What Is A Piping System
What Is A Piping System?
A Piping System is an assembly of various components put together with a proper method of joints, functionally to transport fluid from its source to destination. The different components put together are defined as piping components.
A piping system is a network of pipes, fittings and valves intended to perform a specific job i.e. to carry or transfer fluids from one equipment to another. The plumbing network supplying water at your home is a common example of a piping system. Other more rigorous examples include steam piping in a power plant, milk piping in a dairy, paint piping in a paint manufacturing plant, oil piping in a refinery, so and so forth.
Components of a Piping System
The most common components of a piping system are pipes, pipe fittings and valves.
Pipes: Pipes are long cylinders used to carry or transfer fluids. The most common fluids are water, oil, steam, air, milk or finished products like paints, juices. Other uncommon examples include pulp, acids, alkalies, chemicals etc.
Pipe Fittings: Pipe fittings are used to connect lengths of pipes to construct a long piping system; commonly used fittings are flanges, elbows, tees, reducers, expansion bellows etc.
Valves: Valves are used to stop, divert or control fluid flow. Common valve types are gate valves, globe valves, butterfly valves, ball valves, control valves; the selection is based on intended function and application.
In addition, a number of devices like strainers, traps, expansion loops are necessary for keeping the fluid clean and in good condition, and to accommodate expansion/contraction due to temperature variations.
Pipe is a tubular product used to convey a fluid. The pipe sizes are generally identified as nominal bore (NB) or nominal pipe size (NPS). Pipes have fixed outside diameter (O/D) and variable inside diameter based on the thickness selected.
The codes used for pipe selection are
ASME B 36.10- Welded and seamless wrought steel pipes
ASME B 36.9- Stainless steel pipes
The different sizes and thickness which are available are specified in above standards.
Thickness of pipe is generally designated by schedule no. and the corresponding thickness specified.
Types of pipes:
Pipe produced by piercing a billet followed by rolling or drawing or both.
They are used for high pressure applications.
a. Electric fusion welded (EFW): Pipes carrying a single or double longitudinal but weld joined wherein coalescence is produced by manual or automatic electric arc welding in the preformed tube.
b. Electric resistance welded: (ERW): Pipe carrying ongitudinal but weld joined wherein coalescence is produced by heat obtained from resistance of the pipe to flow of electric current in a circuit of which the pipe is a part and by application of pressure.
3. Forged and bored:
Pipes prepared by forging and then boring to the desired thickness.
Various methods of pipe joints:
1. Butt weld pipe joints
2. Socket weld pipe joints
3. Screwed pipe joints
4. Flanged pipe joints
5. Spigot socket pipe joints
Material for Piping Systems
Piping system may consist of a variety of materials including mild steel, stainless steel, aluminum, brass, copper, glass or plastic. Usually, pipe fittings and valves are made of the same material as the pipe. The material selection as well as pipe sizing depends upon parameters like nature of fluid, pressure, temperature and flow rate.
ASTM A53- welded and seamless pipe, black and galvanised.
ASTM A106- Seamless cs pipe for high temperature services.
ASTM A672- Electric fusion welded steel pipe for high pressure service at moderate temperature services.
ASTM A312- Seamless and welded steel pipe for low temperature services.
A409-welded large diameter austenitic steel pipe for corrosive and high temperature services.
ASTM A358- Electric fusion welded austenitic chrome -nickel steel pipe for high temperature services.
Low alloy steel:
ASTM A335- Seamless ferritic alloy steel pipe for high temperature services.
ASTM A691- Carbon and alloy steel pipe, electric fusion welded for high pressure service at high temperature.
Low temperature carbon steel:
ASTM A333- Seamless and welded steel pipe for low temperature services.
ASTM A671- Electric fusion welded steel pipe for atmpospheric and low temperature services(sizes >=16in NB)
A piping system is generally considered to include the complete interconnection of pipes, including in-line components such as pipe fittings and flanges. Pumps, heat exchanges, valves and tanks are also considered part of piping system. Piping systems are the arteries of our industrial processes and the contribution of piping systems are essential in an industrialized society.
