Combining the problems and on-site use experience of double block and bleed plug valves in the field, the technical characteristics and usage methods of this product are sorted out, providing a reference for the users and maintainers of this valve.
3.1 Features of the DBB plug valve
No friction, small operating torque and long service life
Active and reliable sealing
DBB and online real-time seal detection
Online maintenance
With center cavity pressure relief devices
With a drainage structure
3.2 Installation requirements for the DBB plug valve
1) Before installation, check whether the double block and bleed plug valve and related equipment are damaged, and ensure that the inside of the valve cavity and the pipeline are clean and free of foreign matter.
2) The DBB plug valve should be correctly positioned so that the pipeline’s flow direction is consistent with the direction of the arrow on the side of the valve body to ensure the normal operation of the relief system; the DBB valve should be installed vertically to ensure the effectiveness of the sewage device.
3.3 Pressure relief of valve chambers
When the DBB plug valve is closed, the middle cavity of the main valve is a closed cavity. When the ambient temperature rises, the pressure in the middle cavity of the valve body may increase. Therefore, the double block and bleed plug valve is equipped with a relief device. According to the working conditions of the on-site valve, it can be divided into one-way relief and two-way relief.
3.3.1 One-way relief
The pressure relief in the valve cavity can be automatic and manual. The pressure relief device mainly includes a manual relief valve K1 (normally closed), isolation valve K2 (normally open) and an automatic relief valve. Manual relief valve K1 is used to manually relieve the pressure in the middle chamber and can also be used to check the sealing performance of the main valve when it is closed. When the pressure in the middle chamber is higher than the inlet pressure by a certain value (about 0.2MPa), the automatic relief valve opens and releases the pressure in the middle chamber to the main valve inlet. The middle chamber can also be manually and directly released through the manual relief valve at the upper end.
Figure 5 Outline and schematic diagrams of one-way relief devices
3.3.2 Bidirectional release devices
The pressure relief in the valve cavity can be automatic and manual. The pressure relief device mainly includes a manual relief valve K1 (normally closed), a reversing valve K2 (normally open) and an automatic relief valve. Manual relief valve K1 is used to manually relieve the pressure in the middle chamber and can also be used to check the sealing performance of the main valve when it is closed. When the pressure in the middle chamber is higher than the inlet pressure by a certain value (about 0.2MPa), the automatic relief valve opens and releases the pressure in the middle chamber to the main valve inlet. The middle chamber can also be manually and directly released through the manual relief valve at the upper end. If the valve flow direction changes, you need to manually operate the reversing valve and adjust the flow direction of the reversing valve according to the on-site instructions to ensure that the pressure in the valve cavity is always released to the inlet side.
Figure 6 The appearance and schematic diagram of the two-way relief device
3.4 Sewage discharge
After long-term operation, impurities in the medium will precipitate and accumulate in the main valve of the DBB plug valve. If impurities are not removed in time, it may hinder the action of sealing slide, resulting in the valve not being closed well, opening and closing torque increasing, and even the valve being stuck. Therefore, the sewage should be drained regularly. When the sewage is discharged, open the manual globe ball valve on the lower end cover of the DBB plug valve to drain discharge the impurities, as shown in Figure 7. If necessary, connect the discharge pipe to the outlet of the manual ball valve.
Figure 7 The structure of sewage discharge
3.5 Online leakage detection
When the valve is closed, close the K2 isolation valve in Figure 5 and open the K1 manual relief valve (for a two-way relief device, the K2 reversing valve in Figure 6 is in the closed position and open the K1 manual relief valve). If there is internal leakage, the liquid will continue to flow from the drain port. If the liquid stops flowing after a few minutes, it means the internal seal is good. The main valve’s sealing can be checked online in real-time through the manual globe valve at the upper end of the relief device.
3.6 Checking or replacing the sealing slide
As the main sealing element of the double block and bleed plug valve, the sealing slide will inevitably suffer from leakage due to impurities and rubber aging after long-term use. Therefore, the sealing slide needs to be checked and replaced after the leakage is discovered. It is not necessary to remove the entire valve from the pipeline to remove the sealing slide due to the tapered plug of the double block and bleed plug valve. It is only necessary to drain the upstream and downstream pipelines of the double block and bleed plug valve so that the double block and bleed plug valve is fully open. Remove the lower end cover; remove the sealing slide from the plug and inspect it; replace it if damaged, as shown in Figure 8.
