B31.1 Piping
B31.1 PIPING
Summary
This white paper has been prepared with the purpose of describing the piping functions of a typical Power Plant. In particular, we describe how the spring-type pipe supports fit into this facility, and the activities they carry out.
There are many pipes that connect various equipment in a power plant. These pipes are to be supported properly at regular intervals. There are two roles taken by elements in this procedure. The restraint by definition is a structural element used to constrain or limit the thermal movement of a piping system. Examples, are the rigid strut and the snubber.
On the other hand a support can be defined as structural element of assembly that is required to absorb the piping system weight loads and contain the sustained longitudinal stress within allowable limits. In order to take care of the movement and at the same time to offer proper support, a combination of rod hanger, clamp shoe, variable spring hanger, and constant spring hangers can be provided.
B31.1 Power Piping is the standard for the type of piping material described in this document. The standard is very widely used. However, we find that some engineers know little about spring supports.
This white paper attempts to bring some of this not well known information to the attention of interested readers in the piping industry.
Reason for piping and how various personnel are involved
Piping is the primary conveyance with which fluids such as steam, and other and utility fluids are moved from one point to another point within a Power Plant.
This piping will connect one piece of piping or equipment to another. Pumps will normally be used to boost the flow of liquids from one location to another. The high-pressure nature of steam means that the primary professional involved is an engineer, and they will complete the Power Plant layout.
The primary drawings the process engineer will prepare are material and energy balances which are process flow diagrams. Once the material and energy balances are settled the process team will prepare Piping and Instrument Diagrams (P&IDs). Afterward, P&IDs are issued to the engineers of the other disciplines.
Guiding Documents
ASME B31.1 Power Piping Code provides rules for piping most often found in electric power generating stations. The document is also used in industrial and institutional plants, geothermal heating systems, and central and district heating and cooling systems.
MSS SP-58 is the set of fabrication criteria and guidelines for pipe hanger mechanisms to ensure pipes maintain the combined weight of the piping involved and the fluid that passes through them.
Power Layout
Most modern Power Plants have been laid out and designed by using a computer program. Most commonly 3D modeling software will be employed.
Avoiding structural clashes makes 3 D modeling a crucial tool for piping design.
When the process engineers and managers are deciding piping layout, a major concern is personnel safety. Careful consideration of hazard segregation will result in built-in safety. Equipment needs to be laid out so as to minimize piping runs.
Units are judiciously placed close to their raw material receiving point or finished product dispatch points so as to minimize energy consumption by using gravity flow feed.
Points, where there may be toxic gas releases, are carefully identified, and the arrangement of the piping will be such as to reduce the hazard of a leak or spill.
Role of the piping engineer
Piping engineers carry out the selection of proper materials and lead the way in preparing piping design specifications and approval of piping system components. The piping engineers develop specifications for purchasing the pipes, insulation, painting, and the actual installation. Their work provides major contributions to the entire Plant and creates the piping layout. Further, the engineers coordinate with all the disciplines.
What constitutes a piping system, and why is safety a concern?
A piping arrangement will include the pipes, valves, flanges, strainers, instrumentation, gaskets, hoses, steam traps, vents, and off-line elements
The piping system should be put in carefully and maintained. Think of all those high temperature contents transported throughout the facility and consider the large number of joints that are distributed throughout the Power.
Appropriate Materials
The materials used in the piping system provide safety with adequate strength when used in high-temperature service.
Selection would consider:
- Previous experience
- Corrosion and erosion
- Changes in concentration and content as the fluid moves through a long piping system.
- Whether piping is on the rack or routed below.
Piping material specifications must be developed for the correct category. Normally, for a Power Plant, ASME B31.1 would apply.
Minimum Wall Thickness
It is necessary to get the correct minimum pipe wall thickness, and it is determined in accordance with the piping design codes such as ASME B31.1, which provides with adequate strength when used in high-temperature service.
Stress analysis
When detailed stress analysis is carried out the piping will reveal the load under alternate conditions. These circumstances would include heat up and cooldown, settlement, and hot and cold conditions of equipment such as turbines, compressors, and pumps.
Connecting Points
The weakest points and those most liable for leakage are the connecting joints. Leak prevention by design begins with the relative ranking of the pipe and valve threaded or flanged connections.
The integrity of flanged joints depends upon the gaskets. Flange bolts have a small amount of elasticity in them, and they provide a uniform force when the gaskets remain intact. Flange leaks due to gasket failures normally would not happen unless there were excessive pressures or temperatures.
When piping is subjected to vibrations due to connected equipment or flow, it becomes open to alternating stresses. When using small bore piping, after a high number of fatigue cycles, a joint can eventually fail due to fatigue.
Valves
Control Valves and isolation valves play key roles in balancing flow and in achieving safe design goals. Isolation valves are installed so that they can be used manually or remotely, depending upon the process needs. Fire-rated valves are mandatory for tanks and other equipment which could cause severe and sustained fires.
Check valves are installed when it is necessary that flow can only move in the desired direction.
