Need help reading 4-20ma pressure transducer

[tiopaeng]

I have an AB 1500 LSP and I need to maintain a steady pressure of 80psi +- 10% from an analog 4-20ma pressure tansducer.

I also have a 4-20ma output proportional vent valve to exhaust pressure.

Can anybody please help me out on this project.

I'm a newbie and this is my first project.

Appreciate all your help,
Thank You,
TioPaeng

 

[Stationmaster]

You have picked a difficult project for a "newbie".

Are you regulating air pressure or water pressure? Am I to understand that you have a pump or fan running at a constant speed, and that you plan to regulate pressure by venting away the excess?

GENERALLY speaking you will need a 4-20ma analog input into the PLC to interpret the signal from the transducer, a direct acting (as pressure goes higher, the vent valve opens wider) PID command in the PLC, and a 4-20ma output from the PLC to the proportional vent valve.

Do you understand PID loops? http://learncontrol.com/pid/description.htm

Stationmaster

 

[leitmotif]

VERY important question was what are you regulating water air etc etc?
NEXT important what kind of flow rate are you regulating?

IF you can get them designed for the material and within your desired flow range I would consider using two stage regulation where first stage regulates to 90 then second stage would regulate to 80. With this setup you should get a tolerance well within your desired 10%.

Dan Bentler

 

[tiopaeng]

I am regulating air pressure in a pressure vessel. When filling, let's say from 0-70psi I want the valve to open 100% and gradually strict the flow so it does not get over pressured above 80psi. Once it's pressurized to 80psi I need to maintain it with a tolerance of +-10% anywhere from 1-24hrs. If in case the pressure goes above 10% the proportional valve opens and vents it out gradually and maintains air pressure.

I spoke with my local AB distributor and he's setting me up with the material list.

I know this is a difficult project and I appreciate all inputs. I'm also buying the Process Control Solutions book by John Shaw.

Thanks,
TioPaeng

 

[Stationmaster]

Also, what is PRODUCING the air pressure? At what volume is the air going into the tank? Is there flow leaving the tank? What volume of air is leaving the tank for use (if any).

I am all for automation with PLC's and 4-20ma signals, but there is a wide variety, an entire industry, of mechanical, pneumatic regulators that would control air going into the tank and pneumatic relief valves that would act as a safety against overpressure. The reason I'm asking so many questions about your application is that I'm not sure analog/PLC control is the right choice for an air tank. Only if your volume is so high that a system of pneumatic regulators would not be available or feasible, would I entertain the idea of analog control.

At the very least, you will need a mechanical/pneumatic SAFETY relief valve, ANYWAY, I should think.

Stationmaster

 

[Ken Moore]

So.... let's see if I understand correctly.
You have a pressure vessel, with a pressure transducer which outputs a 4-20mA signal. You have a proportional valve on the air inlet, and a proportional valve on the vessel vent. Is that correct?

Do the proportional valves have positioners? Do they have position feedback?
Are they air to open with I/P's?
What size is the inlet valve? What size is the vent valve?
What is the volume of the vessel?

If the line sizes are small in relation to the vessel volume, and considering that you have a huge tolerance, simple deadband control with discrete valves would probably work, it all depends on the line sizes.

Need more information.

Edit:
Have to agree with stationmaster, air going through a pressure requlator and turned on/off by a discrete vavle should do the trick, buy a high end requlator and your troubles are over.

Would be very simple sequence:
1. open discrete air inlet
2. wait until vessel >= desired pressure.
3. start timer(s)
4. wait until timer is done.
5. close inlet, open vent.
6. wait until pressure is down to desired level.
7. close vent
8. cycle complete.

2nd Edit:
If the fill rate through the requlator is to slow, you could have a bypass that opens at the beginning and closes when you approach setpoint, then the regulator can finish the fill and maintain the pressure.

Is there going to be anything going on inside the vessel that will cause the pressure to go up or down? Heating, cooling, reaction, etc.....

 

[blubb0r]

is this some kind of a test ?
Which forum is the best ?

http://forums.mrplc.com/index.php?showtopic=12367

 

[tiopaeng]

I don't have the hardware yet because this is a new project and I'm still putting the inventory together whatever is recommended in this forum. However the vessel is operational with manual control valves and we want to automate the controls.

Incoming air is 120psi on a 2" pipe and produced by a 50hp screw IR compresser with air receivers and there is no requirements as to how long it would take to fill the vessel around 4'W x 12'L. The vent valve can be any size as recommended.

