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					Block 12 Pipeline Ancillaries                        Isolation Valves - Linear Movement Module 12.1

Module 12.1
                                Isolation Valves -
                                Linear Movement

The Steam and Condensate Loop                                                             12.1.1
Block 12 Pipeline Ancillaries                                                              Isolation Valves - Linear Movement Module 12.1

                            Isolation Valves - Linear Movement
             Isolation valves are a key component in any fluid system as they are used to stop the flow of fluid
             into a particular area of the system. They are also sometimes used to manually control the flow
             of the fluid. The European standard EN 736-1:1995 distinguishes between isolating, regulating
             and control valves as follows:
             o    Isolating valve - A valve intended for use only in the closed or fully open position.
             o    Regulating valve - A valve intended for use in any position between closed and fully open.
             o    Control valve - A power-operated device which changes the fluid flowrate in a process control
             Isolation valves are used in a wide variety of different applications where on / off type control is
             required, these include:
             o    Diverting process media.
             o    Flow isolation to:
                  - Facilitate maintenance
                  - Allow the removal of equipment
                  - Allow the shut down of plant
             A multitude of different types and designs of isolation valve have been developed in order to
             meet this range of applications and the diverse operating conditions in which they are used.
             Valves are commonly classified into two groups (see Table 12.1.1), according to the operating
             motion of the closure device (or obturator):
             o    Linear movement valves - The obturator moves in a straight line. Included in this category are
                  gate valves, globe valves, diaphragm valves and pinch valves. These valves are covered in
                  greater depth within this module.
             o    Rotary movement valves - The obturator rotates about an axis at right angles to the direction of
                  flow. Ball valves and butterfly valves are the two most important rotary valves associated with
                  steam applications and are covered in greater depth in Module 12.2, Isolation Valves - Rotary
             Table 12.1.1 Obturator motion in the basic valve types
              Valve movement                            Linear                                          Rotary
              Operating motion
              of the closing device                 Straight line                          Rotating about an axis at
              (obturator)                                                            right angles to the direction of flow
                                      At right angles to        Longitudinal to
              Direction of flow                                                          Through the                 Around the
                                    the operating motion the operating motion
              in the seating area                                                         obturator                   obturator
                                       of the obturator        of the obturator
              Basic types                 Gate valve              Globe valve            Ball valves              Butterfly valve

                                       Flow                                                                   Flow
                                                                  Flow            Flow

 12.1.2                                                                                         The Steam and Condensate Loop
Block 12 Pipeline Ancillaries                                                    Isolation Valves - Linear Movement Module 12.1

Linear movement valves
Linear movement valves have been developed from the early forms of sluice gates used to control
the flow of water in irrigation channels. Since then, a large number of different designs and types
have been developed for use in almost every type of flow application. Although linear movement
valves are characterised by straight-line obturator movement, the flow of the fluid may be at
right angles to this movement (as in the case of gate valves), or in the same direction, as with
globe valves. The main feature of the linear movement valve is that tight shut-off may be achieved
by tightening down the obturator on a threaded stem.
Gate valves
Gate valves are probably the most common valves in use today due to their widespread use in
domestic water systems, but it should be noted that their popularity in industry has declined in
recent years. However, they are still used where an uninterrupted flow is required, because the
gate fully retracts into the bonnet, creating a minimal pressure drop, when the valve is in an
open position. Gate valves are specifically intended for use in isolation applications.
A gate valve consists of four main components, the body, bonnet (or cover), gate and stem. A
typical gate valve is shown in Figure 12.1.1.



                                                                        Gland follower

                                                                        Gland packing



                                                                        Wedge shaped gate
                                                                        Seat ring

                                 Fig. 12.1.1 Typical wedge gate valve

The gate, which slides between the seats, is lifted in a direction at right angles to the flow until
clear of the flow path. The fact that the gate fully retracts into the bonnet ensures that the
pressure drop across the valve is low.
Gate valves are divided into a number of different classes, depending on the design of the gate
and its seating faces.
Solid wedge gate valve
The gate is wedge shaped and it seats on corresponding faces in the valve body. The mechanical
advantage of the activating thread, together with the wedge angle, enables adequate seating
forces to be applied against the fluid pressure without excessive handwheel effort. The seat can
sometimes be coated with PTFE to assist a high integrity shut-off. A typical solid wedge gate valve
is shown in Figure 12.1.1.

