Equipment I

Piping and Auxiliaries

Piping and Auxiliaries Introduction

In this module you will learn about piping materials and pipe fittings. We will also examine the effects of corrosion on piping and methods used to protect piping.

The purpose of piping is to move fluids throughout a process facility. Because piping is so widespread in chemical manufacturing facilities, it is important you understand piping, its auxiliary equipment, and how process fluids are transported in a plant.

Piping is a hollow cylinder typically constructed from metal or plastic.

Piping delivers process fluids including air, gases, liquids, slurries, and even finished product throughout a unit.

Chemical plants often use pipes constructed of many types of metal alloys. Alloys are a mixture of at least one metal and one other chemical element. A wide variety of alloy pipes are used in chemical facilities.

Plastic is also used as a piping construction material. Plastic, glass, or ceramic may be used as a lining material for metal piping.

The piping used in chemical manufacturing applications is chosen based on many considerations, including: the pipes compatibility with the process material, the temperature of the process material, thermal expansion, and the pressure exerted on the piping. The size of the pipe is determined by the desired flow rate and fluid velocity.

Failure to use the correct piping material, thickness, size, or gasket material can result in leaks and potential injuries. Piping installation and testing are also critical. Improper bolt torquing and failure to properly pressure test the system after installation can result in leaks in new installations.

As a Chemical Operator, it is not your responsibility to select piping materials, however your knowledge of the piping materials used in your area can help keep everyone safe. Operators play a key role in identifying piping problems, isolating leaks, and notifying the right personnel to assess the situation and repair the problem.

By knowing how and where to access P&IDs and design specifications for piping materials, you can help ensure the right materials are being used during process repairs or modifications.


Corrosion Basics

Managing corrosion is an important part of keeping a process safe. Corrosion is the oxidation of a material due to electrochemical reactions.

For example, rust is the corrosion product of iron as it reacts with oxygen and forms an iron oxide. Oxygen present in water can also lead to corrosion of iron. Piping can corrode on the inside or the outside surfaces. Although some stainless steel and nickel-alloy pipes are more resistant to corrosion, all metal will corrode depending on chemical exposure, temperature or other service conditions.

Most metal pipes rely on their ability to form a thin surface oxide layer that provides protection from further corrosion. You will learn more about specific types of corrosion later in this module.

Next, we will examine commonly used piping materials in more detail.


Piping Materials - Metals

Carbon steel pipes are the most commonly used piping materials. Carbon steel pipe is popular because it is less expensive than some other options, is durable, and is workable due to the ability to weld the steel. However, carbon steel is not compatible with all process fluids. The elements in carbon steel forms a less adherent oxide layer than the elements used in some other alloys.

Carbon Steel piping will often be identified as CS on P&IDs. Individual companies will have their own piping material designations that you will learn on the job.

Stainless steel is a higher grade of steel, known for its resistance to corrosion when exposed to the atmosphere and to some chemicals. The corrosion resistance for stainless steel comes from the addition of chromium to the steel. The chromium forms a chromium oxide which is very resistant to corrosion.

Stainless steel alloys like 304 or 316 contain higher levels of chromium and other alloying elements like nickel and molybdenum.

Other alloy pipes, such as nickel alloys, or exotic materials such as titanium or zirconium, may be used in specific chemical manufacturing applications where other piping might fail. Like other alloys, these pipes depend on the formation of a suitable metal oxide for corrosion resistance.

While carbon steel and stainless steel pipes are iron-based alloys, the term alloy is used commonly in the chemical industry to refer to nickel-based alloy piping. Most nickel alloy piping relies on additions of chromium and molybdenum for corrosion resistance. Nickel-based alloy pipes are used to meet the specific demands of some chemical processes. Although the characteristics vary depending on the type of alloy pipe, they can be stronger, more resistant to corrosion, or able to withstand higher temperature ranges than other piping options.

Designations for nickel-based alloy piping include a variety of brand names. P&IDs will show the type of alloy being used and the designations will vary from company to company.

Galvanized pipes are used in some chemical processes. Galvanization is a process of coating a pipe with a thin layer of zinc to help prevent corrosion. Galvanized pipes are typically iron, carbon steel, or aluminum.

