Control Using Wireless Throttling Valves

Various combinations of wired and wireless transmitters and valves are required to address different process control applications:

Wireless transmitter plus wired valve
Wired transmitter plus wireless valve
Wireless transmitter plus wireless valve

Ideally, a wireless valve communicates the valve position (i.e., the target position that the valve accepted and is working to achieve) with minimal delay. However in some cases, significant delay may occur between the times the valve receives a command to change position and it communicates its response to the controller. Additionally, communications from the controller to the wireless valve may introduce significant and variable delay in the valve’s response to control action. To allow the PID (proportional-integral-derivative controller) to automatically compensate for the delay introduced by wireless communication, a new HART command has been proposed for control that utilizes a wireless throttling valve. This new wireless command supports the inclusion of a “time to apply” field with the output value communicated to a wireless valve. This added field specifies a time in the future when the output value takes effect. The time to apply value is selected to ensure that the valve receives the output communication before this future time, as illustrated in Figure 2.

Thus, when the time to apply command is used to communicate with a wireless throttling valve it is possible for the controller to calculate the implied valve position. To ensure that the calculated implied valve position accurately reflects the target position in the valve, a new output command to change valve position can be issued to the gateway only if the gateway has received confirmation of the last communication.

Flow Lab Test of Wireless Control

Closed loop control of a liquid flow process was evaluated using both wireless and wired flow measurement and wired or wireless access to a throttling valve. A special switch box was used to select whether the target valve position to the throttling valve was provided by the controller’s 4– 20mA output or wirelessly from the controller/gateway using a modified wireless HART valve positioner. Communications with the wireless valve was implemented using the command that allows a time to apply to be specified. The external reset input of the PIDPlus was modified to compensate for the delay introduced by the time to apply command. The flow lab used in these tests of wireless control is shown in Figure 3.

The primary objective of the tests conducted in the flow lab was to measure and quantify the deviation of the control parameter from setpoint as a measure of control performance and to measure total valve travel when using wireless transmitters and/or valves. In addition, communication statistics were collected during these tests. The control modules used in these tests were designed to allow the wired or wireless transmitter to be selected as the flow measurement used in control. Also, the modules were designed to allow the wireless or wired valve to be selected in each test run as the manipulated parameter of the liquid flow control loop.

The control achieved using a wireless valve and wireless transmitter was comparable to that of a wired transmitter and wired valve for the battery of tests conducted. PID tuning was set based strictly on the process gain and dynamics and was never changed throughout the wireless tests. This illustrates that the PIDPlus tuning is not impacted by transmitter and valve update rate and delay introduced by communications. When a wireless transmitter was used with PIDPlus, the number of changes in valve position was reduced by a factor of 47 since the output of the PIDPlus only changes when a new measurement is received or the set point is changed. Changing the wireless transmitter update rate from 8 seconds to 16 seconds had minimal impact on control performance.

Wireless Control of a Divided Wall Column

To further evaluate and demonstrate closed loop control with a wireless throttling valve, the Separation Research Program at the University of Texas (UT), Austin, TX is using prototype wireless throttling valves to evaluate control performance. This cooperative industry/university program performs fundamental research of interest to industry. A current area of research is control of the divided wall column (DWC) process. The divided wall column (DWC) process design can provide savings in energy and capital cost compared to a conventional column design. However, very little has been published on DWC control design. Therefore, a project has been initiated to study and document DWC operation and control based on a test that used a 6–inch-diameter divided wall distillation column (Figure 4).

As part of the DWC tests, closed loop flow and temperature control is being evaluated using both wireless and wired flow measurement and wireless and wired throttling valves. Four flow control loops and two temperature control loops on the DWC are designed to utilize wired or wireless input and wired or wireless throttling valves. Mass flow transmitters function as wired inputs to the control system. THUM adapters are installed on the flow measurements to provide a wireless input that is accessed in the control system through a wireless HART gateway.

The control modules used in DWC tests are designed to allow either the wired or wireless transmitter to be selected as the flow measurement used in control. Also, the modules allow the wireless or wired valve to be the manipulated parameter of the flow and temperature control loops used to evaluate wireless control. During each test run, the integral of absolute error (IAE), total valve travel and communications statistics are calculated.

In late October 2014 the divided wall column was initially brought on-line and soon after the control using wireless flow measurements and wireless throttling valves was placed in automatic control, as shown in a screen capture in Figure 5.

Further testing is currently being conducted to compare control performance using wired vs wireless field devices.

Temperature control (2 loops) – Using wireless temperature transmitter and wired/wireless throttling valve with an 8 second update rate and a 16 second periodic update rate.
Liquid flow control (4 loops) – Using wired/wireless flow transmitter and wired/wireless throttling valve with an 8 second update rate and a 16 second periodic update rate.

Summary

Field tests have demonstrated that the control performance achieved using a wireless valve is comparable to that achieved using a wired valve and that installation time and cost may be reduced using wireless field devices. With the proper equipment, the tuning that is used for a wired installation can be applied to an installation that uses wireless field devices independent of the communication update rate.

References

T. Blevins, M. Nixon, D. Chen, W. Wojsznis – Wireless Control Foundation – Continuous and Discrete Control for the Process Industry, 2014

F. Siebert, and T. Blevins, “WirelessHART Successfully Handles Control”, Chemical Process, January, 2011

Authors:

Terry Blevins is an ISA Fellow and principal technologist in the applied research team at Emerson Process Management. Reach him at Terry.Blevins@Emerson.com.

Kurtis Jensen is instruments product manager, field instrumentation at Emerson Process Management. Reach him at Kurtis.Jensen@Emerson.com.

Stephen Briggs is on the technical staff of the process science and technology center at the Center for Energy and Environmental Resources at the University of Texas, Austin. Reach him at briggs57@mail.utexas.edu.