Worker safety, efficiency and the cost of operations, and most recently, new methods of control, are key focal points in operating valves. One factor that affects all areas is the role and level of human involvement in the processes.
Opening or closing valves can be completed either by manual input or automated devices driven by various energy sources. Manual operators are simple, inexpensive and require little peripheral planning beyond the installation and orientation of operators in the process line. Automated devices, on the other hand, require input energy systems, control systems, additional installation space and infrastructure for support, operation and maintenance.
Two concerns considered during selection of manual operators are the effort required to operate the valve and the number of turns some valves require. A lot of effort and a high number of turns can result in personnel fatigue, safety concerns, excessive time for operation and the need for multiple personnel. Also under consideration in selecting manual operators are the valve’s expected frequency of operation and the physical location of the operation, such as whether it might be high in a superstructure or situated in an inhospitable environment. Both also present challenges to humans.
Designers have to weigh all of these factors in their decision matrix to receive the most productive yet acceptable selection of how a valve should be operated. Two aspects that primarily define operator selection are human factors and economic factors. Human factors can be defined as the human capability to cycle the valve in a safe, timely and economically sound manner. These factors require considerations such as the work needed to be done (turns and rim pull) to operate the valve, the environment in which the valve is located, the time required to complete the task, and the health and safety of the personnel involved. Economic factors include the cost of the actuator as well as the cost of infrastructure, which could include wiring, controls system, power required and ongoing maintenance to support automated solutions.
THE SPECS INVOLVED
Specifications for the highest values personnel should exert on levers or handwheels to operate a valve are defined in the industry, with current API specifications limiting pull to 360 newtons (80 pounds-force). Mechanical advantage can be used to decrease the pull required to open or close the valve by increasing the length of the lever or diameter of the handwheel mounted on the valve. However, the maximum lever length or handwheel diameter is also limited by industry specifications.
As valve torque increases, maximum limits imposed by industry standards result in levers transitioning to gear units to increase mechanical advantage. However, this increase in mechanical advantage comes with the disadvantage of increasing the number of turns to move the valve across the full stroke distance.
The higher number of turns results in longer time required to cycle the valve at a constant number of revolutions per minute. With significant gear reduction, the number of turns required for full cycle can number in the hundreds. This increased number of turns leads to a greater opportunity for accidents or injury to personnel due to repetitive motion and fatigue. Companies will limit the rim pull and number of turns to reduce the risk. Once established limits are exceeded for turns, the valve is generally required to be automated.
Communicating what human factor limits might be imposed on valves can provide suppliers the opportunity to recommend the best value of operator for manual valves. Until recently, the valve industry was limited to levers, bevel gears and traditional worm gears for manual cycling. Once these devices exceeded worker safety limits, a valve purchaser’s only choice was to select an automated solution. However, new devices available in the marketplace extend the range of manual operators. These devices can reduce initial capital costs, reduce site design complexity and minimize operating expenditures. The devices include high-efficiency gear operators and portable drivers coupled with well-designed arrestors.