In industry, piping is used extensively for the safe transportation and transfer of fluids from place to place. This transfer and movement of fluids is a controlled procedure requiring piping, valves, fittings and methods of protecting the equipment from corrosion, erosion, expansion, contraction and physical movement beyond allowable limits.
The transmission of fluids can be divided into two broad groups:
- The safe movement within the plant, of process materials - Raw Materials, Intermediate Products and Finished Products.
- Provision of the necessary services fluids for the operation and control of plant equipment and its safety; such as - Dry Instrument Air, Utility Air, Cooling Water, Potable Water, Steam Supplies and Inert Gas Supplies… etc.
TRANSMISSION OF FLUIDS
Piping is manufactured in many sizes, materials and thicknesses.
The size of a pipeline normally refers to its Inside Diameter (ID), and this factor depends upon the maximum expected VOLUME of Fluid to be transported.
The material of construction of a pipeline depends upon various factors such as :
- The type of fluid to be carried - Its Temperature, Pressure, Corrosive and Erosive Properties and the life expectancy of the Pipeline … etc.
- The Thickness of pipe material depends upon the maximum expected Pressure of the transported fluid and, again, its corrosive and erosive properties and the life expectancy of the Pipeline … etc.
Some examples of piping materials are: - Cast iron, mild steel, carbon steel, stainless steel, aluminium, copper, plastic, fibre-glass and concrete. Other special substances are used depending upon the nature of the fluids.
An important factor in material selection is, of course, the COST. However, the selection should put safety before cost.
Other factors to be considered in pipeline construction are; - the weight of the pipe with its contents and the fittings, the thermal expansion due to internal and external temperatures and the erosive and corrosive conditions of the environment through which the pipeline is passing - i.e. whether it is above or under the ground or in water ..etc. Factors of this nature also govern the protection of the pipe material - coating types, wrapping, lagging and Corrosion Protection methods (CP). In many cases, a Corrosion Inhibitor is injected into the fluid as it enters the pipeline.
Where a pipeline is carrying very hot or very cold materials, it will be lagged with a suitable material which prevents heat loss or heat gain and help to protect personnel from burns - (a cold burn can be just as painful as a heat burn). Also, liquids which have a high viscosity are usually transported in lagged piping and may also have steam or electrical 'tracing' running inside the lagging to maintain the liquid fluidity.
Also in hot or cold systems, such as steam or cryogenic fluids, the pipeline is constructed with 'Expansion Loops' which allow the length of the pipeline to vary as the temperature varies. The pipe will also be supported by 'rolling' type devices which will allow the expansion and contraction to take place with little or no friction between the support and the pipe - thus preventing external wear.
These aspects of piping are discussed later.
Figure : 1
JOINING OF PIPELINES
Piping may be joined by various methods :
- Butt-Welded Joints. ('A')
- Welded Flange Joints. ('B' & 'C')
- Screwed Flange Joints. ('D')
- Unions. ('E')
(See Figures : 2 & 3)
Other methods of joining pipes are used according to materials of construction and safety requirements.
Supports must carry the weight of the pipeline and its fittings and contents. At the same time they must allow the line freedom of movement during expansion and contraction. The supports must be placed close enough together to keep the stress forces within the permissible limits. Figure : 4, shows two types of support called 'Brackets' or 'Hangers'. Note the rollers which allow the longitudinal movement of the pipe.
TYPICAL EXAMPLES OF PIPE SUPPORTS
PIPELINE ANCHOR POINTS
These are points in a pipe-work system where the pipe is firmly fixed to a support. They protect weaker sections of the system by limiting the movement of the pipeline due to weather conditions - i.e. wind, additional weight of materials in the pipe, and vibration. They can also be used to direct any thermal expansion forces into sections of pipe designed to absorb these forces as in the expansion loops or bends discussed below.
Figure : 5, shows two examples of anchor points used on a pipeline.
