POWER FOR SOLENOID VALVE

Solenoids are inductors. There is a holding current (power) and inrush current.

The power supply needs to match the required voltage of the solenoid, provide the holding current, and also stay reasonably stiff to maintain voltage during short inrush current.

It depends on the solenoid inrush current whether the 36w power source has enough power to survive turning on. A capacitor across the power supply can help provide extra energy during the inrush when the solenoid is turned on.

Holding current is actually higher than initial current for a solenoid, since the solenoid appears like an inductor. It's not the same as (say) a DC motor, which will have a higher initial (inrush) current (unless it's stalled).

They do create counter EMF each time they move.

    This is my solenoid valve voltage rating

your valve works with 230V AC, it will not work (to specification, if at all) with a 24V power supply

And you could make yourself a science experiment to see why this is the case. Get an electromagnet (any electromagnet, could be a relay coil, or a spool of wire) and by applying different voltages, try to approximately see what difference in force you're seeing. Try half the voltage, then a quarter of the voltage, and then a tenth of the voltage. What you'll  observe will make it very clear.

shabaz to be clear can you emphasize me the causes of solenoid valve not function when testing? Can you advise me the solution to these problems/causes?

The electronic portion (assuming it is a solenoid version and not a motor) is just a coil of wire, so ordinarily that could outlast a lifetime, although the winding can get damaged. You can do a very basic, very partial test by checking with a multimeter to see if it has gone open circuit. Other than that, the item is mechanical, so there can be plenty of mechanical problems (accidentally knocked plunger, stuck plunger due to dirt, etc. 

Since it is a gas safety valve, it's best to replace it with a new modern one. Otherwise, if it ever fails and there is damage or someone is injured, the person that repaired and re-installed it will be the one that will be imprisoned.

Obviously that's an expensive part, but if you're just looking to experiment with a solenoid valve (and don't intend to install it in it's normally intended application) you can get plenty of small low-voltage (e.g. 24V) devices for very little cost.

Is it possible to fix and repair the plunger that being dirty or rusting? Because when there have gone open circuit, the voltage and Amp is enough, and have supplied but the solenoid valve cant trigger to close valve. 

Personally I think no, it should be thrown away in a condition that no-one repairs it. I know there's a strong desire to repair and re-use, but in a critical application the $100 is not worth the risk to lives (and there would be a high risk). Everyone knows of gas explosion stories in their own towns or neighbourhoods, so it's not rare for things to go badly wrong. This would increase the risk a lot.

Our solenoid valve rating voltage and power is specification 24VDC and 18W. The solenoid valve(normally open) we tested was supply with 24VDC, after connect with the coil, the voltage was drop to 7VDC. Why this happen and what is advise to these problem? Is this power source need to continuously supply 24VDC into the solenoid valve?

Afiq You have overloaded your 24V power source.  The solenoid that you have provided a picture of, as Anthocyanina stated, is made to be powered with 230V AC.  Using a lower voltage source will not provide sufficient power to create the magnetism required to move the plunger.  Also, if the coil is made for AC and not DC, putting DC into it will also not provide sufficient power to create the magnetism required to move the plunger.

Afiq You have overloaded your 24V power source.  The solenoid that you have provided a picture of, as Anthocyanina stated, is made to be powered with 230V AC.  Using a lower voltage source will not provide sufficient power to create the magnetism required to move the plunger.  Also, if the coil is made for AC and not DC, putting DC into it will also not provide sufficient power to create the magnetism required to move the plunger.

Alright. Well note kmikemoo . I get your point.