Galvanized piping material designations on P&IDs may use several prefixs, depending on the company.

Aluminum pipes can also be used in chemical processes.

Designations for aluminum piping on P&IDs may include these prefixes.


Piping Materials - Plastic & Lined

Pipes can also be constructed of non-metallic materials. Plastic piping is resistant to many corrosive process fluids, but the service needs to be chosen wisely.  Plastic piping is generally not used for high temperature or high pressure applications.

Pipes can also be constructed of more than one material. Lined pipes typically are steel pipes combined with a plastic, glass, or ceramic lining. Lined piping may be used with some corrosive process fluids. The more commonly used liner material is a plastic made of polytetrafluoroethylene, also known as PTFE.  PTFE lined pipe is sometimes used with fluids that have high solids content and high potential for plugging.  The slick PTFE surface prevents solids from accumulating on the pipe surface.

It is important for operators to be able to identify piping materials of construction from the designation on the P&ID. This table contains some of the most common types of piping construction materials. Consult with someone if you dont understand the designation on a P&ID.

Carbon steel pipes are commonly used for steam, water, and air utility service applications. They are used also for process feeds and gases.  Stainless steel pipes can be used for higher and lower temperature process materials than carbon steel. They are used also for acidic and basic process materials.  Nickel-based alloy pipes may be used in high temperature applications like boiler house lines or highly corrosive process materials.

Galvanized steel or aluminum piping may be used for solid materials handling such as powders and pellets.Plastic pipes are used for low pressure applications and corrosive process materials.Lined pipes can be used for acids, other corrosives, or other hazardous or high fouling materials.


Corrosion Types Part 1 of 2

Next, we will examine corrosion and how each type can result in specific damage to piping.  Temperature can also be a major factor in piping corrosion. Typically, the higher the temperature, the higher the corrosion rate. A good rule of thumb is that for every 20°F increase in temperature, the corrosion rate doubles. Interior pipe corrosion can also be influenced by the interaction of the pipe with the chemical process materials it is transporting. Fluid velocity, pressure, suspended solids, microbes, and the presence of oxygen can influence corrosion.

There are several types of corrosion, which can result in specific damage to piping. Corrosion may take the form of uniform, microbial, galvanic, pitting, erosion, crevice, selective leaching, or stress corrosion cracking.

Uniform corrosion occurs across the entire surface of a pipe due to a chemical or electrochemical reaction with a fluid. This is one of the more easily managed forms of corrosion because it allows for planning of equipment life, inspections, and replacements.

This is a corrosion coupon, which is a test piece of metal that has been inserted into the process to check for corrosion. The exposed surface of this coupon has been uniformly corroded.

Microorganisms present in process fluids may also influence pipe corrosion. Typically such process fluids are waters, hydrotest fluids, sewers, or stagnant waters contaminanted with organic matter. The bacteria come in many varieties that live in aerated or non-aerated environments, and need sulfates to eat, or live off of organic acids.

Here you can see the results of microbial corrosion on a pump impeller.

Galvanic corrosion occurs when two different metals are in contact with an electrically conductive fluid. When the electrical current is flowing, material from the less corrosion resistant metal or alloy is oxidized and corroded more actively compared to the more corrosion resistant, or noble, metal.

This galvanic scale shows the relative corrosion activity of different metals if they are touching and in contact with an electrically conductive fluid. This scale is based on galvanic reactions in seawater, a highly electrically conductive fluid. Galvanic corrosion can cause the more active metal to significantly corrode, while the less active metal is not significantly corroded. Note that if the fluid in contact with both metals is not conductive, galvanic corrosion will not occur.

This image shows galvanic corrosion occurring on a metal pipe in contact with an elbow pipe fitting of a different metal.

Pitting corrosion results in holes or pits in metal pipes. Pitting occurs when there is a localized break in the protective metal oxide. The oxide is unable to reform and corrosion creates a pit that can undercut the protective oxide adjacent to the pit. Typically chlorides have this effect on the 304 or 316 stainless steels at ambient temperatures. Generally, once a deep pit is formed, it can never be cleaned out or reform a protective oxide layer.

