THE 2nd JOB OF VALVES: THROTTLING THE PV FLOWRATE

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MORE FUNCTIONS OF VALVES

ISA Symbol for Valves.

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ... just learned that there are dozens of Valves but all of them perform one of four jobs. PTOA Segment #252 featured Valves with the job of starting and stopping the PV Flowrate.

This PTOA Segment features the members of the Valve family that perform the second function of Valves:

Globe Valves throttle the PV Flowrate between a maximum and minimum amount.

Valves that regulate/throttle the PV Flowrate.

The next PTOA Segment features the 3rd and 4th jobs of Valves:

Valves that prevent backflow.
Valves that safely discharge excess gas Pressure and relieve excess liquid Pressure.

VALVE FUNCTION #2:
VALVES THAT REGULATE OR THROTTLE THE PV FLOWRATE

What kind of Valve should be used when the entire PV Flowrate is not needed all of the time, but rather the PV Flowrate needs to be regulated or throttled to a percentage of maximum flow?

Regulating the PV Flowrate between a maximum and minimum amount is the job of the Globe Valve, Butterfly Valve, Diaphragm Valve, and Louver.

Globe Valves (aka Angle Valves)

As shown, the flow through this Globe Valve is closed. A negligible amount of fluid would leak through the seated flow element.

Unlike the straight flow-through pathway of the Gate Valve and either of the quick shut-off Valves featured in PTOA Segment #252, the flow pathway through the Globe Valve changes direction.

As shown, the regulated amount of fluid flowing through this Globe Valve is approximately 50% of maximum PV Flowrate.

The fluid enters the lower half of the globe-shaped body and then a regulated amount of the PV Flowrate is allowed to flow upward into the upper-half of the globe-shaped body and thence out of the Valve.

The amount of PV Flowrate that flows through the Globe Valve is determined by the position of the typically plug-shaped flow element.

Since the Globe Valve forces a change in the flow direction, a Globe Valve creates significantly higher Pressure Drop than flow through a Gate Valve does, even if the Globe Valve is in the fully open position.

Refresh your browser if necessary to observe how this FBV Inc. gif illustrates a Globe Valve throttling the PV Flowrate.

Globe Valves are often manually operated by a Hand Wheel. Because of the difficulty opening and closing a Globe Valve against significant PV Pressure, Globe Valves are seldom used in sizes over 12 inches even though they are manufactured to a 16-inch Nominal Pipe Diameter.

This is a great graphic depicting the typical Control Valve with Bypass piping. Note the upstream and downstream Gate Valves used for "blocking in" (aka isolating) flow through the Control Valve.

One common industrial service of a Globe Valve is the role of Bypass Flow Valve. The nearby graphic illustrates the Typical Bypass piping around an Automatic Control Valve:

A Gate Valve is on either side of the Automatic Flow Control Valve so that the Automatic Control Valve can be isolated ... aka "blocked in" ... when necessary.

The Gate Valve that precedes the flow into the Automatic Control Valve is the "upstream Block Valve" and the Gate Valve that the process stream flows into after exiting the Automatic Control Valve is the "downstream Block Valve."

The Control Valve Bypass is the piping on the lower half of the diagram. A Globe Valve is used for manually throttling flow while the Control Valve is not in service.

The Bypass Globe Valve is on a separate line that tees off the process service line before the upstream Block Valve and reconnects to the process service line further downstream of the downstream Block Valve.

The PV Flowrate is swapped to the Bypass Flow Globe Valve by opening the Globe Valve gradually as the normal flow path through the Automatic Control Valve is gradually "blocked in."

Swapping flow from the normal route through the Control Valve to temporary flow through the Bypass Globe Valve must be done gradually. Ditto when swapping back to normal flow path.

While swapping the flow path to and from the Bypass Flow Globe Valve, the Outside Process Operator will be in constant radio contact with the Board Operator, taking direction on how much more to open up or close the Hand Wheel to maintain the optimal PV Flowrate.

The nearby gif illustrates how a Bypass Flow Globe Valve is put into service so that a Steam Trap can be removed for maintenance. Once the Steam Trap is repaired and replaced, the normal online flow path is reestablished and then the Bypass Flow Globe Valve is closed.

This rapid-replace procedure would not impact a steam condensate system as much as it would rock a main flow process service. Process Operators should remember to gradually open and later gradually close the Bypass Globe Valve.

Guess what?

Gate Valves, Ball Valves, and Plug Valves are bidirectional. Flow could go through them in either direction.

When the Valves that start and stop flow arrived at the process facility and were thence piped into the facility's Piping Network, any of them could have been piped in without attention to the PV Flowrate direction.

Otherwise stated, the Gate Valve and quick shut-off Valves are bidirectional.

The Globe Valve IS NOT bidirectional and must be piped into the Piping Network as specified on the external surface of the Globe Valve.

