classic | mobile
 

Search...

SA Instrumentation & Control Buyers' Guide

 

A deviation matching approach to pressure override control
September 2017, System Integration & Control Systems Design


Seeing the words ‘deviation matching’ as part of a control philosophy might let the reader’s mind wander toward compressor control, and specifically load balancing, as that is where the term is so often used. Here however, this familiar concept was applied in a novel manner to achieve pressure override control of a liquid header, an application very far removed from compressor control.

The process layout is illustrated. There is a feed of lighter and heavier hydrocarbons into the respective headers, from which the material can flow into any of the four buffer drums shown. The control objective is preferential feeding of lighter hydrocarbons into the buffer drums labelled 1A and 1B, while feeding heavier hydrocarbons into the buffer drums labelled 2A and 2B. This should happen with buffer drum level and header pressure protection in place.

The handles for material distribution are the flows of lighter and heavier material into each buffer drum. The basis for preferentially sending lighter material to the 1A/1B side is to stagger the level controller targets, such that lighter material is taken into the drum while heavier material is backed out when enough feed is available. When not enough light material is available, heavies are taken in for level protection. On the 2A/2B side the opposite is true. The control problem is made more difficult owing to the fact that availability of feed is variable. The flows out of the buffer drums are also variable. The level control problem is therefore well defined, and through correct manipulation of the flows into each buffer drum, the level control problem is relatively easy to solve.

Pressure protection

The problem of pressure protection is however more difficult. Generally one would expect P1 and P3 to be equal (and similarly P2 and P4 should also be equal). There is however a large distance between the buffer drums (highlighted by the dashed line in the figure), such that pressure losses in the lines become significant. There are also various piping bends, check valves, and other equipment in the lines, which all make the pressures read different values, and which also vary dynamically (i.e. one cannot assume that the difference between P1 and P3 is constant, even though these pressures are related).

In general one would like to base the pressure overrides on a specific header from a single measurement. For example, if P1 and P3 were equal, one might use P1 to cut back on all the flows of heavier hydrocarbons when the pressure drops. A uniform cutback is desired to ensure that feed is not cut back on one side, while it is increased on the other, and vice versa. In such a scenario the control valves are colloquially said to be fighting. In this case, even though P1 and P3 are related, they are not the same, and the standard pressure override solution does not work. The problem is aggravated by the absence of hard-wired electrical signals from the 1A/1B side to the 2A/2B side. Communication does exist between the sides, but transferring and duplicating valve openings cannot happen directly as the communication reliability cannot be guaranteed.

Pressure overrides operate with similar deviations

Conceptually, when performing compressor load balancing one monitors the difference between the current operating point and the surge point, which may be referred to as the deviation (i.e. the current distance from the surge line). Load balancing is achieved when all compressors operate with similar deviations. When performing pressure override control, the operating point can similarly be characterised based on the distance from the pressure override limit (i.e. the current distance from the limit). For the pressure override, the deviation is the difference between the override limit and the current pressure reading. Load balancing in the pressure override scenario happens when the pressure overrides operate with similar deviations.

When pressure override deviations are matched, the underlying PID controllers (with the same tuning) will act in the same direction, and will not fight each other. This is therefore a novel way in which to achieve coordination between the pressure overrides without having to use the same process reading (e.g. only P1). This application does require transmitting the pressure readings and override setpoints to achieve the coordination. When the communication fails however, the pressure overrides are not moved, and pressure protection still exists (the valves will still move on both sides).

The end result of this control installation was that material could preferentially be routed to the desired buffer drums, while maintaining level and pressure protection through coordinated overrides.

