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

[JustSomeRoark]

Gents,

It seems to be common practice amongst my 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. 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 cannot 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? Does opening the valve across a large differential pressure creates high vibration or any other operational upset?

Any feedback on this would be deeply appreciated.

Regards.

 

[bimr]

Liquids possess a large hammer effect because they are "incompressible", but gases can also possess a hammer effect large enough that may cause damage.

Noise and vibration will also occur.

There may also be dynamic torque issues with the valve if the valve is not designed for the pressures. The valve may break.

 

[dcasto]

Its more of a torque required to open it. But the 15 to 30 isn't that much differential on a 4" valve, on a 24" valve its a lot. As stated above there is either water hammer or in most cases, what is downstream. In a facility where a sudden surge is going to lift the bed in a reactor, slam the trays in a tower, send fluid into a system with a restricted outlet. It takes a little thought of whaere the fluid is going next too.

 

[Compositepro]

I've never encountered this requirement but, since you specifically mention full port ball valves for on/off use, my guess is that it is to prevent wear or damage on the valve seals. Full port valves have smaller seal area and if it leaks a little when closed, it will no longer be effective as a shut-off valve.

 

[georgeverghese]

Standard practice is to configure the facility to enable a low dp across safety shutdown and critical maintenance isolation valves prior to opening to eliminate the risk of tearing up the seat trim material. At the least, this should be practice for all ball valves DN80 (3inch NB)and larger. A larger dp of up to 6bar may be permitted for metal seated ball valves, if I remember correctly.
The actuator on a remote fail close operated valve will of course be designed to develop the torque to open the valve with the full mechanical design pressure of the piping system as diff pressure across the valve (at the lowest normal operating instrument air supply pressure).

 

[LittleInch]

George, as usual, has it right IMHO.

Looking at the OP

What is the rationale behind this?
The rationale is that ball valves are intended for isolation, not for starting, stopping or controlling flow. The issue is one mainly of damage to ball and seats during opening or closing with a large DP. Metal seats are better until you add soft inserts or lip seals which just get destroyed when you open with a large DP, whether this is gas or liquid. You need to think about the point at which the valve is 0.1% open, i.e. when the open part of the ball is just appearing and in essence you have a small orifice or hole. The velocity through that hole can be very high if you've got a high differential pressure and can damage both the ball and the seat.

The reality of life though is that ball valves are used with large differentials, but just don't expect them to be bubble tight afterwards.....

Is it only aimed to prevent oversizing the actuator?
I've only once or twice seen it where the power of the actuator wasn't sized to allow it to open the valve under FULL DP X 1.5 shaft torque.

Does opening the valve across a large differential pressure creates high vibration or any other operational upset?
See above - the key issue for the valve is seat and ball damage due to high velocity. What happens to your d/s equipment is a wholly different matter. Also for gas especially, you can get local cooling, liquid drop out / hydrate formation etc and liquids can vapourise and create gas bubbles in that initial opening period.

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

 

[crshears]

Very enlightening discussion; thank you.

Full port valves have smaller seal area and if it leaks a little when closed, it will no longer be effective as a shut-off valve.

The way I was taught:

[1] The term "leak" applies to the unintended escape of any medium from a system designed to contain it, and as such leaks can be from any component.

[2] If an isolating or other valve, when intentionally closed, is failing to completely prevent the flow of a medium through it, that valve is described as "passing."

Different terminology, anyone?

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[georgeverghese]

While all emergency shutdown valves are critical isolation valves, not all manual block valves are safety critical isolation valves. In some cases, one of a set of block valves may be used as a "sacrifical valve" that is deliberately permitted to take high dp, since some other valve elsewhere in the line may be assigned to be the critical isolation valve.