Before explaining what pig traps are, it is appropriate to explain what Pipeline pigging is. It is only after this that one can understand what pig traps are and what they are used for.
Pipeline pigging is a technique used in cleaning and inspection of the inside of pipelines using a special device referred to as pig (pipeline inspection gauge). The PIG moves freely inside the pipeline; driven by the pressure of the product in the pipeline. The first generation of pigs were made from straw wrapped in wire and they made a squealing noise while moving through the pipe, hence the name PIG was adopted for them. The term "pipeline inspection gauge" was later adopted. More recently, “smart” pigs are been equipped with on-board computers for higher accuracy and efficiency. PIGs come in different shapes and sizes and with different fittings depending on what they are being used for.
It is to be noted however, that PIGs cannot be used in pipelines that have butterfly valves.
Having shed some light on what a PIG is, we can now go back to the original topic of explaining what a PIG trap is. PIG traps are special installations used for inserting (launching) PIGs into the pipelines and taking them out (receiving) after successful run without interrupting the course of fluid flow in the pipeline. They are located at each end of a pipeline an on very long pipelines, they are located intermediary. They are designed with a closure (door) for access, an oversize barrel, a system of valves for draining liquids and venting out gas, a pig signaller, a reducer and a neck pipe for connecting the trap to the pipeline. Figure a and b shows a typical illustration of pig traps (launchers and receivers)
The choice of pig traps depends on the type of pig to be used and pipeline design conditions.
Because Pig traps are manufactured, or sold by a company with an exclusive patent or trademark; users have to draw up their own specification and give to the manufacturers to produce. However, a very important factor of in the selection of a pig trap is the opening / closure safety. All closures must have a built-in safety lock which prevents them being opened while the trap is pressurized.
Pigging of pipelines is very necessary and must be done from time to time. However, if done wrongly, pigging could lead to pipeline damage and hazards occurring. All the processes -from preparation stage (including setting of valves, draining of pipes etc) to the tracking stage must be carried out with utmost caution to avoid unwanted incidents occurring. The following are basic procedures that are involved in operating PIG traps.
Maintenance of Pig traps should be carried out according to the manufactures instruction. The maintenance should be carried out regularly and should include checking the conditions of the main body of the trap, the equipment such as the pressure gauges, pig signaller, bleed lock etc. to ensure they are working and to carry out repairs or replacements of any faulty parts. Other general maintenance involves simple greasing of valves, checking of pressure gauges and ironically cleaning of the internal surfaces of the trap.
2.0 BLOCK VALVE
Basically, valves are pipe fittings and in pipelines, they are regarded as Emergency Flow restricting devices (EFRD) or sectionalizing valves on gas and liquids in pipelines. They are used to regulate, interrupt or divert the fluid flow by opening, closing or partially obstructing various passageways. They are made of metallic elements such as aluminium and copper as well as metallic alloys such as brass, bronze, steel, cast iron, ductile iron, and stainless steel; or from variety of plastic materials such as Acetyl polymers, Polyvinyl chloride (PVC), Chlorinated polyvinyl chloride (CPC), Polytetrafluoroethylene (PTFE), Polypropylene (PP), Polyvinylidene fluoride (PVDF) and both categories vary in terms of valve size, pressure rating, number of ports, and flow. Connection types consist of bolt flanges, clamp flanges, union connections, tube fittings, butt welds, socket welds, and internal or external threads.
The different types of valves commonly used in mainline transmission include Ball valve, which is good for on/off control, Butterfly valve particularly for large pipes, Choke valve usually placed around or inside another cylinder which has holes or slots, Check valve or Non-return valve which allows the fluid to pass in one direction only, Diaphragm valve a sanitary valve predominantly used in the pharmaceutical industry, Gate valve mainly for on/off control, Globe valve which is good for regulating flow, Needle valve for gently releasing high pressures, Piston valve, and Plug valve for on/off control.
A gate valve is a type of block valve. It consists of a flat piece of metal enclosed in a body attached to the valve operators. The movement of the metal in the up and down direction opens and closes the valve respectively. In this type of valve, the stem is usually long and threaded.
