of tape.

Screwed/Threaded Pipe Connection Pros and Cons

The main benefit of the Screwed/Threaded Pipe Connection is the ease of fabricating a Piping Network; the two lengths of pipe segments that are to be joined together can be easily tailored to fit.

The big problem with Screwed/Threaded Pipe Connections is the potential for line leakage. An over-pressured process line will leak ... perhaps even blow out ... at the Screwed/Threaded Pipe Connection. The more connections, the more potential places for a disruption.

For this reason, the utilization of Screwed/Threaded Pipe Connections is limited to smaller diameter pipe that transports fluids with low PV Pressures and PV Temperatures.

Screwed/Threaded Pipe Connections are more widely used in the "upstream" Exploration and Production of crude oil which requires drilling. Drill pipe can be coupled together to whatever depth is found to be needed. Drill pipe is also used in mining.

Drill pipe inspectors verify the integrity of drill pipe threads.

 

WELDED PIPE CONNECTIONS

Welding two metals together requires heating the metals to a temperature that causes the metal atoms to melt and blend together. A properly made weld by a true craftsperson is as strong as the pipe segments that are welded together. Hence, the Welded Pipe Connection is the go-to connection technology used when the process service requires high PV Pressures.

The Welded Pipe Connection is also the only pipe joining technique that is leakproof and maintenance free. Both the Screwed/Threaded Pipe Connection and the Flanged Pipe Connection have metal-to-metal interfaces that require attention to proper sealing and intermittent maintenance.

The three most common Welded Pipe Connections are:

• The Butt Weld.
• The Socket Weld.
• The Sleeve Weld.

 

The Butt Weld

The Butt Weld joins two pieces of pipe with the same diameter together.

The weld is made at the interface where the two pipe segments are to be united.

The Butt Weld is used to connect all sizes of pipe and is most popular when joining 2" and larger pipe.

Which of the many brilliant PTOA Readers and Students who are slogging through this PTOA Segment noticed that the above paragraph used a Nominal Pipe Diameter reference of "2 inches and larger pipe?"

 

These brilliant PTOA Readers and Students might also consult the nearby table to deduce that the actual Inside/Inner Diameter of the Nominal Pipe Diameter of 2 inches is 2.221 inches for Schedule 40 pipe (because 2.375 - 0.154 = 2.221) or 2.157 inches Schedule 80 pipe (because 2.375 -0.218 = 2.157).

Voila! The concept of Pipe Schedule has been applied to clarify the actual size pipe that might successfully incorporate a Butt Weld Pipe Connection.

 

The Socket-Weld

A pipe with a smaller diameter can be inserted into a pipe with a larger diameter and attached with "Fillet Weld." This procedure describes a Socket Weld. The Socket Weld and Butt Weld are shown in the nearby graphic.

Socket Welds are limited to 2"in (Nominal) Diameter pipe and smaller. Why?

Recall that the purpose of joining two pieces of pipe together is to have continuous, uninterrupted fluid flow. Fragments of weld metal can project into the flow area of the pipe when a weld is not carefully made. Fragments that break free and enter the flowing area can seriously damage pipe, valves and pipe fittings. The potential for metal fragments to interrupt fluid flow increases above pipe sizes of 2 inches.

 

The Sleeve Weld

The Butt Weld described above joined two sections of pipe with the same diameter.  A second way to join two pipes with the same diameter together is to place the pipe sections end-to-end and insert this interface into a separate, surrounding section of pipe called a Sleeve.

Metal Sleeves ... and hence Sleeve Welds ... are typically associated with temporary repairs on a section of a corroding pipeline. The Sleeve (attached via Sleeve Welding) will prevent leakage should the internal corrosion worsen to cause pipe failure. The Sleeved pipe section should be cut out and replaced with new pipe via Welded Pipe Connections at the next maintenance opportunity.

Fillet Welds are made at the left and right side of the Sleeve. If a clamshell type Sleeve has been used, a horizontal Butt Weld or two seal the clamshell in the closed position.

