PIPES FITTINGS AND VALVES
Excerpt from “Practical Manual of Chemical Plant Equipment”, by Schmidt
PIPING
Pipe used in the chemical industry can be made of steel, steel alloys (like stainless steels), cast
iron, plastic, aluminum, copper and glass. It is not uncommon to find almost all of these
materials used in the same plant. The pipe chosen is usually the most economical of those
materials that will resist the corrosion and temperature of the fluid. Then the appropriate wall
thickness is chosen to withstand the pressures of the process.
Steel Pipe
Steel (carbon steel or black iron) is the most commonly used type. Up to 12 inches size of pipe
refers to the nominal inside diameter of the pipe; over 12 inches, the size is the nominal outside
diameter.
Wall thickness is identified by a schedule number. Eleven schedule numbers have been adopted;
5, 10, 20, 30, 40, 60, 80, 100, 120, 140, and 160. The larger schedule numbers refer to heavier
wall thicknesses.
In pipe up to and including 12 inches in diameter, the standard pipe that is most often used is
schedule 40; schedule 80 is the next most popular. In one-inch pipe of low-carbon steel for
example, of which the outside diameter is 1.315 inches, in schedule 40 the wall thickness is
0.133 inch thick, whereas with schedule 80 the wall is 0.179 inches thick. Schedule 160 is seen
in some high pressure processes, but the other schedule numbers are not popular.
Stainless Steel Pipe
Stainless steel is popular because it has good resistance to corrosion due to the presence of Cr, Ni,
Mo, Mn, etc. Stainless steel is classified by a number which identifies its composition. See
Table I
TABLE I Composition of Popular Stainless Steels
SS Number
% Cr
% Ni
% Mo
% Mn
316
16-18
10-14
2-3
---
304
18-20
8-10
---
2
410
11.5-12.5
---
---
---
A good way to distinguish 300 series steels from 400 series is that the 300 series steels do not
have magnetic properties.
It is difficult to get a tight seal by threading stainless steel pipe, so it is usually welded or
screwed and welded.
PIPES FITTINGS, VALVES AND PUMPS (SCHMIDT)
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Copper Tubing
Soft copper tubing is used extensively in most chemical plants to connect instruments to an air
supply and to carry air signals from one instrument to another. Generally this is purchased as 50
ft rolls of soft copper. The malleability facilitates the bends and turns needed to make instrument
connections in tight spaces.
Glass and Lined Steel Pipe
Glass pipe is resistant to the corrosive effects of almost all chemicals (except HF, for example)
and has the advantage of permitting one to see what is going on in the pipe. It is a good choice
for laboratory drain lines. Smaller sizes can withstand considerable pressure. Glass pipe is
relatively expensive and must always be supported to prevent vibrations that would cause
leakage at its joints. It is also less impact resistant than other pipe materials.
Steel pipe with a lining of glass, Saran [poly(vinylidene chloride)], Kel-F
[poly(chlorotrifluoroethylene)], or Teflon [poly(tetrafluoroethylene)] has all the advantages of
glass pipe except the transparency. The lined steel pipe is much stronger than glass pipe. This
type of pipe is very expensive but provides very good service. Valves and fittings similarly lined
are available.
Plastic Pipe
Plastic pipe is becoming very common.

ABS (acrylonitrile butadiene styrene terpolymer) is used for Drain, Waste, Vent pipe
(DWV) in plumbing applications.

PVC [poly(vinyl chloride)] is used for electrical conduit as well as for fluid flow.

CPVC [chlorinated poly(vinyl chloride)] is used for hot water piping

PP (polypropylene) is used in underground sewers and above ground process piping.
PP is practically inert to all solvents and remains flexible at low temperatures, i.e., low
brittle temperature (low Tg) .
All the plastics listed are thermoplastics and cannot be used with hot process fluids. 100–150°C
is the maximum use temperature for most plastic piping. At temperatures above this, the piping
will sag, melt or distort. Plastic piping must be supported throughout its entire length, for
example, by a steel channel.
