Pneumatic Level Controller and Pressure Controler

What are you trying to control?

Liquid level in a Vessel(Filter Separator)

A pneumatic control valve is designed to operate between two supply air pressure levels so yes, probably.

Your question is not clear, at all.

Is this rhetorical, or do you have a real valve?

If you have a real valve, does it have an operating manual, or a known manufacturer?

This is something I am designing. Not in existince yet. Need to purchase correct equipment.

The P&ID has a Level Controller(pneumatic) shown to connect and control a Level Control Valve(pneumatic).

I guess my question is if I need an additional Pressure Controller on the pneumatic Control Valve, to act like a positioner.

There will be no electrical signal involved.

You need something to tell the level controller what to do, and when to do it. "act like a positioner" is hardly an apt way to characterise the function.

You question indicates a lack of understanding of the system and how to control it.

Maybe some else can help you.

In this case, I think your wrong. The Level Controller reads the level in the vessel and sends the signal to the Control Valve, to control weather it is to be open or closed. Therefore controlling the liquid level in the vessel.

Good luck.

Can you provide a little more information on your system? Maybe a sketch? If you are trying to control level, what makes you think you might need a pressure controller?

Hope this helps

This appears to be the 'boot' on a gas knock-out drum to collect any condensate and allow it's removal. Is that correct? The coalescer element (I assume that's what the x's denote) at the top of the boot is a little confusing as to its purpose. If that's not what's going on, please disregard the rest of this. Is there also a coalescer on the gas outlet?

Where I used to work, the Foxboro 43AP was the standard field controller. Technically, it is an indicating controller. Here's a link to the product specification:

http://resource.invensys.com/instrumentation/documentation/eib/pss/pss_3-1b3a.pdf

If you use it:

1. The air supply to the controller must be limited to 30 PSI maximum, so a filter/regulator is almost always required.

2. The controller output range is 3 - 15 PSI and connects directly to the control valve. (Unless a positioner is being used.) The control range of the control valve must also be specified to that range. The control action of the controller and valve must be selected and the 2 must match, i.e. does an increase in controller output correspond to an increase in level, and therefore the increase in signal need to open the valve?

3. The diagram you attached only shows one process connection. Level in a closed process is usually measured via differential pressure, which requires 2 connections, with the level being controlled between those 2 levels. Then the span of the controller must be specified or calibrated to match. If the fluid is water, the span would be equal to the distance between the 2 taps. If the fluid has a different density from water, the span would be ratio'ed. Is some other means of measurement being proposed? If so, you'll have to disregard most of this.

4. Are the 2 taps on the left side of your diagram going to a level gage? If so, good; if not a visual gage is always a good idea to verify operation of the control loop.

Hope I haven't confused things too much - if so, Iris will have to straighten things out.

bigg,

it is a boot and your reply was not at all confusing. thanks for your comment, it will be of great value.

No you don't require a separate pressure controller, depending on what type of level sensor you have. Only when the application is perhaps critical, may you need to play double jeopardy with a separate / back-up control.

Your schematic seems like a standard water knock-out to me, where the level controller merely reacts to your process set-point and sends a signal to the control valve to open or close. The schematic does not show specifically how the level value itself is sensed and communicated to the level controller, and whether your level control scheme is for PID or D-gap control.

Common methods for sensing such 'bootleg' levels (depending on the process fluids) are the DP cell, or also externally mounted (between the tappings on the boot), a displacer type level controller, which has an integral control function which will control the valve directly. Pneumatic DP cells are essentially obsolete, so your best choce is the displacer type. Displacer types are specific to the fluids being measured, so be aware that if you are intending to measure an interface between two different density fluids, the SG's of these fluids will need be specified to the instrument supplier/ manufacturer.

Where the correct power supplies are present, the DP cell method would be cheaper to implement, but with a cost implication in this case, at the valve positioner. Hope this helps...

On the basis that the best way to do level control is to get rid of the tank, why not put the outlet connection where the level sensor is, and get rid of the control system?

Sort-of-like an overflow weir?

Looking at the P&ID, this is apparently just a high level shut off. If your level controller has a relay output, you can run a solenoid feeding the control valve and turn the air off to it at high level. The control valve is drawn a bit like a modulating valve, but with just a level sensor at the high point, this is not likely the intention. If it is, then you would need an I/P interface from a 4-20ma or 0-10V output from ther controller.

Thank you for your comments bigg, hilton, phys, and pwslack. between all of you, my question is answered and you have shed light on other options.

You're welcome.

However, I do have one concern. You need to make sure that the vessel engineer provides the connections you need. If you need 2 process connections, tell him/her now. From experience, I can assure you he/she won't appreciate having to make a last minute field change because the *&^^%&^% instrument engineer wasn't paying attention.

Unsolicited advice:

Regardless of what field you specialize in, it helps to develop a broad enough knowledge of the other fields around you so that you appreciate how you actions affect others. This has to be tempered on the other hand, so that others don't think you are trying to dotake their job.

I completely agree. thanks again.

Adam the scehmatic you showed is enough you do not need anything else unless you want more precise control of the valve position, in which case you would need a spoitioner between the Controller and the Valve

Hi,

Your post was interesting. Did you apply your schematic in real life?

Yes, used in real life. Only thing that changed is the connection size for LC went up to 2" because we used a float type Pneumatic Level Controller, and those typically are only offered in minim 2" connection.

Plus since the float type inserts into the boot, we removed the 1" ball valve that you currently see in the diagram above. Works just fine.