For more information contact Johan Maritz, SAIMC Secunda branch, +27 (0)82 856 3865, johanmaritz260@gmail.com, www.saimc.co.za


Credit(s)
Supplied By: SAIMC
Tel: 086 107 2462
Fax: 086 651 5238
Email: admin@saimc.co.za
www: www.saimc.co.za
Share via email     Share via LinkedIn   Print this page

Further reading:

  • SAIMC: From the President’s desk
    February 2018, SAIMC, SAIMC
    A quest for smooth transition Someone once said: “The first responsibility of a leader is to define reality. The last is to say thank you. In between the leader is a servant.” My ‘thank you’ moment has ...
  • SAIMC: Durban branch
    February 2018, SAIMC, SAIMC
    Annual dinner One of the highlights of the year is always the annual dinner which was held last year at uShaka Marine World, the fifth largest aquarium in the world. On the night of 25 November 2017, ...
  • SAIMC: Secunda branch
    February 2018, SAIMC, SAIMC
    The branch hosted its annual year-end function at Graceland Casino and Country Club on Friday 17 November 2017. This was an event which everyone looked forward to and proved a great way to strengthen ...
  • Batch size one production
    February 2018, Siemens Digital Factory & Process Indust. & Drives, System Integration & Control Systems Design
    Bürkle based in Freudenstadt specialises in the field of press lines and coating technology, supplying turnkey production plants to its customers. To comply with the requirements of one customer requesting ...
  • Technical support encourages custom applications
    February 2018, Countapulse Controls, System Integration & Control Systems Design
    Countapulse Controls has survived by instituting technological innovation in the face of socio-economic uncertainty. In its more than 60 years in business, this leading supplier of sensing solutions has ...
  • SAIMC: From the President’s desk
    January 2018, SAIMC, SAIMC
    Dear industry colleagues. Welcome to 2018. Of course as I am writing this it is still 2017 and we are all wondering if there will be an ANC elective conference? Who will be the successor of Jacob Zuma? ...
  • SAIMC: Durban branch
    January 2018, SAIMC, SAIMC
    On 22 November, members of the SAIMC gave talks to some students at Mangosuthu University of Technology about the role of the SAIMC in bridging the current gap between education and the requirements of ...
  • SAIMC: Secunda branch
    January 2018, SAIMC, SAIMC
    November technology evening    On Thursday 2nd November the Secunda branch held its last technology evening for 2017 at the Honeywell Hub in Secunda. Hennie Grobler from Yokogawa presented on the topic ...
  • SAIMC: Zambia branch
    January 2018, SAIMC, SAIMC
    At the recent SAIMC Zambian branch annual banquet at Kitwe’s Edinburgh Hotel, keynote speaker Oratile Sematle (then-President of the SAIMC) said with more investment in technologies, countries like Zambia ...
  • SAIMC: Tshwane branch
    January 2018, SAIMC, SAIMC
    The last function of the year was a meal at Cheers Restaurant kindly arranged by Mark from DesSoft. Approximately 20 people attended, including a dignitary from MERSETA. Thanks to all who attended. During ...
  • Customer centricity and a sterling team pave the way for CSS
    January 2018, Control Software Solutions, System Integration & Control Systems Design
    A customer-centric approach has allowed Control Software Solutions (CSS) to attain a high percentage of repeat business from its growing client base. This Level 2 BBBEE company has a solid 15 years of ...
  • New improved plant information management system
    January 2018, Yokogawa South Africa, System Integration & Control Systems Design
    Yokogawa has released the ExaquantumTM R3.10. This latest version of the Exaquantum plant information management system comes with stronger security and improved compatibility through the support of OPC ...

 
 
         
Contact:
Technews Publishing (Pty) Ltd
1st Floor, Stabilitas House
265 Kent Ave, Randburg, 2194
South Africa
Publications by Technews
Dataweek Electronics & Communications Technology
Electronic Buyers Guide (EBG)

Hi-Tech Security Solutions
Hi-Tech Security Business Directory

Motion Control in Southern Africa
Motion Control Buyers’ Guide (MCBG)

South African Instrumentation & Control
South African Instrumentation & Control Buyers’ Guide (IBG)
Other
Terms & conditions of use, including privacy policy
PAIA Manual





 

         
    classic | mobile

Copyright © Technews Publishing (Pty) Ltd. All rights reserved.