Controlling Our Water Systems
Editor’s Note: This article is an excerpt of one chapter in Cylinder and Vane Actuators and Control—Design and Installation, which was produced by the American Water Works Association last year. What appears here in Part I of this article is an explanation of the types of actuators, how they’re used and torque considerations. Part II of this column, which is on www.VALVEMagazine.com, continues this discussion by explaining further the types of characteristics the different types have.
WHAT ACTUATORS ARE USED
The primary function of cylinder actuators is to produce linear motion to restrain or operate the closure member of valves and gates. A cylinder actuator consists of a pressure-retaining barrel bolted between head and cap ends (Figure 1). The piston includes seals on its outside diameter to contain pressure between the piston and either end of the cylinder. Important dimensional information for cylinders is the bore or inside diameter of the barrel, the stroke or maximum travel of the piston and the rod diameter, which changes with cylinder bore and operating pressure.
When supply pressure is directed through one of the ports and applied to one side of the piston, the opposite side vents to drain, creating a differential pressure across the piston. The differential pressure is applied over the area of the piston, resulting in a force or thrust on the rod. It is important to recognize that the areas on either side of the piston are not the same. The piston area on the left side of the figure is reduced by the cross-sectional area of the rod. Hence, a cylinder will produce more thrust when pressure is applied to the cap end than the head end.
In a linear cylinder actuator, the head of the cylinder is rigidly mounted to the valve body and the rod is attached to the valve stem or closure member to mechanically create thrust, operating the valve or gate closure member in a linear manner. Thrust is the linear force on the valve stem or slide gate, generally expressed in units of pound-force (lb-F) or Newton (N). During lifting (opening) of the gate, the head end of the cylinder will be pressurized, with reduced area and reduced thrust. A quarter-turn cylinder actuator converts the linear cylinder thrust into torque using one of the mechanisms shown in Figure 2 (page 40).
Torque is a twisting or turning movement on the valve stem or shaft, generally expressed in foot-pounds (ft-lb) or newton-meters (N-m). The required torque must overcome the frictional and flow-induced forces in the valve to permit mechanical operation of the valve. The lever mechanism uses a special cylinder whose head pivots about a trunnion or post. The cylinder rod is attached to a simple lever with a rotatable pin. As the cylinder rod extends, it pushes on the pin end of the lever, thereby rotating the lever 90 degrees. The link-and-lever mechanism is similar except a link is used between the rod and the lever to provide an additional mechanical advantage when the link is perpendicular to the lever.
This mechanism produces high lateral side loads on the cylinder rod, so the pin must be supported along its travel by a guide slot in the actuator housing or with a support bar. The rack-and-pinion mechanism has a straight gear rack attached to the cylinder rod. As the cylinder rod is extended, the rack rotates the pinion gear. This mechanism also produces high lateral side loads on the cylinder rod, so the rack must be supported along its travel. The scotch-yoke mechanism consists of a cylinder rod and pin that travel in a slot contained within the lever. In most scotch–yoke mechanisms, the cylinder rod is extended across the housing to provide support for lateral side loads.
Another version of the rack-and-pinion mechanism (Figure 3) consists of linear racks that mate with a rotating splined-pinion gear inside the cylinder. As air pressure is applied to either end, the pistons in the actuator are sealed against the cylinder housing, driving the two racks and rotating the pinion gear. This type of rack-and-pinion actuator is relatively compact and typically used on smaller valves.
Vane actuators (Figure 4) act like a lever and directly produce torque through a 90-degree arc to rotate the valve stem when there is supply pressure on one side of the vane. The vane is enclosed in a pie-shaped housing and has resilient seals around its periphery to maintain the differential pressure necessary for converting the applied pressure to torque. Hence, the vane actuator is compact and has only one moving part.
Slide-gate cylinder actuators are required to generate thrust to lift and lower the slide gate (see Figure 5, page 42). The thrust should be provided by the gate manufacturer with consideration given to the weight of the gate, the weight of the stem and the friction on the frame due to the differential pressure acting over the area of gate. Thrust values can vary greatly depending on the types of seats and gate wedges employed in the valve design, so the thrust requirements from the manufacturer are needed to properly size the actuators. Typically, the maximum thrust required to operate a vertical slide gate occurs while opening the valve, during the release from the wedges and lifting the weight of the gate. The force is, in part, empirically determined by the manufacturer and normally presented in the following equation (AWWA C560). The 1.5 factor applied to the P1 term represents the thrust needed to overcome the friction from the wedges used to seat the gate.
