Stop'n'go
High and low
Fast and slow

("Stop 'N' Go," by Peter Kent, 1981)

 

THE FUNCTION AND FORM OF VALVES

The PV Flowrate's direction and magnitude are managed by Valves, hardware components which are piped into the facility's Piping Network either by welded pipe connections (featured in PTOA Segment #249)  or by flanged pipe connections (featured in PTOA Segment #250).   

Brilliant PTOA Readers and Students ... meaning those who are reading the PTOA Segments in the intended, sequential order ...were introduced to the Gate Valve and Check Valve in PTOA Segment #164.

Although there are many, many types of Valves there are only four functions by which each type of Valve can be classified:

• Starting/Stopping the PV Flowrate, including quick-shutoff Valves.
• Regulating/Throttling the PV Flowrate.
• 

  A Check Valve prevents backflow.

  Preventing Backflow.

• Relieving the PV Pressure built up in liquids and gases.

 

The job that the Valve performs is determined by its flow element.

This PTOA Segment features the types of Valves that start/stop the PV Flowrate. The next PTOA Segment features the remaining three jobs of Valves. 

 

THE CRITERION FOR VALVE SELECTION
INCLUDES THE PRESSURE DROP THAT WILL BE CREATED.

Matching the optimal Valve Type to the process service is not random.

The decision-making regarding what type of Valve to use for a process service depends upon fluid characteristics and the required process conditions.  Fluid characteristics were featured in PTOA Segments #237 through PTOA Segment #240.

For example, some Valves will not work well with slurries; some Valves are better than others sustaining a high PV Pressure and PV Temperature process service.

Although the selection of a Valve for a process service depends upon processing conditions and fluid characteristics, the foremost consideration when matching a Valve to a process service is limiting the Pressure Drop that is created as fluid flows through the Valve's hardware.

Brilliant PTOA Readers and Students learned in PTOA Segment #165 how fluids can only flow if a ΔP exists and that the fluid only flows from the area of higher pressure to lower pressure. The ΔP is eaten up by friction caused by the fluid's flowing through Piping, Valves, and the Pipe Fittings that were featured in PTOA Segment #251. 

The Pressure Drop created by a fluid flowing through Piping, Pipe Fittings, and Valves is predictable and the subject of an upcoming PTOA Segment.

 

 

VALVE FUNCTION #1:
VALVES THAT START AND STOP FLOW

Gate Valves

Gate Valves are either opened all the way or closed all the way. Ergo, they are selected for process flows that are normally full flow but on occasion the flow must cease (for example, flow must cease for a Turnaround).

When fully open, the fluid flows in a straight line with minimum "resistance to flow." Otherwise stated, the least amount of Pressure Drop is created as the fluid flows through a Gate Valve.

The ''Gate" of a Gate Valve is the wedge-shaped flow element that is attached to a Stem.

When the Stem and its attached Gate are extended into the process stream piping and "seated" between Gate Seats, the flow through the Gate Valve is stopped.

Alternatively, the Stem and its attached Gate are contracted out of process stream piping and into the Bonnet of the Valve's Body when the Gate Valve is open.

The opening and closing of the Gate Valve is performed manually by the Outside Process Operator by turning a Hand Wheel.

Turn "rightsy to tightsy" (aka "close") and "leftsy to loosey" (aka "open"). Big caveat: Propane tank valves are exactly opposite.

The nearby graphic illustrates a Fully Open Gate Valve and a Closed Gate Valve. Notice how the Stem is fully visible above the Hand Wheel when the Gate Valve is open and below the Hand Wheel when the Gate Valve is closed.

The Outside Process Operator can quickly ascertain which Gate Valves are open (full fluid flowrate) or closed (no flow) by noting the position of the Stem.

Gate Valves are more compact than either of the quick shut-off Valves featured next (Ball Valves and Plug Valves). 

This ISA symbol for a Gate Valve can easily be mistaken for a bowtie. Some CAD systems add a short vertical line where the two triangles of the bowtie meet.

 

 

Ball Valves (a quick shut-off Valve) 

Some process stream applications require quickly stopping or starting 100% flow. Ball Valves can do the job.

Nobody should be surprised to learn that the flow element in a Ball Valve is a Ball.

 

Because the Stem of a Ball Valve is attached to a Lever, the Stem of the Ball Valve is shorter than the Stem for a Gate Valve.

There is no "rightsy-tightsy, leftsy-loosey" business to remember with Ball Valves; the Ball is not raised or lowered as the Gate of a Gate Valve is. The Process Operator simply moves the Lever from the fully open to the fully closed position, or vice versa.

The ISA Symbol for a Ball Valve is similar to a Gate Valve but adds a circle in the middle of the Valve.

A Ball Valve could have multiple ports to swap 100% flow sequentially to more than one direction. The nearby photo shows a 3-Port pneumatically actuated Ball Valve.

When sequenced by Programmable Logic Controllers (PLCs), Ball Valves can be used in batch process operations to emulate continuous production.

In summary, for low-to-high PV Pressure applications (with minimum torque), Ball Valves can provide a quick and bubble-tight shutoff.

Ball Valves are popular in sanitary applications like medical, pharmaceutical, biochemical, brewing and food and beverage processing.

