Pneumatic control valve | PLCS.net - Interactive Q & A

Hello experts,

There is a control loop with PID controller and pneumatic actuator that controls the position of the valve. Pneumatic actuator has positioner and is controlled with 4 to 20 mA.

There is one thing I don't understand and need your help and it is not that much related to controls or PLC.
On the nameplate of the pneumatic actuator is written max 6 bar, and in the installation of instrumentation air there is about 5.5 bar. However in the datasheet of the pneumatic actuator, there is so called "control pressure range" and "control force".
I don't understand what this actually means. For example, I have specific actuator with 60 mm full stroke (marked in the attached picture), and it's control range pressure is: from 0.2 up to 1.6 bar.

I don't understand why there is a range in the column "from" and there is a range in the column "to". From: 0.2-1.0 bar and To: 0.8-1.6.
I have Sipart PS 2 positioner, and I'm not sure if I need to order filter pressure reducing valve.

What is confusing to me is the fact that actuator is suitable for max 6 bar, but it's control range is only about 1.5 bar. I'm not sure how to interpret this.

As far as I know, most instrumentation air installations are 6 bar, and SIPART supports up to 7 bar, but pressure range on pneumatic actuator is only 1.5 bar.

I hope there are experts here who can clarify this to me.

Thank you!

I suspect this is your situation: There are two parts to your valve actuator. There is a cylinder that is able to exert a large force to overcome valve friction, hydrodynamic forces, etc. while moving the main valve disc or globe. That large force uses normal shop air at around 5.5 bar (80 psi) to move the cylinder's piston.

The air to the cylinder is controlled by a pneumatic positioner. It uses 0.2 to 1.4 (3-15 psi) and feedback from the cylinder to provide analog positioning. The cylinder moves until the position feedback matches the pneumatic signal, which in turn is regulated by the 4-20 mA I/P signal conditioner.

If you are supplying 6 bar (90 psi) air to the positioner you will need a pressure regulator to drop the pressure. Hello Tom and thank you for the reply. Yes, there is a pneumatic actuator that needs to develop a large force to move the valve. Actuator is air to close, spring to open. Therefore air is supplied to the actuator only on one side. This pressure will develop force to overcome spring force, and all friction and other forces including the resistant force of the fluid (water). What confuses me is the fact that 0.2 bar and 1.6 bar are mentioned as control range of the actuator. This data is not in positioner's manual. I think of 0.2 bar as pressure needed to overcome spring force in order to start moving the acruator. In that way 1.6 bar is enough for actuator to extend full stroke. I think it is only to overcome spring. The pressure in application must be greater than that because there are many other opposing forces. I believe your functional description is correct.

I suspect that the pressures stated on the actuator spec/label are for bench operation, the pressures needed to create the force needed to fully close against the spring pressure.

The PS2 positioner measures position, not pressure, so it allows its internal valve at the Y1 port to stay open as long as necessary to allow supply air to continually flow, chargin the actuator's volume above the diaphragm with more and more air, which creates a higher and higher pressure on the diaphragm until the valve stem moves to a position that matches the 4-20mA demand signal position, at which point the Y1 output valve closes.

The positioner needs a higher supply pressure than the nominal working pressure in order to overcome flow forces and stiction in the valve.

No need to diminish the supply pressure; the position will only use air as needed to create the pressure needed to position the valve stem.

All:

I am working on P&ID's on an LNG project. I have globe valve w/ electro/pneumatic postioner(actuator). It also has a hand jack. The valve tag is HV-XXXX
A 3 way solenoid valve is feeding the actuator.

Could any one tell me the use of the electro/pneumatic positioner? Would a diaphragm actuator work?

Also I have the same knd of system mounted on a butterfly valve?

What are the advantages of butterfly over globe valve?

Any help would be appreciated.

Regards,
npf
This response also applies to thread181-.

I simply love the tag number prefix scheme. It is unusually compliant with ISA 5.1. The H prefix denotes hand or manual operation and prefixing all devices with H keeps one loop number with all devices together.

The HY is a position transmitter and HZI displays the actual valve position not just the controller output signal a.k.a. implied valve position.

The FC suggest that the valve closes upon loss of instrument air or 4-20 mA signal. TSO suggests tight shutoff. Typically control valves do not provide the positive shutoff required for block valves such as shutdown or isolation. The valve symbol is a butterfly valve. Some double and tripple offset valves can be throttled and provide tight shutoff. TSO typically requires Class V or VI shutoff per FCI 70-2 / IEC 534-4.

HZSO and HZSC are position switches. The O and C may not be exactly per ISA but they are the normal industry standard for open and closed position switches. The switch is wired to the SIS may could use position verification in the logic. A link such as TIA 485 or Ethernet permits the position to be displayed on the DCS screen.

HIC is the operator throttling adjustment from the DCS screen. The solenoid HY is actuated by the SIS to trip the valve closed upon the process event as identified in a logic interlock scheme labeled I-2.

The symbol reflects a diaphragm positioner and handwheel. If very large it could actually be a spring opposed piston. The valve tag is HV- etc. but the data sheet must direct the seller to provide nameplates with all of the tag numbers and to reflect them on the vendor drawings. Don't hold your breath as the sellers rarely comply with specifications. However, you may never know unless you are fortunate enough to follow the project to the field for construction support, better still for commissioning and STARTUP.
Nothing left to say after JLSeagull brilliantly dissected the control logic.

The other question:
>>What are the advantages of butterfly over globe valve?<<
You get a lot more capacity for a given diameter with a butterfly valve.
You get a lot more diameter for a give number of (currency units) with a butterfly valve.
MOST butterfly valves shut off tightly. Most globe control valves leak about 1/10% of capacity when closed.

Butterfly valves in general can't handle the differential pressures that Globe valves can handle.

Although butterfly valves are marketed with diaphragm actuators, the diaphragm actuators' torque characteristics are not well matched to butterfly valves. Better matched: Rack and pinion actuators. Best matched: Scotch yoke actuators. Diaphragms also lack stiffness and the stroke is short compared to the other constructions.
JL Seagull:

Thanks for your valuable inputs. I still have some doubts. As per the sketch the solenoid sends pnuematic signal to the control valve( per the instrument line symbology). If thats the case the electropnuematic positioner will convert the electric signal received from the HIC to pneumatic signal and perform the desired action.

So solenoid as well as HIC would intiate the action and send signal. Would both occur at the same time?

Also could you suggest any references/documentation on Solenoid that would help my understanding as I am new to the instrument industry.

Your comments would really be appreciated.

Regards,
npf