2.1 DESIGN FEATURES OF A BLOCK VALVE
Block valves come in different shapes and design such as round body and rectangular; rising stem and non-rising stem, floating slab gates and wedge or expanding gates, all manufactured according to set standards. Manufacturers always indicate the temperature and pressure ratings of the valve and also provide information on the wetted materials in a valve. When choosing a valve, the engineer should ensure that the rated maximum temperature and pressure are within the range of the pipeline he intends to use the valve and that the wetted materials are not affected by the fluid passing through valve.
He must also ensure that the valve has been tested by hydrostatic pressure as required by standards. Generally, the design features of a block valve include the following:
Disc:
This is a movable obstruction inside the stationary body that restricts flow through the valve port. They are usually flat rectangular or flat circular. They should be properly sized and coated to ensure bubble tight seal/ blockage all the time
Seat:
The interior surface of the body which contacts the disk to create a leak tight blockage of flow. The sealing surface between the gate and seal are usually planar. Block valves should be double seated.
Stem:
This transmits motion from controlling devices to the disk. It is typically a threaded rod attached to the disk which passes through the bonnet and attached on the other end to a controller. It could be either non-rising stem or rising stem depending on the availability of space, opening by turning stem left or right and provided with operating nut or hand wheel with the word "Open" and an arrow cast in the metal to indicate direction of opening.
The stem stuffing box should be equipped with proper sealing mechanisms which are replaceable with the gate valve fully open and subjected to fully rated working pressure.
Bonnets:
These are leak proof covers or closures on the body of the valve. They could either be screwed on, union or bolted type, or pressure seal types, depending on the nature of pressure in the pipeline.
Ports:
Although not a separate entity from the body in a block valve, it is the passage for fluids through the valve. To allow for minimal flow loss, the port must be well smoothed with no depressions and cavities.
Connections:
Connection types are mostly flange type but exceptions could be made depending on the requirement of the user. By-pass arrangements should also be installed to allow for maintenance on the valve
2.2 OPERATION
Block valve have low pressure drop and low flow resistance and are normally used in situation of infrequent operation or as mainline valves at valve sites, meter station or compressor/pump stations. They are mainly employed on larger pipelines for on/off services. They are only operated in fully opened or fully closed position and are utilised when fluid flow direction is unchanged. They must never be used to regulate flow because when partially opened, flow through the port does not change evenly with the stem travel. Also, the disk vibrates due to high velocity of the fluids. This could cause the seat to wear and bring about leakage in the valve.
2.3 MAINTENANCE:
If unused for a very long time, block valves can become stiff and hard to operate. There might also be collection of residues at the bottom of the valve body and prevent full closure. Inspection and maintenance activities need to be carried out regularly the block valve to enhance its operation and reliability and extend the valve seal life.
Typical maintenance guidelines for block valves are:
- Fully open and fully close the valve to check vertical gate travel.
- Proper oiling or greasing of the stem to prevent stiffness.
- Passage of steam or inert gas through the valve to remove any accumulated debris.
3.0 OFF TAKES:
Natural gas is transported from the gas fields by the oil and gas producing companies to distribution companies and to some large industrial customers. For effective monitoring and controlling of the natural gas passed into the pipeline and to ensure that all customers receive required delivery promptly, sophisticated control systems are required to monitor the gas that travels through lengthy pipeline network. Centralized gas stations along the pipeline has been established that collect, assimilate/ and manage data received from monitoring and compression stations.
Off takes of gas occur when more amount of gas is pumped into the transmission main so as to balance the demand of gas by the customer without necessarily resulting into shortage due to pressure loss during transmission. For gas to be safely transported through a pipeline system, the gas must be sent (input) into the network through an entry point and make arrangements for the exit (off takes) of the gas else where. These gas inputs andoff takes must be in balance within certain thresholds.
When there is difference between collective inputs and off takes, remedial steps are taken (either to increase or decrease the amount of gas in the network) in order to maintain overall equilibrium of the system.
3.1 OPERATION AND MAINTENANCE:
Off takes must be designed to incorporate compression station with adequate and well designed branching within the mainline. The control of activities of the off takes should be done by high capacity computers with real-time capability fro efficient monitoring and analysis. Mostly pipeline operating companies employ a sophisticated and versatile programme called Supervisory control and data acquisition (SCADA) for their pipeline operational planning an surveillance.
