Split range - change to Manual 1

[Jack Nicholson]

Hi everyone. I have a question. would you please help me?
We have a split range controller, which send PIC signal to both valve (310-PV-023 A/B).

PID:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1607955101/tips/PID_total_2017_l2e6wy.pdf[/url]

Control Strategy:

When we switch the controller to Manual, we have to open valve A first, and then open valve B, as illustrated above. It's possible to change in the way that we can action valve A and B simultaneously (in other words cancelling the split range) in manual mode. But don't you think that there would be some safety or process issue then?

 

[georgeverghese]

There may be operational issues, but the safety issues related to both valves being open at the same time should have been addressed during the plant detailed design phase.

 

[IRstuff]

Presumably, SOMEONE knows WHY A must precede B, and by how much, and likewise, presumably, that same SOMEONE knows what will happen if A doesn't precede B. Beyond that, you've not described the process, so anything else would be absurdly speculative.

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[danw2]

What am I missing? Assuming the control signal is 4-20mA, with 50% output at 12mA, then
- at 20mA output PV-B is 100% open
- PV-A is 100% open at 12mA, so it remains open or drives open for any signal greater than 12mA.

What prevents a manual mode signal of 20mA from keeping or driving both valves open at once?

 

[Jack Nicholson]

Danw2, as a matter of fact, the instrument department told us that they can provide situation that we can drive both A and B separately.

 

[Jack Nicholson]

IRStuff, this two control valve are used to throttle the inlet gas from 60 barg to 30 barg and then headed to demethanizer column.
The control philosophy of this two control valves are progressive valve sequencing as described below:

https://control.com/textbook/control-valves/split-ranging/

PID:
[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1608008543/tips/PFD_2016_total_f4iohe.pdf[/url]

 

[georgeverghese]

Nothing exotic about this scheme - this term progressive valve sequencing is just to improve control rangeability. Typically, a single CV gives a range of 20:1 on flow. Two valves arranged in parallel with this scheme gives a total range of 400:1. But the smaller valve must be selected so it caters for flows in the low range. If you are having to open both valves on startup, it may be that (a) either you dont really need split range or (b) the low range valve is too small for actual startup flows. Check that you are operating at the required low flows during startup as recommended in the Operations Manual.

Dont see any safety issues with both valves open in this scheme you have - that is what will happen at 100% controller output in any case.

 

[LittleInch]

Well the P&ID doesn't tell us a lot, but it appears you have two identical valves ( I think george thinks they are two different valves?)

Put it appears that the controller is controlling on upstream pressure and flow?

Unless your valves are different - are they?, then it appears to be just a way of f=getting a little more granularity in the control loop, especially at the bottom end.

So if they are identical valves with identical trim then yes - operate as a pair, but the control might be a little bit more "coarse".

But if they are different trim / capacity, then it sounds like a bad plan to change something without knowing what the impact will be - almost certainly less fine control.

And why are switching the controller to manual?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[IRstuff]

The control philosophy of this two control valves are progressive valve sequencing as described below:

You missed the point of the question. WHY in the process are the valves sequenced? The mechanics and electrics are moot. What happens in YOUR process if the valves are not sequenced as designed? ONLY YOU can even come close to answering the question.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[georgeverghese]

Error in my previous prost: "Typically, a single CV gives a range of 20:1 on flow"
Correction: Typically a single CV gives a controllable turndown on Cv of 20:1

 

[Jack Nicholson]

Dear IRstuff.
These two valves are sequenced, due to large range of gas flow (these valves should control feed line pressure) from 0 to 300 ton/hr. So if we open or close these valves separately, I don't think any problem would happen, personally. But I just want to know your idea!. BTW thanks.

Dear littleinch.
There are aslo some occasions when operator wants to close valve A completely (for example because valve A is not working or has some mechanical issues) and open valve B instead.

 

[LittleInch]

Jack,

My reading of your P&ID is that the pressure signal and pressure control of your valves comes from the upstream side only.

However this doesn't really impact your issue, but if you have a wide flow range then operating these in parallel and at the same time (i.e. both are open say 20%), then your fine control over low or high flow is reduced.

Personally I don't think control valves are good for much more than working between 10 to 90% open so if you have two valves acting together your low flow might get unstable and "jumpy".

Your control system should be able to cope with one valve being manually shut off or forced closed as it will just move onto the second valve. But you will clearly then limited in flow.

All depends how much of the time you're operating <10% or >90% of max flow.

But all you have to worry about really is operability, not safety.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jack Nicholson]

Quote"" Your control system should be able to cope with one valve being manually shut off or forced closed as it will just move onto the second valve. But you will clearly then limited in flow"".



Yes, this is a very big instrumental fault. If we want to open valve B, we have to open valve A first!.
Fin control on low or high flow rate is manageable, based on my operational experience.