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.
Balancing secure access control and fire safety
Editor's Choice Access Control & Identity Management Fire & Safety
In modern building management, few topics create as much tension as the intersection between security access control and fire evacuation safety. Nichola Allen of G2 Fire sheds light on this delicate balance.
Read more...Submersible pump protection ifm - South Africa
Valves, Actuators & Pump Control
With ifm’s remote monitoring solution you can visualise the condition of your submersible pumps and diagnose issues before they become serious problems.
Read more...Electrically operated, energy-efficient double diaphragm pumps
Valves, Actuators & Pump Control
BMG has introduced the EVO 210 series electric double diaphragm pump range, offering a quieter, more energy-efficient alternative to conventional air-operated diaphragm pumps for chemical processing, mining, wastewater treatment and general manufacturing applications.
Read more...Internationally engineered valves support uptime in tough conditions Bearing Man Group t/a BMG
Valves, Actuators & Pump Control
BMG’s Fluid Technology division works closely with industry in sub-Saharan Africa to ensure that all valves, strainers and flow control equipment supplied meet the exact requirements of every application and also conform with stringent industry standards.
Read more...Loop signature Part 2-5: Interactive control systems Michael Brown Control Engineering
Fieldbus & Industrial Networking
Feedforward control was explained in the previous loop signature articles. One of the examples used was feedforward control of load changes on a heat exchanger when variations occurred in the flow of the process fluid through the exchanger.
Read more...Hygienic processing with butterfly valves and Bürkert control technology Bürkert South Africa
Valves, Actuators & Pump Control
Among the standout technologies shaping modern hygienic processing environments are hygienic butterfly valves, especially when seamlessly integrated with Bürkert actuators, control tops and positioners.
Read more...Controlling access for people and vehicles IDEMIA STid Security Technews Publishing
Editor's Choice Access Control & Identity Management Asset Management Industrial (Industry) Mining (Industry)
When it comes to access control, the security requirements of mines and the industrial sector are similar, requiring a layered approach that combines physical barriers, digital authentication, and continuous monitoring to protect personnel, assets, and operational continuity.
Read more...Cryogenic valve limits leakage and energy loss
Valves, Actuators & Pump Control
Emerson has introduced the Fisher IC2 cryogenic top-entry control valve, ideal for cold box applications in air separation units, hydrogen liquefaction and LNG production.
Read more...Paxton launches new phone-based security system: Solo Paxton
News & Events Access Control & Identity Management
Paxton has officially unveiled Solo, a phone-based, cloud-hosted access control system. As part of the launch, installers can claim a free Solo starter kit from Paxton, allowing them to trial the system and see how it can work for their business.
While every effort has been made to ensure the accuracy of the information contained herein, the publisher and its agents cannot be held responsible for any errors contained, or any loss incurred as a result. Articles published do not necessarily reflect the views of the publishers. The editor reserves the right to alter or cut copy. Articles submitted are deemed to have been cleared for publication. Advertisements and company contact details are published as provided by the advertiser. Technews Publishing (Pty) Ltd cannot be held responsible for the accuracy or veracity of supplied material.
printer friendly version