PDF

Control Valve Sizing for Flashing Services

1. Basic Math and Excel Skill
2. Basic understanding of Valves
3. Fisher FSM Software
4. Have a computer
5. Have a desire

How do you calculate control valve size?
Designing and sizing a control valve is a step-wise procedure which consists of the followings:
1.specify body/bonnet type, material, rating, connection type.
2.Size a control valve by FSM software so that Cv will be calculated.
3.Checking the condition(Flashing, cavitation)
4.determine trim charachteristic, material, type and leakage
5.specify actuatur type, failure mode and positioner
6.Select packing
7.Select a proper flow direction.

How do you calculate Cv for a control valve?
The equation for calculating the valve flow coefficient is Cv = Q √ (SQ/P).

How do I choose a control valve type?
When the pressure drop is very high or there is the risk of accumulation of solids, or the fluid velocity is extreme an angle valve is used. Charachterised ball valve is used when the fluid tends to crystallize or where a high Cv is required. Butterfly valves are used for services with large flow rate and low pressure drop (less than 5 bar). Single-seated globe valves is the standard type in sizes bellow 8 inch in non-severe services where the shutoff pressure and pressure drop can be handled cage guided globe valve shall be used for more rough services, balanced trim can be used for bigger sizes.

In this part you understand how to specify body/bonnet type, material, rating, connection type.

Overview

Valve Body: The main pressure boundary of the valve that also provides the pipe connecting ends, the fluid flowpassageway, and supports the seating surfaces and the valve closure member. Among the most common valve body constructions are: single-ported valve bodies having one port and one valve plug; double-ported valve bodies having two ports and one valve plug; two-way valve bodies having two flow connections, one inlet and one outlet; three-way valve bodies having three flow connections, two of which can be inlets with one outlet (for converging or mixing flows), or one inlet and two outlets (for diverging or diverting flows). The term “valve body”, or even just “body”, is frequently used in referring to the valve body together with its bonnet assembly and included trim parts. More properly,
this group of components should be called the valve body assembly.

Bonnet: The portion of the valve that contains the packing box and stem seal and can provide guiding for the valve stem. It provides the principal opening to the body cavity for assembly of internal parts or it can be an integral part of the valve body. It can also provide for the attachment of the actuator to the valve body. Typical bonnets are bolted, threaded, welded, pressure sealed, or integral with the body. This term is often used in referring to the bonnet and its included packing parts. More properly, this group of component parts should be called the bonnet assembly.

Control Valve End Connections
The three most common methods ofinstalling control valves into pipelines are by means of screwed pipe threads, bolted gasketed flanges, and welded end connections.

Valve Body Materials
Body material selection is usually based on the pressure, temperature, corrosive properties, and erosive properties of the flow media. Sometimes a compromise must be reached in selecting a material. For instance, a material with good erosion resistance may not be satisfactory because of poor corrosion resistance when handling a particular fluid.
Some service conditions require use of exotic alloys and metals to withstand particular corrosive properties of the flowing fluid. These materials are much more expensive than common metals, so economy may also be a factor in material selection. Fortunately, the majority of control valve applications handle relatively non-corrosive fluids at reasonable pressures and temperatures. Therefore, cast carbon steel is the most commonly used valve body material and can provide satisfactory service at much lower cost than the exotic alloy materials. Specifications have been developed for ordering highly corrosion-resistant, high-alloy castings. These specifications represent solutions to problems encountered with some of those alloys. These problems included unacceptable corrosion resistance compared to the wrought materials, weldability issues, and unacceptable lead times. These alloys are also difficult to cast. The specifications include a foundry qualification process, dedicated pattern equipment, pattern alloy qualification, heat qualification, and detailed controls on raw material, visual inspection, weld repairs, heat treatment, and nondestructive testing.

In this part you get know how to Size a control valve by FSM software

You become familiar with flashing services and its impact during design and normal operation

Overview

Flashing damage is characterized by a smooth, polished appearance of the eroded surfaces. To review, flashing occurs because P2 is less than Pv. P2 is the pressure downstream of the valve and is a function of the downstream process and piping. Pv is a function of the fluid and operating temperature. Therefore, the variables that define flashing are not directly controlled by the valve. This further means there is no way for any control valve to prevent flashing. Since flashing cannot be prevented by the valve the best solution is to select a valve with proper geometry and materials to avoid or minimize damage. In general erosion is minimized by:

Preventing or reducing the particle (liquid droplets in this case) impact with the valve
surfaces Making those surfaces as hard as possible

Lowering the velocity of the erosive flow

Selecting a valve with as few fluiddirectional changes as possible providesthe least number of particle impacts.