Although stainless steel can be quite corrosion resistant, if carbon steel or iron pieces are embedded into the stainless steel surface, this will create a localized area where the stainless steel surface can not reform the oxide layer.  If used with corrosive process fluids, the carbon steel will corrode away and the stainless steel surface will rapidly develop pitting corrosion since it is unable to get oxygen.

This image shows a pitted stainless steel surface.


Corrosion Types Part 2 of 2

Erosion corrosion results from the flow of high-velocity corrosive process materials, or entrained solids, that strip off the protective oxide layer. The metal is unable to reform the protective oxide layer against the erosive fluid. The forces of erosion and corrosion can result in significant grooving, channeling, or pitting.

This erosion in a copper pipe occurred from high velocity river water.

Crevice corrosion occurs in shielded areas of a pipe, such as at a welded socket joint, a threaded pipe joint, or the interface of a gasket and a flange, or a bolt washer and a flange. The shielded area creates a stagnant flow area that initiates and accelerates corrosion under the crevice. Usually, this volume under the crevice becomes an electrically attractive location for chloride or sulfate ions which break down the protective surface oxide. Oxygen needed to reform the protective oxide layer is unable to diffuse into the crevice and corrosion proceeds.

Selective leaching, or dealloying, corrosion results from the selective removal of one metal component from an alloy. This weakens the piping. A common example is the selective leaching of zinc from a brass alloy and the redeposition of pure copper in the area of the attack. Another example is the selective leaching of a cast iron pipe which removes the iron but leaves the graphite flakes.

This image shows dealloying of a brass impellor with the copper plating out in the pits but the zinc from the brass has been leached away.

Stress corrosion cracking (SCC) creates spider-web like cracking in susceptible metals. It is caused by tensile stress on the pipe, in addition to corrosive process fluids, and is influenced by temperatures. For instance, 304 or 316 stainless steels are susceptible to chloride and to caustic stress corrosion cracking. The usual rule of thumb is that chloride SCC in 300 series stainless steels starts to occur at 140°F. However, large concentrations of chlorides can lower the start temperature to 50°F.

Operators are the first line of defense against corrosion and leaks. You should conduct visual inspections in your area of the plant for signs of corrosion. Mechanical integrity programs are also in place which include periodic thickness checks at specified locations to help provide early indication of corrosion. Piping systems may be designed with an allowance for corrosion based on the life expectancy of the piping system.

Corrosion is most common at welds and other pipe joints, downstream of pumps and valves, and at pipe corners and other turns.

Insulation soaked with water or other fluids usually contributes to corrosion problems. Wet, hot insulation over carbon steel produces large amounts of exterior surface rust and thin walls. Wet, hot insulation on stainless steel is a major factor in chloride stress corrosion cracking. Wet insulation needs to be removed and replaced in a timely manner.  Always report any corrosion problems you find.

Allowing process material to sit immobilized in process lines can contribute to corrosion or other safety problems, like over-pressurization. Overpressurization can occur from solar heating of the fluid inside the pipe or from heat tracing on the pipe. As the fluid heats, it wants to expand in volume but is restrained by the pipe so that the pressure builds up rapidly. You should be aware of the risk to the piping systems in your process before isolating process materials in a process line.

Piping specifications are found on the piping and instrumentation diagram. The P&ID also typically shows the types of piping, the chemicals used, and the line size. The P&ID can help identify basic information needed for discussions with maintenance for process issues you may observe in your area.


Pipe Fittings 1

Pipe fittings are piping components used to connect pieces of pipe. Pipe fittings are used in the manufacturing industry to connect different sizes of pipe, connect straight and curved pipe, and connect equipment to pipes for processing.

Here you see pipe fittings commonly used in the chemical industry. Fittings shown here include a: bushing, union, coupling, bell reducer, elbow, nipple, tee, wye, cap, and plug. Lets take a look at each fitting and how it is used in the chemical industry.

Bushings are used to join pipes of different diameters. The threading inside of the bushing is referred to as a female connection. In contrast, the threading on the outside is referred to as a male connection. The inside of the bushing fits a smaller diameter pipe than the outside of the bushing.

A union is used to join piping and is designed for quick and straightforward disconnections. A pipe union, which usually contains female threads, allows two pieces of pipe, with male threads, to be joined. Quick disconnections are important when pieces of equipment or sections of pipe need to be removed or repaired for service.

