Does a solenoid valve allow continuous run-time for very long durations?
That looks like the same part as the CDK 4F0/1/2/3 series of solenoid valves.



There is no duty cycle limit on the coils listed in the datasheet. It would be very unusual for them not to be continuously rated. Note that they are solenoid - pilot operated rather than direct solenoid so they will be quite low power - 1.8 W according to the data sheet. You should be able to hold your hand on the coil when they've been powered for an hour.

Starting current and holding current

Note that the AC models have a higher starting current than holding current. This is because the inductance of the coil increases as the solenoid is pulled into the coil. Higher inductance means higher impedance and lower current. Since DC is not affected by inductance after the initial switch-on rise time the starting current and holding current are determined by the coil resistance only.

As a result of the above AC powered solenoids (and relays / contactors) have a built-in power saving advantage over DC. However, the very wide adoption of 24 V as the standard industrial control systems supply voltage means that we live with the power penalty.



DC solenoid power-reduction trick

Just because it came up in the comments ...







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A power-economiser circuit for a DC relay or solenoid. Full voltage is applied to the coil initially through its own normally closed (NC) contact but as it energises the direct connection is broken and the voltage-dropping resistor feed takes over.



Pilot operation


  I have one more question which could be slightly off topic. I tried removing the solenoid connection part which were held by two screws. All I could see apart from the two screw holes was small 3 holes. I thought these solenoid valves actually had some "valves" that opened under magnetic field when activated. I was quite surprised when I noticed the inside with the solenoid to just have 3 holes and how does it control. When I tried connecting to a 24V DC I didn't see any visible movement apart from the click. Do you have any idea how it might be working?




Figure 2. 5/2 solenoid valve animation. Source: ZDSPB.com.

Explanation



Figure 3. Annotated for reference with text below.

This valve has five ports (1) to (5) and two positions (left and right). Hence, 5/2 valve.


Pressure is applied at (1) and exits at (2) when the solenoid is off and (3) when on.
(4) and (5) are the exhaust ports. Having two makes the spool (11) design very simple.
(6) is the solenoid. This moves the actuator (7). Note that this is small and requires low power to move it compared with a direct acting solenoid which would move the spool (11) directly, and have to overcome seal resistance, etc. 
When the pilot is off mains air from (1) via (8) is fed into (10) to drive the spool to the right - the normal position. Output (3) will be energised while output (2) is vented at (5).
When the solenoid is energised the pilot actuator (7) moves right to shut off the air to (10) and vent the left side of the spool (11) at (13) into exhaust (4). Mains pressure at (12) then moves the spool (11) to the left, port (2) is energised and port (3) is exhausted at (4).
Note that while energised air pressure is applied to both ends of the spool but the surface area at (10) is greater than that at (12) so the spool moves right.


All that to answer your question: the split between main block and the pilot section in your valve may be a little different to the animation. Most likely the three holes are:


The mains air supply to the pilot (8).
The pilot itself, to push the spool (10).
The pilot exhaust (13).


Note that there are many ingenious variations of these valves. Some might just use the spring at (12) and not have pilot air assist. In some the solenoid moves a tiny soft rubber diaphragm to allow air into (10).





Figure 4. The underside of the pilot valve.

(1) and (2) will be the pilot valve pressure supply and drive to the spool. How do we know? Because (3) has no seal gasket and the only place leaks don't matter is on the exhaust so (3) must be the exhaust port (13) on Figure 3.