EXPANSION LOOPS (or BENDS)
Most materials, when heated or cooled, will expand or contract. The degree of expansion (or contraction) depends on :
- The Temperature Change.
- The Original Size of the Material.
- The Co-efficient of Linear (length), and Cubical (volume), Expansion of the Material.
As mentioned earlier, hot or cold pipeline contents and extreme changes in ambient temperature, will cause the expansion or contraction of the pipeline. In this case, we are mainly concerned with the change in length of the pipeline. (The longer the line, the greater the change in length).
For this reason, expansion loops or bends are incorporated into some pipeline constructions in order to minimize the forces set up by the changes in length.
(See Figure : 6)
Figure : 6
- Static Electricity : The transfer of materials by pipeline can cause static electricity to be generated. This can build up to high voltages which, in turn can discharge causing electric sparks or give a nasty shock to personnel working near the pipe. All piping should therefore be 'earthed' to prevent this electrical build up.
- Another hazard with pipe-work is leakage of material due to damaged gaskets between flanges, leaking valve stems and release of fluids during maintenance operations. Leaks should be reported and dealt with as soon as they are discovered.
- A further possibility, is a pipeline rupture, due to over-pressure, corrosion, heavy vibration or physical damage due to a vehicle accident or other heavy blow to the piping.
Where the possibility of over-pressure is present, for example, a section of pipe between two closed valves or an operational upset, 'Safety Relief Valves' are fitted to deal with the problem. In the case of a liquid-full section of isolated piping, the pressure increase for only a small temperature increase, can be tremendous. Such a situation should never be allowed to arise - the forces involved can be extremely high and very dangerous. In these cases, the piping must be adequately protected by vent / relief systems.
Where a very long pipeline is constructed, across an expanse of desert for example, the constant patrolling of the pipeline for fractures, ruptures or other problems, would not be practical. In general the pipeline conditions are monitored in a control room in a plant which may be at some distance along the pipeline. In these cases, pipelines are often fitted with an automatic isolation system which consists of a series of special valves set at intervals along the length of the pipeline.
These valves are activated by the continuous and prolonged pressure drop which occurs when a line ruptures. Two valves will close to isolate the affected section.
Following is an example of a type of automatic shut down and isolation valve system which would help to minimize fluid escape to atmosphere in the event of a pipeline rupture.
(See Figure : 7 and the following pages describing 'Shafer' automatic isolation systems )
Figure : 7
'SHAFER' AUTOMATIC LINE-BREAK VALVE
'Shafer' Line Break Valves are fitted at intervals along the length of a particularly long pipeline.
The function of these valves, is to close automatically in the event of a 'Line-break' (rupture of the pipeline). Should such an event occur, the Shafer valves will 'sense' the large, sustained pressure drop in the affected section of pipe. Two valves will trip 'closed' and isolate the section involved.
Following is a description of the operation of a 'Shafer' valve - please refer to the diagrams together with the notes.
FIGURE: 8 -NORMAL OPERATING 'OPEN' POSITION
In this diagram, all systems are at normal operating conditions. Small, short-lived pressure fluctuations - up to about 30 Psi, (passage of spheres or compressor trips etc..), will not affect the Shafer valve. The valve control systems will remain equalized with the valve in the 'OPEN' normal operating position.
The parts of the system are:
- G.1. - Power gas cylinder.
- G.2. - Pipeline gas cylinder.
- H.1. - 'Closing' hydraulic oil cylinder.
- H.2. - 'Opening' hydraulic oil cylinder.
- D. - Diaphragm.
- R.O. -Adjustable orifice - The setting of this orifice determines the closing of the Shafer valve - (Psi Pressure drop/minute).
- P.1. and P.2. - Double poppet valves.
- V.1. and V.2. - Hydraulic oil transfer 'Latch' valves.
- V.3. - Pipeline gas to Power gas cylinder 'G.1.'
- V.4. - Pipeline gas to gas cylinder 'G.2.'
- V.5. - Check valve (to maintain maximum pressure in Power gas cylinder).
