Selection Between Ball and Globe Valve
What is the criteria used when selecting a ball or globe valve in the piping system of propane bulk storage installation? Such valves would be 2", 2-1/2" and 3" size fitted in the liquid and vapor transfer lines (loading and unloading). I get the impression that it is a matter of preference by the engineer or availability by the supplier.
Oh my god, you are not serious here? If you had said "the choice
between ball, gate, and plug valves seems to be a matter of opinion or
availability" this would have been a reasonable question, but the valves
you mentioned have very different purposes.
A ball valve is
intended for on/off service and should not normally be throttled (they
have terrible throttle characteristics).
A globe valve is designed for throttling and even fully open will impose a dP.
If
a particular line should always be fully open or fully shut, then a
ball valve is often a very good choice (the further choice between a
floating ball and a trunnion ball depends on the process). A valve to
control flow within a stream should have good throttling
characteristics--if the required position changes frequently then a
regulator is a better choice, but if it changes infrequently then a
manual globe valve can occasionally be the best choice.
The specification of either a throttling or block valve requires that the design engineer have knowledge of the performance
of each sort of valve that could be used. To think that a ball valve
and a globe valve are interchangeable is amazingly dangerous.
If you are just speaking about isolating valves (on-off) the globe valve is at a disadvantage on almost every parameter.
Shutoff:
a metal-seated globe valve will require use of a cheater to torque the
seats together. Ball valves can be expected to have resilient seats and
to shut off tightly.
Convenience: Globe valve requires 4 or 5
turns of the handwheel to open or close. Ball valve: 90-degrees. (Ball
valves were initially developed for USNavy. Tactical advantage to
being able to yank a valve shut while at a dead run, vs having to stop
and grind away at a handwheel.) Somebody over-eager to close a ball
valve may yank it shut too quickly and cause liquid-hammer.
In fairness: Larger ball valves need a gear-operator.
Position of a ball valve is easy
to read. Lever is aligned with the pipe when the valve is open.
Position indication of a globe valve is on a small plate inside the
yoke.
Environmental: Rising stem in globe valve drags packing
particles out, and environmental dirt in, every time the valve is
cycled. ALso if you figure 4 turns to open, globe valve packing sees 16
times the wear on each stroke. Ball valve packing stays
tighter-longer.
Installation: Globe valve is taller and heavier and usually longer F-F, than a ball valve of the same size.
I started to say something about Globe valves having a fire-safe advantage, but ball valves are offered in firesafe meeting API standards.
Capacity:
Globe valves have a convoluted s-shaped flow path. Ball valves are
straight through. Even a "regular" port ball valve has no more
friction loss than an elbow, while "full" port ball valves have loss no
gtrater than an equivalent length of straight pipe-probably less since
the ball is machined to better surface finish than the inside of the
pipe.
After all being said in the previous two answers: within the probably
normal pressure range and the normal sizes you have mentioned, ball
valves would be readily commercial available in different quality,
standards and constructions.
At least up to two inch sizes a price/performance/lifetime comparison would be in favour of ball valves, and I believe also for the larger dimensions in your range.
I
would suggest you start having a look at all SS ball valves, floating
PTFE seats, three-piece (midsection to swing out for inspection or
change) construction , welding or screwed ends, perhaps flanged for
larger sizes or if not three-piece.
If unsuitable or to high in price: two piece construction and CS house.
For
automated valves for on off, you could have a look at solenoid
(globe-construction) on/off valves with forced lifting. Theese valves
will open if downstream pressure increases above upstream pressure and
will relief overpressure backwards if propane resting in downstream
pipeline are heated and increases in volume/pressure.
Drawback is the pressure loss over the globevalves, as mentioned by others, if the main pressure is low.
For electric operated valves (solenoid valves) check if EX protectioon is required (sometimes it is).
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