This fitting is a coupling, which is used to connect two pieces of pipe together. Couplings can be used when a flange or union are not practical.

The bell reducer is designed with one end larger than the other, which is a transition to change the piping diameter. This fitting is used to sustain a certain amount of pressure over a long distance when several branch lines will be draining the pressure. Most pipe fittings are not shown on P&IDs, however bell reducers are represented on P&IDs with this symbol.


Pipe Fittings 2

Elbow pipe fittings are used to change the direction of the pipe run. They can have a short or long radius depending on the need of the process and are normally designed in 45 or 90 degree angles.

This fitting is a nipple and can be described as a short section of pipe with two male threaded ends used to connect fluid components. A nipple is typically no longer than six inches in length. A close nipple is the shortest nipple in a given pipe size.

This fitting is a called a T due to its shape. The T fitting can be used to join two flows of fluid together or split one flow into two.

A wye fitting joins two pieces of pipe which, when connected, will create a union with a 45-degree angle.

A cap covers the end of a pipe to terminate a process flow. A cap normally has female threads.

A plug also terminates a run of pipe but has male threads. Plugs and caps are represented on P&IDs with this symbol.


Piping Connections

Pipes and pipe fittings are commonly connected three different ways: threaded connections, welded connections and flanged connections. The connection type is selected based on the type of pipe, construction material, and process fluid.

Threaded connections are formed using threaded pipes and pipe fittings. Threaded connections can be used with a variety of piping materials to form an effective seal for some processes.

Welded connections are created when two pieces of pipe, with or without fittings, are welded together to form a new piece of piping. Welded connections can be used when two pieces of pipe are the same diameter size or different sizes. When the pipes are the same size, the weld is called a butt weld. Another type is a socket weld where the male end is inserted into a fitting and welded. Welded connections are often desirable due to the strength and security of a welded piece of pipe.

Flanged connections are plate-like fittings connected to a pipe to create a bolted connection. A flange can be as simple as two pieces of pipe connected with bolts, flanges, and a gasket. Or, these connections can have more complicated designs to meet specific needs. Flanged connections are normally used in situations where access may be needed at a later time. Or flanges may be used if the process conditions do not allow for welded connections. Additionally, flanged connections can be added when a mechanical break in a process line is needed.


Gaskets

Flanged connections use gaskets to form a seal to keep the connection from leaking. The gasket must be evenly compressed around the flange so that it will not leak. During installation, the bolts around the flange are torqued evenly.

The general gasket types found in chemical processing facilities include sheet gaskets, metal ring-joint gaskets, spiral wound gaskets, and jacketed gaskets.

Sheet gaskets are the simplest type of gasket. Sheet gaskets are cut from a sheet of material such as paper, rubber, silicone, metal, graphite, or plastic. Sheet gaskets sometimes cannot meet high temperature and high pressure demands found in chemical manufacturing processes.

Metal ring-joint gaskets may be used in high pressure processes. Ring gaskets are constructed typically of a solid metal that is softer than the flange material. The piping flange is designed with a groove to hold the ring gasket. The gasket deforms slightly to create a tight seal.

Spiral wound gaskets are constructed of metal and metal winding strip wound around layers of plastic, graphite, or other filler material. They are used typically with raised-face flanges. Processes requiring higher temperatures and pressures may require spiral wound gaskets rather than sheet gaskets.

Jacketed gaskets combine a gasket of graphite, plastic, or other material covered with a jacket of metal on one or both sides of the gasket. Jacketing the gasket provides improved resistance to temperature, pressure, and corrosion. Double jacketed gaskets are used often in heat exchangers.

Gaskets are selected based on the pressure, temperature, and chemical properties of the process fluid. The type of gasket chosen for use in a process depends on the pressure, temperature, and chemical characteristics of the process fluid, as well as the level of protection needed to ensure safety for employees and equipment.

Gasket failure within a chemical process facility is costly and can be dangerous. Chemical plants regularly perform preventative gasket replacement when process units or process equipment are down for repairs. Most  gaskets are deformed from bolting pressure between the flanges and should be replaced when the flange is opened.

When inspecting piping flanges, you should watch for and avoid sprays, drips, and puddles. You also may observe folded or misaligned gasket material when inspecting a flange. You should report improper gasket installations and leaks so they can be fixed.