Sliding-stem angle valves are traditional solutions which provide such a flow path. Some rotary valves, such as eccentric rotary plug, and segmented ball valves, also offer straight-through flow paths. Valves with expanded flow areas downstream of the throttling point are beneficial because the erosive velocity is reduced. For those areas where the fluid must impact the valve surfaces, at the seating surfaces for example, choose materials that are as hard as possible. Generally, the harder the material the longer it will resist erosion. Fluids that are both flashing and corrosive can be especially troublesome. Flashing water in a steel valve is an example of the synergistic result of both corrosion and erosion. The water causes corrosion of steel and the flashing causes
erosion of the resultant, soft, oxide layer; these combine to create damage worse than either individual mechanism would. The solution in this case is to prevent the corrosion by selecting, as a minimum, a
low-alloy steel.

In this part you learn how to determine trim charachteristic, material, type and leakage

overview

Trim: The internal components of a valve that modulate the flow of the controlled fluid. In a globe valve body, trim would typically include closure member, seat ring, cage, stem, and stem pin

Flow Characteristic: Relationship between flow through the valve and percent rated travel as the latter is varied from 0 to 100%. This term should always be designated as either inherent flow characteristic or installed flow characteristic

Inherent Flow Characteristic: The relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel with constant pressure drop across the valve.

Installed Flow Characteristic: The relationship between the flow rate and the closure member travel as it is moved from the closed position to rated travel as the pressure drop across the valve is influenced by the varying process conditions.

Seat Leakage: The quantity of fluid passing through a valve when the valve is in the fully closed position and maximum available seat load is applied with pressure differential and temperature as specified.

Know how to specify actuatur type, failure mode and positioner

Overview

Actuator*: A pneumatic, hydraulic, orelectrically powered device that supplies force and motion to open or close a valve

Actuator Assembly: An actuator, including all the pertinent accessories that make it a complete operating unit.

Fail-Closed: A condition wherein the valve closure member moves to a closed position when the actuating energy source fails.

Fail-Open: A condition wherein the valve closure member moves to an open position when the actuating energy source fails.

Fail-Safe: A characteristic of a valve and its actuator, which upon loss of actuating energy supply, will cause a valve closure member to be fully closed, fully open, or remain in the last position, whichever position is defined as necessary to protect the process and equipment. action can involve the use of auxiliary controls connected to the actuator.

Positioner:
Actuator and positioner design must be considered together. The combination of these two pieces of equipment greatly affects the static performance (deadband), as well as the dynamic response of the control valve assembly and the overall air consumption of the valve instrumentation. Positioners are used with the majority of control valve applications specified today. Positioners allow for precise valve assembly response, as well as online diagnostics when used with a conventional digital control system. With the increasing emphasis upon economic performance of process control, positioners should be considered for every valve application where process optimization is important. A positioner can be thought of as a high proportional gain device. When combined with an actuator and valve, the assembly will ideally behave like a first order or underdamped second order system, depending on use and intended performance. A digital valve controller has additional tuning parameters, such as derivative gain, which largely exist to remove undesirable characteristics and further tune the assembly to the desired performance. Many positioners also include an integral capability to remove any offsets between valve set point and position. Under most process control situations, this feature can be turned off to avoid the possibility of forming slow process oscillations, as the offset between valve position and set point is typically handled by the process controller. Once a change in the set point has been detected by the positioner, the positioner must be capable of supplying a large volume of air to the actuator, making the assembly move in a timely and controlled action. This ability comes from the high-gain positioner and is a function of integrated pneumatic booster within the positioner. This pneumatic booster is typically comprised of a relay or spool valve. Typical high-performance, two-stage positioners use pneumatic relays. Relays are preferred because they can provide high gain that gives excellent dynamic performance with low steady-state air consumption. In addition, they are less subject to fluid contamination. In addition, some large or high-friction actuators may use additional external boosters to meet specifications, such as stroking speed.
Positioner designs are continuing to improve by decreasing air consumption and advancing the diagnostic capabilities accessible to users. In addition, features have been added to support advancing industry safety requirements such as safety instrumented systems (SIS) and optimized digital valves.

In this part you are taught how to Select packing

Overview

The selection of stem (or shaft) packing for a control valve must be done taking into account very important factors such as:
temperature of process fluid
characteristics of fluid
working pressure

The temperature of process fluid is the most important parameter which affects the selection of type of packing. It fixes in fact which materials can be used for packing construction independently from other factors.
For example pure PTFE cannot be used for continuous duty when temperature is higher than 200 °C as well as pure graphite is mandatory when temperature exceeds 250 °C.
The characteristics of fluid may be determinant in the selection of valve packing when controlled fluids are highly abrasive or viscous. In these particular cases the lifetime of packing can really be very short if special reinforced packing rings are not used.
High working pressures (above 100 bar about) make necessary to select stiffer packing rings which are not easily subject to be extruded or to be excessively packed. Pure PTFE V-rings for example can be used up to a maximum working pressure of 150 bar, while for higher pressures glass loaded PTFE V-rings have to be selected.
On vacuum service packing rings have to grant a perfect sealing to avoid the reduction of the efficiency of various connected equipments.
The necessary qualifications of a valve packing for a control valve, are in order of importance as follows:

1) sealing capability
2) low friction
3) lifetime
4) wide range of use

In last part the trainer teaches how to Select a proper flow direction.