F = (Dw × f × A × H) + (1.5 × P1 )+ P2
F = total force required to open, lb (N)
Dw = weight density of water, 62.4 lb/ft3 (1,000 kg/m3)
f = friction factor of slide against the seat = typically 0.35
A = area of the gate opening, ft2 (m2)
H = differential head of water at gate centerline, ft (m)
P1 = weight of slide, lb (kg)
P2 = weight of stem, lb (kg)
When cylinder actuators are sized for slide gates, a sizing safety factor of at least 1.3 should be applied to the overall thrust.
Globe valves (Figure 6) are linear valves that require operating thrust calculations similar to those for a slide gate, except that there may be flow-induced thrust greater than the differential pressure force, depending on the shape of the flow path and the flow conditions. As with the slide gate, the maximum thrust is generated during the unseating condition where the differential pressure is the greatest. For flow over the globe plug, unseating would be the maximum force. But for flow under the globe plug, the differential pressure would be trying to unseat the plug.
Quarter-turn actuators generate torque to rotate the valve stem. Torque is defined as rotational twist or turning moment on the valve stem or shaft, expressed in units of foot-pounds (ft-lb) or Newton-meters (N-m). As Figure 7 (page 42) shows, the torque produced by a wrench is found by multiplying the applied force by its distance from the bolt. Valve torque is generated from several factors within quarter-turn valves that relate to valve design. Some of the torque components and their source include:
- Seat torque—friction between the seal and the seat
- Bearing torque—shaft friction due to differential pressure on the closure member
- Packing torque—seal friction around the valve stem or shaft
- Dynamic torque—flow-induced torque occurring in the mid-range of valve operation due to unbalanced differential pressures across the closure member
- Hydrostatic torque—unbalanced static pressure force across the closure member when only one side of the pipe is full of water and the shaft is horizontal
- Shaft-offset torque—unbalanced pressure across an offset closure member
- Center-of-gravity torque—unbalanced gravity forces acting on a nonsymmetrical closure member with a horizontal shaft
These torque components are combined to calculate the two required maximum torques for actuator sizing. Break torque is the amount of rotational resistance that must be overcome to initiate valve motion when the valve is in the closed position.
The torque may be higher during seating or unseating of the valve, depending on the valve design. The break torque is typically the sum of the seat, bearing, packing, hydrostatic, shaft-offset and center-of-gravity torques. In general terms, break torque is proportional to the square of the valve size (diameter) and the differential pressure. Run torque is the amount of rotational resistance that must be overcome to sustain motion during valve travel.
For larger valves, dynamic torque can play a significant role, and because it varies with valve angle, the maximum run torque is often reported with the valve angle at which it occurs. The run torque is typically the sum of the hydro-dynamic, bearing, packing, shaft-offset and center-of-gravity torques. In general terms, run torque is proportional to the cube of valve size and the square of flow rate. Hence actuators for large valves at high flow conditions will usually be sized on run torque. Run torque is combined with other friction torques to produce the total valve torque, which varies with valve position (Figure 8). AWWA Manual M49, Butterfly Valves: Torque, Head Loss, and Cavitation Analysis, provides the methodology for predicting valve operating torque and represents the current method used for the water industry by many valve manufacturers.
For a continuation of this discussion, go to www.VALVEMagazine.com, where you’ll find information on position characteristics, sizing considerations and more. For information on this report, go to www.awwa.org.
Actuators in the oil and gas industry operate many types of valves, and they must do so with great precision and accuracy.
Since the early days of water and wastewater treatment plants in the U.S., municipalities and industries have continued to expand and upgrade existing plants and distribution infrastructure as well as build new facilities.
The issue is that assemblies are not currently treated as “engineered” items — they are often produced by slapping the actuator on the old (or new) valve specified by the piping requirements without much understanding of the specifics of the interactions between valve and actuator connections through the drive train (coupling).