 

 

OTHER WAYS TO JOIN MATERIALS

Soldering and Brazing

Soldering and Brazing join non-ferrous metals.

Brazing is performed at temperatures below the melting point of the mating surfaces. Otherwise stated, at a temperature range much lower than the temperatures required for the metal Welded Pipe Connections described above. Brazing is used in refrigerant piping systems.

The Soldering process uses lower temperatures than Brazing.One common use of Soldering is in the joining of electrical wires.

 

Bell-and-Spigot Joints

Bell-and-Spigot joints connect cast iron pipe that is used to transport non- pressurized water, specifically waste water.

The "Bell" part of the Bell-and-Spigot joint connection is situated on the length of pipe that gradually expands from a smaller diameter to a larger diameter, hence creating a sideways bell shape.

The "Spigot" is the length of pipe that fits into the smaller diameter of the "Bell."

A water-tight seal is made at the interface between the two pipes by a method that is compatible with the service of the pipe.

The next PTOA Segment is devoted to the Flange Connection and the many phenomena associated with the Flange Connection.

 

TAKE-HOME MESSAGES:  A pipe is sized by referring to its diameter ... yet there are three ways to express a pipe's diameter:

• "Inside/Inner Diameter" refers to the inside diameter of the pipe as measured from the interior surface to the diametrically opposite interior surface of the pipe.
• "Outside/Outer Diameter" refers to the outer diameter measured from the external surface to the diametrically opposite external surface of the pipe.
• "Nominal Diameter" is a casual reference to a pipe's diameter. However, the reference infers that the Inside/Inner Diameter is meant when the Nominal Diameter is 12 inches or less. The Outside/Outer Diameter is meant when the Nominal Diameter is greater than 12 inches.

Pipe Schedules specify the Outside/Outer Diameter and wall thickness to which a pipe is manufactured. Common Pipe Schedules are Schedule 40 and Schedule 80. Increasing Pipe Schedule infers a thicker walled pipe. Thicker walled pipe can endure greater PV Pressure yet will accommodate somewhat less maximum flowrate.

Pipe segments, valves, and pipe fittings are joined together by one of the 3 methods listed below. In order of connection strength, the three "pipe joining methods" are:

• Screwed (aka Threaded) Connections
• Flanged Connections
• Welded Connections

Which pipe joining method is used depends upon the degree of access needed to the process in the long term. Screwed/Threaded Pipe Connections and Flanged Pipe Connections are pipe interfaces that can be used for access into the process line. Welded Pipe Connections do not allow process line access. Equally important in the selection process are the required metal characteristics of the pipe to sustain the desired fluid flowrate as the fluid experiences a range of PV Pressures and PV Temperatures. 

The ISA Symbol for a Welded Connection is simply a process line without markings, perhaps just a notation of pipe size. The ISA Symbol for a Screwed (aka Threaded) Connection is a single vertical hash mark over the process line. The ISA Symbol for a Flanged Connection are two parallel vertical lines drawn over the process line with the area between the lines may or may not be left vacant.

Screwed (Threaded) Connections are simple to install. The limitations of Screwed/Threaded Pipe Connections are the tendency for leakage upon over-pressuring. Ergo, Screwed/Threaded Connections are limited to smaller size pipe diameters in process services at low PV Temperatures and PV Pressure. A noteworthy exception is the use of Screwed/Threaded) Pipe Connections in the commercial drilling processes.  

Welded Pipe Connections make one continuous pipe which ensures no leakage, no interruption in flow, and no flow-related maintenance. The most popular welds the Process Operator will observe in an industrial setting are listed below. Both Socket and Sleeve Weld Connections require Fillet Welds.

• The Butt Weld
• The Socket Weld
• The Sleeve Weld

Brazing and Soldering are used to connect non-ferrous materials. The Bell-and-Spigot joint is used for cast iron metal pipe in service of transporting non-pressurized water.

©2024 PTOA Segment 0249

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

 

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