The Steam and Condensate Loop                                                                                         12.1.3
Block 12 Pipeline Ancillaries                                                          Isolation Valves - Linear Movement Module 12.1

             Flexible wedge gate valve
             Although there are several types of flexible wedge gate valves, they all make use of a flexible
             two-part disc, which is shaped like two wheels on a very short axle. The flexibility of the disc
             ensures tight seating over a wide range of temperatures and pressures.
             The most common type of flexible wedge gate valve used in steam applications is the parallel
             slide valve. The two plates that constitute the gate are held against the seat by a spring, encased
             between them. The fluid pressure moves the upstream disc off its seat, and the force is transferred
             onto the downstream disc, thereby ensuring a tight shut-off. The high degree of flexibility in the
             gate allows for expansion and contraction when subjected to temperature variations, making it
             suitable for use in steam systems.
             Globe valves
             Globe valves constitute a major class of linear movement valves; they have become more popular
             than gate valves as there is a wide variety of configurations available to suit most applications.
             The movement of fluid through the valve seat is longitudinal to the operating motion of the
             obturator; this means that for a valve in which the inlet and outlet are horizontally opposed, the
             fluid must follow a changing course. The main advantage of this arrangement is that a globe
             valve opens more rapidly than a gate valve as the disc only needs to move a small distance from
             its seat to allow full flow. This is an advantage when there is frequent operation of the valve. The
             disadvantage is that the fluid has to change course, increasing the resistance to flow and generating
             turbulence. This results in a higher pressure drop across a globe valve than a gate valve.

                                                                                                      Stem seal



                                                                                                      Valve seat
                                Valve disc

                                              Fig. 12.1.2 A conventional globe valve

 12.1.4                                                                                     The Steam and Condensate Loop
Block 12 Pipeline Ancillaries                                                               Isolation Valves - Linear Movement Module 12.1

Globe valves are less likely to leak than gate valves, which means that they can be used for higher
pressure or higher volume applications, for example in steam systems, or where fluid loss can be
hazardous or costly. The increased cost of globe valves over gate valves is therefore offset by the
additional safety they provide, and a reduced chance of fluid loss.
The pressure of the fluid acting over the area of the disc generates an axial load on the stem. This
makes closing the valve difficult, so much so, that it limits the size of a standard globe valve to
DN250. On high differential pressure closed systems, balancing plugs can be used to overcome
this effect, allowing valves with a nominal diameter of up to 500 mm to be used (Figure 12.1.3(a)).
The balancing plug contains a pre-lifting plug that acts as a pilot valve. When the valve is opened,
the pre-lifting plug opens first, allowing the medium to pass through it at a controlled rate
(Figure 12.1.3(b)). This reduces the differential pressure across the valve, enabling the disc to be
easily lifted off its seat (Figure 12.1.3(c)). To assist closing of the valve, isolation valves fitted with
a balancing plug have to be fitted in reverse so that the top of the plug is acted on by the
upstream pressure.

                                            Valve spindle
                                            Pre-lifting plug ‘A’
Pilot valve seat
                                                          Main valve plug ‘B’

                         Main valve seat
                        (a) Valve closed

                                                                                (b) Pilot valve open
                                                                      reducing pressure drop across the valve

                                                   (c) Main valve open

                                 Fig. 12.1.3 Schematic of a typical balancing plug valve

The Steam and Condensate Loop                                                                                                    12.1.5
Block 12 Pipeline Ancillaries                                                   Isolation Valves - Linear Movement Module 12.1

             Piston valves
             One of the main disadvantages of linear movement valves is the fact that their seats are prone to
             damage from dirt and wiredrawing, and therefore, depending on the application may require
             regular maintenance. Although these seats are replaceable in theory, it usually involves significant
             time and cost, and it is often more advantageous to replace the entire valve. To overcome this
             problem, piston valves have been developed.
             The piston valve is a variant of the conventional globe valve, with the traditional seat and cone
             replaced by a piston and lantern bush. The piston is connected to the valve stem and handwheel,
             and passes through two sealing rings that are separated by a lantern bush. When assembled, the
             two sets of sealing rings are compressed around the piston by the load exerted along the stem.
             The upper set of sealing rings acts as conventional gland packing, and the lower set acts as the
             seat. Furthermore, the large sealing area between the piston and rings assures a high level of
             shut-off tightness.
             The piston valve is not designed for throttling duties and must be used in the fully open or closed
             positions. When the valve is fully opened, only the bottom face of the piston is exposed to the
             fluid as the rest of the body is protected by the upper sealing rings. This means that the sealing
             surfaces (the sides of the piston) are protected from erosion by the fluid flow.