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PIPE FITTINGS
Once piping has been selected, the choice of fittings is next in importance. Fittings are used to
change direction of flow, connect different sizes or types of pipe, or connect piping to other
hardware. Learn to recognize the common pipe fittings shown below
90° elbow
45° street elbow
tee branch
Y-branch
cross
Elbows (ells) are usually 90° or 45°. The street elbow has a female fitting on one end (top in
diagram) and a male fitting on the other end. Thus a street elbow threads into another fitting at
its male end and a pipe threads into the street elbow at its female end. A tee-branch fitting
connects three sections of pipe at right angles, while a Y-branch fitting joins three pipes in a
Y-shape. A cross fitting joins 4 sections of pipe.
straight
coupling
reducing
coupling
union
bushings
Couplings are open (female) at both ends. They simply join two sections of threaded pipe. A
straight coupling joins two sections of the same diameter pipe, while a reducing coupling joins
two sections of pipe with different diameters. A union also joins two sections of pipe, with the
difference that a section of pipe joined with unions can be removed from the middle of a long run
of pipe without dismantling other sections. In order to replace a middle section of pipe joined by
couplings, one must start at the end of the run and remove each section in sequence until the
desired section is reached. A union is made of three pieces; the central piece is a threaded nut
that holds the two ends together. Bushings are similar to couplings except that one end is female
and the other is male, whereas with couplings, both ends are female. Bushings (like distillation
glassware adapters) join hardware of different sizes.
This cut away of a union shows its 3 parts. The central nut
that joins the two threaded ends.
union
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Nipples are short sections of threaded (male) pipe useful for making connections. It is difficult
to thread both ends of a short section of pipe, so nipples are usually purchased from
manufacturers.
close nipples
pipe plugs
standard nipple
pipe caps
Pipe plugs are male fittings that
close off the end of a pipe.
Pipe caps are the female
counterpart to pipe plugs.
flanges
Flanges serve a function like that of
unions and couplings. They are used to
join sections of pipe. Like unions, they
can be inserted or removed in the middle
of a long run of pipe. Flanges are most
commonly used large diameter pipe;
(> 6 inches and more often > 12 inches
diameter).
Flanges are threaded and/or welded to the ends of the
pipes to be joined, a flat (donut-shaped) gasket is placed
between the flanges (to create a tight seal), and the two
flanges are bolted together as illustrated in the diagram
above.
bolted
flanges
PIPES FITTINGS, VALVES AND PUMPS (SCHMIDT)
1 = flanged tee,
2 = flanged cross,
3 = flanged 90°
elbow,
4 = flanged 45°
elbow
Answers:
Various fittings can be purchased with flanged ends as show below. For practice, name the
fittings shown below.
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Joining Tubing
Common methods of joining tubing (e.g., copper or steel) include the use of compression fittings,
flare fittings and soldering (sweating)
Compression fittings for joining tubing
Compression fittings use a nut and metal or plastic
compression ring (or collar).
The compression nut is slid over the end of the tubing (a),
followed by the compression ring (b).
The compression nut is then threaded onto the desired
connection, e.g., the nipple in (c) and tightened to make a
sealed joint.
Two sections of tubing are joined in this fashion.
.
cut away
of a
compression
fitting
compression
nut
compression
ring
Flare fittings for
joining tubing
Flare fittings require the use of a flaring tool. A flaring nut
is first slid over the end of the tubing. The tubing is then
clamped in the correct-sized hole in the flaring jig and a
tapered bit is tightened against the open end of the tubing
[see (a)] expanding (flaring) the end. The flaring nut is
tightly threaded onto a fitting creating a sealed joint.
cut away of flare fitting
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Hard copper tubing for water lines are often joined by soldering. The
surfaces to be joined are polished with a fine sand paper that removes
surface oxidation. A thin film of soldering paste (flux) is wiped over the
surface. The parts are assembled and heated with a torch. When hot, a
soft tin-lead alloy (solder) is pressed against the hot joint. The solder
melts and is drawn into the joint (by capillary action). When heat is
removed, the solder quickly hardens.
soldered joint
Plastic tubing (PVC and ABS) are glued to fittings. The glue contains a solvent (e.g., toluene)
that dissolves some of the surface of the thermoplastic. A thin film of the adhesive is applied to
both surfaces to be joined. The pieces are fitted together with a twisting motion that ensures that
the glue covers the entire surface of the joint. It is best to work in a well ventilated area to
minimize inhalation of the odours. The glue sets instantly. Once joined, the adhesion is
permanent; the parts cannot be separated.
