Every valve needs a means by which it can be operated (i.e., cycled or actuated). There are a variety of options to achieve this, including handwheels, levers, gears, and actuators.
Manual operators, such as gears, are relatively inexpensive and require little peripheral planning beyond the installation and orientation of operators in the process line. Gears are simple machines that utilize a series of mechanical parts to increase efficiency – the mechanical advantage that the user gains.
Torque is a function of force and distance – the required force (rim pull) required to open or close the valve can be decreased by increasing the length of the lever or diameter of the handwheel mounted on the valve. The industry defines specifications for the highest values personnel should exert on levers or handwheels to operate a valve. Current API specifications limit pull to 360 Newtons (80 pounds-force). The maximum lever length or handwheel diameter also is limited by industry specifications.
The tooth design and tooth contact can influence the efficiency of the gear as well. Strategically designing the shape of the gear tooth reduces the amount of contact between teeth and the bending stress on the teeth, increasing durability.
Worm Gears (see Figure 1)
Standard worm gears can be configured to fit almost any partial-turn or multi-turn valve (e.g., ball valves, butterfly valves, etc.). Worm gear operators create a mechanical advantage in torque, which reduces the amount of work and/or the number of turns required to cycle a valve.
High-performance/high-efficiency worm gears (see Figure 2)
- The double-enveloping design of MAXTORQUE high-performance worm gear creates multiple start points between the gear wheels, increasing the torque capacity when compared to a single enveloping design. (see Figure 3)
- These gears can be integrated easily into an electric actuator package to reduce the required size of the actuator. They also increase efficiency due to the increased mechanical advantage.
- High-performance worm gears offer a simple solution for large, high-pressure valves that are operated infrequently, but where timely, safe operation is important – such as pipeline isolation or at pig launchers and receivers. High-performance gear units also carry a distinct advantage in subsea applications where it is imperative to cycle a valve as quickly as possible in order to maximize productivity for divers or remotely operated vehicles (ROVs).
- High-performance gears have a much higher level of efficiency than standard worm gears with the same level of safety against back-driving. The higher efficiency results in a significant reduction in the gear ratio, which reduces the required number of turns while remaining below acceptable rim pull requirements. Optimizing for number of turns and/or rim pull can provide a manual operator with a solution for applications where traditional worm gears would exceed rim pull or number of turn limits.
- The disadvantage of high-performance worm gears is cost. But, while they are more expensive than a traditional gear operator, high-performance worm gears remain significantly less expensive than automating the valve with an actuator.
Bevel Gears (see Figure 4)
Bevel gears are used to increase thrust efficiency to operate rising stem valves such as gate valves and rising stem ball valves. They utilize the same basic principles of a worm gear, but affect thrust instead of torque. The two gear wheels in a bevel gear come together (most commonly) at a 90-degree angle (see Figure 5), and allow the transfer of movement to a new angle. The mechanical advantage of a bevel gear is due to the gear ratio and increased efficiency.
Miter Gears (see Figure 6)
A miter gear typically is a 1:1 ratio bevel gear. These gears generally are used to change only the input shaft direction, while a bevel gear is used to change both ratio and shaft direction. Since a miter gear has a ratio of 1:1, it doesn’t reduce required torque input into the gear operator.
Spur Gears (see Figure 7)
Spur gears are added to existing gearboxes to increase mechanical advantage. However, this increases the overall gear ratio, which increases the number of required handwheel turns.
Dual-input gears are comprised of two gears in one housing, with two input shafts and gearing. During the first 10% to 20% of travel, one gear works to overcome the break torque. Once through the break phase, the operator engages the second gear for the run torque. Because the run torque is less than the break torque, utilizing different gears for this operation can reduce the workload and closing time.
Manual Override Gears
Manual override gears are used in conjunction with an actuator to provide a means of manual operation if the power supply to the actuator fails.
There generally are three designs to these gear types:
- The first design utilizes a worm that is engaged only when override is required.
- The second design, called a loss-motion override, has a coupling that is always engaged with the actuator, but the valve is free to rotate 90 degrees. In this design, when override is needed, the gearbox rotates until it engages with the key, then rotates back to operate the valve.
- The third design utilizes planetary gears, which allow input from an actuator or manual means to rotate the valve without engaging or disengaging. Compared to a loss-motion override, the planetary gear design reduces the number of required turns to operate the valve by half.
Selecting a Gear
There are three forces to consider when sizing a gear for a valve – break torque, run torque, and seating torque. The break torque usually defines the size of the gear and is the highest torque value, followed by seating torque. The run torque, which is the torque required while the valve is cycling, usually is the lowest torque value and also is the torque used for the majority of the valve’s operation.
Gears are especially well suited for applications where a portable driver is not readily available, where utilities are not present for an actuator, or where design simplicity is a priority.
Knowing your application and safety standards is key to selecting a gear.
The first step in identifying the best gear type for your application is to know what type of valve will be operated. As stated earlier, worm gears are designed to provide torque for partial-turn or multi-turn valves such as ball valves, butterfly valves, globe valves, and plug valves. Bevel gears provide thrust for rising valves such as gate valves and rising stem ball valves.
The environment in which a valve will be placed impacts the materials used to construct the valve. Keeping the environment (e.g., snow, rain, ice, sand, etc.) out of the gearbox helps ensure that the gears do not stick or become damaged during operation. Special lubricants and materials can be used to construct the gear wheels to help prevent this, in addition to housing construction considerations.
The frequency of operation also will impact the gear selected. More and more, worker safety factors, such as maximum handwheel size and turns to close, are being imposed. Valves must be automated as high-torque applications begin to exceed the manual actuator’s ability to meet acceptance criteria.
Fortunately, innovators like Cameron are finding ways to make manual actuation design better. For example, two-speed auxiliary spur options offer a higher ratio use through the valve run torque and a lower ratio selection to meet the maximum torque requirement of the application.