Butterfly Valves used for flow adjustment in raw water service experiencing valve seat damage 4

[UTK_Civil]

I have a design build low bid contract in progress where the designers are trying to use butterfly valves to control (adjust) flow in the process. The services is untreated raw water used for cooling in a heat exchanger. Past experience with similar systems in at our facility has shown butterfly valves to be poorly suited for flow adjustment given our poor water quality. The problem is that damage to the valve seat usually occurs making the valve unsuitable for control or isolation in a short period of time. I am looking for code or industry studies that supports my position that butterfly valves should not be used with raw water to adjust flow. Any help would be very much appreciated!

 

[itsmoked]

I doubt you'll find it.

More useful would be understanding what the damage is. That would help you understand why that type valve may be a a bad choice or some other part of the design is precipitating the damage to otherwise fine valves.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[btrueblood]

No, I disagree smoked. Butterfly valves are not really meant for flow control, the characteristic curve is pretty nonlinear as compared to other types, and you can end up with the valve in a near-closed position and fluttering/chattering of the seat occurs. The caveat to my opinion, is that you might be able to use a butterfly IF the rangeability you need is pretty limited (maybe a 2:1 range of flow rates), and differential across the valve in flowing position is fairly low (i.e. you have control of system differential via other means, like pump speed control). To meet those requirements means somebody better do a really good job of valve selection. Too often, the valve is spec'd and then spec'd again as requirements are passed along, with each party padding the valve size a bit to "make sure" it will deliver enough flow. The end result is an oversized valve that has to throttle too close to the shutoff position, and thinks go south as the OP has experienced.

a quick pick of 3 1st page hits from a google search for "can you use a butterfly valve for flow control":

http://www.valveuser.com/325-using-butterfly-valves-for-control.htm

https://blog.craneengineering.net/when-can-butterfly-valves-be-used-as-control-valves

https://automation.isa.org/tip-10-b...lves-with-positioners-are-not-control-valves/

edit: grammar and clarity.

 

[bcd]

You need to get to the root cause. Is it the raw water, incorrect control valve size, incorrect valve type selected versus the expected use, or a combination of all. The seat damage will confirm which it is.

Butterfly valves are successfully used for control in some pretty demanding services every day around the world. Any generic statement saying butterfly valves are ill suited for control is flat-out wrong. But if inappropriately sized, or the wrong type selected for an application, there will be problems as you are experiencing.

Most common sizing problem is using a valve the same diameter as the mating pipe. It is common to use a butterfly valve at least one size smaller than the pipe. Ideally, the valve should throttle in the 30 to 60% open range. You can go a little more closed or open depending upon the valve, but not much more.

 

[UTK_Civil]

Thanks everyone for the information!

BCD,

Can you explain the need to have a smaller butterfly valve than line size? Most of our valves are the same size and upstream and downstream line size.

 

[LittleInch]

I agree with Bt and bcd.

In general yes, BF valves are not the best at flow control, but if sized right with the correct seals (metal maybe?) they can be used for relatively low pressure drops or low valve CV requirements.

If however your raw water is full of dirt, sand or similar or you operate at high velocities in your pipe, then many valve types will fail.

Most control valves will operate by effectively reducing the flowing area leading to local high velocities. If your water is particularly dirty then this will erode most surfaces, especially steel. You may need particularly hard materials to cope with this.

The lower valve size acts to increase valve CV for the same opening percent and hence you get a bigger pressure drop at the same percent.

For a BF valve if your required opening is really much less than about 25% open then your valve is too big or your pressure drop too big.

If your pressure drop is realtively constant then you might be able to use a flow restriction device like an orifice or set of orifices then add your BF to "trim" the flow.

If it's variable from 0 to ?? then you probably have the wrong valve type.

It's all about design for the particular fluid and situation. There is no great catchall.

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

 

[itsmoked]

Thanks bt, learned a lot in this thread.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[UTK_Civil]

Thanks everyone for the great information. I will try and update later on outcome!

 

[hacksaw]

Flow control with BF valves is not a problem and used routinely, but more importantly what do you mean by "raw water"?

 

[casflo]

An additional point to take into account in a throttled valve, is that it must be free of cavitation. A butterfly valve with an opening of less tan 30%, usually may have cavitation. The cavitation may damage the valve and the downstream pipe.
Chek your valve pressure drop and the cavitation coefficient of the valve.