Tuning surge tank level control loops

This type of PID loop has a different objective than most, which calls for a different strategy.

12/03/2013


A typical surge tank configuration with its level control loop. Courtesy: DataforthWhile almost all PID loop tuning rules that have been developed aim for fast response, even the IMC and Lambda rules fall short of providing good tuning settings for a surge tank control loop, so we need to consider a different strategy for this application.

As shown in Figure 1, a surge tank is placed between two processing units to absorb flow rate fluctuations coming from the upstream process, so that the flow rate to the downstream process changes less rapidly. To do this, the tank level has to go up and down; therefore, the level controller must allow this movement and not try to hold the level close to its setpoint. The controller should simply keep the surge tank’s level between its upper and lower limits, and do this with the least possible amount of change to its output.

The level-averaging method for controlling the surge tank level is preferred by most operators and engineers. This method minimizes control valve movement during disturbances, keeps the level between its limits, and brings the level back to setpoint over the long term.

The tuning rule describes how to tune surge tank level controls using the level-averaging method and is designed for use on a non-interactive controller algorithm with its integral time set in minutes, such as the algorithm in the Dataforth MAQ20 system.

Tuning procedure

To tune the controller for level-averaging control, the residence time of the surge tank, the greatest expected change in flow rate, and the maximum allowable deviation from setpoint must be known.

Residence time (tres): Residence time is the time it will take the tank to drain from 100% to 0% level if there is no incoming flow and the outlet valve is 100% open. It can be calculated by dividing the volume of the vessel between 0% and 100% of its level measurement span (V) by the maximum flow rate with the outlet valve wide open (Qmax):

tres = V / Qmax

The same unit of measure must be used for volume in V and Qmax, and Qmax must be expressed in volume per minute.

Change in flow rate (Δfmax): The greatest expected change in flow rate should be expressed as a percentage of maximum valve capacity. If available, historical trends of the level control valve’s position can be used to find the largest change the controller output has made over a relatively short period of time.

Maximum deviation from setpoint (ΔLmax): The maximum allowable deviation of level from its setpoint should be specified as a percentage of the full span of the level measurement.

Tuning equations

A surge tank control loop responding to disturbance using the level-averaging method on a non-interactive controller algorithm with its integral time set in minutes, such as a Dataforth MAQ20 system. Courtesy: DataforthOnce the three quantities described above are available, tuning settings can be calculated for the controller:

KC = 0.74 Δfmax / ΔLmax
TI = 4 tres / KC
TD = 0

Where KC is controller gain; TI is integral time in minutes; and TD is derivative time. The typical response of a surge tank tuned with this level-averaging tuning rule is shown in Figure 2.

Although Ziegler-Nichols and most other tuning methods for level controllers are not suitable for use on surge tanks, the level-averaging tuning rule will keep the tank’s level between its upper and lower limits with the least possible amount of change to the controller’s output.

Lee Payne is CEO at Dataforth.

ONLINE

For more information, visit:

www.dataforth.com

Subscribe to Process & Advanced Control eNewsletter at www.controleng.com/newsletters



The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
Each year, a panel of Control Engineering and Plant Engineering editors and industry expert judges select the System Integrator of the Year Award winners in three categories.
Doubling down on digital manufacturing; Data driving predictive maintenance; Electric motors and generators; Rewarding operational improvement
2017 Lubrication Guide; Software tools; Microgrids and energy strategies; Use robots effectively
Prescriptive maintenance; Hannover Messe 2017 recap; Reduce welding errors
The cloud, mobility, and remote operations; SCADA and contextual mobility; Custom UPS empowering a secure pipeline
Infrastructure for natural gas expansion; Artificial lift methods; Disruptive technology and fugitive gas emissions
Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Research team developing Tesla coil designs; Implementing wireless process sensing
Commissioning electrical systems; Designing emergency and standby generator systems; Paralleling switchgear generator systems
Natural gas engines; New applications for fuel cells; Large engines become more efficient; Extending boiler life

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
The maintenance journey has been a long, slow trek for most manufacturers and has gone from preventive maintenance to predictive maintenance.
Featured articles highlight technologies that enable the Industrial Internet of Things, IIoT-related products and strategies to get data more easily to the user.
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Maintenance Manager; California Oils Corp.
Associate, Electrical Engineering; Wood Harbinger
Control Systems Engineer; Robert Bosch Corp.
This course focuses on climate analysis, appropriateness of cooling system selection, and combining cooling systems.
This course will help identify and reveal electrical hazards and identify the solutions to implementing and maintaining a safe work environment.
This course explains how maintaining power and communication systems through emergency power-generation systems is critical.
click me