The main reason for gasket failure is uneven or insufficient load on the gasket. Other reasons for gasket failure include using the wrong size or type of gasket, the gasket material becoming brittle over time, or erosion. It is very important that a gasket is replaced with the proper size and type of gasket. Misalignment, folded gasket material, or failure to torque the flange properly during installation could cause a process leak resulting in a discharge or injury. Also, after replacement, care should always be taken when repressurizing the line to ensure employees are not in the line of fire for a potential leak.


Asbestos

In the past, gaskets containing asbestos material were often used because of its effectiveness in certain applications and low cost. While most asbestos gaskets have been replaced with other materials, some may  still be found in high pressure or high temperature systems.

Asbestos may also be present in pipe insulation, tank insulation, or insulation sprayed on surface materials. The mere presence of asbestos is not a hazard if undisturbed. However, if asbestos insulation is disturbed then it should be reported for maintenance or removal by the appropriate trained personnel.

Asbestos is an inhalation hazard, and if inhaled can result in lung diseases such as lung cancer, mesothelioma, and asbestosis. As a result, working around asbestos requires specific training, personal protective equipment, and pre-work set up. Personal protective equipment for asbestos exposure would include an air purified respirator with a HEPA filter or an air line respirator.

As a chemical operator, you need to be aware of asbestos hazards in your work area. You should recognize asbestos hazard signs like this example. Your process unit may also have other hazard signs that you will learn about in your job specific training. You should always ask if you are unsure about any asbestos hazards in your area.


Other Piping Equipment

Other common piping equipment include blinds, blanks, and flow orifices.

Process blinds and blanks are solid discs used to stop the flow of liquid through a process line.

Blind flanges are used to terminate a process flow at the end of a pipe.

During shutdowns, blanks are installed between flanges to isolate a run of pipe or a piece of equipment so maintenance can be performed. During startups, chemical operators will walk down a run of pipe to ensure all blanks have been removed. This is one of the safeguards used to ensure a safe startup of the process unit. As a chemical operator working in and around piping and auxiliary equipment, installing and removing blinds and blanks will become a common practice when starting up or shutting down a unit.

A blank is thin (1/8th of an inch). Typically, blanks can be identified by the yellow, bent end protruding from the flanges.

Flow orifices are also inserted between pipe flanges. Flow orifices are sometimes used in piping systems to restrict the system flow and pressure. It is important operators understand the piping components used in their process areas. Flow orifices typically are stamped to indicate the size and type of the orifice. They also have a hole drilled through the end.


Pipe Movement

In a process facility, moving fluids and temperature changes can cause the pipes to move.

This pipe movement is controlled through the use of expansion joints, expansion loops, pipe hangers, and pipe supports.

An expansion joint can be made of metal, rubber, or fabric depending on the process fluid, and allows for both the expansion and movement of piping.

Expansion loops also control pipe movement by allowing for expansion and contraction during temperature changes. Piping will expand or contract as the ambient temperature outside the piping changes and when the temperature of the fluid inside the piping fluctuates.

Pipe supports are structural devices designed to suspend piping in overhead locations and control the movement of pipe. The three types of piping supports include pipe shoes, pipe hangers, and pipe clamps.

Pipe shoes are attached to the bottom of the pipe. The shoe prevents the pipe from rubbing on another piece of structure due to pipe movement.

Pipe hangers are structural devices designed to support pipes from the ceiling or separate pipes from other pipes.

Pipe clamps help support vertical pieces of pipe by clamping onto the pipe and resting on the floor or another support.


Pipe Insulation & Tracing

Pipe insulation is material wrapped around piping to prevent process fluid heat from escaping to the atmosphere.

Pipe insulation is also used to prevent process fluid from freezing if the temperature outside the pipe is extremely cold. Insulation saves energy and improves process production. Insulation around hot pipes also protects personnel from accidental burns.

A trace is wire or tubing, utilizing heated elements, that wraps around a piece of pipe to keep the process material from freezing. Steam and electrical tracing are commonly used.

An electric heat trace is a coil of heated wire. An example would be electrical heat tracing on fire sprinkler lines in cold climates.

Steam tracing is a small diamete