Our control valve vendor is proposing to use two control valves in series for a service since the pressure drop is high. (Splitting the total pr. drop across two valves). In that case, how to control these valves from DCS?

Murthy

 

Two pressure transmitters and two pressure control loops. Control the intermediate pressure with the first control valve, control the final pressure with the second control valve. We use this system on an oxygen pressure reducing station.

Rob

 

The vendor has good experience (i.e. no valves-fighting each other problems) with a series setup?

???? Ask the vendor to provide customer references where they've done this SUCCESSFULLY with DCS control, DCS brand, and function blocks used. Then you also contact the references.

 

Sounds like cascade control would be what you are looking for. You set up 2 PID loops but so they don’t work against each other you turn the PI values of the first valve to about 0.5 of what a well tuned system should be and the second valve you tune as normal. You should really look up Cascade control there is a bit more to it then that.

Hope that helps.

 

Murthy

Did you try asking your vendor???

In series will not work to reduce the drop as the pressure drop will be determined by the first valve. I would be talking to them about this and make sure you as the customer undersand what they are telling you, how and why it works and why it can not be done with a single valve.

MJH

 

the intent here is not to disparage the vendor but - perhaps questioning his or her credentials and approach to theory might be in order - putting stuff in series with regard to actual point control should make one nervous - and if he or she does not have the answer to your DCS question you/your company might want to consider finding another vendor (providing of course no one in the company is on the ENRON/Arthur Andersen program of ethics with regard to vendors) - and what is being controlled in the process? Explosive stuff? Nerf? Ask the vendor to show you the math - and hopefully your supervisor will back you up - unless of course you are the final decision maker - and hopefully will not default to Space Shuttle approach to engineering as all too many in industry have been doing for the past 20 years. And if the vendor says that "we feel this will work" definitely run away - there is no emotive component in physics and science/engineering. Thanks for reading the correspondence.

 

Are you sure the valves will be in series? Sounds like they should be in parallel. What type of DCS are you using?

 

Murthy,
Please provide more details related to the service the valves are put to... Flow, pressure control etc.... If they are purely for pressure control, then you may have to set up two PID loops for each valve.... PID_1 giving output to the first valve and attaining an output pressure well within the limits of valve specs (i.e., within the DP range of the valve).... Then PID_2 can be set up in DCS to control the second valve to finally acheieve the desired downstream pressure.... If the application is flow control then it's a very common application where the upstream valve stabilizes the inter-valve pressure and the downstream valve regulates the process flow.... Inasmuch, for flow to be stable, it's necessary the upstream pressure (or commonly referred to as the inter-valve pressure) be maintained very accurately....

Rahul

 

Two solutions are good for you if want to have two valves in series. As your vendor told you, cavitation and excessive vibration can lead to the necessity to have two control valves in series. There are many ways you can mess things up with two valves in series and that is the reason why people have been so sceptical about this.
There are three solutions:

Solution 1 - the easiest: One PI controller with output to the two valves:
Solution 1 will work only if you are okay with having the intermediate pressure float. If this is not the case, revert to solution 2 or 3. In this strategy, the output is send to the two valves - REALLY EASY

Solution 2 - no too hard: Two PI controllers: one for valve 1 and one for valve 2
In this scheme you are controlling the intermediate pressure. Be aware that if you are not using decoupling, you will need to de-tune one of the controllers. The problem with this strateagy is that often you have no choice than to decouple valve 2 because valve 1 is choked (valve 1 sets the flow). By de-tuning valve 2 (the downstream valve) you might fail to meet your control objectives. If you cannot recouple your system by detuning valve 1, or you need a good control on both the intermediate pressure and the outlet pressure you need to revert to Tandem Control Strategy.

Solution 3: Tandem Control
Tandem Control is a mix of the two solutions: your fast controller controls both valves (solution 1)
a slow controller trims the output of valve 1 by multipying its output to the output of the fast controller sent to the upsteam valve.

You need references? We have worked on GE turbines, Pipeline in the US, Steam Systems,... you name it. All these solutions have worked VERY well. You just need to know what you are doing.

Good luck,

Ben Janvier, Process Control Consultant,
[email protected]; 514-229-5268

Do I need to control my intermediate pressure at a fixed value or can I leave it float.