Figure 8 Online maintenance
3.7 Adjusting the direction of the actuator
In the pipeline’s construction and installation process, double block and bleed plug valves require on-site adjustment of the actuator’s position due to the influence of pipeline layout and peripheral equipment, especially for electric double block and bleed plug valves. According to the actual situation on site, the direction adjustment of the actuator of the double block and bleed plug valve can be carried out in the following two situations:
1) The electric actuator does not need to be reset: First, open the valve to any rotational position; remove the bolts connecting the operator and the main valve; rotate the operator and actuator at an angle of 180 at the same time, and then tighten the bolts connecting the operator and the main valve. There is no need to reset the electric actuator.
2) The electric actuator needs to be reset: After removing all the bolts at the lower end of the operator's reduction box, push the manual operating lever of the electric actuator and rotate it counterclockwise to rotate the reduction gearbox counterclockwise until it reaches the required angle and then find the position corresponding to the bolt hole. Tighten the bolt and reset the switch position of the electric actuator.
3.8 Replacing the valve stem packing
The valve stem seal of the double block and bleed plug valve is sealed with graphite packing. Since the seal here is dynamic, the sealing packing needs to be replaced after long-term use to ensure the reliable operation of the valve. Follow these steps to replace the graphite packing without removing the main valve from the pipeline:
1) Release the pressure at both ends of the DBB plug valve.
2) Release the inner cavity pressure.
3) Manually operate the plug valve and remove the cylindrical pin.
4) Remove the operator mounting bolts and operator.
5) Remove the packing gland and replace the inner and outer O-rings.
6) Remove the packing and replace it carefully.
7) Install the packing gland.
8) Install the new operator in reverse order.
9) Reset the valve’s opening and closing position.
10) Check the valve’s operation.
4. Conclusion
With the rapid development of China's oil and gas pipeline storage and transportation industry, double block and bleed plug valves have been widely used for their long service life, zero leakage and convenient maintainability. Therefore, higher requirements have been put forward for the use and maintenance of plug valves. Mastering the correct use of forced sealing double block and bleed plug valves can give full play to its excellent performance and provide a good guarantee for the safe production of enterprises.
3.1 Features of the DBB plug valve
No friction, small operating torque and long service life
Active and reliable sealing
DBB and online real-time seal detection
Online maintenance
With center cavity pressure relief devices
With a drainage structure
3.2 Installation requirements for the DBB plug valve
1) Before installation, check whether the double block and bleed plug valve and related equipment are damaged, and ensure that the inside of the valve cavity and the pipeline are clean and free of foreign matter.
2) The DBB plug valve should be correctly positioned so that the pipeline’s flow direction is consistent with the direction of the arrow on the side of the valve body to ensure the normal operation of the relief system; the DBB valve should be installed vertically to ensure the effectiveness of the sewage device.
3.3 Pressure relief of valve chambers
When the DBB plug valve is closed, the middle cavity of the main valve is a closed cavity. When the ambient temperature rises, the pressure in the middle cavity of the valve body may increase. Therefore, the double block and bleed plug valve is equipped with a relief device. According to the working conditions of the on-site valve, it can be divided into one-way relief and two-way relief.
3.3.1 One-way relief
The pressure relief in the valve cavity can be automatic and manual. The pressure relief device mainly includes a manual relief valve K1 (normally closed), isolation valve K2 (normally open) and an automatic relief valve. Manual relief valve K1 is used to manually relieve the pressure in the middle chamber and can also be used to check the sealing performance of the main valve when it is closed. When the pressure in the middle chamber is higher than the inlet pressure by a certain value (about 0.2MPa), the automatic relief valve opens and releases the pressure in the middle chamber to the main valve inlet. The middle chamber can also be manually and directly released through the manual relief valve at the upper end.
Figure 5 Outline and schematic diagrams of one-way relief devices
3.3.2 Bidirectional release devices
The pressure relief in the valve cavity can be automatic and manual. The pressure relief device mainly includes a manual relief valve K1 (normally closed), a reversing valve K2 (normally open) and an automatic relief valve. Manual relief valve K1 is used to manually relieve the pressure in the middle chamber and can also be used to check the sealing performance of the main valve when it is closed. When the pressure in the middle chamber is higher than the inlet pressure by a certain value (about 0.2MPa), the automatic relief valve opens and releases the pressure in the middle chamber to the main valve inlet. The middle chamber can also be manually and directly released through the manual relief valve at the upper end. If the valve flow direction changes, you need to manually operate the reversing valve and adjust the flow direction of the reversing valve according to the on-site instructions to ensure that the pressure in the valve cavity is always released to the inlet side.