The inlet port of the Globe Valve must direct incoming flow to the lower part of the Globe Valve body and flow from the upper part of the Globe Valve body must be able to exit through the outlet port and continue in the desired direction.

The flow through these Globe Valves is from the right side to the left side.

This purposeful Valve architecture results in the instantly recognizable globey, lumpy Globe Valve body exterior visible in the nearby photo.

This Globe Valve has been converted into an Automatic Control Valve. There is not a Handwheel; the Globe Valve is throttled automatically.

Unlike a Gate Valve, a Globe Valve can be automatically actuated and thus converted into an Automatic Control Valve. An upcoming PTOA Segment features Automatic Control Valve phenomena.

Globe Valves can be used for automatically regulating the PV Flowrate or the PV Pressure.

Globe Valves can also be modified to perform the jobs of the Check Valve and the Pressure Relief Valve (featured topics in the next PTOA Segment).

Butterfly Valves

The ISA symbol for a manually-actuated Butterfly Valve.

Although Butterfly Valves can perform as quick-shutoff Valves in some applications, they are primarily known for throttling/regulating the PV Flowrate.

The flow element of a Butterfly Valve is a circular Disc or Plate that is mounted on a Stem which is sometimes called a Shaft.

The ISO symbol for an automatically actuated Butterfly Valve.

The Stem/Shaft can be rotated by hand with a Lever, or the Butterfly Valve can be actuated automatically.

The components of a Butterfly Valve. Note the Seat is a round ring.

When the Disc or Plate is rotated to align with the cross-sectional area of the pipe, flow to the other side of the Disc is blocked. While in this closed position, the Disc/Plate is seated in the O-shaped ring ... the Seat.

Rotating the Disc/Plate 90° from the closed position allows the maximum PV Flowrate to flow by the Disc/Plate.

PTOA Readers and Students may recognize the quarter-turn quick-shutoff feature of the Butterfly Valve is similar to that of the Ball Valve. Indeed, Butterfly Valves can operate as quick shutoff Valves.

Refresh your browser if necessary to view this great gif illustrating flow through a Butterfly Valve.

When the Disc/Plate is positioned at an intermediate angle between fully open or fully closed, the flow is throttled/regulated between the maximum (totally open) and minimum (totally closed) PV Flowrate. As would be expected, a significant Pressure Drop is notable with the Disc/Plate is in an intermediate position.

The Damper in the stack of a fired heater is a Butterfly Valve.

Due to the Pressure Drop, Butterfly Valves historically had limited service in the process industries and were typically limited to a few low PV Temperature and low PV Pressure applications.

The damper located high up in the stack of a Fired Heater is a Butterfly Valve. Fired Heaters were introduced in PTOA Segment #22 and featured in PTOA Segments #69 through #73. The damper would be closed (horizontal) while starting up the heater and eventually opened up to establish draft.

Furthermore, the Butterfly Valve historically was limited to process services characterized as high volume but relatively steady PV Flowrates.

Advances in Butterfly Valve technology expand its application into high Pressure process service.

Technological advances that improved seating reliability and improved manufacturing standards have made it possible to expand the application of Butterfly Valves into higher PV Pressure process services. Newer, higher-quality, higher-performance Butterfly Valves are used in the oil and gas industries for regulating flow.

This Butterfly Valve is automatically actuated.

Butterfly Valves applications also include wastewater treatment, regulating the high PV Pressure and high PV Temperature of water and steam process service, and the movement of corrosive, viscous and oily slurry fluids.

Still, Butterfly Valves are not used where precision throttling/regulation of the PV Flowrate is required. Precision flow regulation is the domain of the Automatic Flow Control Valve featured in an upcoming PTOA Segment.

Diaphragm Valves for Corrosive Process Streams

The Diaphragm is contracted upward to completely open the Valve and extended completely downward to close the Valve. Intermediate throttling positions create Pressure Drop.

The job of the Diaphragm Valve is to regulate the flow of a corrosive process fluid. A flexible Diaphragm flow element is extended into direct contact with the corrosive flowing process fluid and must thus be manufactured from materials that resist corrosion.

Eventually, the Diaphragm will need to be replaced ... due to corrosion. However, changing out a corroded Diaphragm is easier and less expensive than replacing an entire Valve.

The nearby component diagram for a Diaphragm Valve illustrates that ... like the Start/Stop Flow Valves (Gate Valves, Ball Valves, and Plug Valves) ... Diaphragm Valves also have a manual actuator (Handwheel) that raises and lowers a Stem.

The Stem is attached to the Diaphragm and the Stem/Diaphragm assembly is bolted such that the bottom of the Diaphragm is in direct contact with the corrosive process stream, yet the top of the Diaphragm is on the dry side of the Bonnet.

Note the Body of the Diaphragm Valve includes a Weir. The corrosive fluid that flows into the Diaphragm Valve must flow over the Weir before the fluid can exit the Diaphragm Valve.

Refresh your browser if necessary to view this great gif which illustrates flow through a Diaphragm Valve.

