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waseem19 (Civil/Environmental) (OP)
27 Jan 03 3:04
what's the difference between throttle control  and flow control ,

and if i want to buy a flow control valve what information i should provide to the manufacturer (is there anything that he should give me first ).

up to now i know that i should prepare a flow Vs pressure drop curve ,but don't i need something from the manufacturer to get the pressure drop across his valve

the valve should have a linear flow opening relation.

don't worry guys i'm not buying that control valve alone !
valvetech (Industrial)
27 Jan 03 7:58
You should contact the manufacturer and give them the information in regards to flow and pressure drops that you wish to maintain. the manufacturer will then be able to tell you what valve you need after doing the appropriate sizing calculations.
TD2K (Chemical)
27 Jan 03 11:06
The manufacturer does not give you a pressure drop, you need to supply this to them along with the required flow.  Basically, for a liquid, you need to supply the flow, the inlet and outlet pressure, the temperature, the fluid's vapor pressure and critical pressure.  You do this for a min/normal and maximum sizing case to ensure the valve has the correct turndown range.

For a gas, you need to supply the flow, the inlet and outlet pressure, the temperature, inlet compressibility, MW or SG.  As above, you do this for a min/normal and max sizing case.

For both, you need to identify parameters like is the fluid sour, entrained solids, etc so the manufacturer can recommend a valve.

For a given valve, if you have a flow and %open, the manufacturer can tell you the dP but when sizing it, the dP for the valve is set by the piping system it is installed in and the valve manufacturer has no idea what that looks like.
waseem19 (Civil/Environmental) (OP)
28 Jan 03 0:39
Dear TDK ,

thanks for your comment, i'm just not sure if i completely understand what you mean by (min/normal and maximum sizing case to ensure the valve has the correct turndown range).

do you mean min/max/normal flow condition , i have a two tank system ,the flow will vary (if i don't have a control valve ) depending on the water level in the feeder tank, so do i need to provide 3 curves ( flow Vs dp ) to the manufacturer. so what i need to do is fix the water level at a certain point and then start changing the flow required through my control valve and read the corresponding dp ,and that is my first curve ,then lift the water level to another value and start changing the flow through the control valve again and that is my second curve ,,,  
and the same for the third one………..
is what i said correct ?
hacksaw (Mechanical)
28 Jan 03 8:41
waseem19,

suggest that you put together a hydraulic profile of your tanks, and establish what fluid levels you want to maintain.

once done, then you have to decide about what surge capacities you need, the amount of free board, etc. These will help you understand the series of questions raised by TD2K.

developing a control strategy also figues into the valve sizing requirement, but first you have got to quantify your process design requirement.
TD2K (Chemical)
28 Jan 03 12:43
Waseem, let's say you are pumping from a tank, through a pump, through your control valve, through a bunch of piping to another vessel (it's easier to follow this if you make a sketch).

The inlet pressure to your valve will be set by the water level in the tank plus the head produced by the pump at the flow.  Downstream of the valve, the pressure will be the pressure drop through your piping plus the pressure in the downstream vessel. If you have elevation changes, that also needs to be included.

Now, if you increase the flow, the head from the pump will drop (if it's a centrifugal) leading to a lower inlet pressure to the valve.  The pressure drop through the line will increase which when added to the downstream vessel's pressure, will set the downstream pressure.

Basically, you want to come up with a maximum sizing case for the valve (for its Cv), a normal Cv sizing case and a minimum Cv sizing case.  That way, you can select a valve that will operate for all of your operating cases.

Factors that will affect the sizing cases in addition to the flow rate are the first tank's level (since a higher level increases the suction pressure to the pump) and the pressure and/or level in the second downstream tank.

For maximum sizing case, it's likely with the first tank at minimum level, maximum flow rate and maximum pressure/level in the second tank.  Normal is, well, whatever you think normal operation will look like.  Minimum sizing is likely with the first tank full, minimum flow and the downstream vessel at minimum pressure/level.

The valve doesn't set the pressure drop, that is set by the system and the equipment in that system.

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