Iron/Steel Fittings
Screwed fittings (such as elbows, tees, etc.) are available for steel
pipe in cast iron; 25 lb (‘light’), 125 lb (‘standard’) and 250 lb
(‘heavy’) size. Fittings for welding are available for both butt
welding and socket welding, and come in light, standard, and
double extra strong weights.
These standard fittings are commonly used with schedule 40 pipe;
the extra heavy correspond to schedule 80 pipe. Iron pipe up to 2
inches diameter is usually joined by screwing to threaded fittings.
From two inches up to 12 inches in diameter it may be screwed or
welded, but over 12 inches it is generally welded and/or flanged
(bolted together with flanges).
Stainless Steel Fittings
Stainless steel is very hard and is difficult to thread it and obtain a
tight connection. It is generally screwed and back-welded or
simply welded.
As it is difficult to thread both ends of a short length of pipe,
factory-made short lengths called nipples are purchased in length
up to 6 inches in length.
Copper Fittings
Soft copper tubing used to connect instruments is generally joined by compression or flared
fittings. A flaring tool must be used to shape the tubing end in order to used flared fittings.
These fittings are available in ¼, 3/8, ½, and 1-inch sizes but ¼ inch is most common.
Glass Fittings
Glass fittings can be purchased in the shapes desired, such as tees, reducers, ells, etc. and are
fastened using gaskets of Teflon and metal flanges.
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Plastic Fittings
Both screwed and socket-type fittings may be used with plastic pipes. For some plastics (like
ABS and PVC) a quick-setting adhesive (‘solvent cement’) can be used to glue fittings to pipe.
Lined Pipe Fittings
Lined pipe fittings are available but are expensive. They are usually joined by flanges, with the
lining extending onto the face of the flange.
VALVES
Gate Valves
gate valve
Gate valves are only used for on-off control; either fully open
or fully closed. Gate valves have a gate, disc, or wedge that
can be raised or lowered. The direction of flow through an
open gate valve is straight through (no change of direction) and
so the pressure drop is minimal. This is an excellent valve for
stopping flow but not for throttling it. A partially open gate
valve would have a high velocity of flow at right angles to the
gate. This would cause vibration and erosion of the machined
faces of the disc and prevent the disc from seating properly to
stop the flow completely.
Gate valves may be installed with flanged, screwed, or welded
connections.
Globe Valves
In the globe valve, the direction of flow is changed so that it
runs essentially parallel to the seating surfaces. This reduces
erosion of the seating surfaces, but by changing direction of
flow several times the pressure drop through the valve is
considerable. The globe valve is designed so that if four turns
of the stem are required for full opening to closing of the valve
it will permit about one-quarter of the flow to pass for each
turn of the stem. Gate valves are larger and heavier than others.
Both seat and disc of the globe valve are designed for relatively
easy replacement. The disc can be made of a plug of metal
making contact with a metal seat or the disc may be made of
some composite material such as impregnated rubber.
globe valve
hand wheel
stem
gland nut
gland
packing
bonnet
disc
seat
Globe valves are not suited to handling slurries. Solid
depositions may interfere or prevent valve operation.
Globe valves must be installed so that flow impacts the bottom
of the disc (as shown in the diagram), not the reverse. Their
flow is thus described as unidirectional.
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angle valve
Angle Valves
Angle valves are a variation of the globe valve but the outlet is
at right angles to the inlet. This design is simpler to
manufacture and reduces the weight and size compared to the
globe valve. In addition, the pressure drop is less than the
conventional globe valve.
needle valve
Needle Valves
The needle valve is another variation of the globe valve except that
the end of the stem is tapered to a point instead of ending in a flat
disc. This furnishes precise control and is excellent where a small
stream is desired and a large drop in pressure is acceptable. One
common application of this valve is in sampling, especially where
the sample is being taken from a line under high pressure. They are
also used for fine flow control in rotameters and pressure taps.
This needle valve
controls flow
through a
rotameter.