- C.P. - Shafer valve operating cylinder and piston
- L. -Control arm (lever) - For manual re-opening of the valve (or to check valve operation).
- M. - Manual operated hydraulic pump.
The power storage tank G.1. contains gas at the highest pressure the pipeline reaches. The spring loaded check valve V.5 allows higher pressure into tank G.1. (through shuttle valve V.3.), but will not allow Power Gas pressure to fall when pipeline pressure decreases.
Gas tank G.2 also contains gas at pipeline pressure, but this can change as line pressure fluctuates. The gas line from tank G.2 contains the adjustable orifice RO , and is connected to the pipeline (upstream and downstream sides of the Shafer valve) through shuttle valve V.4., which is centralized.
Each side of orifice - 'RO', a tapping is taken to the top and bottom of Diaphragm 'D'. The diaphragm is spring-loaded to hold it 'DOWN' during normal operation.
The rate of pressure drop and time delay to valve closure is set by the adjustment of the restriction orifice 'RO'.
In conjunction with the diaphragm, a control arm 'L' and double-poppet valves P.1. & P.2. activate the Power gas system from tank G.1 to the top of the two hydraulic tanks H.1. and H.2.
With the Shafer in the fully open or fully closed position the control arm is horizontal and all pressures in the system are stabilized.
Tanks H.1 and H.2. contain hydraulic oil for the activation of the Shafer valve operating piston - 'CP'.
The oil from tank H.1. feeds the 'Closing' (top) side of the valve operator piston, while the oil from tank H.2. feeds the 'Opening' (under) side of the piston.
While the Shafer is fully open (or fully closed), the two hydraulic tanks are open to atmosphere through the two poppet valves P.1 and P.2.
Connected into the hydraulic oil systems, two piston operated latch valves V.1. and V.2. operate to stabilize the hydraulic system when the valve is fully closed or fully open.
In the hydraulic oil system, a hand operated hydraulic pump 'M' is installed for manual operation to open the valve when no gas pressure is available - as after a prolonged shut-down - and has an 'OPEN - NEUTRAL - CLOSE' selector.
FIGURE: 9 -LINE BREAK - SHAFER VALVE CLOSES
This diagram shows the action of the Shafer valve when a 'Line-break' (rupture) occurs in the 'X' section of the pipeline.
The rapid and sustained pressure drop in the pipeline is sensed by the orifice 'RO' by the difference between the pressure in tank G.2 (original pipeline pressure), and the falling pressure of the burst pipeline.
This pressure drop causes diaphragm 'D' to rise. The diaphragm stem contacts the control arm 'L' and pushes it upwards. This action operates poppet valve 'P2' which closes tank H.1. atmospheric vent and opens the power gas line from tank G.1. into tank H.1.
The increased pressure in H.1. forces the hydraulic oil to pass to the top of the operating piston to close the valve. At the same time, the high pressure oil also opens valve V.1. which allows the oil from BENEATH the piston to transfer to tank H.2. through valve V.1. The top of tank H.2. is vented to atmosphere through Poppet valve P.1. to allow for the displacement caused by the rising oil level.
With the Shafer valve now closed, the 'X' section of pipe will continue to depressure due to the line break.
The 'Y' section will contain a good pressure which will move the shuttle valves V.3. and V.4. across due to the D.P. between 'X' & 'Y', closing the impulse line from the damaged 'X' section by valve
FIGURE: 10 - AUTOMATIC RESET
Due to the D.P., valve V.3-A will open allowing the 'Y' section pressure to pass to the check valve 'V.5'. If tank G.1. pressure is lower than the 'Y' section of pipeline, V.5. will open to add pressure to G.1.
Meanwhile, the pressure of the 'X' section of piping will fall to zero due to the line break and/or a de-pressuring operation.
Tank G.2. will also depressurize through the orifice 'RO'. and valve V.4. The DP across the orifice will fall to zero and the diaphragm will return to the bottom of its casing by the action of the spring.
