I'm about to purchase several solenoid valves (will be used as an ON/OFF valve). My questions are:
1. can someone inform me more on the solenoid valve. i.e. why we use it, when to use it and when should we not use it
2. The flow I sent the vendor is in slpm, and the fluid is gas (don't ask me why the designer of this unit loves to use slpm instead of mass rate). the vendor came back with Cv value and when I looked at their website, the Cv value is based on water. Can someone tell me how to interprete a given value (Cv), to check whether the quoted valve will be the right size to use?

Can I still use a valve program, plug in all the conditions, enter the Cv and let the program calculate the max flow? Then, compare the calculated flow to the operating flowrate to make sure is not undersize?
Does it matter that I use a Cv of fluid (water) to check wheter the valve is good for a vapor type of flow?

1) Why do we use solenoid valves...Because they are a cost effective and reliable way of turning electrical energy into linear force.

We use them when we want to move something. Valves, switches, latches, many mechanisms can be moved with solenoids.

Their use is limited by the application and cost. In automotive applications, over a certain power, solenoids tend to become quite large and the cost becomes prohibitive.

Some explosive applications mean the solenoids are not viable for obvious reasons. However, there are many hazardous applications that do have solenoids and solenoid valves.

There are solenoid valves on the atlantic seabed and the the are solenoid valves on space craft. Solenoid valves are ubiquitous.

Solenoid used with valves can be digital. i,e on or off...or proportional meaning for they move a precise amount for a given electrical current input.

But that's a lesson for another day...

2)Answering that question is tricky.

The Cv value is for water flow with a delta P of 1 PSI. A valve will handle much much more gas with a delta P of 1 PSI.

It might be better if you tell us the the type of gas that you are using, as well as the flow and pressure rate required and any other important details.

If flow calculations are very important: ask the supplier to give the whole calculation (with dimensions) to give an exact comparison between competitors. Companies tend to differ in use of calculation pressure/data inputs, not giving the whole story.
 
Some companies gives out Cv's for air, putting in 'normal pressures' to recalculate the Cv which actually have to be based on water. This is only OK if you give the data for the calculation.

To obtain a good, unrestricted flow, largest Cv value for else equal valve dimension and power requirements, lifetimecost etc. is a better choice for on/off valves.

For gas solenoid valves in general: be aware of any Ex, relief and fire protection requirements.

For long 24VDC lines be aware of effect loss in the lines. The solenoid will require a minimum current, depending on make and construction.

In addition to above, general on solenoid valves:

a) Solenoid valves only to be used for clean fluids. Any fluid with particles, dirt, silt (in water), too high viscosity, fluids (solvent) that will give a crust when drying is not suitable for a solenoid valve.

(Better to use a Y-pattern globe valve or membrane valve for those applications. Enthusiastic engineers tend to overrate solenoid valves when they have used especially larger 2/2-ways successfully)

b) Market divided roughly in four types/areas :i) standard common solenoid valves for air,  ii) standard solenoid valves for hydraulics iii) common standard solenoid valves for water - all these three types within normal pressures and distribution line sizes.

c)The rest of the market: iv) All other types of solenoid valves - 'special and process valves'. For theese types you need more process and fluid and electrical data than for picking out almost any other actuated valve. This section is vast, with a large number of specialized types world-wide.

For gas you could also for larger valves end up with  valves with forced lifting. Forced lifting for a 2/2-way: first part of the stem movement opens for eqalizing of pressure over and below the disc, rest of the stem movement lifts the disc. Theese valves(ball valve) will open 'wrong way' for back pressure higher than the feeding pressure.

You will need to put in separate check valves to avoid this. In some cases (propane) this backward opening is used positively, when lines for instance are sun heated, to equalize line pressure sending the expanding gas back into the system.

Also if several smaller lines are attached to a larger feeding line or tank, you have to look at lay-out and possibly checkvalves to get the distribution you want.

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