This needle valve
controls
flow/pressure to a
manometer.
Note the small
handle.
Ball Valves
The conventional ball valve, as the name implies, has a
spherical plug that controls fluid flow. A hole is machined
through the centre of the ball and a mere quarter of a turn of the
handle changes flow from full open to full closed.
As with gate valves, there is very little pressure drop through
ball valves when they are fully open. Their handle is usually
just a short straight section of steel.
¼ -turn
ball valve
There is no place where solids can collect to hinder opening
and as a result ball valves are suitable for handling slurries.
Ball valves generally obtain tight seals and can be used as on-off valves.
If the opening in the ball is asymmetric like the one shown on the right,
the ball valve performs well for throttling service.
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Plugcock Valves
The plugcock valves are similar to ball valves except that the
plug is conical rather than spherical. As with ball valves, there
is very little pressure drop through these valves when they are
fully open.
There is no place where solids can collect to hinder opening
and as a result plugcock valves (like ball valves) are suitable
for handling slurries.
Unlike gate, globe and needle valves, both ball valves and
plugcock valves go from full open to full closed in a mere 90°
turn. Their handle is usually just a short straight section of
steel.
typical 2-port
buret stopcock
3-port buret
stopcock
plug valve
3-port plug valve
(side view)
3-port plug valve
(top view)
The plug valve (like a stop cock in a buret) is unique. It can have several ports (holes). Turning
the valve partial turns can direct flow in any of several directions. This is called ‘multi-port
construction’. Some buret stop cocks operate this way; ¼ turn fills the buret and another ¼ turn
empties the buret. One multi-port plug valve can replace 2, 3 or even 4 straight-through valves.
Butterfly Valves
For large pipes, the butterfly valve is frequently most economical. It is lightweight and compact.
In this type of valve the opening gate rotates (pivots), rather than moving up and down.
butterfly valve
A butterfly valve
mounted between
pipe flanges
Butterfly valves handle
slurries well (without
plugging). Their straightthrough design minimizes
sediment build up and gives
low pressure drop.
They are most often used
for handling large flows of
liquids or gases at low
pressure. Simple butterfly
valves are used in air ducts
of domestic forced air
furnaces.
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Check Valves
Check valves are used only to prevent back flow; they are ‘one-way’ valves. There are several
types of check valves but all limit flow to one direction. The swing check, the lift check and the
ball check are common. For tight seating, check valves require high line pressure. Against a low
pressure, they have a tendency to leak. Check valves are easy to distinguish from other valves.
They have no external handle because they are self-actuating.
swing check
valve
lift check
valve
ball check
valve
Diaphragm Valves
Diaphragm valves are opened and closed by a valve
handle that lifts or depresses a flexible (elastomeric)
membrane (diaphragm). The diaphragm protects the
bonnet (main section, housing the stem of the screw
and packing) from the liquid being controlled. Thus
diaphragm valves are especially suited to pumping of
corrosive fluids. In the Saunders diaphragm, the
bonnet is completely isolated by a continuous
elastomeric diaphragm. The change of direction of
flow is minimal and so the pressure drop in these
valves is also small. They handle slurries well.
high weir
Saunders
diaphragm valve
cut away of swing
check valve
diaphragm valve
flow-through
Saunders
diaphragm valve
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Safety Valves
Any vessel or tank that is not open to atmosphere, or at
least connected to another that is open to atmosphere
should have a safety valve. No other valve should be
placed between the tank and the valve or between the valve
and the atmosphere.
safety relief
valve
spring
Two major safety valves are in common use, the relief
valve and the rupture disc. The former is reusable, the later
must be replaced after each use.


disc
The relief valve is similar to globe valves except
that it has a spring that holds the disc against the
seat. If the pressure in the tank exceeds a
predetermined maximum, the valve will compress
the spring as it opens to release pressure (allowing
gas to escape). As the pressure drops, the spring
will force the valve closed again. For small
changes in the pressure-relief setting, an adjustment
handle is provided.
seat
The rupture disc is generally a thin piece of metal
held between two flanges. The discs are designed
to break (rupture) within a few percent of the rated
pressure. After failure, a new disc must be installed.
