March 2005Valves, Actuators & Pump Control
Access Control & Identity Management
Readers occasionally e-mail Michael Brown with questions after reading his articles. Michael felt - and I agree - that by publishing some of the discussion, it would probably be of value to others who have similar control loop related problems that they are looking to solve. Here is one such exchange that revolves around knowing just how far open (or closed) a valve really is. - Ed.
Richard Mackay writes
Sir, reading your article about non-linearity of loops reminded me of an experience I had last year tuning the density control loop for a DMS plant at an old diamond mine near Pretoria.
We got the water addition valve stroking OK: Closed, 25%, 50%, 75%, 100% ...all checked out OK when looking at the I/P converter controlling the valve. But when we tried to control on density - which is a slow loop (as you can only add water, not remove it) - we found that PID control was impossible.
I guessed by looking at the trend that the flow was only starting when the positioner was far from closed. After much acrimony I tested the valve the next day and found that it only started flowing at 25%!
On calling the valve supplier, his response was that they can change the position of the seat of the valve. I do not know why a proportional valve would require this feature but I have learned now not to trust the positioner to inform me of the flow through a valve.
Michael Brown's response
I am not an expert on valves, and I am not sure what you mean when you said that you judged the valve position by the I/P converter output, and then you also talk about judging the valve position from the positioner.
Are you referring to looking at the little pressure gauges on the valve positioner combination? If so, then I never trust them. Generally one can get an idea of what is happening with valve position by actually looking at the valve stem. However this is also fairly rough, and as you did, you can judge best what is going on by the process response, which is easy to do if you have a measurement of the flow through the valve. This also allows you to check installed flow characteristics through the valve.
Judging what is happening in a valve on a process like a DMS density, would be relatively easy to do if the water addition is on the pump suction before the density measurement. However, many of the mines add the water into the tank which feeds the pump. This is a bad thing as it turns the process into a very slow integrating process, and it takes a long time to correct density variations. If this was happening on the mine where you were working then it would indeed have been very hard to judge what the valve was doing from the process response.
I have just had a similar problem with DMS density control, and have just come back from a diamond mine where they were adding water into the tank via a splitter box that was set up completely incorrectly. Control was almost impossible. The water addition flow was going from zero to full from about 48 to 57% of the controller output range, making the control almost ON/OFF.
Navigating the complexities of privileged access management
Editor's Choice Access Control & Identity Management
Privileged Access Management and Identity Access Management are critical pillars of modern cybersecurity, designed to secure access to sensitive resources, enforce principles like least privilege, and implement just-in-time access controls.
Read more...Paxton opens second experience centre Paxton
News & Events Access Control & Identity Management
Security technology manufacturer, Paxton, has opened a new experience centre in Cape Town on 12 February in partnership with its exclusive distributors, Reditron and Regal Security.
Read more...High-flow solenoid valve Emerson Automation Solutions
Valves, Actuators & Pump Control
Emerson has introduced the new ASCO Series 327C solenoid valve, which features a direct-acting, high-flow design that provides superior flow-to-power ratio. It permits high flows at minimum power levels, making it ideal for use in power plants, refineries, and chemical processing facilities.
Read more...Case History 197: Bad reboiler temperature control. Michael Brown Control Engineering
Editor's Choice Flow Measurement & Control
It is very important that reboiler temperature controls operate well in petrochemical refineries, or the product quality can really suffer. I was asked to check such a control in a refinery where they were having problems with one of these controls.
Read more...Gallagher Security expands Digital Badge Programme
News & Events Access Control & Identity Management Training & Education
Following a successful launch and roll out across Australia and Papua New Guinea in 2023, Gallagher announced its Digital Badge programme is now available to channel partners and end users across the rest of APAC IMEA.
Read more...Integrated Control Technology and Ingram Micro sign distribution agreement
News & Events Access Control & Identity Management
Integrated Control Technology, a global manufacturer of intelligent electronic access control and security solutions, announced it has extended its presence in the Middle East and Africa region by entering a distribution agreement with Ingram Micro.
Read more...Proportional cartridge valve series is the fastest in its class
Valves, Actuators & Pump Control
Parker Hannifin is introducing the enhanced version of the proportional cartridge throttle valve series, TFP. The new pilot valve further reduces the step response times by up to 30%, making the TFP the fastest valve in its class.
Read more...Empower individuals to control their biometric data
Information Security Access Control & Identity Management Security Services & Risk Management
What if your biometrics, now embedded in devices, workplaces, and airports, promising seamless access and enhanced security, was your greatest vulnerability in a cyberattack? Cybercriminals are focusing on knowing where biometric data is stored.
Read more...Security industry embraces mobile credentials, biometrics and AI
AI & Data Analytics Access Control & Identity Management Integrated Solutions
As organisations navigate an increasingly complex threat landscape, security leaders are making strategic shifts toward unified platforms and emerging technologies, according to the newly released 2025 State of Security and Identity Report from HID.
Read more...Nice launches DC Blue Astute garage door motor Nice Group South Africa Technews Publishing
News & Events Access Control & Identity Management Perimeter Security, Alarms & Intruder Detection
Nice Systems SA has launched the Nice DC Blue Astute, a garage door motor for the South African market featuring a pre-installed lithium-ion battery instead of traditional lead-acid batteries.
printer friendly version