Many piping and valve engineers are familiar with the American Petroleum Institute’s API 6D standard for pipeline valves. A lesser-known standard is API 6DX, which covers actuator sizing and mounting kits for pipeline valves. This second standard defines requirements for mechanical integrity between the valve and actuator and provides actuator sizing for the valves produced based on the API 6D standard. Meanwhile, a third standard, which covers mechanical integrity and sizing of actuators and mounting kits for pipeline valves, is the International Organization for Standardization’s ISO 12490.
This article lists key considerations based on API’s and ISO’s standards and the industrial practices that can provide strong and sufficient integrity between valves and actuators. Generally speaking, that includes proper design of the mounting bracket, drive shaft and bolting connectors.
A common practice in pipeline design is to install a mounting bracket: a mounting kit or coupling between the valve and actuator. A coupling, as automation mounting hardware, acts as an intermediate support and connector between the valve and actuator.
Figure 1 shows a pneumatic-actuated ball valve with a coupling between them. (Note: electrical actuators, in most cases, can sit directly on the valve without a coupling.)
Couplings come in different shapes and sizes such as circular, cubic and others. The question is, what parameters should be considered in designing couplings so high integrity is established between the valve and actuator?
ACTUATOR AND OTHER LOADS
API 6DX section 7.6 requires that mounting kits be designed to withstand all loads from the actuator. It specifies that kits withstand 1,1 times the actuator loads (ni European measure). Stresses generated in the mounting bracket from the 1,1 times actuator loads shouldn’t exceed 67% of yield strength.
Three categories of loads have been specified in these standards:
- Pressure relief valves or any pressure-limiting device on the control panel
- Torque/thrust generated by compressed spring force for spring return actuators
- Torque/thrust generated by electrical actuators
The mounting kit and coupling also should withstand other loads, such as wind, snow, seismic and blast loads. This last load—blast or explosion load—depends on the dynamic load (pressure) and the drag coefficient. Dynamic pressure could be equal to 0,3 barg as an example based on a past offshore project. Figure 2 illustrates a plant design management system (PDMS) model with two actuated ball valves in blue and a structure support in yellow and red. The reason for designing and applying the structure support is to protect the actuator from the blast load (since the blast load was not considered here during the coupling design.) A gap is recommended between the support and actuator to provide enough space for actuator air supply tubing passage and for maintaining actuator flexibility.
Other considerations include:
Valve and actuator orientation: The orientation of the valve and actuator is important in coupling design. Valves installed with a horizontal stem can require support between the valve and the bottom of actuator to ensure accurate alignment of the valve and actuator. In some cases, the coupling bolts can be increased, or a material can be selected with higher strength for coupling and bolting connections.
Frequency of cycling and speed of operation: If the valve requires more cycling or higher speed of operation, more stress occurs on the actuator, valve and coupling.
Type of valve and actuator orientation: The orientation and type of the actuator may cause more load concentration on the mounting coupling and may require more robust design and selection. Figure 3 shows that no coupling may be required for electrically operated actuators that have through-conduit gate valves because the electrical actuator is light.
Coupling Connections: Dowel pins and bolts are integral to the mounting and must be designed in a way that can properly transfer the loads from the actuator to the valve. Dowel pins or fitted bolting ensure the proper location of the actuator, which prevents misalignment or improper assembly of the actuator on the coupling. The typical valve to actuator interface is based on ISO 5210 or 5211 standards.
Fabrication: Solid couplings without any welding is the preferred choice of bracket design, especially if the actuator is heavy and large. Finite Element Analysis might be done on the coupling to ensure the coupling can withstand the stress values. If welded, full penetration weld is the preferred choice compared to a fillet weld because full penetration provides better joint efficiency.
Material: Different choices of materials are available, such as carbon steel, austenitic stainless steel and 22Cr duplex. Austenitic stainless steel 316 may be more practical for offshore applications as opposed to painted carbon steel. However, 22Cr duplex coupling provides higher mechanical strength compared to austenitic stainless steel 316.
Section 4.7 of API 6DX, states: “Coupling is a driven component (driver adapter, drive tube, drive shaft) that allows transmission of torque and/or thrust from an actuator driving component to the valve shaft/stem.” Therefore, shaft and shaft extension integrity and strength have a positive effect on the valve and actuator integrity.
Figure 4 illustrates the mismatching of two pieces of stem extension between the valve and actuator. These two pieces are not machined correctly at the end so the stem extension (drive shaft) is loosely coupled between the valve and actuator.
Stem extension should be designed in accordance with ISO 14313 and API 6D Standards for Pipeline Valves as per both API 6DX and ISO12490. Those standards also state the output of the actuator should not exceed the stress limits of the valve drive train. In fact, maximum allowable stem torque (MAST) should exceed the torque values produced by the actuator.
CONCLUSION AND RECOMMENDATIONS
The mounting bracket, stem extension and bolting connections between the valve and the coupling as well as the coupling and actuator themselves all play important roles in the integrity between the valve and actuator. Multiple parameters, such as actuator, wind and snow loads, type and size of the actuator, valve and actuator orientation, coupling material, fabrication and other factors should be considered for maintaining the valve and actuator integrity. Additionally, stem and stem extension should be designed to withstand the loads the actuator generates.
- American Petroleum Institute (API) 6D, (2015). Specification for Pipeline and Piping Valves. 24th Edition. Washington, DC, USA.
- American Petroleum Institute (API) 6DX, (2012). Standard for Actuator Sizing and Mounting Kits for Pipeline Valve. 1st Edition. Washington, DC, USA.
- International Organization for Standardization (ISO) 12490, (2011). Mechanical Integrity and Sizing of Actuators and Mounting Kits for Pipeline Valves. 1st Edition. Switzerland.
- International Organization for Standardization (ISO) 14313, (2007). Pipeline transportation systems-Pipeline Valves. 2nd Edition. Switzerland.