Each valve controls flow by blocking the flow path with a solid surface. Different valve designs are used to keep this surface sealed after being opened and closed multiple times.
In a Metal Seat Lubricate Plug Valve, the key is grease. The lubrication system does more than reduce friction between plug and valve body. It also helps to create seals, update seals during operation, and protect sealing surfaces from fluids in valves.
To understand how the system works, look at three steps: how the sealant gets into the valve, how it works under pressure, and what happens when the lubrication system is not maintained.
Practical role the Lubrication System
The system has three main functions. Each job can fail in a different way.
The first task is hydraulic sealing. Plug and valve body using controlled clearance processing. The gap is tight enough to keep the parts steady and wide enough for the plug to rotate. This gap can become a leak path. Sealant fills the void. Because the sealant is under pressure, it pushes against the metal on either side. This creates a hydraulic barrier. This valve can be closed tightly even if both sealing surfaces are metal.
The second task is friction control. Clean and dry metal surfaces can get stuck when they collide. Stainless steel also produces bile. Small surface points can stick together and tear. This can cause the plug to get stuck and damage the sealing surfaces. A layer of lubricant keeps the metal surfaces apart. The stopper can then be rotated through the grease rather than directly at the metal.
The third task is corrosion control. The sealant helps prevent processing fluid from entering the small gap between the stopper and the valve body. This is important in water, chloride or hydrogen sulfide services. Excluding these fluids helps prevent crevice corrosion and spotting within the valve body. If the valve is not removed, these problems may not be visible.
Anatomy of the Injection Path
Sealants do not accidentally reach sealed areas. It flows down a special road. High-pressure connectors on the valve body are connected to the internal channel. This passage leads to the sealant grooves on the stopper.
These grooves are machined into a the plug's sealing surface. They can be in crossover or spiral patterns. The design helps spread the grease around the circle rather than in one spot.
Check valve is located between the external connector and the internal channel. It allows sealants to enter under injection pressure. When the pressure of the pipeline is higher than the injection pressure, it can also prevent the flow of processing fluid back into the pipe.
If check valve fails, it may cause the pipe to enter the leak path. Operators should check the injection port after each service operation.
Pressure and hydraulic wedges
The main performance advantage of metal seat lubricating plug valve is the pressure difference between the sealant and process fluid. Pressure is higher when sealant is injected than pipe pressure. A manual or pneumatic grease gun can deliver about 5,000 to 10,000 psi grease at the test site. Higher pressure pushes the grease into small gaps on the mating surfaces. It also helps flush out liquids and solids.
The grease forms a hydraulic barrier between the metal surfaces. When the plug is turned on or off, it compacts both surfaces and helps maintain the seal closed. The seal can also be changed during use. If the valve starts leaking, the operator can inject more sealant during pipeline operation. The leak may stop within minutes.
This is an important feature of lubricated plug valves. Seat damaged ball valve may need to be closed and removed. A lubricated plug valve can usually be restored by adding more sealant.
How films reduce torque and wear
Sealability is only part of the story of a Metal Seat Lubricate Plug Valve. The effort to operate is also important because a quarter-turn valve can be opened and closed thousands of times a year.
Boundary lubrication helps keep the plug's breakout torque, or the force needed to start rotation, at a stable level. Operators can use torque as a simple way to check valve condition. If the stopper becomes harder to turn before completing a quarter-turn, the lubrication film may be running out. Over time, tracking torque can help detect the problem early. If torque rises, particles may get stuck in the gap even if the valve is lubricated as expected. This requires maintenance before the valve gets stuck.
Lube film also helps protect plugs during throttling. When the valve is partially opened at high pressure, the flowing fluid quickly wears away the dry metal surface. This is called wire drawing. The sealant film helps protect the surface and can be replaced when wear starts. This can extend the life of the cover.
Sealant Chemistry and Service Matching
Sealant chemistry plays a major role in how long the metal seat lubricates the valve. Many of the failures were caused by the use the wrong sealant rather than mechanical problems.
There are different types of sealant according to different conditions of use. Hydrocarbon-resistant sealants are thickeners that are stable in oil, gas and condensates. They should not dissolve or be washed away. Steam-service sealants use different formulas to deal with high temperatures without melting or forming carbon deposits. Acidic and corrosive services require sealants that resist chemicals on the production line.
Valve manuals usually include sealant selection tables. Follow these recommendations unless engineers approve other options. Using the wrong sealant can quickly lead to leaks or operational problems.
Wrong sealants can cause some common problems. Sealants that soften during processing can be rinsed off and leave residue downstream. A sealant that react with fluid can harden inside the valve. This can make the plug difficult to turn and increase stem torque. A sealant that damage the elastomer parts within the check valve may reduce backflow protection and cause an external leak path.
Operation discipline to maintain integrity of the system
Only by following the correct maintenance schedule can the lubrication system work properly. Sealants are slowly depleted during valve operation. Some can enter the flow during rotation, others can decompose on the surface. The injection schedule should match the frequency of valve operation. High-cycle valves can be scheduled in a cyclic manner. Valves that are rarely operated may only need lubrication every few months.
The condition of the valve can also be seen with the use of sealant. If a valve uses more sealant than other valves, it may have a larger gap or a damaged sealing surface. The extra sealant may fill those gaps. Tracking the use of sealant for each valve helps detect problems early.
Temperature also affects sealant performance. Sealant viscosity varies with temperature. Outdoor valves may need a winter-grade compound to pump easily in cold weather. Maintenance personnel must use the correct sealant and follow the correct schedule. Write the sealant type and injection interval into the maintenance program. Keep records and check regularly.
Frequently Asked Questions
What happens if the wrong sealant is used in a Metal Seat Lubricate Plug Valve?
The sealant can dissolve and clean out sealed areas. It may also react with the process fluid and harden inside the valve. Both problems can degrade sealing performance. Hardened sealant also makes it difficult to turn the stopper, increasing the risk of stem damage.
How is sealant injected when the valve is under pressure?
Sealant is injected into the valve body through high pressure tubing and check valve. grease gun rated above pipeline pressure. The check valve stops the flow of processing fluid back into the pipe after injection stops.
How often should the valve be re-lubricated?
Follow valve maintenance schedule. High pressure valves may need to be lubricated according to cycle count. Valves that are rarely used may use a calendar-based schedule. The increase in breakout torque may indicate that the lubricating film is running at a low speed and may require lubrication.
Will the valve leak if the the lubrication system fails?
Yes. check valve failure, wrong sealant or blockage of distribution slot may affect the lubrication system while valve is still sealed. These problems can lead to breakdowns or pressure surge on the next operation.
Source of information
- API 599, Metal Plug Valves-Flanged, threads and Welding Ends
- API 598, Valve inspection and testing
- ISO 5208, Industrial Valves-Metallic Valves Pressure Testing
- ASME B16.34 Valves-flanges, threads and Welding End
- NACE MR0175 / ISO 15156 for materials H2S-Containing Environments
- ISO 9001, Quality Management Systems