The control arm will return to its original, horizontal position. Poppet valve P.2. will spring back to close off the power gas supply and vent tank H.1. to atmosphere.
At this point, the Shafer valves are closed, the damaged section of pipeline is isolated and the control system of the Shafer is stabilized as in Diagram 1.
Work can now begin on the repair of the pipeline.
FIGURE: 11 – RE-OPEN THE SHAFER VALVE
The Shafer valve will NOT OPEN automatically.
With the line break repaired and the pipeline pressurized, it is now time to re-open the Shafer valve.
(It should NOT be opened before the line pressure is equalized each side of the Shafer. If the Shafer is to be opened with the complete line de-pressured, and no pressure is available in the power gas tank, then the manual pump (M) is used).
The line pressure will replenish the power gas in tank G.1., through the check valve V.5.
Tank G.2. will also come to line pressure. To open the valve, the control arm lever is manually depressed (pushed down) and HELD DOWN until the Shafer is fully open.
Manually pushing and holding down the control arm, operates Poppet valve 'P.1.' which moves down against its spring. This action closes the vent to atmosphere from tank H.2. and opens the power gas from tank G.1. into the top of tank H.2. The oil enters the Shafer operator cylinder 'CP' and pushes the piston up to open the Shafer valve. The oil ABOVE the piston operator is transferred back to tank H.1. through valve V.2. which has been opened by the HP oil.
H.1. is vented to atmosphere through poppet valve P.2. When the Shafer is fully open, the control arm is released and returns to the horizontal position. Poppet valve P.1. moves up to close the power gas supply and open tank H.2. to atmosphere. All pressures will stabilize, valves V.1 and V.2 will close and valves V.3 & V.4 will become centralized. The Shafer valve is returned to the original condition as in Diagram 1. and is ready for the next emergency.
With long gas pipeline systems which also contain liquids such as gas condensate and are passing over (or under), uneven ground, the liquid will tend to collect in the low points of the pipeline. These pockets of liquid will cause 'Gas-Locks' in the pipeline, giving rise to fluctuations in pressure and flow.
In order to prevent these gas-locks, the line is periodically swept by launching a 'Sphere' or 'Pig' which is the same diameter as the ID of the pipe. As the device is launched into the pipeline, the flow and pressure of the following fluids, push the sphere or pig along the line sweeping the liquids ahead of it in the form of 'Slugs'. As the sphere arrives at the other end of the line, the liquid is diverted into a liquid separation vessel while the sphere passes into a receiver from which it can be removed from the pipeline and transported back to the launching station again.
In the case of a pipeline carrying liquid, a similar device is used but is generally referred to as a 'Scraper' or 'Pig'. In this case the purpose of the device is to scrape the inside of the pipeline to keep waxy deposits, scale .. etc from adhering to the pipe walls which, in time, would cause loss of efficiency and possible corrosion of the pipeline inner walls.
Figure: 12 -Shows a typical sphere launching arrangement as used in Operations in the Field. Because the sphere launcher is normally held at the same pressure as the pipeline, the system is fitted with various safety devices which will not allow the launcher to be opened until certain isolation and pressure conditions are satisfied.
The following picture shows a sphere launching system as described on the previous pages. Loading of the barrel with up to 8 spheres can only be carried out when the isolation MOV is closed and the system pressure vented down to below 2" water gauge pressure. Pressure above this level will deactivate electrical circuits to prevent operation of the door mechanism.
When the spheres are loaded, the door is closed and the system re-pressured by using by-passes around the valves, until the launcher and the main pipeline are at equal pressure. The isolation MOV can then be opened.
The launch valve is designed with a cavity into which a sphere will just fit. When the launching of a sphere is required, the launch valve mechanism is activated, the valve rotates through 180° and the sphere drops into the main line where it is picked up by the fluid flow and pushed along the pipeline. The Launch valve is returned to its start position and the next sphere rolls into the cavity ready for the next launch.