Piping systems accounts for a significant portion of the total plant cost, at times as much as one-third of the total investment. Piping systems arranged within a very confined area can be a added challenge to piping and support engineers.
Fig. 1 illustrates the magnitude of piping required in a typical chemical process plant.
The initial design of a piping system is established by the functional requirements of piping a fluid from one point to another. The detailed design is decided by criteria such as type of fluid being transported, allowable pressure drop or energy loss, desired velocity, space limitations, process requirements like free drain or requirement of straight run, stress analysis, temperature of fluid, etc. The supporting of piping systems requires a significant engineering, design, fabrication and erection effort. In some cases, special structures (like structural T or inverted L, cantilevers, U portals, pedestals, etc) must be built solely for the purpose of supporting piping systems.
The material to be used for pipe manufacture must be chosen to suit the operating conditions of the piping system. Guidance of selecting the correct material can be obtained from standard piping codes. As an example, the ASME Code for Pressure Piping contains sections on Power Piping, Industrial Gas and Air Piping, Refinery and Oil Piping, and Refrigeration
Piping Systems. The objective being to ensure that the material used is entirely safe under the operating conditions of pressure, temperature, corrosion, and erosion expected. Some of the materials most commonly used for power plant piping are discussed in the following sections.
- Steel – Steel is the most frequently used material for piping. Forged steel is extensively used for fittings while cast steel is primarily used for special applications. Pipe is manufactured in two main categories – seamless and welded.
- Cast Iron – Cast iron has a high resistance to corrosion and to abrasion and is used for ash handling systems, sewage lines and underground water lines. It is, however, very brittle and is not suitable for most power plant services. It is made in different grades such as gray cast iron, malleable cast iron and ductile cast iron.
- Brass and Copper – Non-ferrous material such as copper and copper alloys are used in power plants in instrumentation and water services where temperature is not a prime factor.
Commercial Pipe Sizes
Commercial pipe is made in standard sizes each having several different wall thicknesses or weights. Up to and including 304.8 mm (12 inch) pipe, the size is expressed as nominal (approximate) inside diameter. Above 304.8 mm, the size is given as the actual outside diameter. All classes of pipe of a given size have the same outside diameter, with the extra thickness
for different weights on the inside. For example, if a pipe was designated as 152.4 mm size this would mean that it has a nominal or approximate inside diameter of 152.4 mm. The outside diameter is 168.28 mm. This is a constant value no matter what the wall thickness is. The actual inside diameter of the pipe will depend upon its wall thickness. For a standard wall
thickness, the actual inside diameter of 152.4 mm pipe is 154.06 mm. For an extra strong wall thickness, the actual inside diameter is 146.34 mm.
There are two systems used to designate the various wall thicknesses of different sizes of pipe. The older method lists pipe as standard (S), extra strong (XS) and double extra strong (XXS). The newer method, which is superseding the older method, uses schedule numbers to designate wall thickness. These numbers are 10, 20, 30, 40, 60, 80, 100, 120, 140 and 160. In most sizes of pipe, schedule 40 corresponds to standard and schedule 80 corresponds to extra strong.
Fig. 2 Dimensions and the mass in kg/m of different sizes of steel pipe with varying wall thicknesses
A pipefitting is used in pipe systems to connect straight pipe sections, adapt to different sizes or shapes and for other purposes, such as regulating (or measuring) fluid flow. Pipe Fittings (especially uncommon types) require money, time, materials and tools to install, and are an important part of piping and plumbing systems. Valves are technically fittings, but are usually discussed separately. The purposes of the fittings, shown in Fig. 3 may be generally stated as follows:
- Elbows – for making angle turns in piping.
- Nipples – for making close connections. They are threaded on both ends with the close nipple threaded for its entire length.
- Couplings – for connecting two pieces of pipe of the same size in a straight line.
- Unions – for providing an easy method for dismantling piping.
