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solenoid valve Cv

solenoid valve Cv

solenoid valve Cv

(OP)
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?

Thanks in advance,

RE: solenoid valve Cv

Hello...

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.

Good luck

Adrian

RE: solenoid valve Cv

There are direct action solenoid valves and pilot operated solenoid valves.

Direct action will close against the rated inlet pressure without any minimum DP across the valve.

Pilot operated solenoid valves depend upon a minimum pressure differential across the valve to assist in the action of the solenoid.   Without the minimum DP, the valve is unlikely to actuate, even when energized.   

Make sure you get the right style.

RE: solenoid valve Cv

(OP)
Thanks for all the responses.

The solenoid conditions are:
Fluid: natural gas
Min P: 0 psig
Max P: 130 psig
Max Rate: 300 slpm
Line size OD: 3/4"


The vendor came back with:
Orifice size: 3/4"
Line Size: 3/4"
Cv=5.8 (per catalog, the title shows "Cv value water (gpm)")

So, when I calculated back, with Cv=5.8, with the NG conditions, (130 - 0, makes it a choke flow), the max flow = 15,100 slpm. That is way too big from the max rate we expected. So, do they quoted a valve that's too big? Or in this case it doesn't matter since it's for on/off valve as suppose to a throttling valve?

Another question I have is, if I use a Cv value of water for a gas flow, is that the correct way? The formula to calc Cv gas and liquid are different, so what should I do if the catalog shows Cv for liq, but they claim you can use the valve for liq or vapor? Can you still treat Cv the same, compare it as apple to apple between Cv liq to calc a gas flow?

RE: solenoid valve Cv

Since you are using the valve as a shut off, it does not matter about the size.

If the valve is not physically too big, use it.

In respect of using Cv values, the calculation used does not apply for fluid and for gas. We are talking about two different states of matter. The Cv value is based on water at a specific temperature. The behaviour of water and its viscosity, stability etc...are all known, with gas it they are different.

Generally valves specified for use with gas are much smaller than the fluid control valve with the same flow rate.

You also need to make sure that the valve is gas tight. Gas can get into and through places that liquid cannot.

Adrian

 

RE: solenoid valve Cv

Dear all,

I am currently working on designing a 2 ways solenoid vavle project. Could you please recommend me some good books or resources that I can refer to get a start in that field. Thanks so much in advence.

Minmin

RE: solenoid valve Cv

I have a "Valve Handbook" that I think was published by McGraw-Hill.

It's full of possibly useful application information if you know nothing about valves, but it's of limited to zero use in actually designing valves.

I suggest you start by reverse engineering the valve you're trying to replace.

If you are a student, buy a valve and reverse engineer it, come back here after you graduate, and please start your own discussion instead of hijacking someone else's.



Mike Halloran
Pembroke Pines, FL, USA

RE: solenoid valve Cv

To Hydromech

Could you please provide me some information, where to buy proportional solenoid valves? Especially solenoid valved in SS316 and certified EEx i and EEx d.

Best regards
Carsten Toft

RE: solenoid valve Cv

Carsten...

There are many types of proportional valves with different levels of performance.

Can you give any details of the level of performance you require?

Are you looking for directional control valves or flow control or pressure control..?

Adrian

RE: solenoid valve Cv

Suggestion: Use google and search for solenoid valves natural gas, and you will include a worldwide selection of specialized valves.

The European Kv and the US/Imperial Cv (Cv approx. 1,2 Kv)
are a dimensional depending figures. Correct dimensions to be used. Both based on actual measurements with water on the given valve with given constants, but both figures are independent of the flow medium to an adequate degree of approximation.

Formula
www.ascojoucomatic.com/images/site/upload/_en/pdf1/00011gb.pdf

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 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.

RE: solenoid valve Cv

Don't get hung up on Cv just being for water.  Cv is an index of the capacity of the valve.  Double the Cv, Double the capacity.  
There are separate methods for calculating Cv for liquids, Gases, steam, viscous flow. But once you have calculated a Cv required you can grab a valve with that Cv and expect it to flow about what you require.  

The ugly part is that when the pressure drop exceeds a certain value then the valve chokes, which means really no more than the flow is not as much as predicted by the "non-choked" equation, and you have to do more math. Some of the fudge factors that are valve-specific may not be published, so manual calculation starts to climb rapidly on your "Not Fun Things" list.    Part of the differential pressure does not count, and changing the downstream pressure does not change the flowrate thru a choked orifice.  The moral of this is: Use a sizing program from a reputable control valve company when calculating Cv.  If the programmer is any good at all the program will automatically calculate for critical drop and shift to output the appropriate value.  

What does a soleniod valve do? It allows you to control a big honkin' valve with a tiny electrical signal.  Just flip a switch in the control room, and the solenoid valve opens and lets air into the actuator to move a BIG valve. Then there are permutaions of that, but you get the idea.  

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