Instrumentation, valve hand-wheels, levers, and other equipment requiring checking or manual operation should be installed in safely accessible elevations/locations and should not obstruct access clearances. Equipment must have proper means of access and not require the operator to stand on boxes or to prop himself up by standing on adjacent pipes.
Why Bother With Pipe Supports?
If piping is not supported at regular intervals, it will lead to slumped and bending pipes with stress points at welded joints and flanges. Many times this will result in overstressing and flange leaks. Most likely, it will eventually lead to failure or rupture in the line.
Pipe supports carry the weight of the pipe while allowing it to move under the influence of the process taking place within the Power. The pipe support may comprise several elements, including springs, slide bearings, hangers, or supports. The pipe support generally deals with gravitational issues.
Spring Hangers are a particular type of pipe support containing a spring in the assembly. These supports have a lesser stiffness than a restraint. Spring hangers can accommodate the displacement of piping while still providing the necessary amount of reaction force required to support the piping.
Why the Power Plant Needs Pipe Shoes
Metallic Pipe Shoes are one of the more common restraints and are used extensively with insulated pipe as they provide the needed clearance between the insulation for the pipe and the main supporting steel.
Pipe shoes belong to the rigid support systems category. They provide rest and guidance by limiting movement and they support the pipes to keep them from drooping. They protect process systems in several ways:
- First, they are properly spaced to keep the pipe from sagging.
- Secondly, they can stop pipes from rubbing against dissimilar metals thus stopping galvanic corrosion.
- Thirdly they hold the position of the pipe so that its insulation will not be rubbing or tearing on other parts of the structure.
- Fourthly, they allow the pipes to move without bouncing or shaking. It is very beneficial to reduce vibration which can accelerate wear and tear that will eventually turn into ruptured pipes and disaster.
- Fifthly, the shoe separates pipes from the metal structure thus reducing heat loss.
Properly constructed pipe shoes will stop thermal expansion, which causes friction points and damage. Random movement will be restricted. Pipe shoes are commonly made from structural metal shapes, such as channels and I-beams.
One of the desired characteristics of these shoes have been their ease to fit the available material and their simplicity which makes it possible that so many suppliers can make them. They are inexpensive and have simplicity of installation and production.
Pipe shoes elevate the piping and insulate pipes from beams or other surfaces. Variations of the pipe shoes are welded directly to the pipe in a pipe fabrication shop or in the field using qualified welders.
Shoes may be guided by welding steel strips parallel to the sole plate. This constitutes a traverse restraint to limit the movement.
Rod Hanger
Rod hangers support suspended pipe work and restrict vertical and lateral movement. Versions of rod hangers include 2 bolt clamps for supporting bare pipes and 3 bolt clamps for supporting insulated pipes.
Single rods may be dropped from overhead steel to support a single pipe. Note that rods will swing if subject to horizontal forces. If the horizontal displacement is large, considerable forces can be generated in both the horizontal and vertical directions.
Generally a rod hanger will hand straight, but if the pipe it is supporting moves due to thermal growth, the rod will hang at an angle. The expected angle from the vertical is less than 4 degrees (sin 4 degrees = 0.07 and cos 4 degrees = 0.99).
Note that erection is made more accessible by using a turnbuckle having both left- and right-hand threads to connect to two pieces of rods. This simplifies the adjustment of elevation. Simply using a turnbuckle with right-hand threads to connect two pieces of right-hand threaded rods implies that the rods must be cut to the exact length.
Why the Power Plant Needs Spring Hangers
The metal in the piping will expand with a rise in temperature. The expansion will be in accordance with the coefficient of thermal expansion applicable to that metal, and it will take place on all three axes, i.e.,X, Y, and Z
Moreover, the fluids in these pipelines have a tendency to expand with an increase in temperature.
Since the piping in Power Plants has many bends, there will be points in which the pipe will slightly bow up. When this happens, the pipe will not be supported at that location unless the support moves with the pipe. Steel shoes and a fixed rack will not be effective. Thus, there is a need for spring hangers.
Spring Supports
The spring hangers and spring supports will do the balancing of slight vertical displacements in the pipe system. These components work based on pre-set helical coil springs that exert a variable supporting load over the range of movement according to their specified spring characteristics.
Spring Coils Have Spring Rates
In order to demonstrate the method of calculation, we have randomly selected some data. Please do not think that your equipment will have these same data points.
To find the spring rate, divide the average measured force of 7.0 kgf by the compression distance. We obtain the compression from the measured free length and the fully compressed length: 20.9mm – 10.1mm = 10.8mm.
We normally speak of Newtons so 1.0 kgf = 9.8 Newton and then 7.0kgf / 10.8mm = 0.65 kgf/mm = 6.4N/mm. These values are tabulated in the spring catalog.
Variable Spring Hangers
Variable spring supports (or hangers) use helical coil compression springs to accommodate loads and the associated pipe thermal expansion movements.
Their movements for load and travel follow the spring rates discussed we discussed in the paragraph above and do the balancing of slight vertical displacements in the pipe system.
Constant Spring Hangers
A constant spring hanger provides a non-varying continuous effort for pipes and equipment. Keeping the same stress at a particular location is critical when piping is subjected to vertical movement due to thermal expansion.