- Tees and Crosses – for making branch line connections at 90º.
- Y-bends – for making branch line connections at 45º.
- Return Bends – for reversing direction of a pipe run.
- Plugs and Caps – for closing off open pipe ends or fittings.
- Bushings – for connecting pipes of different sizes. The male end fits into a coupling and the smaller pipe is then screwed into the female end. The smaller connection may be tapped eccentrically to permit free drainage of water.
- Reducers – for reducing pipe size. Has two female connections into which the different sized pipes fit. May also be made with one connection eccentric for free drainage of water.
There are three general methods used to join or connect lengths of pressure piping. These are:
- Screwed Connections.
- Flanged Connections.
- Welded Joints.
Each of these methods has certain advantages and disadvantages and each will be discussed in the following sections.
In this method, threads are cut on each end of the pipe and screwed fittings such as unions, couplings, and elbows are used to join the lengths. This method is generally used for pipe sizes less than 101.6 mm (4 inch) for low and moderate pressures. It has the advantage that the piping can be easily disassembled or assembled. However, the threaded connections are
subject to leakage and the strength of the pipe is reduced when threads are cut in the pipe wall.
This method uses flanges at the pipe ends which are bolted together, face to face, usually with a gasket between the two faces. Flanged connections have the advantage over welded connections of permitting disassembly and are more convenient to assemble and disassemble than the screwed connections. In order to prevent leakage at flanged connections, the flange faces, which butt together, would have to be absolutely flat and smooth. While it is theoretically possible to grind the faces to this condition, it is a time consuming and expensive proposition. Therefore gaskets are usually used between flange faces. Gaskets are made of a comparatively soft material which, when the flanged connection is tightened, will fill in any small depressions in the flange faces and thus prevent leakage.
For more on Flanged Connections, check out:
In this method, the pipe lengths are welded directly to one another and directly to any valves or fittings that may be required. The use of these welded joints for piping has several advantages over the use of screwed connections or flanged connections:
- The possibility of leakage is removed with the elimination of screwed or flanged joints.
- The weight of the piping system is reduced due to the elimination of connecting flanges or fittings.
- The cost of material and the need for maintenance are reduced with the elimination of flanges and fittings.
- The piping looks neater and is easier to insulate with the elimination of bulky flanges and fittings.
- Welded joints give more flexibility to the piping design as the pipes may be joined at practically any angle to each other.
The main disadvantage of using welded joints for piping is the necessity of obtaining a skilled welder whenever a connection is to be made.
Piping must be supported in such a way as to prevent its weight from being carried by the equipment to which it is attached. The supports used must prevent excessive sagging of the pipe and at the same time must allow free movement of the pipe due to expansion or contraction. The supporting arrangement must be designed to carry the weight of the pipe, valves, fittings and insulation plus the weight of the fluid contained within the pipe.
Figure. 4 Various types of piping support
Types of Pipe Support
Rigid supports are used to restrict pipe movement in certain direction(s) without any or limited flexibility in that direction. Main function of a rigid support can be:
Anchor or 3 Dimensional Stop
In this type of support arrangement, pipe is fixed with reference to the supporting structures. Movement in any direction is not allowed. This can be achieved by welding or bolting the support with supporting structure.
Rest or Sliding Support
In this type of support arrangement, pipe is fixed with reference to vertical downward direction. Movement in downward vertical direction, mainly due to the weight of pipe and containing fluid, is not allowed. This support is sometimes also referred as sliding support.
In this type of support arrangement, pipe is fixed with reference of directions other then the direction in which weight of pipe and containing fluid is acting. Limited flexibility can be provided with the provision of guide gap (gap between pipe outer surface and guide plate inner surface).
Both Rest & Guide
In this type of support arrangement, pipe is fixed with reference to vertical downward direction along-with any or all the guide directions.