Thanks for all feedbacks
TioPaeng

 

[tiopaeng]

This is not a test I'm just trying to get all the help I can and I'm serious about it, in fact I just bought the "The PID Control Algorithm" book this morning as suggested, I need to learn how it works. So far this site has given me a lot of information.

I also bought this book "Your Personal PLC Tutor...A Guide to Understanding PLCs" from this site on Friday.


Please keep up the good support,
Thanks, TioPaeng

 

[rsdoran]

I think the reason they are asking so many diverse questions is they can not "visualize" what the application is and why PID is necessary.

For the most part a "tank" can be regulated and maintain a constant pressure with pneumatic components.

Where an issue may occur is when you are "taking" from the tank at a higher rate then it is supplied, in your case you mentioned a 2" supply line, if you have 3-1" outlets then it may be possible the in supply can not maintain in some cases.

Everything is dependent on the application(s) involved, in other words what happens with the air in the tank. In some cases it may be better to allow the tank to pressurize to the supply then regulate the outgoing pressure, just depends on needs.

What is being asked is provide more information regarding what the system is doing and more appropriate information may get provided.

 

[Tom Jenkins]

If you only need to maintain pressure ±10% I wouldn't go to the trouble of PID loop. Use a simple deadband control, like the thermostat in your house. I also wouldn't bother with a proportional valve. Just use a solenoid.

 

[Mickey]

I don't see a need for a PLC at all. Just use a couple of self-contained spring loaded regulators.

 

[Stationmaster]

I don't see a need for a PLC at all. Just use a couple of self-contained spring loaded regulators.

My exact concern from post #5. Unless there is something you are not telling us, the most a PLC would be needed for would be to SEQUENCE the operation by allowing the operator to press a start button to open a discrete valve, or some time clock functions for how long you want it pressurized. The pressure regulation would be better left to (pardon the redundancy) "pressure regulators".

Stationmaster

 

[tiopaeng]

There is more to this operation that's why I'm using a plc but the only part that I'm having trouble is controlling the pressure at +-10%. I need to use a proportional valve to let air in and vent out gradually, other wise I will have a sudden increase or decrease in pressure and I'll be out of tolerance which will show on the chart recorder.

Right now we are regulating air pressure manually through air regulators and an operator constantly monitors the long process.

This operation involves blowers, safety interlocks, cooling, vacuum, air pressure, heating and everything is recorded through a chart recorder. By using a plc would automate the whole procedure.

Thanks all for your responses,
TioPaeng

 

[leitmotif]

I don't have the hardware yet because this is a new project and I'm still putting the inventory together whatever is recommended in this forum. However the vessel is operational with manual control valves and we want to automate the controls.

Incoming air is 120psi on a 2" pipe and produced by a 50hp screw IR compresser with air receivers and there is no requirements as to how long it would take to fill the vessel around 4'W x 12'L. The vent valve can be any size as recommended.

TioPaeng

This operation involves blowers, safety interlocks, cooling, vacuum, air pressure, heating and everything is recorded through a chart recorder. By using a plc would automate the whole procedure.

Getting info from you is like pulling hen's teeth.

OK so here are the facts
1. You have a complicated interactive multiple system.
2. Part of the system requirement is compressed air at 80 plus or minus 8 psi
3. Air supply source is 120 psi and 50 HP.
4. this large tank 4 x 12 foot is apparently acting as reserve supply.

I think you are overlooking doing this the simple way with widely available controls ie standard pneumatic regulators.
What is the compressed air flow demand of the system??
You do not need to vent the tank
- just stop filling
COmpressed air is expensive - by venting you are taking those compressed air dollars and dumping them to atmosphere. If you can afford this waste please send me a check.
BUT you do have to install a pressure relief to prevent tank rupture. I would use two one set for 88 (or 90) and one for tank pressure rating.
Use a line regulator set for 88 psi to fill the tank. Also install a moisture trap since you are dropping pressure from 120 to 80.

Here is what I would do install a line direct from the compressor with a solenoid to feed 4x12 tank. If this line is parallel to others place check valves in all lines. This way you are not compressing to 120 to fill a 88 psi tank you are using the compressor to fill using maximum 88 (plus line drop) discharge pressure on compressor -- will save lots of power and you will still stay within pressure tolerances

Dan Bentler