Max Differential Pressure Across An On/off Valve Prior To Open It

[JustSomeRoark]

Gents,

Disclaimer: Process Eng background here with only superficial knowledge of intimate details of valve design: be gentle!

It seems to be common practice amongst my Process Eng colleagues that a full bore, ball on/off valve shall not be open before ensuring that the differential pressure between the upstream and downstream side is lower than 15-30 psi, so a smaller bypass valve is normally provided to allow the pressurisation of the downstream side. Some colleagues argue that this holds for liquid, but some of them consider this should be followed even for gas at, say, 300 psig (i.e. valve should not be open until the downstream side is pressurised to 270-285 psig).

What is the rationale behind this? Is it only aimed to prevent oversizing the actuator? Let's assume the actuator is sized for full pressure rating. Does opening the valve across a large differential pressure (i.e. larger than 15-30 psi) creates high vibration, noise or any other operational upset? We are concerned about erosion of the ball at lower opening (high velocity, high erosion) or damage to the seats.

Any idea where this assumption of low DP across the valve may come from? Is it still valid?

Any feedback on this would be deeply appreciated.

Regards.

 

[LittleInch]

Actuator design, in the vast majority of cases, is sized for opening and closing under max DP as stated in the data sheet, plus a safety factor, sometimes of 2 or 1.5.

However in practice this rarely happens because of the things you mention.

Normally the issue is damage to the seats, not the ball itself, but both can occur.

Especially if your seat has a "soft seat" insert, this can get chunks of it ripped out when opening under high differential pressures.

The other issue with gas is whether the large DP leads to low temperatures / hydrates or excessive velocity in the downstream equipment.

You can go overboard with these bypasses and usually there is some rationalization to work out when this large DP might appear and how to start up the plant using the least number of bypasses.

Then you need also to look at the volume of fluid needed to pressure up the downstream section. If the volume is large then your little 2" bypass on a 30" valve might take hours for a gas system but seconds for a liquid system...

Is it still valid - very much so.

Do the operators use all these little bypasses.. Hmmmm



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

 

[georgeverghese]

That criterion is valid for soft seated isolation valves >DN50. Metal seated isolation / safety critical valves can tolerate a much higher dp of up to 14bar, per operational guidelines in my former OpCo. Pressure equalisation bypasses are not required for main line valves DN50 and less. Actuators are sized for the full differential nevertheless.

 

[bcd]

There may be situations where the valve seat design is not robust enough to withstand the large pressure difference when operating, but that is not the primary reason for reducing pressure or mandating slow operation. Reason is primarily to avoid water hammer, or a large temperature increase in the down stream pipe. Ball valves can be fast opening and create a large pressure spike (incompressible fluid) or temperature increase (with compressible media) in the downstream pipe that blows out gaskets or melts all the soft parts in the next downstream component. So slowly decrease the pressure before opening, or slowly open the valve. Either way usually works.

 

[mk3223]

Added to the comments above, the dp limitation across the ball valve may be due to the ball valve types. Per one of the design guidelines I used, the floating type ball valve is for the size of =<2" or the dp of <75psig, and the trunnion type ball valve for >2" or =>75 psig.
I believe the guideline is the best practice of the plant operation & reliability, and the similar design is also seen for the globe valves but in the larger dp value than that for the ball valve.

 

[gerhardl]

All above is essential bits of a complicated picture, stating that one should always accurate describe all possible working conditions before and selecting a valve.

If, for instance, you seek a ball valve with full opening and larger size suited for large pressure difference, you could look into double eccentric ball valves. (High price limited liquids)

The point is that every valve problem has a best possible solution if all procession details are known.

In addition to answers regarding sealing damage, cavitation could be a problem if the valve is left open with a smaller opening.