Pressure difference of control valve

[sujins]

When sizing the control valve and after calculate the actual pressure drop of system and maximum allowable pressure drop of control valve, found that actual pressure drop less than max.allowable. Then select the actual pressure drop to size the control valve but the result shown that the cavitation occur. Can we reduce the diff. pressure of control valve and find the suitable size of control valve? But the new pressure diff.will not refer to the system, what can we do next? Or can we use this new pressure diff.?

 

[jsummerfield]

You likely need special cavitation control trim or installation techniques. I may lack the precise description of a valve design instructor. However, consider the maximum allowable pressure differential as related to sonic velocities. Once sonic, the fluid flows no faster regardless of the dp. The vena contracta has the highest velocity, lowest pressure. After the vena contracta the pressure recovers somewhat - depending upon the valve design. Cavitation occurs when the pressure in the vena contracta (after the plug or orifice) is low enough that the liquid flashes. (This is easily predictible in pure fluids by the known vapor pressure. Not so easy in mixtures.) After the liquid flashes in the vena contracta it may condense as the pressure increases. Picture bubbles forming in the liquid, then bursting as the pressure rises. The bursting bubbles are cavitation which can break chunks of metal. There are several solutions to cavitation problems.

Most solution include specially designed valves. Some that take smaller pressure drops in stages. Some permit the vena contracta recovery to be in the center of the valve away from metal. An angle valve may do a similar function with the recovery away from the valve or piping components. If using a valve sizing and selction program that alerts of a cavitation issue, begin selecting special cavitation control valves.

John

 

[sujins]

Normally, when we size the control valve, should we use the actual pressure drop or maximum allowable pressure drop that liquid will not cavitate. Anyone can suggest more.

Thanks.

 

[bmsg]

I am not clear from your queries:

When we have a control valve in our projects, we follow the following steps:

1. The Technologist gives us a maximum allowable pressure drop across the control valve, depending on his piping configuration [the controllability of the valve is best if this drop is a significant portion - say, about 25-30% of the overall pressure drop in the line - but we are not going into this now].

2. We size the control valve with the maximum allowable DPand maximum flowrate.

3. We select a valve which has a Cv somewhat higher than the calculated Cv - so that the valve opens, say, 70-80% at the maximum flowrate (to keep some cushion).

So, for the selected valve, the actual DP will be LOWER than the max. allowable DP.

 

[jsummerfield]

Use the maximum allow pressure drop. This question relates well with the flow restriction orifice discussion. Once the pressure drop equals the allowable drop, an increasing inlet pressure no longer increases the flow through the valve due to choked flow, right?

John

 

[JimCasey]

> Once the pressure drop equals the allowable drop, an increasing inlet pressure no longer increases the flow through the valve due to choked flow, right? <

Not exactly.

Choked flow (I wish there was a better term)means the DOWNSTREAM pressure is independent of the flowrate.Choked flow is just when the flow no longer follows the Cv=Q*SQRT(DP/GF) curve. If you boost the upstream pressure, you'll still get more flow. If the choked flow is in a partially open valve and you open the valve more, you'll get more flow then, too.

 

[Assumptions]

As always language has a huge effect on things.
I don't know the full background on how the language for control valves has developed but my two cents on the wording for this is, that it is at the least confusing.

Perhaps a better definination for max allowable dP would be max effective dP. In other words above that dP, for the given conditions and upstream pressure, one is not movimg more fluid through the valve.

Thus for a liquid application, if the dP goes above the vendors stated max value for your conditions then one must use the vendor's value for calcs when your installation is above that value.

A good reference would be Liptak for understanding the issue.

Hope that this might help answer your question.