The amount of process fluid that flows over the Weir is regulated/throttled by how far the Diaphragm has been extended into the flow path of the corrosive fluid. The maximum PV Flowrate occurs when the Diaphragm is fully contracted; the PV Flowrate decreases as the Diaphragm is extended further and further into the flow path.

Louvers for Adjusting Air Flow

These Louvers are opened up which maximizes the transfer of heat that is convected from the hot tubes that lie below them.

Louvers are rows of angled slats which can be adjusted to increase or decrease the flow of air through them.

A common use of Louvers is increasing or decreasing the movement of air through Fin Fan Condensers. Fin Fan Condensers were featured in PTOA Segment #38.

Louvers on Fin Fan Condensers are automatically operated; the Control Board Operator can open or close the slats to maintain the target Process Stream PV Temperature.

The positioning of the Fin Fan Louvers is adjustable. The most heat transfer via convection will happen when the slats are fully open (Convection Heat Transfer was featured in PTOA Segments #64 and #65). When less heat transfer from the hot fluid is warranted, the Louvers should be adjusted more toward the closed, flat position.

The Louvers of a small Fin Fan Condenser located at grade might be hand operated by an Outside Process Operator. The Louvers of the larger and significantly taller Fin Fan Condensers in a processing facility will be automatically actuated to change the position of the Louvers.

Gas Turbines (GTs) were featured in PTOA Segments #195 through PTOA Segment #201 .

This Axial Compressor linkage bar is obviously disconnected from the Axial Compressor. The angle of the vanes use adjusted by the light blue linkages, thus the vanes functionally operate as Louvers throttling the flow of ambient air.

PTOA Segment #197 explained the role that the adjustable guide vanes on the Axial Compressor's linkage bar play regarding efficient GT operations. These guide vanes function as Louvers regulating the amount of ambient air flowing within the Compression and Expansion stages of the GT.

The next PTOA Segment features the 3rd and 4th jobs of Valves, preventing backflow and reducing the PV Pressure of a fluids, respectively.

TAKE HOME MESSAGES: Typically, the PV Flowrate of a process fluid does not need to be zero flow or 100% flow but rather regulated ... aka throttled ... to a percentage of maximum flow. Regulating and Throttling flow is the 2nd job of Valves. The Valves that perform this job are:

Globe Valves

The ISA Symbol for a Globe Valve is a bowtie with a black circle in the middle.

The flow path through a Globe Valve guides the process stream to the bottom of the valve's body. The amount of flow that flows to the upper half of the Globe Valve and out of the Valve is determined by how much the (plug-shaped) flow element and its Stem are extended into the flow path.

Globe Valves ARE NOT bidirectional; an arrow on the external Valve Body will indicate the direction that the Valve must be piped into the Piping Network. The Globe Valve can be modified to perform the jobs of a Check Valve and a Pressure Relief Valve. Globe Valves can be actuated manually or automatically. Globe Valves can be modified into Automatic Control Valves used for controlling the PV Flowrate or the PV Pressure.

One common use of Globe Valves is as the Bypass Valve on the Bypass Loop piped around an Automatic Control Valve. The Outside Process Operator gradually opens/closes the Bypass Valve while the upstream and downstream Block Valves surrounding the Automatic Control Valve are gradually closed/opened.

Butterfly Valves

The ISA Symbol for a Butterfly Valve is two vertical lines with a diagonal drawn between them from the lower left to the upper right. A small dark circle (representing a hinge) is in the middle of the diagonal line.

Butterfly Valves have a full-bore Disc supported by an external hinge. When the Disc is completely seated, flow is blocked completely and the PV Flowrate is just what leaks past the barrier. When the Disc is rotated 90°, the flow path of fluid glides by both sides of Disc at maximum flowrate. Partial, regulated flow results in considerable Pressure Drop. Butterfly Valves can be manually or automatically actuated and even modified to function as quick-close Valves. Although Butterfly Valves cannot be used for precision flow control, improvements in seat technology and manufacturing have extended the Butterfly Valve to higher PV Pressure process service. The damper in Fired Heater is a type of Butterfly Valve.

Diaphragm Valves

The ISA Symbol for a Diaphragm Valve is a bowtie with a sunrise drawn on the upper side of the bowtie.

Diaphragm Valves are used for corrosive process service because replacing a Diaphragm is easier and cheaper than replacing an entire Valve. The flexible Diaphragm must be made of corrosive resistant material. The corrosive process fluid must flow over a Weir. The amount of process fluid allowed to flow out of the Valve depends upon how far the Diaphragm is extended into the flow path. Diaphragm Valves can be manually or automatically actuated.

Louvers

Louvers are used to regulate the flow of air at ambient conditions. Louvers are found on Fin Fan Condensers and the Axial Compressor of a GT.

PTOA Readers should recognize that even though Valves are categorized by their main job function, they can be physically modified to perform a different Valve function.

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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