                                                                                                      Upper sealing rings

                                                                                                      Lantern bush
                                                                                                     Lower sealing rings

                                                   Fig. 12.1.4 A piston valve

             If the valve requires maintenance, all the internals can be easily removed by undoing the cover
             nuts and withdrawing the piston. The rings and the lantern bush can then be removed using an
             extractor tool. This operation is simple and can be undertaken without having to remove the
             valve from the pipeline. In general, the piston should never have to be replaced, but the sealing
             rings may wear over a long period with frequent operation.

 12.1.6                                                                              The Steam and Condensate Loop
Block 12 Pipeline Ancillaries                                                           Isolation Valves - Linear Movement Module 12.1

Diaphragm valves
Diaphragm valves constitute the third major type of linear movement valves. The stem of the
valve is used to push down a flexible diaphragm, which in turn blocks the path of the fluid. There
are two different classifications of diaphragm valve based on the geometry of the valve body:
o   Weir type - A weir is cast into the body, and when closed, the diaphragm rests on the weir,
    restricting the flow (see Figure 12.1.5 (a)).
o   Straight-through type - The bore runs laterally through the body and a wedge shaped diaphragm
    is used to make the closure (see Figure 12.1.5 (b)).

                                 Diaphragm                                Diaphragm

                                    Open                                   Closed
                                                   (a) Weir type

                                 Diaphragm                                Diaphragm

                                    Open                                   Closed
                                              (b) Straight-through type

                     Fig. 12.1.5 The weir type (a) and straight-through type (b) diaphragm valves

The main advantage of a diaphragm valve is the fact that the diaphragm isolates the moving parts
of the valve from the process fluid. They are therefore suitable for handling aggressive fluids and
for those containing suspended solids. In addition, as the bonnet assembly is not exposed to the
fluid, it can be made from inexpensive materials such as cast iron, thereby reducing the overall
cost. The development of new diaphragm materials enables diaphragms to be used on most
fluids. Their application is however limited by the temperature that the diaphragm can withstand
- typically less than 175°C. Diaphragm valves are generally used on process fluid applications.

The Steam and Condensate Loop                                                                                                12.1.7
Block 12 Pipeline Ancillaries                                                          Isolation Valves - Linear Movement Module 12.1

             Linear movement valve stem options
             Linear movement valves are available with a number of different stem arrangements:
             o   Rising / non-rising stems - If the stem is rising, it will move vertically upwards when the
                 valve is opened, as opposed to only rotating, as with a non-rising stem. The rising stem indicates
                 the degreee of valve opening, which in turn roughly reflects the amount of flow through the
                 valve. Valves with rising stems do however require more space above the bonnet to
                 accommodate the stem in the fully open position. The use of non-rising stems is recommended
                 on gland packed valves, as they reduce the wear on the packing.

                                (a)                                                            (b)

                                         Fig. 12.1.6 Rising (a) and non-rising (b) stem valves

             o   Inside / outside stem screws - On a stem with an outside screw, the actuating threads on the
                 stem are situated outside the valve body and are not exposed to the process fluid. As screw
                 threads are particularly susceptible to corrosion, outside screws should always be used on
                 fluids with corrosive or erosive properties. They are also beneficial where the valve is frequently
                 exposed to large temperature variations, as the expansion and contraction of the stem may
                 cause binding of the threads inside the body.