 

Ball Valves are also used for the movement of air, gas, and hydrocarbons. The high PV Pressures and high PV Temperature services found in gas transportation, crude oil refining, and tank farms will employ automated Ball Valves. Ball Valves with the highest PV Pressure ratings are used in underground and subsea fluid flow systems.

 

Plug Valves (a quick shut-off Valve)

Plug Valves also start or stop 100% flow very fast.  Just a 1/4th turn of the Lever starts or stops 100% of the flow in low PV Pressure, low PV Pressure process service (with modification to the Plug, some Plug Valves can operate in higher temperature process services).

The quick shut-off feature of both Ball Valves and Plug Valves comes with a notable disadvantage; both Ball Valves and Plug Valves cause more water hammer.

The Plug in a Plug Valve is cylindrical or tapered like a cone, not wedge-shaped as the Gate in a Gate Valve is nor like the Ball in a Ball Valve. And here's a big difference in Ball Valve and the Plug Valve flow elements: 

A typically rectangular opening is bored into the Plug of the Plug Valve.

The opening is at least 70% of the connecting pipe's inner diameter. When the Plug Valve is open, fluid flows straight through the opening and out through the Valve's outlet port.

When the Plug is rotated 90° (a mere quarter of a turn), the rectangular opening in the Plug is no longer aligned with the inlet port and outlet point, hence flow stops immediately. Depending upon the materials of manufacture, the seal can be bubble tight which is an advantage over Gate Valves.  

Because Plug Valves can achieve a bubble tight seal, they perform better than Gate Valves in low PV Pressure applications (in fact, Gate Valves can leak).

However, Plug Valves are not practical in high PV Pressure applications due to the increased torque needed to rotate the Plug; even with an automatic actuator, the power cost to operate a a Plug Valve is significantly greater than that for a Gate Valve.

As PTOA Readers and Students would suspect, the intentional constriction in flow caused by boring an aperture that is less than the process line diameter creates a significant Pressure Drop. The Plug Valve would only be preferred over the Gate Valve when the process service requires completely starting and stopping flow frequently. Plug Valves with openings closer to the inner pipe diameter accommodate more PV Flowrate and create less Pressure Drop.

Because the Stem of the Plug Valve is not exposed to the flowing fluid, Plug Valves can handle corrosive process streams better than Gate Valves or Ball Valves can.

In summary, Plug Valves are ideal for low-to-moderate PV Pressure service when a quick, tight shutoff to flow is critical.

Plug Valves are found in the transportation of slurries like mud and sewage and also for corrosive fluids. Like Ball Valves, Plug Valves can have multiple ports and be automatically actuated to swap flow directionally.

The next PTOA Segment features Valves that are selected to perform the following jobs:

• Regulating/Throttling the PV Flowrate.
• Preventing Backflow.
• Relieving the PV Pressure built up in liquids and gases.

 

TAKE HOME MESSAGES: The PV Flowrate is managed by Valves. Although there are many types of Valves, each Valve performs one of four jobs:

• Starting/Stopping Flow (including quick-shutoff Valves).
• Regulating/Throttling the PV Flowrate.
• Preventing Backflow.
• Relieving the PV Pressure built up in liquids and gases.

 

The flow element in the Valve determines which type of job the Valve will perform.

The selection of each type of Valve found in a processing facility is not random; the characteristics of the flowing fluid are considered as are the changes in PV Temperature and PV Pressure of the process stream. Most importantly, the Pressure Drop that will be caused as the fluid flows through the Valve impacts the Valve selection. Pressure Drop reduces the flowing PV Pressure of the fluid. The flowing fuid must maintain sufficient PV Pressure to be able to flow from an area of high PV Pressure to an area of lower PV Pressure.

Valves that Start and Stop Flow:

• Gate Valves with a flow element "Gate."
• Quick-Shutoff Valves:
  1. Ball Valves with a flow element "Ball."
  2. Plug Valves with a rectangular, tapered flow element that has a bored-out aperture which is at least 70% of the pipe's diameter.

 

The hardware components of Start and Stop Flow Valves includes an external Handwheel or Lever and a Valve Body that encloses the Stem, Flow Element (Gate, Ball, or Plug), and Seats.

The Gate Valve is operated manually by turning the Hand Wheel. The wheel is turned clockwise (aka "to the right") when closing and anti-clockwise (aka "to the left') when opening.

The Stem position of the Gate Valve reveals whether the Gate Valve is opened or closed. When fully open, the Gate Valve has lower Pressure Drop than either of the Quick-Shutoff Valves.

The Quick-Shutoff Valves can be actuated manually or automatically. Both types of Quick-Shutoff Valves can be automatically actuated in an on/off sequence dictated by a Programmable Logic Controller. 

Gate Valves have a small amount of leakage when seated. The Quick-Shutoff Valves can be manufacture to seal bubble-tight.  Both types of Quick-Shutoff Valves have an increased incidence of water hammer compared to Gate Valve operations.

The ISA Symbol for a Gate Valve looks like a bowtie. The ISA Symbol for a Ball Valve looks like a bowtie with an empty circle drawn in the middle of the bowtie. The ISA Symbol for a Plug Valve looks like a bowtie with a tapered rectangle (aka trapezoid) drawn in the middle of the bowtie.

©2024 PTOA Segment 0252

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

 

 

 

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