Spring supports are used to support a load and allow simultaneous movement. Spring supports use helical coil compression springs (to accommodate loads and associated pipe movements due to thermal expansions). The critical component in both the type of supports are Helical Coil Compression springs. They are broadly classified into,
Variables Effort support
Variable effort supports also known as variable hangers or variables are used to support pipe lines subjected to moderate (approximately up to 50mm) vertical thermal movements. Variable effort supports are used to support the weight of pipe work or equipment along with weight of fluids while allowing certain quantum of movement with respect to the structure supporting it. Hot load is the working load of the support in the “Hot” condition i.e. when the pipe has traveled from the cold condition to the hot or working condition. Load Variation (LV) or Percentage variation =[(Hot Load-Cold Load) x 100]/Hot Load or [(Travel x Spring Rate) x 100]/Hot Load. Normally MSS-SP58 specifies max Load Variation ( popularly called LV) as 25%.
Constant effort support
Constant effort supports are used to support pipe lines subjected to large vertical movements typically 150 mm or 250 mm. For pipes which are critical to the performance of the system or so called critical piping where no residual stresses are to be transferred to the pipe it is a common practice to use CES. In a constant effort support the load remains constant when the pipe moves from its cold position to the hot position. Thus irrespective of travel the load remains constant over the complete range of movement. Therefore it is called a constant load hanger. Compared to a variable load hanger where with movement the load varies & the hot load & cold load are two different values governed by the travel & spring constant. A CES unit does not have any spring rate.
In the case of steam piping, it is necessary to constantly drain any condensate from the lines. If this is not done then the condensate will be carried along with the steam and may produce water hammer and possibly rupture pipes or fittings. In addition, the admission of moisture carrying steam to turbines or engines is most undesirable. Various devices are used to remove this condensate and moisture from the lines and these are discussed in the following sections.
Steam separators, sometimes called steam purifiers are devices which, when installed in the steam line, will remove moisture droplets and other suspended impurities from the steam. To do this, the separator either causes the steam to suddenly change its direction of flow or else it imparts a whirling motion to the steam. Both of these cause the moisture and other particles to be thrown out of the steam stream.
The purpose of the steam trap is to discharge the water of condensation from steam lines, separators and other equipment without permitting steam to escape. In addition, most traps are designed to discharge any air present in the lines or equipment. Steam traps should be installed in lines wherever condensate must be drained as rapidly as it accumulates, and wherever condensate must be recovered for heating, for hot water needs, or for return to boilers. They are a “must” for steam piping, separators, and all steam heated or steam operated equipment.
Most piping systems are used to convey substances that are at temperatures much higher than that of the surrounding air. Examples would include the main steam piping and feedwater piping. In order to reduce the amount of heat lost to the surrounding air from the hot substance, the piping is covered with insulation. The insulation not only retains the heat in the hot lines but also prevents the temperature inside the process plant building from becoming uncomfortably high. In addition, insulation of hot pipe lines will prevent injury to personnel due to contact with the bare surfaces of the pipe.
In the case of piping which carries substances at a lower temperature than that of the surrounding air, insulating the piping will prevent sweating of the pipe and consequent dripping and corrosion.
A material suitable for use as an insulation should have the following characteristics.
- High insulating value.
- Long life.
- Vermin proof.
- Non corrosive.
- Ability to retain its shape and insulating value when wet.
- Ease of application and installation.
Some of the more common materials used for piping insulating are discussed in the following sections.
- Diatomaceous Silica – This material is bonded with clay and asbestos and is used for temperatures up to 1030ºC.
- Asbestos – Pipe covering sections are molded from asbestos fibre and are used for temperatures up to 650ºC.
- Calcium Silicate – This insulation is made from silica and lime and is suitable for temperatures up to 650ºC.
- Cellular Glass – This material is glass which has been melted and foamed and then molded into pipe covering forms. It can be used for temperatures up to 430ºC.
- Magnesia (85%) – This material is composed of magnesium carbonate with asbestos fibre. It is available in molded form for pipe covering and also is supplied in powdered form to be mixed with water to form an insulating cement which is used to cover pipe fittings. Magnesia pipe covering is suitable for service up to 315ºC.
