Currently we developing existing plant where all control valves have no input signal on positioners. Means operator does not exactly know did valve opened or not. But on the other hand there're a lot of other sensors like level, temperature and etc near these valves and through them it is clear if valve opened or not (i.e. temperature changes when it open and etc).
The question is did anybody face this kind of issue? Did you always install signal from positinoners? From economic point of view it seems expensive to me. Or I missed something?
To use positioners or not will depend on the type, size and service of your control valves and the type of actuator on the valve.
If you need a piston operator on a valve for failure action, i.e. fail open, closed or locked, you will need a positioner. Speed of operation, power required or characterization may require a positioner. Small valve sizes, small trim sizes and low pressure processes will not require a positioner. Every valve application will have to be properly sized and specified to determine positioner requirements.
My experience in a large Utility Plant is that most valves had positioners.
Thank you very much for your reply!
Yes, we use valves with positioners (or actuator has a positioner). This is a control valve, not only on/off. Sorry if I was unclear somewhere.
The thing is that 4-20 mA signal goes to control valve (close, open) and no signal goes from positioner to DCS. Thus what really goes there is not shown on the screen. It just shows that you closed this vavle for 80% (for example). But is it really closed or not we don't knot, we just can see how parameters like flow, temperature or pressure change, then we can conclude whether is it OK or not.
I don't know is there some requirement on safety or other rules that we need to install it. This plant equipped with HART, thus there are a lot of wires. And additional 100m for wire, license, service work, cards and etc. I thought it is increasing the costs for about 800-1000 USD for each control valve. Or I'm mistaken?
These days the cost for feedback is relatively inexpensive and the benefits probably outweigh the added cost over the life of the product. I would be inclined to have it included unless your company just cannot afford it.
Thank you for your reply!
As I wrote in previous reply I calculated my costs as:
- additional costs on wires (average 100m to each valve)
- these wires need licenses (I guess supplier did not provide them to us)
- new cards will be needed; (not expensive)
- may be new space in panels, in the worst case a new panel it there won't be any place.
- service, wiring (man hours will be increased, then payment also)
I'm sorry, if my calculations are wrong, but I would really appreciate if you tell me where I was wrong.
Valve position monitoring is only used in rare cases. Use of a "positioner" is another matter and used only where required.
Your question was unclear if it was about using positioners on control valves or using position feedback from a positioner to your DCS as an input.
In my experience position feedback or limit switches on valves are only used where required or for special applications.
That being said with modern smart positioners such as Hart or Fieldbus, you can get position feedback data by using smart AO cards in your DCS without the cost of extra wiring and I/O channels.
I'm sorry if my question was unclear.
The question is: usage of feedback from valve to DCS.
Currently feedback is not used, but we're thinking about to install it in the future. But to be honest, technically it doesn't have sense to do it (because we can see how other parameters changed if valve opened or closed even for 15%, i hope i was clear now).
Thus, our idea was if it is not expensive and has some real reason, then in the future we could install it (HART is used in the plant). But at the moment I see it expensive, because: cost of wire, equipment (may be new cards will be needed. because there were used general AO/AI cards, so we will be needed to wire them to AI card), licenses (for wires, cards and so on), service work (manhours will be increased).
Please, could you tell me, are my cost estimations wrong?