The discs can be obtained in different metals,
carbon, or plastic coated materials.
rupture disc
Steam Traps
The steam trap is a type of valve. It holds the steam back and lets condensate pass. They are
commonly used in industrial evaporators.
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WORKBOOK FOR PIPES, FITTINGS AND VALVES by Schmidt
STEP 1
1. State seven different materials of which industrial pipes are constructed.
2. State the four factors that determine the selection of the type of pipe to be used.
3. Of the types stated in 1, which is most commonly used and state two other names for this
type of pipe.
4. The wall thickness of pipe is identified by a ‘schedule’ number. State the schedule
numbers that are available.
5. Of the schedule numbers in 5, which three are used most commonly?
6. In older terms, the wall thickness was given as ‘standard’, ‘extra strong’, and ‘double extra
strong’.
Standard pipe corresponds to schedule …………… ,
extra strong to schedule ………….., and
double extra strong to schedule ………………… .
7. Why is stainless steel pipe popular in chemical industry?
PIPES FITTINGS, VALVES AND PUMPS (SCHMIDT)
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8. State three types of stainless steel that are commonly used and list the metals of
composition other than Fe.
a) ……………………………………………………………………………………………………
b) ……………………………………………………………………………………………………
c) ……………………………………………………………………………………………………
9. What is the most common method of joining stainless steel pipe?
10. Some industries make extensive use of Cu tubing, while others will use relatively little.
However, there is on use of copper which is common to many processes. State this use.
11. Glass pipe is very resistant to corrosion and has the advantage of visible flow. State two
disadvantages of this pipe.
12. State four different linings that are available in lined-steel pipe.
a) ……………………….
b) …………………..
c) ……………….. d) …………..……
13. Plastic pipe is being used more commonly in industry
a) State its advantages
……………………………………………………………………………………………………
b) What are some disadvantages of plastic pipe?
…………………………………………………………………………………………………..
c) List the chemical names of 3 types of plastic pipe. ……………………………………….
…………………………………………., ……………………………………………………..
14. What are three weights of fittings that are available for iron pipe?
15. State three ways of joining iron and stainless steel pipe.
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16. What are the two common methods used for joining soft copper tubing? (Note: These
methods work well on soft copper tubing but hard copper has soldered fittings).
17. GATE VALVE
a) The pressure drop across the fully open valve is ………………………………………
b) The type of service of a gate valve is ……………………………………………………
c) Explain why this valve should be used either fully open or fully closed.
……………………………………………………………………………………………………
……………………………………………………………………………………………………
18. GLOBE VALVE
a) Direction of flow through the valve is …………………………………………………….
b) The pressure drop across the fully open valve is ………………………………………
c) The type of service of a globe valve is ……………………………………………………
19. NEEDLE VALVE
a) What is the difference between the needle valve and the globe valve?
……………………………………………………………………………………………………
b) List two applications of needle valves that a technologist might encounter
……………………………………………………………………………………………………
20. PLUGCOCK OR BALL VALVE
a) The pressure drop across the fully open valve is ………………………………………
b) The type of service of a ball valve is ……………………………………………………
c) Explain what is meant by ‘multi-port construction’ re: plug valves. ………………….
……………………………………………………………………………………………………
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21. BUTTERFLY VALVE
a) What is the difference between the application of a butterfly valve and a globe valve?
…………………………………………………………………………………………………
b) Can butterfly valves handle slurries without jamming? ……………………
22. CHECK VALVE
a) List the 3 main kinds of check valves …………………., …………………… , ……………..
b) State the single purpose of a check valve?
…………………………………………………………………………………………
23. DIAPHRAGM VALVE
a) The pressure drop across the fully open valve is ………………………………………
b) The type of service of a diaphragm valve is ……………………………………………
c) What is the unique advantage of the diaphragm valve compared to other valves?
…………………………………………………………………………………………
24. SAFETY VALVES
a) What is the function or purpose of a safety valve?
……………………………………………………………………………………
b) What are two major types of safety valves?
……………………………………………………………………………………
c) What are the advantages of one type over the other?
……………………………………………………………………………………
25. STEAM TRAPS
a) What is the function of a steam trap?
……………………………………………………………………………………
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