By Gobind Khiani – Consulting Fellow-Piping/Pipelines
During this process, natural gas, which is primarily methane, is cooled below its boiling point, and certain concentrations of hydrocarbons, water, carbon dioxide, oxygen and some sulfur compounds are either reduced or removed. LNG is less than half the weight of water. If spilled on water, it will float and then immediately vaporize if under atmospheric pressure.
LNG Uses
LNG has been used as a clean burning alternative in power plants and in thousands of vehicles for decades. With the substantial growth of the LNG fuels industry around the world, the opportunity to use LNG as a cleaner and lower-cost vehicle fuel in transportation fleets and heavy horsepower equipment continues to grow.
LNG Benefits
LNG has many benefits when compared with other fossil fuels. It is less expensive, has fewer emissions, and is safer and less likely to ignite.
Cost
On average, LNG costs about 50% less than gasoline or diesel fuel and delivers the same power and performance. According to the Federal Reserve Economic Database, in 2010, the U.S. spent USD $552 billion on oil alone, and 61% of that was imported. A 50% savings could bring many benefits.
Emissions
Natural gas is the cleanest burning fossil-based fuel with 50% lower CO2 emissions than the next best option. LNG burns almost completely, leaving only a small amount of carbon dioxide and water behind (CH4 + 2O2 = CO2 + 2H2O).
LNG produces up to 90% lower emissions than gasoline or diesel. LNG’s reduced amount of greenhouse gases makes it the cleanest internal combustible fuel for our environment. It also burns cleaner inside engines, resulting in fewer oil changes and less maintenance.
By replacing the traditional diesel engine of one 18-wheeler with an LNG engine, its carbon footprint reduction is equivalent to removing 324 automobiles from the road.
Safety
LNG is lighter than air. In the event of a spill, LNG disperses quickly, unlike petroleum-based fuels that pool on the ground and create a fire hazard. It also has a higher ignition temperature, making it less flammable than gasoline or diesel. In addition, it is non-toxic, non-corrosive and will not contaminate ground water.
LNG benefits have increased the demand for this cleaner burning fuel and associated production and distribution equipment.
It is imperative to manage these equipment and prepare for preventative maintenance on butterfly valves used in this application.
Special Conditions for Safe Use of Butterfly Valves
The following factors must be carefully considered to ensure the valve is compatible with the atmosphere in which it is applied.
Design Considerations
The system designer and/or end user should formally address each item and carefully document the reasoning behind specific measures taken to ensure continued compliance throughout the life of the butterfly valve.
Material Considerations
Titanium is not to be used in Group I, mining applications and Group II Category 1 equipment, due to the potential of ignition from sparks caused by mechanical impacts. Please use best engineering judgement including industry codes and compliance for details regarding material limitations.
Temperature Considerations
The surface temperature of the butterfly valve is wholly dependent on the ambient temperature in combination with the temperature of the process medium. The maximum surface temperature of the butterfly valve may be calculated from the maximum ambient temperature plus the maximum process medium temperature as shown below:
Equation 1 – Surface Temperature Calculation
Ts(max) = Ta(max) + Tp(max)
The engineer on record is responsible for ensuring the maximum temperature, either inside the valve body or on the external surface. It should remain well below the ignition temperature of the atmosphere. Additional protective devices may be required to ensure a sufficient thermal safety margin, including but not limited to thermal shutoff devices and cooling devices.
For operating temperatures above 200°C (392°F), manufacturers recommend thermal insulation of the valve body.
Static Electricity Considerations
Where the process medium is a liquid or semi-solid material with a surface resistance of more than 1 G-ohms, special precautions should be taken to ensure the process does not generate electro-static discharge. This may be done by ensuring the flow rate of the process media remains below 1 m/s or providing sufficient discharge points along the process path to eliminate electrostatic build-up. Consultation to EN 50404 is recommended. Appropriate grounding may be necessary using grounding straps or other means.
Stray Electric Current Considerations
When a valve is used near sources of high current or magnetic radiation, a secure bonding to earth ground should be made to prevent ignition due to inductive currents or a rise in temperature due to these currents.
Filtration of Process Medium Considerations
Special consideration should be made regarding the filtration of the process medium if there is a potential for the process medium to contain solid particulates. The process medium is recommended to be filtered to allow particles no greater than 1.0 mm in diameter through the valve assembly where there is a high probability of solid particulates. Larger particulate sizes may be deemed appropriate based on the possibility of particulates within the process medium and the area classification. The decision regarding filtration levels and limits should be well documented by the engineering team and/or end user to ensure continued compliance through the life of the valve.
Preventative Maintenance on Butterfly Valves
When replacing the seat and/or seal ring in a dead-end service or if the piping on the body seat side is removed, then depressurize the line and partially open the disc before loosening any of the valve trim fasteners.
1. Clean the valve, removing any grit or scale. When handling, care should be taken not to scratch the seal ring, seat and gasket faces on both sides of the valve.
2. Replacement seat, disc seal ring, and other parts are available from authorized sales and service locations.
Recommended Lubricants
• PTFE Thread Lubricant for non-oxygen service.
• PTFE Thread Lubricant Krytox GPL226 or equal for oxygen service.
Manufacturers provide “kits” for our cryogenic triple offset butterfly valves – a seat replacement kit, a seal replacement kit, and a stem packing kit. They consist of the following:
Seat Replacement Kit: Seat, seat gasket, and seat retainer (upper and lower) cap screws.
