Proper Actuator Sizing for Rotary Control Valves - Valve World

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Keywords: Actuator sizing • Rotary Control Valve • Safety

C O N T RO L & S M A RT V A LV E S

Proper Actuator Sizing for Rotary Control Valves In the sizing of rotary control valves, cost pressure leads to the actuators being sized with the lowest possible safety factors so that smaller actuator sizes can be used. The problem is that no applicable sizing standards to guarantee reliable actuator sizing exist. This results in the actuators being either oversized or undersized. This first part of the article, will show how rotary valves can be sized reliably.

By Domagoj Vnucec, Nadine Wetzstein & Dr. Jörg Kiesbauer, Samson AG

1. Latest developments It is common practice when sizing actuators for rotary valves to use the particularly friction-laden static torques that occurs near the valve‘s closed position as the relevant parameters to determine the required torque. Using this method while taking into account different effects that influence the valve‘s friction behavior may have been justified in the past as additional safety factors for the calculated torques, which have been gained from experience, were used as an additional basis. Additionally, the sized valves were mainly used as shut-off valves, which means that the differential pressure at the fully open valve that needs to be considered is considerably lower than near the valve‘s closed position. As a result, the valve‘s function could be ensured without exactly knowing the flow torque and by applying certain safety factors. Nevertheless, sizing actuators with indiscriminate, excessive safety factors leads to the actuators being oversized. As a result, different working groups, including user groups, have tackled this problem. Their work, however, only

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takes into account the friction-laden static torques that are relevant mainly in the valve’s closed position. Due to cost pressure and larger plants, however, rotary valves are increasingly used in throttling service, which means that high differential pressures occur across the valve’s entire rotational angle range. This may lead to considerably higher torques existing at larger opening angles due to dynamic flow effects than in the closing area. When taking into account the torque behavior – in particular of pneumatic rack-andpinion actuators or scotch yoke actuators, which show minimum torques in certain rotational angle ranges – it becomes evident that a rotary motion is only possible at certain opening angles (see Figure 1). Not taking the dynamic torque into sufficient account thus results in the actuator being undersized. As a result, the described method of sizing actuators is to be considered unreliable.

2. Calculation model To size actuators for rotary valves effectively, it is necessary to know the individual torque characteristics. This includes an in-depth analysis of the different components of the required actuator torque. Static or dynamic torque components dominate depending on

the application and actuating range the rotary valve is used in. Static friction torques occur mainly in the closing area, while the torque in the opening area is caused by dynamic flow effects. In the following, we will present a model for reliably determining the total of all torque components for all rotary valves. The static friction torques are made up of the bearing torque TB, the packing torque TP and the seat torque TS. The share of the seat torque is at a maximum, particularly in the valve’s closed position. The direction of the static torque always opposes the operating direction of the valve’s closure member. The dynamic flow torque TF, however, is determined by the differential pressure acting on the closure member in a flowing fluid. Apart from the differential pressure, the flowing medium can also have an effect on the valve-specific torque behavior. Depending on the direction of flow, the flow torque can cause the valve to open or close. As a result, the directions of the static and dynamic torques differ depending on the valve’s direction of action. Equation (1) shows the composition of the required actuator torque TTot. TTot

St Sa ( TB

TP

TS ) TF

(1)

Figure 1. Typical torque behavior of actuators.

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C O N T RO L & S M A RT V A LV E S

Figure 2. Standardized static seat torques.

The bearing torque TB and the packing torque TP are approximately constant across the valve’s entire rotational angle range and mainly determined by the material-specific coefficient of friction (see equations (2) and (3)). TB

f ( N,

B

, mDK , dW , g )

TP ⫽ f ( n, f P , dW )

(2) (3)

The seat torque, in contrast, is valvespecific and depends on the rotational angle. Figure 2 shows measured static seat torques relating to the maximum for two different arrangements of the closure member. In the ball valve for example, the seat ring has contact with the closure member across the entire opening range due to the central arrangement of the ball in the valve body. As a result, the seat torque acts across the entire rotational angle range. However, the seat torque increases near the ends of the range, particularly in the valve’s closed position. This is due to the percentage of contact area between the seat and closure member. Other valve types, such as butterfly or rotary plug valves, have a double-eccentric closure member. This allows the closure member to break away from the seat quickly so that contact only occurs in the closing area. This ultimately causes the seat torque to be at its maximum in the valve’s closed position and to disappear completely at smaller opening angles. The exact opening angle where the transition to this diverging seat torque

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behavior occurs, however, depends on the geometry of the seat and closure member. This friction behavior is taken into account in the associated equation (see equation (4)) through factor α, which depends on the opening angle, as well as the materialdependent coefficient of friction fS. TS

f

(

, f S , f M , DS , TS,dyn )

(4)

