Electro/Pneumatic Postioner

[npf]

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

 

[danw2]

An actuator physically moves the valve stem.

The electro-pneumatic positioner receives an electrical control signal (probably 4-20ma) from the controller and converts it to a pneumatic signal which drives a pneumatic actuator.

Mechanical linkage between the valve stem and the positioner supplies feedback to the positioner so that the positioner can vary its output to the actuator in order to get the valve stem at the position demanded by the control signal.

Dan

 

[JLSeagull]

The HV tag suggests that the valve is manually controlled. The electro-pneumatic converter suggests that the operator will have a throttling adjustment on the DCS operator workstation to adjust the valve. The solenoid valve could be associated with an interlock to automatically force the valve to a selected position (fully open or closed) in the event of a measurement related status.

 

[npf]

Dan:

Thanks for your response. Just to make things clear, I have attached a sketch, HV-1000.
If I undertsnad correctly we receive pneumatic signal from the controller (for eg 3-15 psig) that would cause the valve to close or open.

If possible could you in short expalin whats going in that loop.

Your help would be appreciated

Regards,
npf

 

[JLSeagull]

This response also applies to thread181-27567.

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-1000 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.

 

[npf]

JL Seagull:

Appreciate your explaination on the control scheme. It improved my understanding. I have some doubts

In your reply you mentioned HIC as the operator throttling adjustment, how does that work actually?
I saw some Solenoids actuated by DCS and some by SIS. What difference does it make?

Your comments would be appreciated.

Regards,
npf.

 

[JLSeagull]

The operator has a bargraph display on the DCS screen. He selects the output and enters the desired percentage open or uses slew keys to raise or lower the output. The DCS has a 4-20 mA analog output that connects to the current to pneumatic positioner. I would assume this to be a smart "HART" digital valve controller.

Separately, the SIS trips the solenoid valve, closing a line in the pneumatic energy and vents the diaphragm as an override trip.

 

[JLSeagull]

The DCS may activate solenoid valves not associated with a safety shutdown. The SIS should activate solenoid valves if they are part of the safety shutdown logic.

 

[npf]

JL Seagull:

I appreciate your help.I got a better understanding now.

Regards,
npf

 

[JimCasey]

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.

 

[npf]

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

 

[JLSeagull]

Download the engineering information available at

http://www.ascovalve.com/Applications/Products/EnggInformationApproval.aspx

This will describe the operation of a three-way solenoid valves. For a throttled pneumatic signal avoid the pilot type solenoids.

Assume that the analog output from the I/P converter connects to a spring return diaphragm actuator with a three-way solenoid between the diaphragm and the converter. The three-way solenoid such as the ASCO series 8320 permits air to flow through the valve from the pneumatic converter to the valve diaphragm in one state (energized) - and blocks the source and vents the diaphragm in the other state (de-energized). Thus, with the solenoid valve energized the HIC can adjust the control valve; and de-energized the spring moves the vented actuator to the failure position.

 

[JLSeagull]

I mentioned to avoid the pilot operated solenoid valves. The required feature is described on the ASCO 8320 series models as "No Minimum Operating Pressure Differential required." If the output to the actuator is only 3-4 psig then a valve that requires a 5 psi differential will not work as you need.

 

[npf]

JL Seagull:

Thanks. Is that the reason why I do not see any instrument air supply connection to the 3 way valve?

Regards,
npf

 

[JLSeagull]

Yes. This is the normal arrangement.
- but -
Perhaps some applications might require a regulated air supply applied to the actuator instead of venting the actuator. In this case the connections would be the I/P transducer output and the air supply. This would be unusual.

If you need a shutdown valve, my preference is a separate valve instead of a solenoid valve to trip a control valve. Most control valves are suitable for throttling; but lack positive bubble-tight shutoff characteristics.