- Glass Fibre – This is glass that has been processed into fibres and then formed into pipe covering sections which are suitable for temperatures up to 190ºC.
- Plastic Foams – These are plastics that have been processed into a foam during manufacture and then formed into pipe covering sections. They are available for temperatures as low as -170ºC and as high as 120ºC.
Piping Flexibility Requirement
Piping is used to convey a certain amount of fluid from one equipment to another. It is obvious that the shortest straight path for the pipe seems to be most economical and viable in the first sense. There can be many reasons;
- Shorter the pipe, lesser the capital expenditure required in procurement, welding and erection.
- Shorter the pipe, lesser will be the pressure drop making it more suitable for the proper operation.
- Shorter the pipe, lesser will be the number of supports required to support the pipe.
Still, as a piping engineer, we hardly see pipe routing following shortest straight path. Why ??
The biggest reason for that is that the direct shortest layout generally is not acceptable for absorbing the thermal expansion.
As the pipe temperature changes from the installation / ambient condition to the operating / design condition, it expands or contracts depending upon the difference between installation and operating temperature. In the general term, both expansion and contraction are called thermal expansion.
When a straight pipe connected end to end with equipment’s expands, it has the potential of generating enormous force and stress in the piping system. However, if the pipe routing is flexible enough, the expansion can be absorbed without creating undue force or stress. Let us understand this with the help of an example.
Figure 5 shows what will happen when a straight pipe directly connected from one point to another is subjected to change in temperature. First, consider that only one end is connected and the other end is loose. The loose end will expands an amount equal to ΔL = α L ΔT
ΔL = change in length or thermal expansion (in)
α = linear expansion coefficient (K¯¹)
L = original length of pipe (in)
ΔT = change in temperature (K)
However, since the other end is not loose, this expansion is to be absorbed by the piping. This is equivalent to squeezing the pipe to move the free end back an ΔL distance. This amount of squeezing creates a stress of the magnitude S = E (ΔL/L) and the force required to squeeze this amount is F = A S
E = young’s modulus of elasticity, psi
A = pipe cross section area, in²
F = axial force, lbs
For checking the magnitude of such stress and force, lets take a real life example. Consider a pipe of standard wall thickness with,
Material = ASTM A53
outer diameter (O.D) = 6 in
L = 100 ft = 1200 in
T1 =70 F (Installation), T2 = 270 F (Operating)
α = 6.33 x 10-6 in/in-°F
E = 27.5 x 106 lbf/in2
Then, ΔL = (6.33 x1 0-6 in/in-°F)(1200 in)(270°F-70°F) = 1.52 in
F = AEα (ΔT) = (5.581 in2)(27.5 x 106 lbf/in2)(6.33 x 10-6 in/in-°F)(270°F-70°F) = 194,315 lbf
Now, one can imagine the magnitude of force produced in pipe following shortest straight path. The result will likely be failed anchors, a buckled pipe or both. If the pipe routing is flexible enough, the stresses will remain well below the yield point of the steel. It is clear that the straight line direct layout is not acceptable to most of the piping and flexibility has to be provided.
Pipe’s Natural Flexibility
Providing the proper flexibility is one of the major tasks in the design of piping system. Piping flexibility are provided in many different ways. The simplest method is to take advantage of the pipe’s natural flexibility.
Pipes bend, even under their own weight. The longer the pipe, the easier it is to bend. If a pipe is bent within its elastic limit (no permanent deformation), it will behave like a spring and return to its original shape after the load is removed. If the elbows and anchors on a pipe system are arranged to allow free movement of pipe under effect of thermal expansion, the forces will be much less than a straight run. Figure 6 shows how thermal expansion of horizontal pipe leg is accommodated in the deflected shape of vertical pipe.
The anchor loads and stresses are much less than in the straight pipe case,but there are some restraints for this approach.
- This layout introduces moment (torque) loads on the anchors.
- The pipes also move in one direction, which may not be acceptable due to space constraints.
