Why is a bypass line required for the control valve?
A bypass line may or may not be required during installing of a control valve in the piping of a process plant. Unless otherwise required, the necessity of installing a bypass line is determined by the P & ID creator (process engineer) in consideration of safety, operability, maintainability, economy, etc. Also, the process licensors and customers may be determined by the above requirements.
Therefore, in this article, I shall explain when a bypass line is necessary and whether it needs to be installed. In addition, we will also explain the points to keep in mind when installing a bypass line. By clearly stating the criteria for the necessity of bypass lines in a book that summarizes the concept and further describing it in the P & ID legend sheet (lead sheet), the process design department and the piping design department have a common understanding and are surely reflected in the design.
When installing a bypass line is required?
Bypass lines are often required at the control valve during plant operation. In a large process plant, it takes time to start up or shut down the entire process. Before the regular operation time, the production volume of the product (fluids such as steam or gas) needs to escape from the prime mover. So, installing a bypass line to the control valve can continue the operation.
Supplement: As for the pressure-equalizing line and pressure-equalizing valve installed around the high differential pressure control valve, the purpose of the installation is different from the above, so the consideration explained in this article is not necessary. In such a case, when performing maintenance of the control valve, close the block valves before and after the control valve, remove the control valve, and manually adjust the opening of the bypass valve to adjust the flow rate and control the operation of the plant.
When is a bypass line not required to install?
Depending on the fluid to be handled and the operating concept of the entire plant, troubles (leakage, erroneous operation, etc.) caused by installing a bypass line may not be tolerated. In such cases, the bypass line will not be installed even if the control valve has to shut down the plant for maintenance. In addition, from the viewpoint of the economy, there are cases where a bypass line is not installed to reduce the amount of piping work and the construction cost.
The above is decided by comprehensively considering operability and maintainability, construction cost, and operating cost, so it is necessary to consult with customers and related departments in good consultation. If a bypass line is not installed, an operation handle (hand-wheel) may be provided on the control valve as shown above to enable manual operation on-site if necessary. However, when selecting a control valve, consider whether the operation case at the low opening and high opening during unsteady operation is covered without bypass (whether the control valve is of the type with such rangeability).
In addition, as shown in the above figure, the control valve, including the cutoff circuit, often does not have a bypass line. We are considering the risk that dangerous fluid will flow out of the bypass line even though the broken circuit is working due to the opening due to the erroneous operation of the bypass line.
Points to be considered during installation of a bypass line
The flow coefficient (Cv) value
– of bypass valve ・ Countermeasures against the erroneous operation
– of bypass valve ・Operability of the bypass valve
Value of Bypass valve flow coefficient (Cv)
Since it is necessary to adjust the opening of the manual valve of the bypass line, the globe valve is basically selected. In principle, the same Cv value as the control valve is selected. However, depending on the operating case, it is necessary to select a valve that covers the low opening range and high opening range as necessary. The Cv is a universal capacity index and is simply defined in terms of U.S. gallons of water per minute at 60°F (or 15℃) that will flow through a valve with a pressure drop of 1 psi (or 6894.76 N/m2).
Countermeasures against the erroneous operation of the bypass valve
If the bypass valve is inadvertently opened by operation during operation, there is a risk that the operation will be disturbed, leading to plant failure or emergency stop. It needs to be designed properly. For example, removing the manual handle of the bypass valve during plant operation and using the Locked Open / Close specification are effective measures to prevent erroneous operations. It is also important to keep in mind the effect of opening the bypass valve by mistake and letting it flow back through the valve. If backflow is unacceptable, installing a shut-off valve on the upstream side is necessary.
Bypass valve operability
It is also necessary to think about the field indicators (pressure gauge, thermometer) necessity for operating the bypass valve should be installed in a place that can be seen from the operating position. For that purpose, it is necessary to consider the on-site target indicator by noting “In View” on the P & ID and clearly stating the relationship with the target bypass valve. It is also important to secure a space between the actuator and accessories and the bypass line, considering the maintainability and operability of the control valve.
Summary
In this article, I explained the idea of the necessity of the control valve bypass line. Here is the summary of the article-
A bypass line may or may not be installed when installing a control valve in piping. Unless otherwise required, the necessity of installing a bypass line is determined by the P & ID creator (process engineer) in consideration of safety, operability, maintainability, economy, etc. Also, the process licensors and customers may determine the above requirements. By clearly stating the criteria for the necessity of bypass lines in a book that summarizes the concept and further describing it in the P & ID legend sheet (lead sheet), the process design department and the piping design department have a common understanding and are surely reflected in the design.