Max flow velocity in a butterfly valve

[GeraldMan]

Hi there,
I was wondering about the operating limits of a butterfly valve (lets say water type) in the aspect of max flow velocity.

For example a PN10 valve of that type has a 3m/s limit (constant) but for higher flow velocity the graph(Y-axis: diff. pressure and X-axis max permitable flow) becomes exponential decay. Does that mean that the valve can work at these higher flows with lower pressure limits?

And is there a need for reinforcement in some parts (ex. disk) to have a safe operation?

 

[gerhardl]

I am not quite sure what you actually are asking, but to give correct answer for any particular flow/presssure conditions for any valve, some comments:

1. If you increase velocity above the one given by the manufacturor, you will lose any guarantee, and possibly extend wear and breakdown chances, if this is not accepted (in writing).

2. Flow, and hence velocity, is determined by difference between upstream and downstream pressure and resistance (pressure loss) over the valve.

3. The pressure loss (resistance) over the valve is a result on valve construction ('construct given obstacels'), flow speed, and throtteling degree (position of closing member), and will together with flow data and constructional strength have to be known to give underlying data for calculation or tests for cavitation conditions or excessive wear, under open, throttling and opening/closing conditions.

All above factors have to be considered. For any particular valve, only the valve construction supplier can give the correct answer of max flow under the given conditions. To further complicate the answer the piping layout upstream and downstream will also influnce the answer, in some cases considerably.

 

[bcd]

You need to review this with the valve manufacturer. There is a term called "Dynamic Torque". It is the torque produced on the valve shaft caused by flow around the disc. This torque varies by disc angle, disc type (concentric vs.offset), valve size, and differential pressure across the valve.

The equation to calculate dynamic torque is = Ctp * D^3 * DP

Ctp is a coefficient that is determined by testing a valve in a flow loop. D^3 is the flow path diameter cubed. DP is the differential pressure across the valve. As the DP increase, the flow rate and flow velocity increases (except when choked flow). The dynamic torque can be large enough to exceed the strength of the shaft or at least the operator trying to turn the valve. It is rare that this is a problem because normal design flow rates in piping systems are low enough to limit noise, cavitation, flow erosion, etc. But in blow-down and other very high flow situations with high DP, dynamic torque should be checked and a knowledgeable and experienced butterfly valve manufacturer has all the information you need on this subject.