A Constant System Works Like a Weight Balance
The weight of 1 Liter of sand is about 1.6kg. In this regard, the volume of a 25kg container of sand in litters is 25÷1.6 = 15.6, so the volume of a 25kg container of sand is about 15.6 Liters.
If you have worked with a weight balance, you will remember that when the weights on both sides are the same, you could move either side to a position up or down, and it will remain there. So for our example, we will fill each bucket of the balance with 15.6 liters of sand to have 25kg on both sides.
As long as the weight on either side remains the same, the balance will remain in whatever position you put it. The constant balancing of the two weights provides the same behavior as a constant spring.
Constant Spring Hangers In Bell Crank Arrangement
What we can observe is that the load P1 times its horizontal distance D from the pivot point equals the spring force F times its vertical distance d from the pivot point.
Restraints
It is essential to have a well-designed, and complete piping layout for trouble-free service in the Power Plant. This is done with restraints as well as with pipe supports.
A pipe restraint is a component of the secondary system installed to protect the piping from forces that come from process phenomena or external influences. The restraint system will typically be passive under normal operating conditions and should not be used to carry the dead weight of the system.
About Hydraulic Snubbers
Snubbers are often used in piping systems but the reason for this is often not fully understood. A Snubber is a restraining device which looks similar to a rigid strut but is actually a mechanism which initially allows slow movement as force is applied, but become very resistant to rapid movements or shocks.
There are two kinds of snubbers, mechanical and hydraulic. Mechanical snubbers operate on a purely mechanical basis with parts turning inside. They offer no resistance to movement until an acceleration threshold is reached and then they begin to seize. Hydraulic snubbers are created with a piston and a double chamber reservoir filled with oil. The stroke of the piston causes the oil to move through a small hole in the wall between the two chambers. When the piston moves too rapidly the hole cannot handle the massive amount of flow and so the snubber acts rigidly and resists rapid movement in the pipe.
These rapid movements may be caused by steam and condensate, turbine trips, sudden safety/relief valve discharge, rapid valve closure and earthquakes. The snubber allows free movement during normal operation but restrains vigorously during sharp abnormal conditions.
Why We Need Snubbers
Snubbers are used in piping systems to reduce hydraulic shock. This is a shock which is internal to the pipe and related to the transmission of fluids and is not exterior shock such as would be sustain by wind caused vibration or seismic shock.
Steam travels at a high velocity in pipes and in some cases causes a noise like a hammer hitting against the pipe is caused by the accumulation of condensate trapped in some of the horizontal steam piping. The velocity of the fast moving steam causes ripples which then can build up into a solid mass or a slug which completely fills the pipe. This slug is carried at high speed by the steam and when it hits a turn (usually at an elbow) the force impacts the elbow like a hammer blow. The force can be so great that the pipe breaks loose from its supports or that the elbow is split. It is a serious problem.
A safety/relief valve has no discharge under normal operation
Another steam related shock is cavitation. The way it arises is that steam travels into water or condensate so that a bubble is formed inside the water and because the water is colder it draws the heat out of the bubble causing it to collapse. This implosion happens suddenly and cause a sharp shock to the system piping.
If cold liquids are involved in larger pipes then another kind of shock can be created by sudden valve closure but this is not usually a problem with in plant piping.
Coating
It is important to make a plan for coating at the project implementation stage. You will need to consider the site location and ambient conditions. Most of the time, chemical Powers and refineries are located close to the sea, which means that the atmosphere is saline.
Since the pipe supports will have a long lifetime, it is less expensive to appropriately coat for a long duration than to spend less and get an ineffective coating.
In the GCC, there are many galvanized products, and this is a good adaptation to the hot, salty atmosphere.
Periodic Checking
Pipe supports are carefully specified, designed and built to perform a critical role. The supports accommodate and control a considerable amount of movement of pipes which can be experienced under different operating conditions. If the supports don’t do their jobs, the movement may become unrestrained or locked and considerable damage can occur to people, piping, capital equipment and the efficient working of the plant.
Regular inspection is necessary to evaluate the status of the individual pipe supports and overall support of the piping system. Safety of the system is an important consideration when it comes time for replacement of individual pipe supports or recalibration of the entire support system.
Visual inspection can be done quickly and in a large plant several Service Engineers can work at the same time. They can spread out so as to work in different locations. Regular visual inspection is convenient and vital, with the assessment of the supports being the key indicator of the satisfactory performance of the piping itself.
Visits have found examples of supports in such terrible disrepair that they cannot possibly perform the task for which they were designed. Sometimes damage occurs in installation due to lack of care in handling.
Many of the problems can be seen, even from a distance, and inspection is recommended when the plant is at both hot and cold conditions. Sometimes we see anomalies and the rule is that, “Anything that just doesn’t look right should be inspected.”
Piping Fabrication and Erection
As pointed out earlier, proper engineering and design need to be rolled up into the Plant layout. Considerations will always consider safety and convenience.
Plant layout will be closely followed by detailed piping design and specification preparation. Piping fabrication, erection, and installation need to closely follow design and specification and any variation must be authorized and approved by the engineering team.