What is the pressure drop across a flow control valve? 1

[Bhupinder1]

From my understanding, a flow control valve is a globe with an actuator on it. From Crane book, I found the equation for the K value of 340 x ft. ft = friction factor. Using this equation I get a pressure drop of 0.018746781 bar. However, my colleague was saying he always uses 1 bar pressure drop for a control valve. So, now I'm confused whether it should be the above value or 1 bar? It's a big difference. Are control valve designed to partially open state?

 

[LittleInch]

You don't appear to have any idea what a control valve is our how it works.

I suggest you do a search for such material e.g. Fisher controls then come back here.

What your colleague MAY mean is that a control valve trying to control flow or pressure needs a minimum of 1 bar differential pressure to work effectively.

But I have no idea as you haven't given any useful information

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

 

[GD2]

Bhupinder1,
Control valves are not easy to spec out and one of the most difficult item in a piping system to spec. If not correctly designed, it creates a lot of headache for operation and maintenance.

I am not sure what you are planning to do, but if you are trying to spec out a control valve, make a data sheet first with all process, mechanical, electrical/utility inputs and send it to a control valve manufacture like Fisher as LI has pointed out.

GDD
Canada

 

[bimr]

Contact a valve manufacturer that makes the type of valve that you desire.

The valve manufacturer will provide the pressure drop at the flow conditions. Another method to determine the pressure drop is from reviewing the valve manufacturer's performance curves.

 

[georgeverghese]

A general rule for specifying the dp for a control valve is for the control valve to consume at least 30% of the dynamically available dp between the 2 units straddling the control valve. It could be 2-5inches of water column on a low pressure fuel gas suppply to burners, or it could be 500psi or more on a high pressure steam or compressor capacity recycle letdown. Also remember that turndown on the control valve capacity is equally important as sizing it for max flow case, so you should specify the flow and dp conditions at minimum opening of the control valve also. Vena contracta pressure drop is greater than permanent pressure drop, so watch out for internal flashing / 2 phase conditions also that can restrict flow capacity and cause erosion on control valve trim. The process engineer designing the plant should always specify duties, response times where critical, mechanical design press / temp conditions and construction details, noise limits are required of the control valve - never leave it all to a control valve vendor. Get a senior process engineer to set up these specs - not a job for newbie engineers.

 

[Orsiz]

The pressure drop across the control valve is up to you as you know the system in which the valve is operating. Search for two points (up- and downstream of the control valve) in your system where the pressure is known. The pressure difference between those two points is the driving force in your system and will correspond to a certain flow rate (which will depend on the control valve opening). Calculate the friction pressure drop in your system, for highest flow rate, to just up- and downstream of the control valve. The difference between those two will be taken by the control valve.

Read the Masoneilan Control Valve Sizing handbook in which they state that a good working rule is that 50% of the friction drop should be available as drop across the valve. Keep in mind that the maximum quantity that a valve should be required to pass is 10 to 15% above the maximum specified flow rate.

 

[GT-EGR]

it sounds like you looked up a line size globe valve and calculated the pressure drop across it wide open. Problem is if you go with this valve, your system may not see any change in flow at all until it closes off quite a bit. This is a concept called valve authority, if you pick a valve with no pressure drop from the start, then closing it a little bit will really not add any significant pressure and therefore flow won’t change either.

This is why you’ll usually see control valves selected at a smaller size than the adjacent pipe.

On the flip side, size the valve too small and you’ll have the opposite problem, where it closes a little bit and flow dramatically reduces.

As for your friends number of 1 bar, that number should be relative to the total system size and pressure drop, pump head available, and control tolerances. So it may or may not make sense. I definitely don’t recommend just using a number because it has worked for somewhere else.

 

[MFJewell]

https://www.eng-tips.com/viewthread.cfm?qid=430771