Pig launchers and receivers are designed to accommodate the removal of liquids and solids from a piping system. Running pigs results in the possibility of considerable accumulation of both liquids and solids in the vicinity of the pig receiver. While there is no one design that represents the “only” way to design this equipment, there are characteristics of effective equipment that can be assessed.
I grade receivers in several categories on a 0-4 scale (i.e., “4” is best in class, “3” is above average, “2” is average, “1” is below average, and “0” is unacceptable). By breaking each category down with objective criteria it is possible to assess the competence of the design. The categories I evaluate for receivers are:
1. Area behind closure: The area behind the closure is used to stage pigs and lifting equipment.
a. Best in Class: There is room behind the closure to extend the barrel for smart pigging, install a removal rack for long/heavy pigs, and to set a crane for lifting a heavy pig. A pig that is inadvertently launched from the receiver to atmosphere would be unlikely to damage any equipment
b. Above Average: There is room behind the closure to extend the barrel for smart pigging, install a removal rack for long/heavy pigs, and to set a crane for lifting a heavy pig. Some amount of equipment/piping is in the line of fire for a pig that is inadvertently launched out of the receiver.
c. Average: Some amount of congestion exists behind the closure, but it is possible to run smart pigs without modifying non-related equipment.
d. Below Average: Non-related equipment would have to be removed to retrieve a smart pig.
e. Unacceptable: The area behind the receiver cannot be modified to retrieve a smart pig without unacceptable expense/disruption of operations (e.g., a third party compressor set within 20 ft of the closure on a receiver).
2. Over Pressure Protection: The receiver can be isolated from the pipeline while full of liquid. In that case, very high pressures can be reached with a relatively small temperature change.
a. Best in Class: Properly designed, tested, and maintained pressure relief valve is installed on the barrel
b. Above Average: Properly designed relief valves are installed, but documentation of testing and maintenance is missing
c. Average: Reliance on formal, written procedures to prevent over pressurization (e.g., procedures that specify that vent valves will remain open when the barrel is isolated)
d. Below Average: Reliance on informal procedures to prevent over-pressurization (e.g., an operator decides to leave a vent open after receiving a pig)
e. Unacceptable: No steps in place to protect the barrel from over pressurization.
3. Design pressure and temperature: The values that the designer used in specifying required strength of the construction materials
a. Best in Class: Receiver design matches the design of the pipeline and all components can be hydrostatically tested to the same test pressure as the pipeline.
b. Above Average: Receiver design matches design of the pipeline, but some components cannot be tested.
c. Average: Receiver design exceeds the design of the pipeline
d. Unacceptable: Receiver design requires the pipeline design pressure/temperature to be reduced.
4. Closure Tell-Tale: Does the closure have a method to warn the operator of pressure in the barrel?
a. Best in Class: There are multiple tell-tales that are an integral part of the opening mechanism and cannot be skipped.
b. Above Average: There is a single tell-tale that is an integral part of the opening mechanism and cannot be skipped.
c. Average: There is a pressure tell-tale that is a part of the opening mechanism, but it can be overridden (e.g., some closures have a chain from the fixed part of the closure to the tell-tale operating mechanism, the mechanism must be operated, then removed before the closure can be opened—the chain can be cut without affecting the operation of the closure)
d. Below Average: There is a tell-tale, but there is no mechanism to cause it to ever be used.
e. Unacceptable: There is no tell-tale
5. Closure Secondary Capture: Does the closure have a way to prevent trapped pressure from converting the closure into a projectile?
a. Best in Class: A capture mechanism allows the closure to move a small distance to release the seal, but prevents it from coming fully open or swinging fully open. Best in Class closures will have both a secondary capture method and a hinge.
b. Above average: Closure has a hinge to limit travel if forced open under pressure
c. Average: Closure has a davit to limit travel if forced open under pressure
d. Unacceptable: Closure is free to become a projectile if opened under pressure
6. Bypass Line Size: The bypass line is used to redirect flow around the side valve.
a. Best in Class: The bypass line is sized to allow the processes upstream of the pig to continue with minimal excess backpressure.
b. Above Average: The bypass line requires noticeable increase in the backpressure of the upstream process.
c. Average: The bypass line requires considerable increase in the backpressure of the upstream process.
d. Below Average: The bypass line requires so much of an increase in the backpressure of the upstream process that some processes fail or go off line.
e. Unacceptable: The bypass line requires so much of an increase in the backpressure of the upstream process that most upstream processes fail or go off line.
7. Bypass Line Position: The position of the bypass line controls how constant the downstream process can be.
a. Best in Class: The bypass line is located far enough into the barrel section to ensure that any pig that could be run in normal operations has exited the throat.
b. Above Average: The bypass is located well into the barrel, but many pigs could cover the line, restricting process flow.
c. Average: The bypass is located very close to the start of the barrel and most pigs can block flow out the bypass
d. Below average: Bypass is located in the throat.
e. Unacceptable: Bypass is located at the start of the throat.