A further factor that influences the seat friction is the process medium. In valve types where there is contact between the seat and the closure member regardless of the opening angle, e.g. in ball or segmented ball valves, the state of the process medium causes the seat friction behavior to vary considerably. For example, the coefficients of seat friction

are lower with incompressible media than with gases, which is due to a lubricating film that forms on the closure member. In addition to the static seat friction, a dynamic seat torque TS,dyn can occur when a differential pressure exists. Closure members of floating ball valves for example, are free to move in the axial direction to a certain extent. When a differential pressure exists, the ball is pressed into a seat ring. Figure 3 shows that the dynamic seat torque increases in the same way as the differential pressure increases. This is caused by an increase in the coefficient of friction between the seat and the ball. Depending on the size and material of the seat ring, the increase behavior of the coefficient of friction can differ. Due to the eccentric design, seat torque components that depend on the differential pressure also exist in butterfly valves. Contrary to ball valves, however, these torque components only occur near the valve’s closed position, i.e. where the butterfly disk and the seat ring make contact. Apart from the friction-laden torques, a flow-induced torque TF has an influence as well. Due to the differential pressure to be let down at the valve, a pressure profile is produced at the closure member accordingly. Figures 4 and 5 use the example of a butterfly valve to show that the flow torque is not constant but depends on different factors that influence it. The flow simulated using CFD (Computational Fluid Dynamics) shows that different pressure profiles appear along the butterfly disk at different opening angles. Moreover, the levers,

Figure 3. Influence of the differential pressure on the dynamic seat torque of a ball valve.

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C O N T RO L & S M A RT V A LV E S

Figure 4. CFD-simulated pressure distribution in a butterfly valve at different opening angles.

which are required for torque calculation, change depending on the opening angle. This results in a flow torque that reaches its maximum at an opening angle of approx. 70°. This course of the dynamic flow torque is represented by coefficient β in equation (5). It needs to be taken into account that this flow coefficient depends on the detailed valve geometry as well as on how the medium flows across the closure member. Figure 5 also shows that the flow torque depends on the existing differential pressure, but that the flow around the closure member, which is determined by the valve design, determines the proportionality between the differential pressure and the flow torque. TF

f

(

, p, a, DDK )

(5)

Under normal conditions, the torque components mentioned so far take into account all essential elements for determining the required total torque of a valve. However, extreme conditions can cause a considerable increase in the static torques. For example, the coefficients of friction increase at very high operating temperatures due to the material expansion of the individual valve components. This is taken into account in equation (1) through safety factor St. In addition, static friction can increase in certain soft-seated valve versions if

Figure 5. Measured flow torques in a butterfly valve.

About the author Dipl.-Ing. Domagoj Vnucec Dipl.-Ing. Domagoj Vnucec is head of the development test bench department at Samson AG, Mess- und Regeltechnik in Frankfurt am Main, Germany. His work experience includes planning and evaluation of flow and acoustic laboratory tests carried out on control valves, application of CFD programs for the purpose of flow calculation, development and optimization of calculation and sizing methods as well as the implementation of sizing software for control valves. He can be reached at: [email protected] Domagoj Vnucec or +49 69 4009 1796.

M.Sc. Nadine Wetzstein M.Sc. Nadine Wetzstein works for the development test bench department at Samson AG, Mess-und Regeltechnik in Frankfurt am Main in Germany. Her work experience includes application of CFD programs for the purpose of flow calculation as well as planning and evaluation of flow and acoustic laboratory tests carried out on control valves. She can be reached at: [email protected] or +49 (69) 4009-2269.

Nadine Wetzstein

Dr.-Ing. Jörg Kiesbauer Dr.-Ing. Jörg Kiesbauer is the board member in charge of Research and Development at Samson AG, MESS-UND REGELTECHNIK in Frankfurt/Main, Germany. Standardization activities: Working Group 9 Final Control Elements of IEC SC 65B, DKE/K 963 Control Valves and ISA SP 75 Control Valve Standards. He can be reached at: [email protected] or +49 69 4009 1300. Jörg Kiesbauer

they are not operated for longer periods of time; this is expressed in safety factor Sa.

Part 2 of this article will be published in Value World March 2013.

Bibliography (1) Kiesbauer, J., Vnucec, D.: Fields of Application for Computational Fluid Dynamics in Control Valve Development, Valve World, KCI Publishing, Volume 15, Issue 3, April 2010, p. 27 to 31 and Volume 15, Issue 4, May 2010, p. 59 to 60, 63 to 64. (2) WIB Guidline: Automated block valves (ABV) Assemblies Part 1: Valve torque requirements, May 2011. (3) Mattick, R.: Operational parameters influencing the torque of ball valves - correct actuator size selection, Valve World Conference 2006. (4) NE 14: Attachment of Pneumatic Part-Turn Actuators to Valves, NAMUR Recommendation, April 2011.

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