The globe-style body is the main pressure-retaining portion of the valve
and houses the closure element. The flow passages in a globe valve are
designed with smooth, rounded walls with no sharp corners or edges,
thus providing a smooth process flow without creating unusual turbulence
or noise. The flow passages themselves must be of constant area to
avoid creating any additional pressure losses and higher velocities. With
two widely spaced end connections, globe-valve bodies are adaptable to
nearly every type of end connection, although the face-to-face is too
long to accommodate a flangeless design (bolting the body between two
pipe flanges, which is commonplace with a rotary valve). With globe
valves, mismatched end connections are also acceptable.
The globe valve’s trim is more than just a closure element (because a
throttling valve does more than just open or close), but rather it is a
regulating element that allows the valve to vary the flow rate against
the position of the valve according to the flow characteristic, which
may be equal percentage, linear, or quick-open (see Sec. 2.2). Typically,
this trim consists two key parts: the plug, which is the male portion of
the regulating element, and the seat ring, which is the female portion.
The portion of the plug that seats into the seat ring is called the plug
head, and the portion that extends up through the top of the globe
valve is called the plug stem. The plug stem may be threaded at the top
of the stem to allow for interaction with the handwheel mechanism.
The chief advantage of the single-seated trim design is its tight shutoff
possibilities—in some cases better than 0.01 percent of the maximum
flow of the valve. This occurs because the force of the manual operator
is applied directly to the seating surface.
Two sizes of trim can be used in manual globe valves. Full trim is the
most common and refers to the area of the seat ring that can pass the
maximum amount of flow in that particular size of globe valve. On the
other hand, reduced trim is used when the valve is expected to throttle
a smaller amount of flow than that size is rated for. If full trim is used,
the valve must throttle close to the seat, as well as in small increments—
which is difficult to achieve with a hand operator. The preferred
method, then, is to use a smaller seat diameter with a matching
plug, which is called reduced trim.
The bonnet is a major element of the valve’s top-works and acts as a
pressure-retaining part, providing a cap or cover for the body. Once
mounted on the body, it is sealed by bonnet or body gaskets. It also
seals the plug stem with a packing box—a series of packing rings, followers
or guides, packing spacers, and antiextrusion rings that prevent
or minimize process leakage to atmosphere. Mounted above the packing
box is the gland flange, which is bolted to the top of the bonnet.
When the gland-flange bolting is tightened, the packing is compressed
and seals the stem as well as the bonnet bore.
Keeping the plug head in alignment with the seat ring is important
for tight shutoff. To maintain this alignment, one of two types of guiding
mechanisms is used: double-top stem guiding or seat guiding.
Double-top stem guiding uses two close-fitting guides at both ends of
the packing box to keep the plug concentric with the seat ring (Fig.
3.19). These guides can be made entirely from a metal compatible with
the plug to avoid galling and can include a hard elastomer or graphite
liner. The ideal arrangement is for the two guides to be located as far
apart as possible to avoid any lateral movement caused by the process
fluid acting on the plug head. The guides, bonnet bore, and actuator
stem must all be held to close tolerances to maintain a fit that will
allow smooth linear motion without binding or slop.
The other common type of guiding in manual globe valves is the
seat-guiding design, where the plug stem is supported by one upper
guide (which also acts as a packing follower). As a second guiding surface,
the outer diameter of an extension of the plug head guides inside
the seat (Fig. 3.20). This means that the lower guiding surface remains
inside the flow stream, so therefore the process must be relatively
clean. The lower portion of the plug head has openings that allow the
flow to move through the plug head to the seat during opening. By
varying the size and shape of these openings, reduced flow and flow
characteristics can be introduced. Because the length between the
upper guide and the lower guide are at a maximum length, lateral
plug movement due to process flow is not an issue and the tolerances
required for this type of guiding are not required to be as close as double
top-stem guiding. This design minimizes any chance of vibration
of the plug in service. When the plug and seat are made from identical
materials, galling may occur during long-term or frequent operation.
High temperatures may also lead to thermal expansion and binding.
The metal seat surface of the plug is designed to mate with the metal
seating surface seat ring, using angles that slightly differ. Normally the
plug has a steeper seating angle than the seat ring. This angular mismatch
assures a narrow point of contact, allowing the full axial force of
the operator to be transferred to a small portion of the seat only, assuring
the tightest shutoff possible for metal-to-metal contact. In most
designs, the seat ring for manual globe valves is threaded into the
body. This sometimes requires a tool to turn the seat ring into a body
with limited space. With threaded seat rings, exact alignment between
the seating surfaces of the plug head and seat ring must require
lapping—a process where an abrasive compound is placed on the seat
surface. The plug is then seated and turned until a full contact is
achieved. Although simple in concept, threaded seats have some dis-
advantages. First, in corrosive or severe services the threads can
become corroded, making disassembly difficult. Second, alignment
between the plug and seat ring require the additional step of lapping
to achieve the required shutoff. And third, in situations where vibration
is present and the seat ring is not held in place by the plug in the
closed position, the seat ring may eventually loosen, allowing leakage
through the seat gasket and/or misalignment of the seating surfaces.
Some globe-valve applications require bubble-tight shutoff, which
cannot be attained with a metal-to-metal seal. To accomplish this, a
soft elastomer can be inserted in the seat ring. In this case, the seat ring
is a two-part design with the elastomer sandwiched between the two
halves (Fig. 3.21). The metal plug surface pressing against the seat
ring’s soft seat surface can achieve bubble-tight shutoff if the plug and
seat-ring surfaces are concentric. Some manufacturers also insert the
elastomer into the plug, which achieves the same effect (Fig. 3.22).
The globe-style body is the main pressure-retaining portion of the valve
and houses the closure element. The flow passages in a globe valve are
designed with smooth, rounded walls with no sharp corners or edges,
thus providing a smooth process flow without creating unusual turbulence
or noise. The flow passages themselves must be of constant area to
avoid creating any additional pressure losses and higher velocities. With
two widely spaced end connections, globe-valve bodies are adaptable to
nearly every type of end connection, although the face-to-face is too
long to accommodate a flangeless design (bolting the body between two
pipe flanges, which is commonplace with a rotary valve). With globe
valves, mismatched end connections are also acceptable.
The globe valve’s trim is more than just a closure element (because a
throttling valve does more than just open or close), but rather it is a
regulating element that allows the valve to vary the flow rate against
the position of the valve according to the flow characteristic, which
may be equal...
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