Pressure Relief Valve Closed Loop Control

Why go to all this fuss when pressure relief valves controlled by nothing more complex than a spring are available?

dear all i am sorry form my heart for wrong technical word ,

in did we need proportional pressure relief valve directly operated to maintain the forward pressure around 30- 55 bar based on set point where is line pressure is 70bar

but we need very fine regulation of forwared pressure. for that we need close loop system most importently based on set point provided by other PLC system.

please help

There should be a way to get your Siemens S7-200 PLC's PID function to work properly. I have never set up the PID function in one of those PLC models myself, but I have done it in other PLC brands. Hopefully the instruction manual gives you an example, but it sounds to me like there is a parameter problem if the PID algorithm's output always goes to zero when PV=SP.

Don't try to program your own proportional-type equation into the PLC. Use the factory PID function. If for no other reason you're going to need fast integral action to properly control this loop (if it's liquid pressure control), and a proportional-type equation won't do the job.

Please don't feel badly about that. Technical terms vary from place to place around the world.

1. A pressure relief is supossed to operate only when system's pressure exceds safe levels, and when it does, it opens fully, not proportionally to the excess pressure. You don't need a close loop control, and using one can violate safety regulations, due to a number of reasons: power failures, and electronics malfunctions + what your local code dictates.
2. If you still want to do so, no matter what, then change to "reverse output"; a dedicated controller (like a temperature controller for example), have the options for reverse or direct output, I know you're not working with a temp controller, but you can divide a constant by the process control output in order to invert it. Just don't call me when you blow up the plant.

Why don't you use the type of valve recommended by PWSlack?.

Regards

Agree with prior two posts.

Pressure relief valves should be free to operate without outside control.

An alarm signal out seems reasonable.

What does your local code and your insurance company have to say about this?

Are you an MBA?

*Facepalm*

What is it you are really trying to do... ?

Don't tell me what you are planning to do, or how you want to do it... first lets start with why..

You must want to control the pressure... correct?

Why do you want to control the pressure?

Why would you choose a relief valve for this function? Like already stated, A relief valve is a safety device, not a controls device.

You are way down the road already... and it might just be in the complete wrong direction... You should stop forward progress with your PLC controlled relief valve and start from the problem you face...

If we go back to the real problem, and why you need to control the pressure we might be able to provide you with a good workable solution that does not require so much work as you are putting into this.

Very common thing I see is people getting the cart before the horse... makes it really hard to steer... and you might just get a bit of horse$h!t in your eye.

I think what you really want is called a downstream pressure regulator.

Yep. ↑ Like that.

Some paragraphs about the process would be useful.

Your problem is that you have only a " Proportional " controler so that is normal to obtain zero when both values are equal. Use an "Integral" controler which will integrate till both values are equal and no further if the sum is zero. As soon as the sum (difference) changes the error leads to an integration process which modifies the output signal till again the result is zero which means values are equal.

Hi Kailash,

As others have already stated, you need a pressure regulating valve, and need to configure it to control downstream pressure.

I know you have mentioned you need very fine control over the pressure, but I have had excellent results with a mechanical self-regulating valve. Essentially the downstream pressure controls the diaphram(s) in the regulator to achieve accurate pressure control. I can't imagine the mechanical action of an incompressible fluid can be slower acting than a pressure instrument signalling a PLC that drives the action fo a valve (suspect a larger lag would exist, especially if you employ a feed back control loop as you describe).

Anyway, just giving you the option of using mechanical pressure regulating valves, as I can recommend their perforance as excellent.

Hope this helps.

Anthony

yes i agree sir that we can use mechanical self-regulating valve but the problem is the set point shall be provided by PLC based on product so set point will change every time process or product temp change so i need loop control through elctrical control refrence. thanks for valuable information i

Hi

I cannot answer your question but Bosch Rexroth can. You are getting some unhelpful comments. The servo pressure relief valve has been made by them for 30+ years so they know what they are doing. If you want precise control of Force (pressure) then you have the right device. PID control must be available.

The unit was specifically aimed at testing machines where movement was negligible but precise variable force was required.

Send them an E mail giving all the details of your equipment and application and you should get a decent response.

Regards

Oliver Dunthorne

Hydraulic Engineer

i respect and admire you sir

as i do belive u understand my requriment i need fine regulated force on machine and calculated force shall be send by PLC as a set point based on process so i cant use mechanical pressure relif device. and i will flollow your advice

Hi Kailash

This rexroth valve what kind of valve is this- electro- Pnuematic, Or motorised. and what type of valve is this butterfly,

as i understand you need a Setpoint based pressure control valve. ( PIC - pressure indicating controller - with direct action output will decrease as error decrease)

I thing S7200 has a good PID controller.

Now two things

1. Your upstream pressure is 70 Barg

2. You want to control downstream pressure to 30-40 Barg.

what is your line sizes is there an increase in size downstream? your valve opening will not be constant- it will be very small opening required to maintain the pressure and will close if pressure increases more than sp. because your upstream pressure is almost double your downstream pressure.

If you still want to maintain amin opening you may put the controller output low at a value say 10 0r 15% instead of 0% ( like output high 100% , output low 0% (make this 10%) or waht ever you want and valve will not close below this value.

The data given is very minimum to say anything, but it is possible.