Mechanical coupling control valves 1

[NghiaPP]

Hi all,
I design a temperature controller for a heat exchanger. A by-pass line around one side is used to control the outlet temperature. The size of the main line and bypass line is large (14"), such as a three-way valve is not desired. A layout with two butterfly valves is suggested. But somebody is concerned the case both valves closed. A minimum stop is suggested for the valve on the heat exchanger line. But unfortunately, in some scenarios the heat exchanger needs total bypass. I heart earlier the same control system with mechanical coupling between two valves such as when one valve fully open, the other will be fully close. One actuator is used to moving both valves. It seems similar to a way three-way valve works. Unfortunately, I don't have any drawing or detailed how it is constructed and operated. I would appreciated if someone can help providing information and experience about the mechanical coupling valves system.

 

[JimCasey]

Mechanically coupled butterfly valves on a tee for 3-way action have been available as long as butterfly valves have been available. 50 years, anyway.

Actuators have evolved to be so reliable that it is more accurate, more relible, and more "tunable" to use a fail-closed butterfly valve and a fail-open butterfly valve on a tee, each valve has its own actuator and a digital positioner. Digital positioners are software-characterizable so you can dial in the bypass valve, for example, to open a little less than the valve that goes through the heat exchanger because the restriction of the two paths is different. This makes a balancing valve unnecessary.

When you lay out the system, be aware that butterfly valves have a lot of capacity and it may be beneficial to use reduced-size valves. In a 14" line you could very possibly control with 10" valves and get better control. Have your control valve supplier look at the application and run it through his sizing software.

 

[NghiaPP]

Thanks for your reply.
The control valves in the question should allow the bypass varied from 0 to 100% in different operating cases
Is there any risk that both valves closed at the same time caused by actuators failure? In this case, a PSV should be installed to protect equipment upstream the valves.

I’m thinking a mechanical arrangement between these two valves such as closing both valves at the same time is impossible, like a three-way valve, it is impossible to block both outlets simultaneously. If possible, the arrangement could eliminate the need of PSV.

 

[ash9144]

Use common air lines and have one valve air to open the other air to close. Set up one as reverse from the other and they will move inversely.

 

[jmw]

Fisher developed a pair of control valves, mechanically linked, to be used for blending.
In this case the valve position is controlled by differential pressure which maintains the outflow at a constant viscosity.
You can find a diagram of the valve by visiting the CBI engineering (Denark) web site and viewing the pdf files on blenders.
The slide valve etc. are superfluous to your needs but the purpose is to show the arrangement of the diaphragm control and that the components are generally available at pipe sixes to say 8 or 10".
see:

http://www.cbi.dk/images/pdf/CBI Mechanical Fuel Blender.pdf

JMW

http://www.ViscoAnalyser.com

 

[JimCasey]

You didn't say anything about what metallurgy you need.

I still like the idea of two valves with opposing action being driven by the same signal. Increase the signal: one valve opens and the other one closes. AN earlier post mentions a pneumatic signal. This would also work nicely with an electronic signal running in series through both positioners. With the butterfly valves you can get any imaginable metallurgy so they can be compatible with any fluid running thru them.

Another way to do it is with a valve made by Leslie. They call it the RVK. It is a 3-way valve with a rotary shutter. They offer it only in Iron and Bronze. It is not very expensive and was designed to control the bypass around a heat exchanger. Works nicely to control the temperature of big marine diesels, too.

 

[NghiaPP]

Thanks for valuable input.
The valves from Leslie (RVK, GTW or DOT) could be a solution to my design. I will investigate further together with mechanical link system.
My control system will be operated at 60 bar, 350 deg.C in hydrogen and H2S environment. Material for the valves shall be SS321/SS347 or higher.
Further search in the web, I found the following links described the mechanical linkage butterfly control valve system:

http://www.bitorq.com/three_way_linkages.html

http://www.pdcvalve.com/accessories/accesories_3-way.html

http://www.conservationsolutions.com/pdfs/spartan/bfly.pdf

I would like to hear if anyone have seen or used the mechanical link system and any experiences about it.

 

[JimCasey]

60 bar, 350C, Hydrogen and H2S

That changes the question markedly.

You need a very specialized valve and it will cost a lot of money.
Butterfly valves are likely to be unsuitable at pressures this high. Possibly the Tyco Vanessa triple-offset butterfly valve could work, but butterfly valves do not like to modulate with much pressure drop.

Forget the Leslie RVK and DOT; Maybe you could use the GTW.

At 350C it would be prudent to use an extended bonnet. And for Hydrogen it would be wise to use a lantern-ring and double packing.
Valtek could offer the Mark 1 3-way with an emissions-sealed bonnet in a NACE material. The Valtek offering would have a cylinder actuator which you would find useful at these pressures. A 3-way globe valve is essentially 2 unbalanced globe valves in a common body, and the actuator forces can get pretty high.

 

[NghiaPP]

I would like to clarify that the operating fluid is at 60 bar and 350.C. These data would affect on material selection, thickness of body and sealing. The pressure drop across the valves shall however be as low as required for a properly size.

 

[NghiaPP]

I don’t think the design temperature & pressure and required material prohibit the use of butterfly valve in this application. The standard system provided by some vendors in the above links is for low temperature and pressure and with some limited material. But the same principle can be applied for a tailored design system with suitable valves.
The bypass system with two butterfly valves has been previously used by us in similar design temperature & pressure and material. The new in this application is that the bypass shall be design for 0 to 100% of the total flow. Previously, we used an actuator for each valve with a mechanical stop for the valve to heat exchanger, since the bypass is only designed for 0% to say 60% of the total flow. Sometime, the butterfly valve to heat exchanger is omitted or just a hand-driven valve.