8. Bypass line orientation: The receiver is there to bring in solids and liquids. If the bypass is oriented properly, then most of the solids can be captured within the barrel and won’t accumulate in the lines to the next process.
a. Best in Class: The bypass valve is located high in the barrel and is above the highest accumulation of solids that could be reasonably introduced.
b. Above Average: The bypass is located high in the barrel, but there are credible scenarios that could result in the bypass flowing some amount of solids
c. Average: The bypass is located low in the barrel and some distance from the throat and will often ingest solids
d. Below Average: The bypass is located low in the barrel and fairly close to the throat and will often ingest considerable solids
e. Unacceptable: The bypass is located in the bottom of the throat and will preferentially ingest solids.
9. Barrel Length: The length of the barrel between the closure and the reducer
a. Best in Class: The barrel is long enough to ensure that any pig that could be run in normal operations has exited the throat and doesn’t reach the bypass.
b. Above Average: The barrel is long enough that most pigs that could be run in normal operations have exited the throat and won’t reach the bypass.
c. Average: The barrel is short enough that most pigs will have the potential to plug the bypass.
d. Unacceptable: There is no place that the pig can lie to allow produced fluids to flow around it.
10. Extensibility: Receivers should be designed for proper operation with maintenance pigs, but they should also be able to be extended for unusual conditions.
a. Best in Class: The barrel length is extendable without modifying piping (e.g. a set of flanges between the bypass and the closure would allow a pup to be installed to receive very long smart pigs). The receiver could be used as a launcher without piping modifications
b. Above Average: Either the barrel is extendable without modifying piping or the receiver could be used as a launcher without piping modifications.
c. Average: Some amount of piping modifications would be required to run a smart pig or to use the receiver as a launcher.
d. Below average: Extensive modifications would be required to either run a smart pig or use the receiver as a launcher.
11. Throat: The throat section is intended to allow a pig to clear the barrel-isolation valve before it enters the barrel and slows down. The throat is also used to seat the pig prior to pressurization in a launcher (or in a receiver used in reverse-pigging), a vent near the barrel isolation valve is used to aid the loading of pigs
a. Best in class: The throat is long enough to hold the intended maintenance pig without excessive compression of the trapped gas. There is a vent on the throat near the barrel-isolation valve
b. Above Average: The throat is long enough to hold the intended maintenance pig, but some amount of force is required to compress the trapped gas. There is a vent near the barrel isolation valve.
c. Average: The throat is too short to allow the expected maintenance pig to be fully loaded in the throat. There is no vent in the throat
d. Below average: There is no throat; the eccentric reducer connects the barrel-isolation valve and the barrel.
e. Unacceptable: There is no throat and the connection between the barrel-isolation valve and the barrel is a concentric reducer.
12. Pig Signal: Pig signals are used to indicate that the pig has entered the receiver.
a. Best in class: Bi-directional indicator that provides a clear indication of pig passage, fails in a way that makes the failure clearly known to the operator, and does not require manual reset.
b. Above average: Bi-directional indicator that provides a clear indication of pig passage, fails in a way that makes the failure clearly known to the operator, and does requires manual reset.
c. Average: Uni-directional indicator that provides a clear indication of pig passage.
d. Below average: Indicator has unclear indication of pig passage (e.g., one pig sig has an LCD that changes a binary value from its current value to the opposite value unless the operator noted if the starting value was a “0” or a “1” he doesn’t know what the “0” or “1” in the display means).. A good pig signal will be also be below average if it is installed in the Barrel where it is unlikely that the pig will hit it instead of in the throat.
e. Unacceptable: No pig signal.
13. Vents and drains: The vents and drains are used to depressurize and purge the barrel.
a. Best in Class: Vents and drains are appropriately sized and located for expected operations. Best in class receivers will have a 2-inch vent located 1 pipe diameter from the flange attaching to the barrel isolation valve (to be used for launching pigs); a 2-inch vent located in the same plane as the bypass valve on the top of the pipe; and a 3-inch drain located in the same plane as the bypass valve on the bottom of the pipe. Drain line is routed to a place where solids can be isolated.
b. Above Average: Has vents and drains per the Best in Class, but vent is located close to the reducer, or the drain is undersized, or the drain goes into a water system.
c. Average: Multiple vents in the barrel, no vent in the throat, or drain directed to a location where the solids can do harm.
d. Below Average: No drain.
e. Unacceptable: No vent or drain.
Scoring. Each category is assigned a “grade” and the grades are summed, and then averaged. The results are:
1. Expert design: Average is 3.5 or higher and no single category scored below a 3.0.
2. Better than Competent: Average is 2.5 or higher and no single category scored below 2.0 or Average is higher than 3.5 and there was at least one category below 3.0.
3. Competent: Average 1.5 or higher or Average higher than 2.5 and more than one category below 2.0.
4. Less than Competent: Average greater than or equal to 0.5.
5. Incompetent: Average less than 0.5.