Seal Replacement Kit: Seal ring, seal gasket, and seal retainer cap screws.
Stem Packing Kit: Bottom plug or bottom plate gasket (depending on whether the valve has a bottom trunnion plug or a bottom plate).
Packing Replacement
1. If the valve is installed, relieve line pressure. Remove actuator from the valve. Remove socket head cap screws and lock washers. Remove mounting bracket or mounting plate, depending on valve size. Note assembly positions of the actuator and the mounting hardware for reinstallation.
2. Remove packing gland retainer nuts and lock washers. Remove gland retainer, anti-blowout retaining ring/split ring and gland ring.
3. Remove all packing, taking care not to scratch the stem or the bore of the bonnet. Do not remove the thrust washer unless further valve disassembly is required.
4. Examine the bonnet packing bore and the stem surface. Clean as necessary to remove any corrosion, foreign matter, and minor surface imperfections.
5. Install new packing rings in valve body packing bore one at a time – first the external ring, then internal rings and last the second external ring.
6. Reinstall gland ring, anti-blowout retaining ring and gland retainer. Re-install lock washers and nuts. Tighten gland nuts utilizing a cross bolting technique to the proper torque value given by manufacturer.
7. Reinstall mounting bracket or mounting plate with cap screws and lock washers. Remount actuation device on top of the mounting bracket.
8. Operate the valve open and closed several times to check for binding and to set the seal rings. Loosen gland nuts and retighten, utilizing a cross bolting technique, to torque values.
9. Install the key(s). Then mount actuator, paying attention that the actuator is properly oriented.
Seal Ring Replacement
Exercise extra care when handling the seat and seal ring to avoid damage to the sealing surfaces. The seal ring may be replaced in two ways: without removing the seat; or replacing the seal ring with the seat removed.
To remove the seal ring without removing the seat, the actuation device must be removed, and the valve oriented in a manner that allows access to both sides.
Note: This procedure is not suitable if the seal ring is to be replaced while the valve is installed in the pipeline. In addition, this procedure is not recommended for large valves where manipulating the valve may be more difficult than removing the seat and installing the seal ring solely from the seat side of the body.
Seal Ring Replacement with the Seat in the Butterfly Valve
1. Remove valve from the pipeline. Remove the actuator from the valve.
2. Clean the surface of the valve with compressed air, blow out all debris around the seal ring retainer and clean out the hex sockets of the seal ring retainer cap screws.
3. Loosen all seal ring retainer cap screws, but leave them in the valve with the seal ring retainer attached to the disc.
4. Using a wrench, rotate the valve stem counter clockwise past the fully open position far enough so the disc can allow seal ring retainer and seal ring removal. Be careful not to over-rotate the stem to the point where the seal ring or disc edge contact the body. Make sure the packing gland retainer nuts are tight enough to prevent the valve stem from rotating on its own under the eccentric weight of the disc.
5. Remove the seal ring retaining cap screws and extract disc seal ring retainer and the seal ring.
6. Rotate the disc as necessary to access the seal face on the disc. Using soft tools and suitable wire brush, carefully clean any remnants of old gasket and foreign matter from the face of the disc. Blow out all threaded holes and the gasket groove with compressed air.
7. Rotate the disc to its previous position to facilitate installation of the seal ring. Place a new seal ring gasket into the groove on the disc face. Place the seal ring onto the disc making sure the alignment line on the disc seal ring matches the locating dimple on the disc face. Place the seal ring retainer over the seal ring. Apply PTFE Thread Lubricant to the seal ring retainer cap screws. Install all seal ring retainer cap screws. The cap screws should be fully threaded into the disc but remain only finger tight at this time.
8. Using a suitable actuator, close and open the valve 2-3 times, only closing the valve to the point where the seal ring engages the seat. Check each time that the seal ring makes full contact without torquing into the seat. Attention should be paid in the closing stroke that the seat does not scratch the seal ring. This will allow the seal ring and seat to be properly aligned.
9. Orient the valve with the seat side facing up. Verify that all four alignment marks (body, seat, seal ring and seal ring retainer) are aligned.
10. Tighten the seal ring retainer cap screws using a cross bolting technique to the torque specified.
11. Reinstall operator or actuator and test the valve.
Gobind Khiani
This article is dedicated to the late Mr. Stan Allan.
REFERENCES:
a) Codes such as ISO, ASME, MSS, API, NACE, CSA, PEP etc.
b) Content provided by Industry colleagues, manufacturers and from various committee’s author participates.
DISCLAIMER:
The article written by author to best of his/her knowledge including enough references provided at the time of writing this, to meet best industry practice.
ABOUT THE AUTHOR
Gobind N. Khiani, a UCalgary alumnus with a BSc in Civil Engineering and MSc in Mechanical Engineering has a proven track record in technical and value engineering and holds a Fellowship in Engineering and an MBA. Currently, he holds the position of Vice Chairman of the Standards Council of Canada. He has done peer review on Emissions Management regulatory documents for Environment and Climate Change Canada and participated in research and development initiatives for Emissions Management and Reduction Programs, Alberta and Canada’s Oil Sands Innovation Alliance (COSIA) – a world-leading innovation alliance that set the model for sharing intellectual-property to accelerate environmental performance.