Knowledge about Hydraulic and pneumatic valves 1

[englearn]

Knowledge about Hydraulic and pneumatic valves

I am a lay man in this filed …could some please tell me the difference between Hydraulic and Pneumatic valves… how do they work? Which one is the latest technology? What are the advantages of one over the other? What are the limitations of the two technologies?

Thanks

 

[davefitz]

I am not an expert in the field of valves or actuators, but my 2 cents worth is:

a) pnuematic- air operated actuator. The plant needs a reliable source of compressed air, usually a with min pressure of 80 psig at the inlet to the valve positioner. For a large plant with many air actuated devices, a central air compressor with drier and filter plus distribution piping is more economical than the laternatives of electric motor actuator or electro-hydraulic actuator.

The air actuated valve must be desinged for relatively low valve stem load, as the power from a air actuator is limited to relatively low levels.

Air is non toxic and nonflamable and does not generate a spark that could cause a fire in areas with possible flammable gases.

b) electro-hydraulic-EH- the valve actuator uses high pressure hydraulic liquid ( at 1500-3500 psig)to drive a piston than will move the valve actuator. The forces that can be developed are much larger ( 25-50 times larger)than with an air actuator, and the speed of action can be much faster. Typical stroke periods of 30 seconds with a large air actuated valve can be alternatively stoked at a stroke speed of as quickly as 0.2 seconds with elector hydraulic actuators. Hysterisis is much lower, and valve position accuracy, resolution and repeatability is much improved over air actuator.

The cost is much higher with EH . If severa EH valves are used, then one central EH pump with accumulator is used, with a distribution and drain piping system to power the valves. Traditional hydraulic fluid is flammable, which leads to serious fire safety hazards to be dealt with . there are now available several non-toxic and nonflammable sythnetic hydraulic fluids that can be used in place of traditional hydraulic fluid. If only one EH valve is requried, then one can use a single purpose EH pump, and also water-based hydraulic fluid can be used to lower the cost.

 

[RGStewart]

"electro-hydraulic-EH- the valve actuator uses high pressure hydraulic liquid ( at 1500-3500 psig)to drive a piston than will move the valve actuator."

Does 3500psig work out to be about 238 bar? I'm used to seeing applications at 450bar (6000+psi).

 

[davefitz]

14.5 psi = 1 bar
3500 psi = 241 bar

The pressure the sytsem operates at may be limited by the type of hydraulic fluid selected. Water - based hydraulic fluids would be at lower pressures, while mineral oil based fluids would be at higher pressures.

 

[Ashereng]

Pneumatic valves run on "gas". It can be instrument air as stated above, and it can also be process gas/HC gas (from your process gas line - common on gas pipelines), nitrogen, propane gas and any other suitable gas.

Hydraulic valves run on "liquids". It can be water, HC/oil, and any other suitable liquids. I have only seen oil hydraulic valves (just an observation).

The electro part is not a requirement for either pneumatic or hydraulic valve.

In remote sites, I see many purely pneumatic systems - pneumatic controllers to pneumatic valves. I see hydraulic systems usually with pneumatic sensors (may be industry specific). The hydraulic valves are usually ESD valves, used for shutdown in case of line breaks (pipelines).

An I/P (current to pneumatic converter) is used in many applications to convert a 4-20 mA signal (from a control system) to an equivalent pressure to the valve acutator. This is usually in plants and facilities where both power and air are available.

An I/H (current to hydraulic converter?) does the same for hydraulic system. I have not seen this personally, so I am assuming that this is somewhat rare. Most hydraulic valves that I have seen are self-contained.

I would not classify either one as latest technology. They are both older than I am so that is saying something.

The limitations are more related to availability. If you have gas/air, you use pneumatic. If you don't, you use a self contained hydraulic valve.

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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