Article: Selecting the Right Rotary Control Valve

Wade Helfer, Rotary Technologist, and Jason Jablonski, Director of Fisher Rotary Engineering, both at Emerson Automation Solutions, recently published an article in the November 2020 issue of Hydrocarbon Processing. The article describes the key features one must consider when selecting a rotary valve, and it’s titled Selecting the Right Rotary Control Valve, please see a summary below. 

Control valve selection can be a complicated task involving a large number of design considerations. Wade and Jason describe the issues:

The primary function of a control valve is to regulate flow. Accurate, consistent and reliable performance of this task affects many important process unit or plant metrics such as throughput, quality and uptime. Valve responsiveness is a key factor for determining process loop performance, but it is often limited by valve construction. Selection of the right valve type and features can be a complex task. 

While reciprocating globe valves are often the valve of choice in control applications, rotary valves can be an attractive alternative due to their compact size, higher flow capacities and lower installed cost. However rotary valves do have limitations which can be exacerbated or overcome depending on design features of the rotary valve itself. There are generally three areas of concern:

  • Design of the rotary valve
  • Design of the valve actuator
  • Design of the linkage connecting the two

Careful attention must be payed to each of these areas to insure the control valve performs as required. 

Rotary Valve Design Concerns

When evaluating a rotary valve, start by examining the valve itself. The valve design incorporates three main features that directly impact performance: friction, flow characteristic, and valve sizing. Each of these items must be carefully considered.

 Reducing friction. The authors describe issues arising from excessive friction:

The drivetrain of a valve is made up of all the components that translate actuation energy (in the forms of electricity, compressed air or hydraulics) to the final control element (disk, ball or plug). Friction and lost motion are the two main factors affecting the ability of a valve drivetrain to control accurately. 

The goal is to minimize friction within the drivetrain, with most of this friction coming from seating forces, shaft bearings and packing. Friction between the final control element and the seal ring is the primary contributor of drivetrain friction. The desire to have a tight shutoff control valve can negatively impact control performance due to the resulting increase in seal friction and resulting stiction as the control element works to open or close against this seal. 

Rotary valves with more than one shaft offset can help a valve respond to changes in the process because the offsets allow the control element to abruptly loosen contact with the seal ring. This reduction in contact eliminates seal friction after only a few degrees opening. Figure 1 shows cross sections of a double-offset butterfly valve in the open and closed positions.

 

Figure 1: Cross-section images of a double-offset butterfly valve, showing disk position and offsets in the closed position from the side (left) and the open position from the top (right). 

Other sources of friction include bushing and bearings, along with packing box friction. Bushing and bearings should employ low coefficient of friction and wear-resistant solutions such as carbon or polytetrafluoroethylene coatings or liners. Packing should use low coefficient of friction materials that seal well but allow the shaft to turn easily. Packing design is a constant tradeoff between sealing ability and friction. More packing load results in less emissions but also higher friction. The goal is to find a packing system with minimal friction that prevents leakage at low packing loads. 

Installed Characteristic. When a rotary valve is integrated into a control loop, the installed response needs to be approximately linear. This response is determined by the response curve of the valve itself, as well as the response of the process. Due to varying pressure drops and different flow rates, the process response is often non-linear. Ideally the rotary valve response characteristic should be the inverse of the process, such that net result is linear. The valve characteristic is a function of the valve design but can be modified by configurations in the control system or positioner.

Valve Sizing. The final design consideration is valve sizing. The Cv of the valve should be chosen such that the valve can provide the maximum flow required, while still controlling at the lowest desired flow rates. An oversized valve will control poorly at low flows. 

Specifying the Right Actuator Type

Rotary valve actuators have three main design features that impact valve performance: torque, dynamic torque, and lost motion. All of these are affected by the type of actuator chosen. The authors describe the challenge of selecting a proper actuator for rotary control valves: 

Different types of actuators exist, all with different degrees of lost motion and resultant levels of control that need to be considered when choosing a valve assembly. A well-engineered control valve should be able to control flow at +/- 1%, with the best valve constructions able to control flow as low as +/- 0.25%. 

Pneumatic spring-and-diaphragm actuators (Figure 2) are simple yet effective constructions, where low pressure (up to 70 psig) air acts on a rolling, molded diaphragm against a spring. This actuator type has very low friction and a consistent torque output. The actuator must convert linear motion to rotary motion, introducing levers and linkage, but pneumatic diaphragm actuators are still considered the best type for control applications because they are capable of controlling flow at +/- 0.5% or less.

