8 replies to this topic

[A mukherjee]

Hi,
Can any one tell me how does a solenoid valve work?Is every on-off valve a solenoid one?What are the advantages a solenoid operated control valve have over other control valves?A detailed explanation is required...
Thnxx in advance...

[djack77494]

A solenoid valve is an electrically actuated on/off valve. The come in two way, three way, and other configurations. The solenoid part of the valve is always in one of two possible states - it's either energized or deenergized. There is a single moving part inside the solenoid, a (typically) iron rod that is influenced by the magnetic field which develops when the solenoid's coil is energized. Thus energizing causes a change of position of the rod which causes a change of position of the valve. These valves are not capable of modulating action. Because they cannot modulate, I would not refer to a solenoid valve as a "control valve". You could have an on-off valve that was not a solenoid valve, so no, not every on-off valve is a solenoid. Unlike a motor operated valve, a solenoid will have a fail position, and when it is not energized, it will be in the fail position. Also, solenoid valves are small and cheap (relatively speaking).

[kybele39]

Dear Mukharjee,

Djack77494 explained the basics in the previous post.


We use them in our applications. To me, the main reason that solenoid valves are used when you need FAST and SAFE operation in your process.

For instance, 2 way solenoid valves are used on the fuel trains as gas shut-off valves (fail closed) and vent valves (fail open)widely when you need low pressure. Automotive and pharmaceutical industry uses them also.

The leading manufacturers and distributors are ASCO, DWYER, MAGNATROL etc. in USA. Their website is full with information (google them).

http://www.valve-sto.../literature.htm

http://www.solenoid-....com/index.html

Enjoy,

kybele

[Propacket]

With apologies, I would not agree with djack77494 that solenoid valve cannot modulate. Now solenoid valves are also fabricated with combined action of on-off valve and control valve. Just take an example of anti surge valve on an axial compressor which switches on the gas flow through the recycle line from discharge to suction in order to prevent a minimum flow at which surging takes place. It only opens when the compressor is close to surging. At the same time, it also controls the gas flow through the recycle line and only sends the necessary flow. There are similar numerous applications in industry.

[fallah]

It only opens when the compressor is close to surging. At the same time, it also controls the gas flow through the recycle line and only sends the necessary flow. There are similar numerous applications in industry.

Antisurge valve is a special flow control valve that normally is closed and opens at the specified set point (activated at low discharge flow in the compressor).

Antisurge valve function is the same as minimum flow control valve for the pumps and never titled as a classic on-off valve.

Edited by fallah, 23 July 2010 - 09:36 AM.

[A mukherjee]

Thnxx a lot to every one....
Anti surge valve is an fail open valve.I completely agree with FALLAH that it is never a classic ON-OFF valve.
Can you guys just stress on how am I getting benefited by using solenoid control valve instead of a normal control valve?

[chemsac2]

A Mukherjee,

I would explain solenoid valves typically used in ON-OFF valve services.

It consists of a coil and a plunger. It has 3 ports: one for instrument air inlet, 2nd is connected to ON-OFF valve actuator and 3rd port is open to atmosphere.

Consider an ON-OFF valve that is open on normal level and closes when level gets low.

During normal level, interlock is healthy and that keeps coil of solenoid energized. Plunger is suspended in coil magnetic field. Construction of solenoid is such that suspended plunger connects 1st port to 2nd port keeping ON-OFF valve open.

On low level, interlock activates and cuts off electric supply to coil. Coil gets de-energized and plunger gets released from coil magnetic field. This action results in closing of port 1 and opening of port 3 such that instrument air from actuator gets released from port 2 to port 3 closing ON-OFF valve.

To re-open the ON-OFF valve, two conditions need to be satisfied. Coil needs to be energized and plunger needs to be brought back in coil magnetic field.

Auto-reset solenoids have secondary magnetic field system which brings plunger back to coil magnetic field. There are other solenoids with field mounted reset switch. For these solenoids, operator needs to go to field and press reset switch to push plunger back to coil magnetic field.

Regards,

Sachin

[A mukherjee]

Thnxx a lot Sachin..just clarify one thing...Is the instrument air is for pressurising the coil?If the plunger is controlled by spring,which is again controlled by electromagnetic field created by solenoid coil,then what is the requirement of IA air?
Thnxx in advance..

[Nikhiln]

Hi,

To make your understanding clearer, first we need to realise the purpose of the solenoid. The valve is still pneumatically the same, it is still kept in open/close position by instrument air. Where the solenoid enters the picture is, providing/releasing this IA from the space above the valve diaphragm. What the solenoid's plunger does is it acts as a mini-valve itself when deenergised, diverting the instrument air from the diaphragm space to atmosphere, based on its physical location wrt to the IA ports (as nicely explained by Sachin). Thus the pressure above the diaghragm is released, which either closes or opens the valve based on its configuration. And the reset will restore the earlier position for next use.

A wonderfully simple explanation is given at Wikipedia. Just replace the process fluid above the diaghragm by IA (and the discharge of the air to atmosphere, instead of valve downstream) and you will get the picture.

http://en.wikipedia..../Solenoid_valve

As for the advantage, it is much faster as compared to conventional valves, which makes it the preferred choice for critical applications where some fluids need to be stopped immediately/others need to be pushed in immediately, for flushing/purging etc. (like reactor trips, for instance)

Cheers!