Figure 6 The appearance and schematic diagram of the two-way relief device
3.4 Sewage discharge
After long-term operation, impurities in the medium will precipitate and accumulate in the main valve of the DBB plug valve. If impurities are not removed in time, it may hinder the action of sealing slide, resulting in the valve not being closed well, opening and closing torque increasing, and even the valve being stuck. Therefore, the sewage should be drained regularly. When the sewage is discharged, open the manual globe ball valve on the lower end cover of the DBB plug valve to drain discharge the impurities, as shown in Figure 7. If necessary, connect the discharge pipe to the outlet of the manual ball valve.
Figure 7 The structure of sewage discharge
3.5 Online leakage detection
When the valve is closed, close the K2 isolation valve in Figure 5 and open the K1 manual relief valve (for a two-way relief device, the K2 reversing valve in Figure 6 is in the closed position and open the K1 manual relief valve). If there is internal leakage, the liquid will continue to flow from the drain port. If the liquid stops flowing after a few minutes, it means the internal seal is good. The main valve’s sealing can be checked online in real-time through the manual globe valve at the upper end of the relief device.
3.6 Checking or replacing the sealing slide
As the main sealing element of the double block and bleed plug valve, the sealing slide will inevitably suffer from leakage due to impurities and rubber aging after long-term use. Therefore, the sealing slide needs to be checked and replaced after the leakage is discovered. It is not necessary to remove the entire valve from the pipeline to remove the sealing slide due to the tapered plug of the double block and bleed plug valve. It is only necessary to drain the upstream and downstream pipelines of the double block and bleed plug valve so that the double block and bleed plug valve is fully open. Remove the lower end cover; remove the sealing slide from the plug and inspect it; replace it if damaged, as shown in Figure 8.
Figure 8 Online maintenance
3.7 Adjusting the direction of the actuator
In the pipeline’s construction and installation process, double block and bleed plug valves require on-site adjustment of the actuator’s position due to the influence of pipeline layout and peripheral equipment, especially for electric double block and bleed plug valves. According to the actual situation on site, the direction adjustment of the actuator of the double block and bleed plug valve can be carried out in the following two situations:
1) The electric actuator does not need to be reset: First, open the valve to any rotational position; remove the bolts connecting the operator and the main valve; rotate the operator and actuator at an angle of 180 at the same time, and then tighten the bolts connecting the operator and the main valve. There is no need to reset the electric actuator.
2) The electric actuator needs to be reset: After removing all the bolts at the lower end of the operator's reduction box, push the manual operating lever of the electric actuator and rotate it counterclockwise to rotate the reduction gearbox counterclockwise until it reaches the required angle and then find the position corresponding to the bolt hole. Tighten the bolt and reset the switch position of the electric actuator.
3.8 Replacing the valve stem packing
The valve stem seal of the double block and bleed plug valve is sealed with graphite packing. Since the seal here is dynamic, the sealing packing needs to be replaced after long-term use to ensure the reliable operation of the valve. Follow these steps to replace the graphite packing without removing the main valve from the pipeline:
1) Release the pressure at both ends of the DBB plug valve.
2) Release the inner cavity pressure.
3) Manually operate the plug valve and remove the cylindrical pin.
4) Remove the operator mounting bolts and operator.
5) Remove the packing gland and replace the inner and outer O-rings.
6) Remove the packing and replace it carefully.
7) Install the packing gland.
8) Install the new operator in reverse order.
9) Reset the valve’s opening and closing position.
10) Check the valve’s operation.
4. Conclusion
With the rapid development of China's oil and gas pipeline storage and transportation industry, double block and bleed plug valves have been widely used for their long service life, zero leakage and convenient maintainability. Therefore, higher requirements have been put forward for the use and maintenance of plug valves. Mastering the correct use of forced sealing double block and bleed plug valves can give full play to its excellent performance and provide a good guarantee for the safe production of enterprises.
Next: Common Valve Applications in the Petroleum Industry
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