   

Figure 2: This spring-and-diaphragm actuator provides very accurate flow control. 

Pneumatic piston actuators provide high stem force in a smaller package by using higher-pressure plant air (up to 150 psig). The piston seals against the cylinder with elastomer O-rings, which have higher friction than a diaphragm and wear over time, resulting in reduced performance. Similar to pneumatic spring-and-diaphragm actuators, the piston actuator must convert linear to rotary motion. 

Rotary vane actuators may be hydraulic or pneumatic. This type of actuator inherently generates true rotary motion, so linkages are not required, but the seal arrangement in the actuator creates significant friction. This type of actuator provides control comparable to a piston actuator. 

Pneumatic rack-and-pinion actuators use dual pistons with O-ring seals, along with a rack-and-pinion gearset to translate linear motion to rotary motion. The seals and gears create significant friction and backlash, allowing only rough control of about +/- 5%. 

Valve Linkage Design Concerns

The final area of rotary valve design is the linkage that connects the actuator to the valve, while allowing a positioner to accurately sense the true valve position. These seemingly simple mechanisms can introduce a number of problems if not carefully designed. The linkage can create significant friction, backlash, and lost motion—making tight control virtually impossible.

Shaft diameter. The diameter of the shaft can create significant problems if it is too small. Wade and Jason describe the problem:

 Angular deflection of the shaft associated with an applied torque is known as “shaft windup,” and it is undesirable because it increases lost motion in the drivetrain. The most effective way to minimize shaft windup is to increase shaft diameter.

Thicker drive shafts are very stiff so when a lever forces the shaft to turn on one end, the valve end immediately moves as well. A thinner shaft will first twist under load before eventually turning the valve. When the valve reverses position, the shaft must first untwist, then twist in the opposite direction before moving the valve. This lost movement and backlash dramatically and negatively impacts valve performance.

 Shaft connections. The connections between the shaft and valve disk and shaft and actuator levers can be another source of lost motion. The authors describe a solution: 

The best way to eliminate lost motion is through zero clearance connections of the drivetrain components, and this feature is imperative for critical control loops. Pinning options exist that use taper pins (Figure 3) or taper keys that accomplish this goal between the shaft and control element.

 

Figure 3: This pinned shaft-to-disk assembly eliminates lost motion at this connection point. 

The actuator end of the shaft also requires carefully designed connections. Better designs employ splined shafts and clamped connections (Figure 4) to ensure there is no lost motion or slippage when the actuator moves the valve.

                                 

Figure 4: This clamped spline connection eliminates lost motion in the shaft-to-lever connection 

Valve couplings. Depending on the actuator and positioner selected, some rotary valves require valve couplings to transfer motion, and if not carefully selected, they can introduce lost motion into the system. Figure 5 shows an example of a new and worn coupler used to connect the drivetrain of the valve to the drivetrain of the actuator. One worn component can seriously impact a valve’s ability to control flow.

   

Figure 5: This Image shows worn (left) and new (right) valve couplers. A worn coupler results in an increase in lost motion as the actuator moves more without resultant motion in the square drive shaft.

 Takeaway

Jason and Wade conclude their article with these thoughts:

Control valves have special design characteristics that allow them to respond to small changes in the process with precise flow regulation. When selecting a control valve, it is important to merge the supplier’s knowledge of the capabilities associated with different valve types, actuator styles, and digital positioners with the end user’s understanding of the process loop. This effort will result in a cost-effective valve assembly that provides optimized loop performance. 

All figures courtesy of Emerson 

About the Authors

Wade Helfer is the Rotary Technologist at Emerson Automation Solutions for its Fisher rotary valves, and is responsible for developing and evaluating new technologies. He has 22 yr of industry experience in the design and evaluation of control and isolation valves for a variety of industries, and is an expert in rotary valve seals, butterfly valve flow dynamics and high-temperature valve design. He completed his BS degree and graduate coursework in mechanical engineering from Iowa State University. 

Jason Jablonski is the Director of FisherTm Rotary Engineering at Emerson Automation Solutions, and has 20 yr of experience in the design, testing and manufacturing of process control equipment. He is a project management professional, an agile certified practitioner, and a member of the API subcommittee on piping and valves. Mr. Jablonski earned a BS degree in mechanical engineering from Iowa State University and an MBA from The University of Texas at Dallas.