Pneumatic Vs Electric actuators selection for valves 1

[pratapsingh]

Hi,
i Would like to know there any thumb rule or guide line by which we can define the selection of actuator type which is most economical on perceptive of EPC. Up to what condition initial cost of pneumatic valves ate cheaper than electric ?. Up to which pressure class pneumatic actuator are cheaper ? up to what size and type of valves pneumatic actuators are cheaper?

 

[LittleInch]

I don't know of any easy solution to this but a lot depends for me on:

No and density of valves in a single location.
Lots of valves in a small plot - pneumatic
Small number spread out - electric

Type of valve
Ball / plug vales - Pneumatic
Gate valves - electric

High pressure rating - hydraulic

Is there any other use for compressed air? - pneumatic

Very irregular opening / closing - electrical

Low quantities of electricity - hydraulic or pneumatic with large accumulators

Operator / Company preference - Depends

ESD valves - Spring Pnuematic

In general a pneumatic actuator will be cheaper than electric on a one to one basis, but the electric cabling is easier and cheaper and you don't need an air compressor (x2), dryers, accumulators and lots of piping. Also the compressors can become the largest power supply on some stations by a long way whereas individual actuators spread the load out a lot more.

I've rarely seen large plants anything other than pneumatic and often seen small isolated stations as wholly electric or valves a long way from the air compressor.

Wellheads and high pressure locations often are hydraulic.


Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[itsmoked]

Really well put Little!

I find that control wise pneumatic is annoying, problematic, and somewhat erratic, as wear will change the operation as do mysterious pressure and flow fluctuations. Electrical you can profile to insane detail and wear is automatically compensated. Somewhere in these annals I detail an egg packing machine I did. It started out being pneumatic but never were the pneumatics able to perform up to the required level, even after weeks of adjustment. Noisy as hell too! We finally switched to electric and the performance improvement was nothing less than shocking. You could draw the motion curves you wanted and the actuator would deliver those profiles flawlessly - and nearly silently too.

Electric is certainly more expensive though. So, to me if you have non-speed requirements that are essentially digital in nature with perhaps minimal and non-critical speed profile requirements use cheaper pneumatics. If you need close dynamic performance for success of something don't waste your time, go straight to electric.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[gerhardl]

Excellent presentation by LittleInch! Comment to Itsmoked: what you mention is common problems by pneumatic actuators, but can be avoided by correct layout, maintenance and operation of the pneumatic system, proper drainage of water first of all, filters for dust and dirt, and preferably oil-free systems.

Any actuator, pneumatic, electric, hydraulic or water hydraulic operated will give problems if end and limit switches and other signal switches are not properly adjusted and connected.

One thing, not mentioned above, is the advanced signal and operational signal systems built in as standard or option in modern advanced electric actuators, giving cheaper control systems overall for total electronic controlled facilities.

 

[Compositepro]

Pneumatic controls are almost universal in chemical plants. They are extremely robust and inherently safe around flammable chemicals. Pneumatic components do not generate heat or burn-up when they fail. Explosion-proof electrical is very expensive and not as robust.

The pneumatic components used for assembly and packaging equipment is an entirely different category, where low cost is important and the operating environment is expected to be relatively clean and non-corrosive.

It was an amazing experience for me to work in a chemical plant back in the early 80's, which had been built in the 50's. Almost everything in the plant was pneumatic except for motors and lights. All of the signal lines were 1/4" PVC jacketed copper tubes that transmitted a 3-15 psi signal up to several hundred feet. It was easy to trace lines and see what was going on, without opening a cabinet or referring to a schematic.

 

[mk3223]

As for the application valve in the Safety Instrumented System (SIS), the pneumatic actuator with spring action is needed to achieve the valve closing time as required to protect the system.