- Geometry can affect this arrangement. If thermal expansion accommodating leg is shorter, the forces and moments will be higher.
Expansion Loop and Expansion Joint
This pipe’s natural flexibility may or may not be sufficient depending on the individual cases. Additional flexibility can be provided by adding expansion loops or expansion joints. In the straight line example discussed above, the stress can be reduced by loops installed as shown below. The idea is to provide some pipe perpendicular to the direction of expansion. In this way when the pipe expands it bends the loop leg first before transmitting any load to the anchor. The longer the loop leg the lesser the force will be created.
But expansion loop also have some limitations.
- Require more space to accommodate the loop.
- Routing length increases. This results in excess material procurement (pipe and elbows) and more pressure drop.
- Difficult arrangement where free drain requirement is there.
In such cases the better method is to use expansion joint. Expansion joints are more sophisticated than the pipe loops which are just extra lengths of the same piping. For this and other reasons, engineers tend to favor piping loops over expansion joints.
However, expansion joints can be used effectively in many applications when they are properly designed. One of the major requirements in the design of expansion joint system is to install sufficient restraints for maintaining the stability.
Advantages of an “Expansion Joint” versus a “Expansion Loop”
- Space is inadequate for a pipe expansion loop with sufficient flexibility.
- A minimum pressure drop throughout the pipe line is required and the absence of flow turbulence from the elbows and piping is required by process flow conditions.
- The fluid is abrasive and flows at a very high velocity.
- There is no adequate support structure to support the size, shape, and weight of a pipe loop.
- The pipe loop is impractical as in an application of low pressure or large diameter.
- Construction schedule does not allow for the man-hours required to install the pipe loop and the piping loop support structure.
- In most cases it is more economical to use an expansion joint instead of pipe loops.
Minimum Pipe Spacing in Pipe Rack
Usually there is a pipe rack running in the center of the plant which carries all the pipes from various sources to destinations alongwith cable trays, instrumentation ducts etc.
Whenever two pipes are running parallel to each other, piping engineer has to maintain a minimum gap between the two pipes. There can be many reasons for this;
- To prevent clash between pipes during construction and erection.
- Sufficient gap to accommodate sideways thermal movement of pipes (For example, thermal expansion of Leg AB in below Figure).
- Sufficient gap to accommodate pipe supports with guide plates.
The basic principle for deciding the spacing the pipes running parallel to each other is:
Center to Center Spacing = 1/2 O.D of the bigger size pipe flange + Insulation thickness of bigger size pipe (if applicable) + 25 mm + 1/2 O.D of the smaller size pipe + Insulation thickness of smaller size pipe(if applicable).
where O.D = Outside diameter of pipe.
Most of the companies consider minimum 300# for calculation of O.D of bigger size pipe flange.
For simplification, most of the companies have prepared standard charts or tables which shows center to center spacing between different size and ratings of pipe without insulation. while 3D modelling, modeler add insulation thickness to spacing’s specified in these standard tables to get the exact center to center spacing.
- Spacing based on min. clearance of 25 mm between flange of one pipe and outside diameter of adjacent pipe.
- All dimensions shown in millimeters.
- If pipes are insulated, insulation thickness has also to be added.
To arrive at spacing between two pipes of different ratings, refer to calculations for both pipes and use the larger of the two distances. For example: To Determine spacing between 150 NB Pipe of 150# rating and a 100 NB pipe of 600# rating.
- For 150 NB pipe and 100 NB 600# flange Rating spacing = 255 mm.
- For 100 NB pipe and 150 NB 150# flange Rating spacing =230 mm.
- Consider the maximum distances from 2 cases. The spacing between the 2 lines = 255 mm
- Flanges and or valves in adjacent lines must be staggered.
- Lines with tapped orifice flanges shall be given special consideration.
- Where sideways thermal movement occurs must be so spaced that in either the expanded or contracted position adjacent pipes maintain minimum clearance specified.
Source: China Piping System Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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