Appendix-2, Appendix-3 and Appendix-4 contains the recommended sequence of operations for the Pig launcher and Receiver in Oil, Gas and Multi phase flow fluids respectively.
Prior to start of pigging operations the operations team shall calculate the required flowrate/ pressure to achieve the optimum pig velocity advised based on purpose of pigging, pig type and other parameters noted in paragraph 2.4.1.3 “ Pigging Speeds”.
The estimated pig travel time shall also be calculated.
The monitoring and control of pigging operation are carried out manually and local to the pig launcher/ receiver by appropriate positioning of valves. The most important locally- mounted instruments are pig signallers; which signal the safe arrival and departure of pigs. The pressure indicators give indication that the pig trap is pressurized or otherwise, as appropriate.
6 SAFEGUARDING SYSTEM AND PHILOSOPHY 6.1 General
Voluntary compliance of personnel operating the plant, pig traps and handling equipment. If these guidelines are contravened due to human error, or malicious malpractice, serious accidents will occur. Accordingly, the need for a safety system that positively controls the entire process, without dependence on human judgement, is MUST.
The many separate operations involved when launching and receiving pigs must be made to follow a safe, predetermined path. This can be done by having an Interlock system between the end closure and various valves.
The SHELL DEP suggests three types of Interlock system: Safety issues are always at the heart of pig trap design. Whilst pigging is a common procedure, typically carried out when a pipeline needs purging, cleaning or surveying, it can involve a high risk of human error, high enough to warrant pig traps being described as ‘primary grade sources of hazard’.
Probably the greatest associated danger is when a trap is opened accidentally whilst still under pressure – usually to insert or remove a pig.
To overcome this danger it is vital that a system is incorporated which ensures that the trap is fully vented before the door can be opened. Venting depressurises the trap and removes the force necessary to propel the pig.
Pig trap systems also involve other more complex, procedures. Even in a basic system, safe operation of the closure require it to be correctly sequenced with certain valves which are incorporated into the trap – these being the pig trap valves, drain and kicker valves.
The pig trap line valve governs the piping connection between the pig trap and the main pipeline whilst the kicker valve is on a secondary piping connection used to move the pig into and out of the trap. Both valves must be closed, isolating the trap from the main pipeline, before the trap is drained. By establishing safety guidelines, a certain level of control over pig trap operations can be imposed but these usually rely on the
¾ Relay based; that is thought of as monitoring device instead of safety device and hence will not be considered;
¾ Micro-processor based; and
¾ Mechanical Key transfer Interlocking
6.2 Safety and Interlock Systems
The microprocessor based safety interlock and mechanical key transfer safety interlock systems have the following advantages and disadvantages that are discussed in the following sections.
6.2.1 Micro-processor based Interlock System Advantages:
• Soft logic.
• No mechanical parts and hence no wear, tear and maintenance problems.
• Changes in the logic is possible i.e. operator intervention possible.
• Addition of future logic possible.
Disadvantages:
• Expensive.
• Hazardous area protection needs to be considered.
• Valve needs to be automated.
• Temperature control for panel is required and hence in remote areas were electricity is not available a passive cooled shelter to maintain temperature suitable for instruments shall be required.
6.2.2 Mechanical Key Transfer Interlock
Advantages:
• Forces the operator to carry out tasks in the correct sequence and hence eliminates the possibility of human error.
• No electric power required.
• Unauthorized by-passing of sequential activities is not possible.
• Simple mechanical device.
Disadvantages:
• Corrosion problems with the key.
• Sand and dust particles clog the grooves of the key.
• Damage to groove / key or loss of key leads to procedural delays.
• Loss of flexibility.
• Frequent vendor intervention would be required for maintaining the key transfer interlocks.
• Operation becomes cumbersome when key transfer interlocking is incorporated between lots of valves.
6.2.3 Selection of Interlocking System
The ‘Sequential Mechanical Key Transfer Interlock system’ is selected. Mechanical key transfer interlocking is developed from the principle that actions performed in the correct sequence are safe but potentially lethal if performed out of sequence.
Therefore, the use of trapped key interlocks in pig trap operations will limit the sequence of valve and end closure operations to a single, unchanging path. By fitting interlocks to all relevant valves, as well as to the end closures, it becomes impossible to load or retrieve a pig without first depressurising the pig trap.
In a typical system, the key can only be retrieved to open the end closure when the vent valve is opened, thereby reducing the pressure within the vessel. This is the simplest form of trapped key interlock. This system can be extended to include interlocks on all isolating valves like pig trap valve, bypass valve, kicker valve, drain and vent valves. It is decided to select the sequential mechanical trapped key interlock between various valves for the pig trap systems.
In addition the pig trap Quick Opening Closure (QOEC) shall be provided with a safety lock device that will not permit the closure to be opened before the internal pressure has been fully released, and that prevents pressurizing the trap before the closure is in the properly locked position, in accordance with ASME Boiler and Pressure Vessel Code, Section VIII, Division 1, Paragraph UG-35.
6.2.4 Interlock Logic
The mechanical interlock system shall be designed to prevent opening of the scraper trap Quick Opening Closure (QOEC) when the kicker/by-pass and main line isolation valves are open. The valves installed in the main line at the launcher/receiver outlet/inlet and in the kicker/by-pass line shall be mechanically interlocked so that the valves cannot be opened if the launcher/receiver closure is open and the trap closure cannot be opened if the isolation valves are in open position.
A typical mechanical key interlock logic between various valves & end closure of pig trap is depicted in the Appendix-9 P & ID 30.78.08.607.
The following Figure – 5 represents a generic logic in graphical format for more clarity.
PREMISSIVE TO OPEN DOOR KICKERLINE, PURGE &
PRESSURING VAVE(S)
& PIP TRAP ISOLATION VALVE(S) CLOSED
INHIBIT KICKERLINE PURGE &
PRESSURISING VALVE(S) PIG TRAP ISOLATION VALVE(S) &
VENT (TO FLARE) VALVES(S) FROM REOPENING
INHIBIT DRAIN VALVE(S) FROM OPENING IF CONNECTED TO CLOSED
DRAIN SYSTEM
Figure 5: PIG Launcher / Receiver Logic Diagram Notes:
1. Barrel Pressure alarm is energised when PIG Trap is depressurized. This signal together with permissive signals from all valve interlocks permit unlocking of closure door.
2. Only required when Drain is connected to the closed drain system
7 SUGGESTED PIGGING SCHEDULING
Date Activity Responsibility
Minimum of 1 week prior to launch.
Pigging sequence schedule prepared and distributed to the relevant parties/authorities.
ADCO to define
Minimum of 2 days prior to launch.
Pig checked: refurbished, replaced as necessary. Locator and transmitter checked, batteries charged.
ADCO to define
One day before Launch
Confirm launch event to the relevant parties/authorities.
Advice expected travel time of pig.
ADCO to define
Pre-launch Line up the pipeline and the receiver.
ADCO to define
8 SAFETY PRECAUTIONS