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Process control systems

Johno12345
Hi,


I'm looking at some plant in the facility where I work, which uses a significant amount of energy. Some of it needs replacing because its worn out and I think its a good opportunity to look at the process and potentially upgrade the controls with a view to saving energy. 


I spoke to our energy advisor, which didn't give me a great amount of confidence.


It basically consists of systems that cool a liquid (cooling towers), move the product hydraulically, and stir the liquid. In total, there are 15 motors up to about 25kW and it all runs star-delta, manually controlled, part has a thermostat but that brings on the whole plant where only a small part could be on. 


I think a lot could be run with inverters, based on the process temperature and outside temperature, along with other inputs.


This is where it gets beyond me, I can describe how I want it to operate, and what I want to achieve, but all the people I speak to seem to fail to grasp the system as a whole, and can only focus on a tiny part, like 2 or 3 motors or pumps.


I'm obviously looking for a specialist but don't know what they would be called, or where to look. Does anyone know where or who i could speak to? 


Thanks
  • 0
    Hi Johno


    It's the kind of thing I'd expect a good BMS to control. Schneider building automation team may offer a solution.
  • 0
    Hi Johno

    You need to produce a flow diagram, with each stage marked, and each input to the stage, such as temperature or flow. Where there is a motor involved you need to define the degree of control that can be applied, for example, a high-speed mixer has a speed^4 characteristic, so much speed reduction would turn out a different product, particularly if the idea is to mix in air. Really you need to talk to the plant designer because there are usually a lot of variables to take into account, many of which may not be obvious. From that point, a PLC and a re-designed control panel are all that is necessary along with drives where significant speed reduction is sensible, and extra measuring sensors as necessary. This is all likely to cost quite a lot of money and a good deal of your effort, but the overall system design should not be too difficult, given the assistance suggested above. The payback period may be long, and you may be able to save quite a lot of money simply by changing the motors for modern high-efficiency ones (with a gain in reliability for a period), and perhaps switching them on and off when required with a relay controller and a couple of sensors. Process control is not really like building controls, it is usually quite complex, and needs to be exactly right to make the correct product specification. Often things work very "closed loop" with the exact result, something like the exact colour of biscuits coming from an oven varying the temperature of sections, for example. The EU is terribly keen on VFDs, but in some cases they have little or no advantage in energy saving where power reductions are not possible. Your best bet may well be to try the cooling tower fans first, which can easily vary with the ambient and product temperatures. You will need to make a lot of measurments to correlate the two, to design the control algorithm. Good luck.
  • Former Community Member
    0 Former Community Member
    Hi Jono,


    I note that a BMS system has been recommended below, I don’t think that is the solution that would fit your needs.


    As Dave Z suggests you need to start with a functional design specification (FDS), this should be as detailed as you can make it and needs to describe the process flow and function of the system.


    It does sound like automating the process could not only save energy, if the right selections are made, but it could also improve product quality, plant reliability and diagnostics, and in the long term, reduce costs.


    This is no small piece of work, and as Dave has rather simplistically said a PLC and re-designed control panel are basically what is required.


    However, legislation would dictate that it is slightly more complex, but, don’t be afraid of the legislation, if you embrace it from the start and used the relevant harmonised standards to assist you in your design, the legislatory aspects could be easily resolved.


    You are looking for a. process integrator I would recommend one that is a partner of one of the large controls companies such as Schneider or Siemens, others do exist.


    Then there is also the functional safety aspects.


    If I were better I could be in a position to advise you on this.


    I do though work with a few integrators dotted around the country, some work nationally, some, regionally, some locally.

    If you want to take this forward, then I am more than willing to try and assist, within the limits of my hospital bed!

    (Can’t sleep due to the pain at the moment, hence the crazy hour for the post!)


     By the way, your FDS should form part of your request for quotation (RFQ).


    As I say, if you want more guidance, please let me know.

    It will help me keep my sanity, though I can’t always respond to things immediately.
  • 0
    Johno12345:


    I'm obviously looking for a specialist but don't know what they would be called, or where to look. Does anyone know where or who i could speak to? 


    Thanks


    I think hat the process design should be led by a Process Engineer who would produce optional Process Flow Diagrams to optimise the system. The selected process would then be illustrated on Piping & Instrument Diagrams (P&IDs) which would describe the required control scheme. At this point the actual control scheme detail design can be started.
    As an “electrical person” I have only got involved in the control system design/commissioning when the P&IDs had been drafted but a few points for your consideration of energy saving are;-
    I agree with others that VSD inverters do not always provide the best solution. They lower the drive full load efficiency and are sometimes used as a means of compensating for an oversized pump/fan selection.
    The selection of the more efficient mechanical equipment is followed by the selection of a potentially smaller and more efficient electrical drive.
    The use of storage vessels can create a means of avoiding the need for continuous circulation/pumping.
    Frequent starting & stopping is usually avoided in continuous process systems to minimise wear but there are ways of soft starting to minimise wear and cut down on machines running on recirculation for extended periods.
    At the end of the proving the functional control/sequencing the Process Engineer/Technicians may optimise the instrument settings to achieve the correct product without over heating or cooling, optimum periods of pumping or agitation etc.

    I’m sure that a Process Engineer would think of a lot more energy saving techniques and, with his Mechanical colleagues,  have the background to provide the financial implications of the alternatives considered.
    Regards

     


  • 0
    Thanks for the advice everyone, no matter the hour!


    I have been away for a few days, hence the late reply.


    I have started drawing out how the systems currently work, its fairly basic, the only rules we have is the start temperature of the tanks. Thats what dictates the product outcome. Some parts have a max or min flow rate, and a flow rate for maximum efficiency, so that is going to dictate some aspects. 


    I have always said its dead basic, but when I draw out the pumps and fans, it stops being basic!  The plant was never really designed as a whole, things have been added on, removed and extended, often using pumps we have had, so are likely oversized. At the moment, its a collection of parts that work independently and I want to bring it together as one system for maximum energy efficiency.



    Ill be back - Thanks

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