4.0 COMPRESSOR STATIONS
Natural gas is transported through a Pipeline under pressure and as the flows continues, it loses pressure due to friction against the inside walls of the pipe. To maintain the natural gas flow at the desired rate, the pressure must be increased. This is accomplished with compressor stations located along a pipeline.
There are various categories of compressor Stations viz
Filed or Gathering stations:
these are used to boost the supply pressure of gas coming from low pressure wells into a transmission or distribution pipeline. They can handle suction of lower than atmospheric pressure up to 750psig, and volumes from a few thousand mscf to millions of mscf per day.
Repressurising and recycling stations:
these are used mainly for processing or secondary recovery facilities. The compressed gas from such stations is usually sent for storage or maintenance in the reservoir. Discharge of such stations exceeds 6000 psig.
Distribution plant stations:
distribution plant stations compress gas coming from a storage into high or medium pressure (20-100psig) and is sent into distribution lines. It also compresses gas and sends it for bottle storage at pressures of about 2500 psig.
Pipeline booster stations:
These areused with gas transmission lines where large volumes of gas are involved with compression ratios less than 2. The pressure range handled by these compressors is between 200 and 1200 psig.
4.1 LAYOUT FEATURES
As stated earlier, different types of compressor stations exist for different purposes. However, the features of these stations are basically the same. A typical compressor station can be split into three major systems
Standards forpiping design of compressor stations differ according to regions. In the United States for example, pipe design is governed by ASME B31.8 which states that the piping must be a class 3 construction and a design factor of 0.5 must be used in the steel pipe design. This is not the case however in the U.K as design here is governed by different standards. Whatever the standards are in any case, the approach used in designing the piping of a compressor station is similar. The following give the approach:
- Determination of pipe size and preliminary station layout
- Pressure drop studies and thermal analysis is carried out
- A stress analysis to determine if the stress in the piping is within the allowable limit and if the stress at compressor flanges are within the acceptable limits.
- Determine the velocities of gas in each of the major systems and ensure that at:
Main gas system, velocity = 12 m/s
Unit gas system, velocity = 25 m/s
Auxiliary gas system = 7.5 m/s
This is to maintain the noise levels in the pipes at 80 decibels to avoid damage to the pipes through excess noise.
-Determine the better of alternatives between burying the pipeline and installing it above ground.
- Designing of drainage system for draining water out of the pipeline after hydrostatic testing.
4.2.2 Components and Equipment
However, additional requirements and features will depend on each project and the specific experiences of the pipeline operator. In fact, compressor selection consists of the purchaser defining the operating parameters for which the machine will be designed. The "process design parameters" that specify a selection are: flow rate, gas composition, inlet pressure and temperature, outlet pressure, train arrangement, for centrifugal compressors: series, parallel, multiple bodies, multiple sections, intercooling, etc., for reciprocating compressors: number of cylinders, cooling, and, flow control strategy; and number of units (Akhtar, 2002).
4.3 OPERATION:
When the natural gas enters the compressor station, it flows through separators used to remove solids and liquids from the natural gas in the pipeline. These separators are provided mainly to protect the compressor from any small debris that has gotten into the pipeline during construction and from water left behind from integrity testing. After the separation, the natural gas is then sent to the compressor(s) for compression to boost their energy. It does this by increasing the pressure. When the pressure of the natural gas is increased, its temperature rises. The natural gas must be cooled to minimize impacts on the pipeline and permafrost. As a result of this, two important processes take place at a typical compressor station. These are gas compression and gas chilling or cooling.
Most compressor stations are automated and equipped with dedicated hardware so that the compressors can be started, controlled and stopped from a central control location regardless of the weather conditions, time of day, or day of the week. The control center also can remotely operate shut-off valves along the pipeline system. The automation system also acts to protect the equipment, facility, and surrounding area in the event that the equipment is not operating as it was intended. In such stations, operators have very little to do other than to continuously monitor and adjust the mix of compressors that are running to maximize efficiency as well as keeping detailed operating data on each compressor station.
In case of emergencies, the automated Emergency shutdown (ESD) controller controls the valve sequencing and responds to fire and gas detection. There are three types of shutdown and emergency conditions warranting them. The table below highlights them.