             Stem thread


                                                                                                              Stem thread


                                (a)                                             (b)

                                          Fig. 12.1.7 Outside (a) and inside (b) stem valves

 12.1.8                                                                                     The Steam and Condensate Loop
Block 12 Pipeline Ancillaries                                               Isolation Valves - Linear Movement Module 12.1

Stem sealing
In order to prevent leakage of the process media from around the stem of a valve, a barrier must
be placed between the fluid and the environment. Stem sealing is usually achieved by one of
two methods, namely gland packing and bellows sealing. Gland packing consists of a polymeric
material, typically PTFE, packed tightly between the stem and the bonnet of the valve, thereby
preventing any process media escaping.

                                                                                         Secondary gland
                                                                                         packed seal


                                                                                         Metal bellows

                                                                                        Rising spindle

                                  Fig. 12.1.8 Bellows sealed valve

In bellows sealed valves, a flexible metallic bellows is used. It is connected on one end to the
stem and the other end is connected to the bonnet, effectively producing a barrier between the
fluid and the environment. This bellows extends and contracts as the stem moves up and down.
The bellows is so effective, it produces a ‘zero emissions’ seal. Fitted to the bellows is an
anti-torsion device, which prevents the bellows from rotating with the stem. Such a device is
essential, otherwise the repeated twisting of the bellows would lead to the failure of the seal.

Although less costly than the bellows sealed valves, the gland packed valve does not produce
such a tight seal as the bellows. If a gland packed valve is not used for a significant period, the
gland packing can stiffen, and leakage will occur the next time the valve is used. The bellows
sealed valve does not suffer from this problem. Furthermore, gland packed valves require
regular re-packing of the gland, whereas a typical bellows requires no maintenance for over
10 000 cycles.

The Steam and Condensate Loop                                                                                    12.1.9
Block 12 Pipeline Ancillaries                                                         Isolation Valves - Linear Movement Module 12.1


             1. What is the main advantage of a gate valve?
              a| They are better than all the other linear movement valves for producing a tight
                 shut-off on steam systems                                                                                      ¨
              b| They can be used in throttling applications as well as for isolation                                           ¨
              c| There is a low pressure drop across the valve                                                                  ¨
              d| They are easily automated                                                                                      ¨

             2. In which of the following applications should an outside, non-rising stem be used?
              a| Where a gland packed valve is used in a corrosive fluid                                                        ¨
              b| Where a bellows sealed valve is used in a steam system                                                         ¨
              c| Where there are no temperature variations of the fluid passing through the valve                               ¨
              d| Where a bellows sealed valve is used in a corrosive fluid                                                      ¨

             3. Why must balancing plugs be used in globe valves that are larger than DN250?
              a| The pre-lifting plug enables more precise control of the fluid                                                 ¨
              b| It reduces the pressure drop across the valve allowing the valve to open easily                                ¨
              c| It allows the valve to be balanced on water circuits                                                           ¨
              d| A balancing plug has to be used with a bellows seal                                                            ¨

             4. What is the main reason for choosing a bellows sealed stem over a gland
                packed one?
              a| A bellows seal will never require maintenance                                                                  ¨
              b| The bellows seal produces a ‘zero emissions’ seal                                                              ¨
              c| Gland packed seals on valves above DN250 are prone to leakage                                                  ¨
              d| All of the above                                                                                               ¨

             5. Which of the following valves should be used where the valve is to be welded
                into a pipeline and rapid seat wear is expected?
              a| A globe valve                                                                                                  ¨
              b| A parallel-side valve                                                                                          ¨
              c| A diaphragm valve                                                                                              ¨
              d| A piston valve                                                                                                 ¨

             6. Why is a diaphragm valve not suitable for most steam applications?
              a| Condensate collects in the weir, increasing the pressure drop across the valve                                 ¨
              b| Diaphragm valves are incapable of producing a tight shut-off above 4.0 bar                                     ¨
              c| The diaphragm valve is only suitable for handling fluids containing suspended solids                           ¨
              d| Diaphragm materials are not suitable for temperatures above 175°C                                              ¨

                                                 1: c, 2: a, 3: b, 4: b, 5: d, 6: d

12.1.10                                                                                    The Steam and Condensate Loop

Shared By:
Description: valve is a device that regulates, directs or controls the flow of a fluid (gases, liquids, fluidized solids, or slurries) by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category. In an open valve, fluid flows in a direction from higher pressure to lower pressure.