sizing and selection of control valve

[kald]

What the factors consider when sizing control valve for oil and water separator outlet , as an example in my process plant , separator oil outlet is 10 inch and installed control valve is 6 inch while the water outlet 8 inch and installed control valve is 6 inch . is the above related to upstream & downstream pressure and flow rate only or there is another fatores? what is the bad effect rather than economic if i want to increase the control valve sizing to decrease pressure losses . another example there is a 4 inch control valve on 12 " discharge of transfer pump which lead huge pressure losses at this control valve so what about if i replace this valve by 8 inch or 10 inch valve

 

[SNORGY]

The problem probably isn't sizing the control valve so much as it is in safeguarding the downstream equipment in the event the valve goes wide open.

Who is right doesn't matter. What is right is all that matters.

 

[don1980]

In order for any control valve to perform its control function, it must cause pressure drop. A valve that's not causing any pressure drop is a valve that isn't performing any control. Pressure drop across control valves costs energy, but that's the price you pay for the ability to control the process. You want to allocates sufficient pressure drop to the control valve, but no more than is necessary.

The pressure on the downstream side of the control valve is completely independent of the pressure on the upstream side. The downstream pressure is entirely dictated by the downstream system. The upstream pressure simply has to be high enough to produce the necessary flow.

In the system you describe, installing a larger control valve is a complete waste of money because it'll have no affect on the pressure drop across that valve, and worse, it'll adversely affect the quality of the process control (assuming the existing valve isn't operating near its full-open position) . A bigger valve will simply have to operate at a lower opening position, and if that position is too low, you'll probably experience poorer control.

Reducing excessive pressure drop across control valves is a good thing, but that has to be done by reducing the pressure on the upstream side.

 

[rotw]

I would define the various operating cases and calculate the corresponding valve cv as basis for the sizing of the valve.
I guess that a valve correctly sized should have a size compatible with your piping inlet / outlet size, usually being one to two size smaller. If its more economical to go for the lower size without compromising controllability and operability, then that should be your choice. The piping installation should be such that it accommodates this.

I found this detailed resource online, if this can help

http://www.bbpsales.com/wp-content/uploads/2017/01/3-Control-Valve-Sizing.pdf

 

[LittleInch]

What are the factors?

Will the control valve do what you want it to, i.e. control flow or pressure or temperature depending what is driving it within the flow ranges you want.

what is bad effect making it bigger? Basically you can loose your fine control as the control valve closes and it has more difficulty to control.

Most control valves need to increase the velocity of fluid in order to control them better so end up one or two normal sizes below the pipe size which is optimized for longer pipe runs. Most control valves try to operate in the range 20% to 70% open to get a good control.

To have a 4" control valve on a 12" pump discharge nozzle would tend to indicate that your pump is far too big or your control valve too small.

You are looking at this the wrong way around. You need to figure pout what flow range, pressure range you want to achieve from your system and then size the equipment appropriately. only if you're trying to de-bottleneck something do you really start to look at equipment and saying is it too big / too small / pressure drop too big even when 100% open.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[kald]

very thanks , from the above the main factor for control valve sizing is the success of this valve size in control process parameters with the minimum cost .
but what we mean by " end up one or two normal sizes below the pipe" in LittleInch post , and " sized should have a size compatible with your piping inlet / outlet size, usually being one to two size smaller." in don1980 post

 

[moltenmetal]

kald: here's what they mean: pipe is usually sized based on an economic optimization between pumping energy and pipe capital cost (material, fabrication, installation, supports, insulation etc.). That optimization is sometimes dumbed down to a range of acceptable velocities for design. And that range of velocities or pressure losses per 100 ft of pipe is often followed even in situations where the process requires you to throw away pressure energy by throttling. The opportunity to reduce line size and save money in those cases is often not taken by the designer out of (usually) irrational fears about erosion, or simply because the high velocity in the line doesn't "feel right" to them because it violates a rule of thumb they've been applying for years without completely understanding WHY. Instead of reducing the linesize to the true optimum in these cases, i.e. the size which makes use of as much of the available pressure drop as possible without itself becoming a controlling resistance, they use the same rule of thumb for the sizing of a line up and downstream of a throttling control valve as they would on a long line between a pump discharge and a tank.

A control valve is sized to give a certain pressure drop at a certain flow even when the valve is fully open. That pressure drop is (usually) selected so that even when the valve is fully open, at the maximum design flowrate, it represents a CONTROLLING fraction of the pressure difference between point A and point B in the flow of the fluid- often roughly 1/3 of the total pressure difference. That determines the Cv of the trim required. If frictional loss in the inlet and outlet lines are low because of sub-optimal line sizing as noted above, the control valve ends up a substantially larger fraction of the total pressure difference, which is OK from a control perspective because it means that the control valve itself will not have to compete with other pressure losses to control the flow. Usually, a trim with the required Cv can be fit into a valve body which is a couple NPS sizes smaller than the line itself, therefore it is not uncommon (in fact it is NORMAL) to see a 3" control valve in a 4" line, a 4" control valve in a 6" line etc. Sometimes, the required valve will be several sizes smaller than the line because the pressure drop required for adequate control is quite high, i.e. when a valve is used to throttle a fluid from a high pressure separator to a drain or tank etc.

Control valves can be fitted with a range of trims which depend on the size of the body. The required Cv may not be available in a larger valve body, and even if it were, you wouldn't want to spend money on a larger valve body just to avoid a couple reducers on either side of the valve in the piping.

 

[kald]

Thanks moltenmetal , your answer make it more clear